F rcadcast Maintenance Nandbcck
F rcadcast
EnUíneerinQ &
Maintenance
Nandbcck
No.852
$19.95
I rcadcast
Eneineerine &
Maintenance
1- landbcck
f
Uv !Patrick S. ìnneean
'73933
MTAB BOOKS
Blue Ridge Summit, Pa.
17214
FIRST EDITION
FIRST PRINTING-SEPTEMBER 1976
SECOND PRINTING-APRIL 1980
Copyright © 1976 by TAB BOOKS, Inc.
Printed in the United States
of America
Reproduction or publication of the content in any manner, without express
permission of the publisher, is prohibited. No liability is assumed with respect
to the use of the information herein.
International Standard Book No. 0- 8306 -6852 -8
Library of Congress Number: 76 -24783
www.americanradiohistory.com
Preface
Radio is a very important part of life today. Since its
beginnings in the early 1920s, broadcasting has undergone
many changes. not only in its style of operation. but in its
adoption of new technologies as well.
Although AM broadcasting is the older service -FM. after
its faltering start in the late 1940s- caught its second wind in
the 1960s. and today it is an important broadcast service. While
different transmission techniques are required of AM and the
FM stations, in every other aspect they are the same. In this
book. they are treated as one
radio broadcast station.
The role of individuals in broadcasting. including
engineers. has also changed. While operational changes and
new technologies have on the one hand, reduced the actual
number of engineers required at a station. the need for greater
knowledge and more maintenance by those remaining has
increased.
From personal experience of 30 years as a broadcast
engineer. I know that when problems develop or it is necessary
to do major maintenance in the middle of the night. the
broadcast engineer needs practical answers to problems
rather than theoretical exercises. In this book. the material is
presented with a practical view, little mathematics, and
hands-on theory concepts. The engineer in the field needs to
"put a handle" on the theory so he can produce practical
-a
results. And in the end, correct results are what count; and not
the theories that led to these results.
My book views the station as a complete system and
encourages the engineer to view his station and technical
problems that develop in the same manner. So that
maintenance is not done haphazardly or inefficiently, methods
are suggested for planned routine maintenance that work for
the engineer. Some general theory is presented on how
equipment works, but the main emphasis is on operation and
maintenance, including methods of performing required
annual audio proof -of -performance tests.
Patrick S. Finnegan
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Contents
1
Typical Broadcast Systems
9
Combined Studio-Transmitter -Separate Studio-Transmitter
Sites-Type of Services-Style of Operation- Stereo Operation
Systems
Systems -Peripherals- Automation
Antenna
Complexity & Maintenance-System Division -The Master
System-Common Divisions -The
Transmitter Area- Antennas
Connecting
2
Maintenance Techniques
3
Planning and Installation
Troubleshooting -Priorities-Develop
-Link -
36
Method- Practical
Problems-Signal Tracing-Routine Maintenance-Updating
a
-
70
-
103
Viewpoint -Audio Connections
Equipment Placement-Rack Space- Wiring -GroundingSurge
Protection -Installation- Identifying Cables -Jack
Fields Jumper Connections -Wire Dress -Fan -In- Lacing
Drawings- Operator's
-
4
Audio Characteristics and Problems
Level Setting -Distortion -RFI- General Causes of Distortion
Impedance
terference
Matching -Pads
5
The Control Room
6
Peripherals
7
Remotes
8
Program Automation
and
Transformers -RF
In-
145
Console- Console Installation- Modifications to ConsoleConsole
Setup -Console
Maintenance- MicrophonesTurntables -Audiotape Machines
190
Newsroom-Recording Booth -Teleprinters -Audio
Systems-Constant- Voltage Systems
Land
Lines -Radio Links -Portable
Interconnecting Link -RFI
The System -MOS Program Controller
Monitor
Equipment
-
227
265
9
The Transmitter
10
Coax Transmission Line
291
Basic
Transmitters -Tuneup-Power Measurement -AM
Power -FM Power- Frequency Measurement -Signal Processing-General Maintenance
-
343
and Antenna
11 Tower
The AM Antenna -FM Antennas -Towers -Standard Lighting-
373
Characteristics -Derating-Selection of Transmission Line
Installation Pointers -Installing Rigid Line -Installing Flexible
Line -Line Maintenance
High- Intensity Lighting-Tower and Antenna
Protection-General Maintenance
Icing- Lightning
Required Inspections
12
407
Inspection- Quarterly
Equipment Inspection -Annual Inspection
Transmitting- Equipment
13
Tests and Measurements
14
Proof of Performance
Lighting -
Equipment- Measurement Basics-Procedures -Test
Equipment Maintenance-Useful Techniques
427
Test
Common Aspects -The AM Proof -FM Monaural
Proof
453
Proof- Stereo
Appendix A
Simple Resistor Branching Pad
503
---
in Tandem -3-Into-1 Arrangement- Bridging Pads
Photoresistor as Gain Control- Recording From Regular
Telephone Line-Cassette or Mobile Receiver Matching
Terminations-Transformers on Jacks -Mike Cord Tester
Capacitors as Timers-Capacitor as Momentary Start Circuit
Diode as Gate -Diode as Transient Suppressor -Alarm Mute
Circuit -Alarm System- Remote Microphone Switch
Turntable Remote Start Switch -Alarm With IC Interface
Audio Tape Machine Automatic Rewind-Relay Power Supply
Pads
Appendix B
Useful Formulas for Broadcast
Engineers
522
Index
526
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Chapter 1
Typical Broadcast
Systems
Every broadcast station is shaped by its market, economic
forces, programming type and style, environment, and many
other factors. These factors result in each station becoming a
unique entity. It is very doubtful that a typical or average
station could be described with any degree of accuracy, but
even though each station is unique. all stations have many
things in common. And one thing all do have in common is the
need for technical maintenance.
Because of this uniqueness of each station, the engineers
charged with the installation, maintenance, and operation of a
station must be able to shape general principles into a form
that apply at that particular station. And to apply the general
principles to that station. the engineer should have an
understanding of its technical system.
In the discussions that follow, some of the factors which go
into the shaping of a station are discussed briefly, and then a
method is explained which can help the engineer bring the
technical operation of his station into better perspective.
The physical arrangement of the station and many
operational factors dictate some of the basic equipment
requirements. These also contribute to the technical
complexity of the station and the maintenance problems.
COMBINED STUDIO-TRANSMITTER
This arrangement is popular at many stations (Fig. 1 -1),
but all can use it. All of the studio and transmitting equipment
9
STATION BUILDING
Fig.
1
-1. Combined studio- transmitter
arrangement. This is the simplest
form and requires the least equipment.
is housed under one roof with the antenna nearby. The control
room, studios, and transmitter are all clustered together for
ease of operation. The transmitter may be located in the
control room itself, or in an adjacent room where it can be
observed through a soundproof window. This arrangement is
the most efficient in operation and requires the least amount of
equipment. Also. the job of maintenance and making
measurements is easier to perform.
There are at least two variations of the combined
studio -transmitter arrangement Fig. 1-2). Although the studio
and transmitter are combined under one roof, the transmitter
may be located in another room or on another floor. The
transmitter is no farther than 100 ft nor more than a floor away
from the operator. Whenever the transmitter is moved away
from direct visual and manual access by the operator,
additional equipment is required and the system's complexity
increases. Maintenance problems increase in direct
1
proportion. The transmitter must have extension meters, and
there must be a wired control so the operator can monitor and
operate the transmitter.
In the second variation, the transmitter may be over 100 ft.
and several floors away. Once the transmitter has been moved
out past the limits described in the first variation, the station
requires special FCC authorization, and a regular remote
control unit must be used. Such arrangements are often found
where tall buildings are available. The transmitter is located
on the top floor and the antenna on the roof of the building. The
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(A)
STATION BUILDING
CONTROL
ROOM
STUDIO NEWS
TRANSMITTER
NOT OVER 100' AWAY
51
(B)
7TH
TALL
BUILDING
2ND
1ST FLOOR
-2. Two variations of the combined studio- transmitter arrangement.
the transmitter is not over 100 feet away nor more than one floor
away. In B, we have the second variation, in a tall building. This requires
FCC remote control authorization.
Fig.
1
In A,
building supplies tower height, and the use of a floor close to
the tower will reduce the length of transmission line needed.
SEPARATE STUDIO- TRANSMITTER SITES
This is a very common arrangement IFig. 1 -3). The
studios are located at some convenient place in the city, while
the transmitter and antenna may be several miles away,
usually out in the country. This is also the most complex of the
arrangements and requires considerably more equipment.
Maintenance and measurements are more difficult to
perform, and travel time often becomes a factor. In those
stations that do not have operators on duty at the transmitter
site. security of the building also becomes a factor. Unmanned
11
r-----
SEVERAL MILES
---
-
TRANSMITTER
V
STUDIOS
CONNECTING
LINK
(TELEPHONE CIRCUITS OR
MICROWAVE)
Fig.
1
-3. A separated studio-transmitter arrangement. This is the most
complicated and entails considerably more equipment and maintenance
difficulties.
transmitters must have an FCC remote -control authorization
and an approved remote -control unit in operation.
The two sites must have an interconnection link. There
must be an audio circuit, a circuit for controlling functions of
the transmitter. and provision for monitoring transmitter
parameters. The interconnecting link is often provided by the
telephone company, but it may also be a station -owned
microwave link.
TYPE OF SERVICES
AM and FM stations. while both engaged in broadcasting,
operate in different services. Everything at the transmitter
and beyond is also different. So the service affects the
equipment required. and maintenance problems are also
different. The audio equipment can be the same for both types
of stations, as long as the FM is a monaural service.
STYLE OF OPERATION
The -live" style of operation will dictate a certain number
of equipment items as well as their arrangement. This type of
operation is used by many stations. Typically, the majority of
programming comes from records played on turntables in the
control room. operated by a combo man lone who also acts as
an announcer). In another live situation. there may be both an
announcer and an engineer performing the work in two
different rooms -that is. an announcer's booth and a control
room working together. The combo arrangement usually
requires the least amount of equipment.
12
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STEREO OPERATION
For FM stations that operate in stereo, the equipment
requirements literally double from those of monaural
operation. And stereo places more exacting demands on
equipment performance and operating practices. This also
creates numerous maintenance problems. There must also be
special stereo monitoring equipment and an FCC approved stereo modulation monitor for the transmitter. And
to convert the left and right audio channels to a composite
signal which modulates the transmitter, a stereo generator is
required.
At this time there are two additional stereo proposals
before the FCC. but as yet there has been no approval. These
are for quad stereo and AM stereo. The quad method uses four
channels instead of the present two. Quad is being transmitted
now, but it is converted to two-channel for transmission and
then converted back to quad at the receiver.
The proposal for stereo on AM is for two- channel system.
The main carrier would be amplitude modulated as is now
done, and the subchannel would phase modulate the carrier.
ANTENNA SYSTEMS
The antenna system of a station is often a very strong
determining factor in how the station is arranged and the
equipment to be used. Besides the technical requirements,
there are often site problems. And when an AM station must
use a multitower. directional antenna system, the equipment,
problems. and maintenance all multiply rapidly.
The FM antenna must have relatively great height
because of the transmission characteristics at VHF The
antenna itself is relatively small, so it must have some tower
or other supporting structure for height. The antenna may be
mounted on a short pole above a tall building, or on its own
tower mounted on the ground. In either case, a transmission
line is necessary to cary the RF from the transmitter to the
antenna. Long lines in not -too- accessible locations make it
difficult to perform maintenance, and when repairs are
necessary. rigging equipment and tower crews must be called
in to do the work.
While the tower is the supporting structure for the FM
antenna, in AM the tower itself is the antenna. Although many
stations can operate with only a single tower, a great many of
13
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them must use a multitower directional system. These
systems require several towers working togeher to obtain the
required radiation pattern. Still other stations use two
different directional patterns, one for day and the other for
night operation. The tower is actually only half of the antenna.
The other half is the ground system underneath the tower,
made up of many copper wires fanning out from the base of the
tower at least as far from the tower as its height. These can be
simple two -tower arrays or multitower arrays. They all
increase the stations complexity.
To demonstrate how various factors can shape the station
and
into a unique individual and affect its operation
complexity. one station with which I had a special problem
required two different directional patterns for its day and
SITE
1
DIRECTIONAL
ANTENNA
FOR DAYTIME
PATTERN
SITE 2
D RECTIONAL
TRANSMITTER
ANTENNA
FOR NIPATTERN
2
transmitter
station with a real problem. It takes two separate
apart.
miles
11
sites and antenna arrays to solve it. The two were located
Fig.
14
1
-4. A
night operations. It wasn't possible to create these two
patterns from the same set of towers at this site (Fig. 1-4).
Consequently. it had to erect an additional multitower antenna
system at another site 11 miles away! There are two different
transmitter sites, one for the day operation and one for the
night operation. Fortunately. there are only a few stations with
problems such as this.
Directional antennas take up a lot of real estate. which is
one of the reasons that most of them are located out in the
country. There are many stations today that operate with a
directional system within the city limits, but in most cases
these stations had origionally built out in the country and the
city grew out past them.
PERIPHERALS
Peripheral operations abound at any station. These are
programming activities that cluster around the core of the
station and make a heavy contribution to the station's
programming. One important area is the news -gathering
operations and the news room. Newsrooms today are often
very well equipped with many electronic items.
Not necessarily associated with news, but often used by
the news department, are the mobile remote pickup
transmitters. Some stations have several of these units. The
remote pickup requires its own peculiar methods of
maintenance, and it generates its own problems as it also
involves a somewhat different technology.
Additional program services -such as connection to a
national. regional. or sports network, and the station's own
remote broadcast arrangements-contribute to the station's
complexity.
AUTOMATION SYSTEMS
Program automation systems, whether used on a
part -time or full -time basis, rapidly multiply the needed
equipment units, and if the system operates in stereo Fig. 1 -5)
doubles its complexity and maintenance problems. When all
the station's programming passes through an automation
system, there is a need for recording booths for local
production purposes. How well these are equipped depends
upon how much of the automated programming is created at the
station. In a station that uses full automation of its
(
15
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programming. there usually isn't a control room as this term is
used in the general sense. One of the production booths must
have some arrangement so that it can be placed on the air as a
substitute for the automation system when the system fails.
COMPLEXITY AND MAINTENANCE
It should be very evident that whenever a station grows
more complex for any reason there is a direct growth in the
number of equipment items in use. And along with this growth
in complexity and equipment numbers, there is a greater
mumber of equipment failures. This is because there are just
more things that can go wrong. Consequently, there is a need
for more and better maintenance.
SYSTEM DIVISION
Although the discussions in this book must, of necessity,
deal in generalities, the engineer on the job must always try
relate these general discussions to the particular station for
which he is responsible (Fig. 1 -5). Even though one station
Program automation systems increase the station's complexity
and maintenance. Shown is a major program system by Systems Marketing Corp. (Model 3060), using a sequential programmer.
Fig.
1
-5.
16
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may have the same equipment lineup and may operate
similarly to another, they will not necessarily have the same
problems. Always remember that each station is unique. Each
of its parts has its own stresses, wear, and possible abuses.
For example. one station may have an operator who programs
the automation computer keyboard with a feather touch, but
another station may have an operator who pounds on the
keyboard as if it were an ancient typewriter. It goes without
saying that the latter station will soon have to replace keys on
the keyboard.
So that engineer can develop a better understanding of his
own station and its operation. the overall system should be
divided into many separate parts. This does not mean that the
equipment should be physically rearranged or even that the
divisions need be written down on paper. However, drawing
the divisions out in simple block diagram form could prove
helpful, especially when new members are added to the staff.
But the divisions should essentially be done in the engineer's
mind.
THE MASTER SYSTEM
This is a term that can be applied to the station's complete,
overall electronic operation, from its microphone inputs to its
antenna outputs. Everything within this system must interface
properly and work together as a single unit to produce the
station's signal. This master system is made up of many parts
that all dovetail together. And this master system can be
divided into subsystems in a descending order of importance
to master system. See Fig. 1 -6.
Major Subsystems
The first division of the master system is the major
subsystem. There can be several major subsystems of equal
importance. A major subsystem can be described as an
operational area in which a number of units work together to
create a somewhat independent product or activity. One
example would be the studio area of a station. All of its various
subsystems work together to create the station's audio. This
product is generally used to modulate the transmitter, but it
can also be used for other purposes. It could, for example, be
fed to a tape recorder. The recorded program could be sent to
other stations or reserved for use at another time.
17
This same thinking can be applied in division of the master
system into other major subsystems according to the
particular station arrangement. But do not have divisions into
major subsystems, than the particular situation warrants.
MASTER
SYSTEM
_1_
_L.__
_1_ _1_
MAJOR
SUB -SYS
MAJOR
SUB -SYS
MAJOR
SUB -SYS
T
T
ETC.
TETO.
MINOR
SUB -SYS
IMIN
S
MIN
BI
MAJOR
SUB -SYS
TETO.
MINOR
SUB SYS
MI
S
M N
I
MIN
S
1 -6. Basic outl'ne of the master system division into subsystems. This
similar to a company's personnel organization chart. Subsystems at the
low end of the stack play the least role in the station's system.
Fig.
is
Minor Subsystems
Each major subsystem can then be divided into minor
subsystems. and, again. each of these ranked according to the
importance of its role within that major subsystem. And each
minor subsystem can be further divided into its own
subdivisions. This dividing and subdividing creates a pyramid
structure, all narrowing from a broad base to the single
master system at the apex.
Many individual items that are low ranked in this
particular arrangement are very expensive, high -quality units
that can stand as master systems in their own right -but in
other situations. A tape recorder, for example. is a complete
unit in itself and can stand alone. But when used in the control
room as a program source, it plays a lesser role and therefore
is ranked as a minor subsystem. Thus, use in the operation
determines the ranking.
18
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Maintenance Aids
Dividing the station into its various parts proves very
helpful to an engineer when problems develop and must be
corrected in a hurry. It also helps the engineer visualize the
total operation and not become confused by an apparent
variety of independent operations. When he understands the
total operation. he can quickly isolate problems to specific
subsystems. Then another minor subsystem may be
substituted. or bypass arrangements can be made so the
station can continue its programming with little interruption.
COMMON DIVISIONS
As was pointed out earlier, each station is unique, but all
have many things in common. In this chapter, we use the
division method and discuss some of the equipment found in
the various subsystems of a station (Fig. 1 -7). In later
chapters, those various equipments are discussed in more
r-
detail.
- - - -- - -MASTER SYSTEM
I
1
I
I
STUDIO
AREA
I
CONNECTING
LINK
TRANSMITTER
AREA
ANTENNA
SYSTEM
I
I
1
1
(ACTIVITY FLOW FROM LEFT TO RIGHT)
Fig. 1 -7. The most common division of any station into four major subsystems.
Any station's master system can be subdivided into at
least three or four major subsystems. Some stations may have
more, but all will have at least this many. These major
subdivisions are: studio area, connecting link, transmitter
area, and antenna system.
THE STUDIO AREA
This major subsystem is composed of a variety of minor
subsystems, surrended by many peripheral subsystems. all
working together to produce the station's programming ( Fig.
1 -8). Further subdivision can be made in this manner: control
room and its minor subsystems; recording booth and its
19
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subsystems; second recording booth (if used); audio
processors (if used at this location) ; peripheral systems,
starting with the newsroom and its subsystems; remote pickup
base transmitter and mobile units; network connection,
regional network, and sports network: local remote lines and
QKT circuits (voice coupler) ; and EBS l emergency broadcast
system) equipment.
This is only a rough division, and each station should set its
own order of priorities according to the actual equipment it
has in use.
Control Room
This should be the highest ranking subsystem in the studio
area of any station that uses a control room. (Automated
stations may not.) In the control room, the highest ranking
equipment item should be the control console. All other
subunits feed into this console. there the programming
material is mixed. blended. and controlled in the desired
manner to produce the finished program product. The console
is often referred to as the control board, or simple board.
Microphone System
Microphones originate a considerable amount of
programming. whether live or recorded. For the live -tape
operation. in which the main announcers work out of the
control room, the microphones rank as a minor subsystem.
There may be one or two located in the control room, one or
two located in an announcing booth, and one or more located in
each studio that is in operation. All of these feed directly to the
console and are controlled by it. Microphones are low -level
devices, so they must have preamplification. The
preamplifiers are generally located in the console itself,
although in some cases they are located in a rack.
Turntables
The second major source of programming is the
turntables. There are a minimum of two turntables in the
control room. It is extremely clumsy for a disc jockey to do a
music show with only one turntable.
Although the term turntable is often applied to this
program source, it is really a system made up of several
subsystems: the turntable itself, drive meter, cabinet,
cartridge and stylus, tone arm, equalizer, and preamplifier.
21
The signal output of the pickup cartridge is a low -level
signal that requires amplification. The usual practice is to
locate the preamplifier and equalizer in the turntable cabinet
itself, close to the tone arm. The equalizer may be a separate
unit, or it may be built into the preamplifier. The preamplifier
brings the low -level signal of the cartridge up to program level
of +8 dB (decibel) or the level desired.
Tape Recorders
Another important program source for the console is the
tape machines in the control room. Tape machines may be of
the open -reel type or the cartridge type. The cassette recorder
is another type that is increasingly common.
In most cases, the open -reel machine is basically a
two -system machine. That is. it is both a recording machine
and a playback machine. It is the playback section of the
machine that is used as a program source for the console. It
may play tapes that have been recorded on this machine, other
machines, or tapes that have been sent to the station. The
playback amplifier can provide normal +8 dB program output
levels.
The recorder section of the machine also has much use in
the control room. The input to the recorder is often a selector
switch so that the recorder can record from many sources.
Cartridge tape machines are newer than the open -reel
machines. and all stations have two or more of these machines
in use today (Fig. 1 -9). Besides the different method of
handling the tape. the configuration of these machines is often
different. That is, many are playback -only machines which
can play only prerecorded tapes. And there are often
multideck machines that incorporate more than one playback
section in the same unit. In the multiple units. some of the
functions are shared. such as the cabinet. drive meter, etc.
Ordinarily, each has its own head, playback amplifiers, and
control circuit. When multideck units are employed, each slot
is often referred to as a tray. For example, a machine may be
a 3 -tray unit or a 48-tray unit. The output of each of these trays
or each unit is normally at program level of +8 dB.
A cartridge tape system is somewhat limited if only
playback machines are used. So there is one or more recording
units in the station, although the control room may have only
one.
22
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Cartridge tape equipment comes in many styles, from the
single tray unit to multitray units such as the24 -tray model shown here.
(Courtesy Systems Marketing Corp.)
Fig.
1
-9.
Peripherals
As mentioned earlier, there
are usually many peripherals
in the studio area. They vary from station to station, not only
in type. but in numbers. Most stations have a newsroom. And
how large this area is depends upon how much the station does
in the news area. A major newsroom has a small console or
mixing arrangement along with open -reel and cartridge tape
recorders. There s a variety of electronic sources used in the
news -gathering process; some may be portable and others are
telephone circuits in the station. Most of these sources are
channeled through the small console or mixer, so that editing
can be done and completed news tapes produced. In many
stations. the newsroom is well equipped so that it acts as a
subsidiary control room for news programming. That is. when
it comes time for the news. the newsroom is switched into the
main control room as though it were a remote program. and
the news is presented from the newsroom itself.
23
There are many other items that are helpful in the
news -gathering process, such as receivers that monitor police,
sheriff, and fire channels. There also may be a base
transmitter -receiver located here for use of the remote pickup
mobile units for news stories.
Remotes
Many program sources originate outside the station and
are brought into the station over telephone circuits, or there
may be remote pickup stations. Each of these is fed into the
console in the control room for the switching, blending, and
other processing needed. Programs may originate at stores,
auto showrooms, fairgrounds, or any place that makes a good
program or news source. These wire circuits are called
broadcast loops and are leased from the telephone company for
the occasion. Some may be equalized, others not, depending
upon the quality of circuit required.
QKT Circuit
This type of circuit has become popular in recent years
because it can be less expensive on a long- distance broadcast
than regular broadcast loops. The QKT, also called a voice
coupler, is supplied by the telephone company. It is a regular
telephone that contains either an exclusion key, or a cutoff
key, and a transformer to connect the station's remote
amplifier to the telephone circuit. The broadcasts are handled
as regular long-distance telephone calls. At the studio, it is
necessary to arrange a connection to the telephone company
circuit for the phone number in use, that is, a connection to the
console.
Signal Processing
There may be several processors in the studio area. These
are devices such as AGC amplifiers, peak limiters, equalizers,
etc. They may be used at any place this action is required. In
combined operations these follow the console output, but in
separated operations they may be placed at either or both ends
of the connecting link.
Processors are also used with production booths, and AGC
amplifiers are often used to control incoming levels from a
remote line. Here again, the use and location of the processor
determines its ranking in your divisions.
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Other Program Sources
There may be a variety of program sources feeding the
console besides the equipment in the control room. Each
station would apply a ranking to the source according to its
importance to the station. That is, if the station carried a very
heavy schedule from a sports network, this network is given a
higher ranking that an occasional remote. Other sources can
be from a sister AM, FM, or TV station the newsroom, or a
production booth acting as a subsidiary control room.
EBS Equipment
are required to have a receiver that receives
the EBS alerts from key stations, and they must also send out
EBS tests each week. The new FCC rules. which went into
effect April. 15. 1976. require that a dual -tone system be used.
That is, two tones -853 Hz and 960 Hz-are transmitted for 22
seconds. The receiver must be in operation all the time the
station is on the air and must set off alarms when the alerting
signal is received. There have been many changes in this
system since its origination.
All stations
THE CONNECTING LINK
The second major subsystem that some stations have is
the connecting link between the studio and transmitter. For
stations with the transmitter in the control room, or in the next
room, the connecting link is simply a pair of audio cables. A
simple arrangement like that is hardly considered a major
subsystem. But many stations have their transmitter out of
sight and out of reach of the operator. The connecting link then
does become a major subsystem.
Whenever the transmitter is far away from the operator,
he must be able to monitor and control it, as well as feed the
audio signal to the transmitter for modulation. There must
always be an audio feed from the studio to the transmitter for
the program audio. So let's consider that first.
Audio Circuit
The audio circuit is perhaps the most important subsystem
of the link. The quality of this circuit must be as least as good
as that required of the rest of the station itself. The
audio -frequency response. distortion, and noise are the basic
characteristics, although line losses are also a factor. A simple
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way to remember the quality of line needed is to consider the
limits allowed when making a proof of performance. All the
system audio must pass through this circuit. These are usually
telephone lines that are equalized. And if the station transmits
stereo, two identical circuits are needed for the left and right
audio channels.
Transmitter Control
The operator must be able to do everything almost) to the
transmitter at a remote location that he can do to it while
standing in front it-at least the controlling functions of
turning it on and off. raisng power. etc. This involves a
remote -control system. There are different models of
remote -control units. and some of these use metallic telephone
circuits. There is a sending unit at the studio which controls
the different functions of the transmitter through a receiver
unit at the transmitter site.
(
Metering
parameters of the transmitter must be
metered. The parameter samples are sent from the unit at the
transmitter site to a receiving unit at the control point, where
they operate one or more meters. These devices usually scan
the samples in sequence. and the unit at the control point is
sychronized to this scan. In the simple units, these are
stepping relays that send DC direct current) voltage samples
back to be measured. A single pair of wires is needed.
All of the important
(
Sophisticated Units
Besides the simple control units, there are many modern,
sophisticated units. These are basically digital devices that
convert analog samples and control functions to digital
signals. These use (frequency -shift keying) of an audio
carrier signal to transmit information over the lines, if
telephone company pairs, or the modulator of an STL
studio-transmitter link) microwave unit owned by the station.
1
STL
As just mentioned, these are microwave links. Such links
are common in television stations and are becoming more
common for FM radio. They are not as yet very popular in AM
broadcasting. When the station uses its own microwave link,
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except for the quirks of Mother Nature on propagation, it
places the control of the connecting link back in the station's
hands. The STL, by the way. only replaces the telephone
circuits that would otherwise be used. There must still be a
remote -control unit for the transmitter.
Monitoring
Transmitter parameters must be monitored -also the
antenna system, tower lights, and building security system if
1
used) .
Transmitter modulation must be monitored by an
approved modulation monitor. When the station is operating
by remote control, the modulation monitor must be located at
the remote -control point. When placed at the remote -control
point, an RF amplifier is often required to increase the RF
signal level so that the monitor can operate properly. When
work must be done at the transmitter site that requires a
monitor. there can be some extra difficulty in doing the
maintenance.
Controlling and monitering the transmitter from any
distance can become a rather complex operation IFig. 1 -10).
When the more sophisticated remote -control units are
employed, and a STL. then other technologies become
involved: digital and microwave. When dividing by the
subsystem method, the rank must be established according to
that which is used at a station. Several of these technologies
can be of equal rank. But always remember that the ranking is
relative: it is designed to help the engineer quickly isolate
problems that develop, as well as aid his understanding of the
overall station operation.
TRANSMITTER AREA
This is the third major subsystem of the master system.
This equipment receives the audio from the studio area.
develops the RF carrier. impresses the audio on the carrier as
modulation, and sends it to the antenna system for radiation.
This area includes systems that could be classed as major
systems. but in a station they work together as part of one
major subsystem.
Transmitting Gear
transmitter is composed of many internal
subsystems. Transmitters differ according to make, model,
Any
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(A)
CONTROL
ROOM
TRANS-
MUTER
PAIR OF AUDIO WIRES
(B)
LESS THAN 100'
I-
CONTROL
ROOM
AUDIO PAIR
TRANSMITTER
IDIRECT WIRE CONTROL CIRCUITS
EXTENSION METERS
NOTE
1.
METERS MUST BE CALIBRATED
EACH WEEK.
NOTE 2. THERE IS MORE DIFFICULTY MAKING
MEASUREMENTS.
Fig.
1
-10. Moving the transmitter out of direct visual view and control by
the operator increases station complexity and requires more circuitry and
routines. In A, the simple system requires nothing more than a pair of
audio wires. In B. there must also be wire control circuits and extension
meters. The further away the transmitter is moved, the more complex it
becomes.
power range, and type of modulation. The transmitter is
usually a self-contained unit that requires only an audio input
and AC power input to provide a modulated RF output to the
transmission line.
AC
Power Input
Low -power units work on 230V AC. single phase, while
high -power transmitter employ three -phase 230V AC. Very
high -power units use 440V AC. this AC power is brought to the
building over high-voltage mains, then stepped down to the
230V by transformers on a pole or on the ground. It is fed to a
main power entrance. where there is fuse protection. Then it is
distributed throughout the building. One of these circuits feeds
the transmitter. In some of the older transmitters (and even
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some newer ones). the transmitter crystal is installed in an
oven; this even usually has a separate AC circuit. The power
feed should be through conduit.
Audio Input
The audio from the control room must be fed to the
transmitter audio input. This audio is usually run through
processors before it gets to the transmitter itself. There
usually is an AGC amplifier to control program levels and a
peak limiter right before the transmitter. There may also be
speech enhancers or other dyamic equalizers in the line also.
The AC and limting amplifiers may be combined in a single
unit or may be separate units. When the transmitter is at a
distance, these processors may be split up. or they may all be
at the transmitter site. Much depends upon individual
preferences and the particular situation. There are different
types of processors used for FM and AM, as well as a different
arrangement when stereo is in use. The same types can be
used, but today the ordinary AGC amplifiers and peak limiters
are giving way to more sophisticated ypes.
Peak limiters, designed for AM. switch the highest peak of
a cycle to the positive modulation side before sending it into
the transmitter. This allows the positive modulation to be
higher than 100%, and at the same time limit the negative side
to no more than 99% modulation. The output of these units are
polarized.
FM peak limiters must contend with the 75 microsecond
preemphasis in the transmitter. So some shape the audio to a
true preemphasis curve by clipping, if need be. so as to attain
a higher degree of modulation without overmodulating. Both
stereo AGC amplifiers and limiters are strapped together so
that they operate as a single unit, although controlling both left
and right audio channels.
Stereo Generator
With stereo, the audio signals must be processed through a
stereo generator before they are fed to the transmitter input.
The generator is a very important subsystem in the stereo
process. The composite output of the generator modulates the
transmitter-not the audio signals themselves. The generator
output contains both audio and supersonic signals that have
been specially processed.
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Monitoring
The monitoring of the transmitter can be classed in two
categories: the modulation and the parameters.
Modulation-There must be an FCC -approved modulation
monitor in operation. The AM monitor has provisions for
monitoring the positive or negative modulation envelopes,
provides audio output for speaker monitoring (after
amplification), and provides outputs for measuring audio
distortion during proof -of- performance measurements. The
FM monitor provides the same capabilities as the AM monitor,
except that the positive and negative modulation is not as
important. The FM station does not use asymetrical
modulation as do many AM stations. If the FM is stereo, there
must be an FCC -approved stereo monitor. This monitor has
many additional capabilities and provides many switching
positions and pads for making the stereo proof measurements.
All monitors will have provisions for remote metering of its
various functions necessary for the FCC -required monitoring
of percentage of modulation. These functions can be routed
over an extension- metering arrangement.
Parameters -Certain parameters of the transmitter must
be metered and logged. When on remote control, or if
extension meters are used, a sample of the parameter must be
provided. Most modern transmitters already have the
samplers built into them. Older transmitters, however. may
not have the samplers. and these must be added. The
output -stage power input -that is. voltage and current-and
the transmitter power output must be logged. There may be
other samplers provided also.
Transmission Lines
Once the RF signal has been generated and modulated, it
is sent to the antenna over transmission lines. Some stations
still use the old open -wire transmission line, but the majority
use coaxial line. Coaxial lines come in different diameters, are
either rigid or flexible, foam filled or air dielectric, and may
be bare on the outside or jacketed. Air systems may use dry
air or gas under pressure. Pressurization may be done by gas
cylinders or an air pump.
The length of the line between the transmitter and tower is
called the horizontal run. This may be suspended on posts or
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buried under ground. When the line feeds an FM antenna, it
must also continue up the tower. This is called the vertical run.
When a transmission line is not properly terminated,
standing waves are set up along the line. That is, some of the
transmitter power is reflected back to the transmitter. The
standing -wave ratio SWR) is the ratio between the forward
and reflected powers. If the voltage of the waves are used in
the calculations, the term is VSWR.
Standing waves on the line can cause damage to the line
itself or to the output stage of the transmitter. FM transmitters use a device that monitors this VSWR and shuts the
transmitter off when VSWR reaches a predetermined value.
1
Cooling Systems
Transmitters must have cooling for proper operation.
There is a subsystem built into the transmitter to provide its
cooling. The air or water temperatures in the cooling system
are sampled. and if the flow stops or decreases, an interlock
shuts the transmitter down.
Transmitters are also designed to function within certain
ambient temperature ranges, that is, operating temperatures.
Either end of this range may be uncomfortable for humans.
But if the ranges of air temperatures vary outside the
transmitter limits, cooling or heating must be added.
ANTENNAS
The antenna system is the final point where the station has
control over its signal. The antenna system is complex enough
to be classed as a major subsystem; this is particularly true of
the multitower AM directional systems. AM and FM antennas
are classed in different categories. Their treatment is
different because of the difference in frequencies. They
require different operating and maintenance practices.
AM Antenna
This may be a single tower, or many towers in an array.
The tower itself is the antenna, so its height has a definite
relationship to the station's carrier frequency. Thus a vertical
antenna is used, with the other half of the antenna system in its
ground system. Primary coverage in AM broadcasting relies
on the ground wave signals. The ground waves are more
limited in distance but are more reliable than sky waves. But
31
sky waves do enter the picture, and that is the reason stations
cut back power at sunset and change to directional patterns,
for the sky wave reaches farther after sunset.
Antenna towers may be either self -supporting or guyed.
When guys are used, they are segmented with insulators so the
guys do not affect the radiation of the signal. The guys are not
part of the antenna; they are only for support.
The antenna may be series fed or shunt fed. There are
some shunt -fed towers around today, but the majority are
series fed. In this method, the antenna must be insulated from
ground.
Coupling Units
When a series -fed antenna is used, a coupling unit must be
used to match the transmission line to the antenna. This unit is
usually a T- arrangement of coils and capacitors which
matches the impedance transmission line to the tower,
cancelling out any reactance of the tower itself.
In a directional system. each tower has a coupling unit,
but there are also power dividers and phasers that divide and
distribute the RF to the different towers in the array in the
amounts needed to obtain the desired pattern. The whole
system is interconnected with coaxial transmission lines.
Antenna Monitoring
In the single- tower, omnidirectional system. the power at
the base of the antenna must be measured as the station output
power. In a directional system. the power to the common -point
feed of the system is measured for power output. And the
phases and base current of each tower must be sampled and
fed back to a phase monitor so that the proper operation of the
antenna can be observed as an operational requirement. The
samplers for this are usually small loops mounted at the
appropriate place on each tower, or on poles away from the
tower, yet close enough to get an adequate sample. The RF
samples are fed back to the transmitter room over
small- diameter coaxial lines.
FM Antenna
This antenna is small in physical size because of the
carrier frequencies (VHF) used. Because of the small size,
many antennas are often stacked one above the other, suitably
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spaced. This arrangement provides power gain. Each one of
the units is called a bay; the whole array is the antenna. A
directional pattern is possible when antennas are stacked, but
the usual pattern is circular (but with increased power gain).
Although each bay may be small, a typical 12 -bay antenna can
measure 100 feet in length.
FM Propagation-Signals at VHF behave far differently
than those in the AM broadcast band. They tend to travel more
in the line of sight. although they do get far over the horizon.
They suffer more propagation losses. To overcome some of
these factors, the antennas are mounted as high as possible
within the limitations set by the FCC. This ordinarily requires
a tall steel tower, unless a suitable building or other tall
structure is available.
Weather Effects on FM Antennas-Weather affects the
FM antenna far more than it will the AM antenna. Heating and
cooling by the air or sun can cause the elements to expand or
contract and detune the antenna. The most serious problem is
sleet or ice forming on the antenna. This will seriously detune
the antenna and cause serious VSWR losses on the transission
line. To overcome this roblem, heaters are inserted in the
antenna elements to melt any sleet or ice. These heaters are
operated from 120V or 230V AC and are usually
thermostatically controlled by a unit mounted near the base of
the FM antenna. In some areas of the country, where icing is a
very severe problem. the entire antenna is often enclosed in a
radome.
Towers
Towers are used as supporting structures for FM antennas
or as the antenna for AM stations, or a tower may serve in both
capacities at the same time. Aside from the electrical
characteristics and consderations as antennas, towers are
large physical structures that require maintenance if they are
to remain standing for many years.
Both for greater visibility and weather protection, the
towers are painted. This painting and the colors used must
conform with the FCC rules. The painting is mainly for
visibility, but it does add increased weather protection. Other
methods. such as galvanizing, will contribute the major share
of weather protection. Weather is not the only thing the metal
must be protected from, for these are other chemical
33
elements. such as salt near the oceans and a variety of
chemical elements added to the air in industrial areas.
Also for visibility, the towers must be lighted, and the
lighting arrangement must conform to FCC standards. The
rules specify a different arrangement for different tower
heights, depending on whether the location is close to an
airport or air lane. The lighting system requires 120V AC, and
there must be one or more flashing beacons and side -marker
lights at different levels. These lights must be turned on at a
certain level of sky intensity (light), which is usually sensed by
a photecell. This lighting must be observed each day for
operation and then logged. If the tower cannot be visually
observed, then samplers must be included that send data back
to the control point to indicate that all lamps are operating
properly. A new method of lighting recently developed is
high- intensity strobe lighting. This type can be seen farther
than conventional lamps.
Lightning Protection -Of course, a tall tower makes a
likely target for lightning. Lightning develops tremendous
forces, and the best we can do is divert it to prevent damage.
Unless some precautions are taken, damage can result the
elements in the tuning section, transmission line, transmitter,
and transmitter building. Some very complex arrangements
have been developed to prevent lightning strikes. Old -time
remedies make use of lightning rods on the tower, ball gaps
across the insulator of an AM tower, and static -drain chokes.
Isolation-The AM tower must be insulated from ground.
But there are many metallic conductors that need to cross the
base-or at least the signals they carry must cross the
base-so special arrangements must be made.
Tower lighting and heaters for FM antennas require 120V
AC. There are two ways to cross the base with AC power. The
first is the large open -ring transformer. There is no connection
except through the magnetic flux of the transformer windings.
The second method feeds the AC power through large RF
chokes.
FM antennas may be mounted at the top of an AM tower.
The coax transmission line must cross the insulator at the
tower base. Again, there are two methods of doing this. The
preferable way is through an isolation unit. There is no direct
connection to this unit. It maintains the impedance
characterisic of the coax line. The second method makes use
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quarter -wave section of the coax line. The
quarter -wave length is figured at the FM carrier frequency.
When directional AM antennas are in use, the RF signals
from the samplers are fed back over small coaxial lines. These
must cross the base. Usually, large coils of the line are formed
in an amount that creates an RF choke at the AM frequency to
prevent shorting out the base. In another arrangement, the
sample loops are on small wooden poles on the ground, but
close enough to get an adequate sample from the tower.
of an insulated
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Chapter 2
Maintenance Techniques
The term maintenance is a general term often used to cover
any technical work on equipment. But there are different types
of maintenance which can be divided into at least three
general categories: troubleshooting. routine or preventive
maintenance, and updating or modification.
TROUBLESHOOTING
Technical problems occur almost daily and may range
from simple operational problems to major equipment
breakdowns. The more units of equipment in a station, the
higher the proportion of equipment failures. There are simply
more things that can go wrong.
Troubleshooting is different from other forms of maintenance in that it is directed at failures in the operation, which
often require attention.
Troubleshooting may be defined as: the application of
general and specific knowledge, in a systematic manner, to an
evident problem. so that data may be collected upon which a
rational judgement can be made as to the solution and
correction of the problem.
Each phrase of that definition has a special meaning and
should be studied carefully (Fig. 2 -1). For a better understanding, then, let's take each of the phrases and explore it
more fully.
36
GENERAL
ELECTRONIC
KNOWLEDGE
APPLY IN
SYSTEMATIC
MANNER
EVIDENT
PROBLEM
SPECIFIC
KNOWLEDGE
COLLECT
DATA
MAKE
JUDGEMENT
Fig. 2 -1. Troubleshooting
SOLUTION
CORRECTION
Application of General Knowledge
The troubleshooter must have a general understanding of
basic electronics. He needs to know how components work and
how they work in circuits. he must have some general
knowledge of circuits and the particular technology in which
the fault lies. For example. if it is an audio problem, he must
understand audio systems, and if it is an RF problem, he must
know general RF technology. Unless the troubleshooter has a
good grasp of the fundamentals, he is groping in the dark! He
could, in fact, cause more damage than he corrects. And in
some sections of the system, such as high- voltage areas of a
transmitter, he could cause himself harm.
Specific Knowledge
Although a good understanding of the fundamentals can go
a long way in solving problems, efficiency dictates that the
37
troubleshooter have a reasonably good understanding of the
particular system and its equipment components. This is not
about systems in general -for example, how transmitters
work-but how a particular one works.
Systematic Manner
The engineer should follow definite routines or procedures
in his search for the fault, rather than jumping around
helter -skelter like a grasshopper. Without definite procedures,
his efforts may be very inefficient, ineffective, and unproductive.
Evident Problem
This means that something in the system is acting abnormally or has failed. Actually. this is crux of troubleshooting. A problem already exists; it has happened and
needs correction. Other forms of maintenance are
anticipatory. but the troubleshooter must chase present
realities.
Data Collection
One of the major aspects of troubleshooting is asking
many pertinent questions and obtaining correct answers.
Questions must be asked of those who were operating the
equipment when it failed, and questions must be "asked" of
the system by checking various meter readings and any other
source that turns up data which helps in the solution of the
problem.
Rational Judgement
Without facts, there can be no judgement-only guesswork. The gathering of data turns up facts and also turns up
many irrevelant bits of information. These must be sorted out
so that only pertinent facts remain upon which to make a
judgement. When asking questions of nontechnical people who
may have been operating the equipment at time of failure,
extreme care must be used in assessing the answers they give
so that real information may be uncovered.
The Solution and Correction of the Fault
In the sense used here, solution and correction are not the
same thing. Solution means that the fault has been uncovered,
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and the engineer knows what it takes to restore the unit to
normal. Correction means the steps taken to get the system
operational again, even though this may be some temporary
arrangement, such as bypassing the unit or substituting with a
spare unit.
DEGREES OF TROUBLESHOOTING
There are degrees of troubleshooting just as there degrees
of severity in problems. Consider a few examples that can help
describe both.
In the first situation, the operator is sitting at the console
ready to switch up a very important program coming in from a
remote location. He is tense and doesn't realize he has his hand
on the wrong key. When the time comes to switch, he throws
that wrong key! Now he panics and calls for help. Technically,
this is a simple problem (although it may not sound that way
on the air). Solution calls for finding the correct key;
correction calls for turning it on and the wrong one off.
In the second case. it is the same situation, but instead of
the wrong key, he does throw the correct key -the console goes
dead (the fuse blows). Technically, this problem is more
severe and requires a little more troubleshooting.
Another example -assume that at the moment he throws
the key, his air monitor goes dead and alarms warn that the
transmitter is off the air! The fault this time is a burned -out
section of rigid coaxial line, and there isn't a spare section in
town. Technically, this is a very severe problem, and it
requires considerably more expertise on the part of the
troubleshooter in finding the fault. Besides that, in this case it
also takes a considerable amount of latent ingenuity -or
friends at a neighboring station with a spare section of line -if
he is to get the transmitter back on the air with little loss of air
time.
Troubleshooting can be translated into its long -term and
its short -term aspects. Quite often, both of these aspects come
into play when a problem occurs.
Short -Term Troubleshooting
In the short -term sense, time is usually a critical factor.
That is, something fails and must be corrected immediately or
within a short time. For example, a main program amplifier
fails, shutting off the program that is on the air. This fault in
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the system must be found in a hurry and corrective steps taken
immediately to remedy the situation. In this case, the defective amplifier is found and is bypassed with a patch cord,
some level adjustments are made for the lack of gain, and the
program is back on the air. Or the unit may be replaced with a
spare unit, which is patched into place, and the programming
resumes. Time is a critical factor in this example. since the
programming must be restored quickly. This is the short -term
aspect of troubleshooting.
Long -Term Troubleshooting
Whenever there is a defective equipment unit which is a
low-priority item and the correction can be done whenever
there are less pressing problems, then long -term aspects of
troubleshooting come into play. Again a defective amplifier
must be replaced with a spare unit or bypassed. But now time
is not the critical factor that it was in short -term
troubleshooting. With the system operating satisfactorily,
using the substitute amplifier, the faulty amplifier can be
repaired when time allows.
Although the short -term aspect took the master system or
at least a major subsystem into consideration while isolating a
defective minor subsystem the amplifier), the same
troubleshooting technique is used to isolated the defective
component or fault in the amplifier. That is, for this problem,
the amplifier now becomes the master system. All of its
circuits and
subsystems.
components
become
major
and
minor
Mental Attitudes
Since time can be an important factor in short -term
troubleshooting, additional elements enter the picture. One of
these elements is mental attitude. Loosely translated, this
means: the engineer's tendency to hit the panic button!
When one has allowed himself be drawn into a panic
situation, his reasoning powers are severely hampered -he
simply cannot think straight. Under such circumstances,
much costly air time can be lost until the fault in the system is
remedied and programming resumed. Besides that, there can
be additional equipment damage caused by the engineer in a
condition of mental panic. He may. for example, begin twisting
every knob in sight, turning switches on and off, and even foul
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up special setup adjustments which can cause other units to
operate far out of tolerance and even burn up components in
them. In fact, anything can happen! To do his job effectively,
the engineer who is naturally excitable must learn to discipline
himself.
Lack of Knowledge
Besides an excitable engineer's natural tendency to panic,
there are other possible causes of panic ( Fig. 2-2) . One cause is
the lack of a good, basic technical knowledge -in particular,
knowledge of the system and the parts with which he works on
a regular basis.
/
EXCITABLE
NATURE
PANIC
/
LACK OF
BASIC
ELECTRONIC
KNOWLEDGE
LACK
OF A
METHOD
F g. 2 -2. Causes of a panic situation.
Another cause of panic can be the lack of a troubleshooting
method. Without some method, the engineer may make much
movement and effort but find it very unproductive of results.
Over a period of time, most troubleshooters develop some
method which they apply to help quickly isolate and correct a
problem. One such method is described a little later on.
Decision Making
When short -term troubleshooting has isolated the fault in
the system, decisions must be made in most cases. The decisions may not always be made by the troubleshooter alone,
but he contributes his share to the decisions. And if he allows
himself to become panicked, he probably won't be able to
make a satisfactory decision or contribute logical information
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to the decision-making process of others who may make the
final decision.
The scope of these decisions are tied to the facts in each
case. He may need to decide alternate routes or alternate
equipment for substitutions, or time may be an element in the
correction and he may need to assess this aspect. For
example. should he try to operate a faulty transmitter with the
PA (power amplifier) plates glowing white hot, wedging in the
circuit breakers, in hopes of staying on the air the last 5
minutes of an important broadcast? Will the tubes be
damaged? The broadcast could be of great enough importance
to warrant sacrificing the output tubes! Another example: A
component has failed and a spare is not immediately
available. Should he try to use one of less rating, and will it
hold up? It may not last long, and the transmitter will be
down again. So, would it take less time to dash off to the parts
store to get a replacement, or take a chance that the low -rated
part will hold up? Many such decisions must be made, and the
engineer needs a cool head. If he doesn't have to make the
desision himself, at least he should provide correct
information on the situation and point out alternatives he can
provide. so that others responsible can make the correct
decision.
DEVELOP A METHOD
Any method eventually helps the engineer find the causes
of problems and correct them. However, whatever method is
used. it should be graded by its efficiency in terms of time,
effectiveness, and productivity of results. And whatever
method an engineer is using, he should review it from time to
time to see if it really does the job for him. Is it really a
method, or just a set of old habits? After all, what good is a
method, when through its use, it takes an engineer all day to
correct a problem that could have been corrected in a
half -hour?
Whatever method you use, review it from time to time
against changing technologies; make whatever adaptations or
modifications that appear necessary.
One Method
The troubleshooting method described here is one that I
have developed over the years It has proven itself successful in
most cases.
42
When an equipment failure occurs, apply first the
technique of taking a mental step back from the equipment.
This mental step back is most important and is especially
helpful to an engineer who tends to hit the panic button. The
step back has the effect of mentally placing one outside the
situation so that he can be objective. It also allows time for the
reasoning machinery to get into gear. This mental step back
should only take a few seconds to accomplish. It is somewhat
akin to the saying, "Look before you leap."
Once the mental faculties are brought into proper
receptive mood, careful observation is made of the events
taking place. At the same time, proceed with the collection of
data and other facts that provide clues. As the data and facts
begin to pour in, the reasoning powers are brought to
bear- sorting, discarding, and isolating data-until a solution
is reached. As soon as a solution is reached, a decision is called
for on the part of the engineer. Since the correction of the fault
may not always be a simple matter, a decision must be made
on what course of action to take. One usually needs to decide
which course of action takes less time for correcting the fault,
selecting alternate routes, substituting equipment, and so on.
Restating this method in condensed steps ( Fig. 2 -3) (1)
Take a mental step back from the equipment, (2) Collect data
and facts by a careful observation of what is taking place, (3)
Arrive at a reasoned judgement based on the facts uncovered,
(4) Make whatever decision is required.
:
The Wide View
Carefully observing events unfolding and collecting data
and facts must be done in a systematic manner.
Once you have taken the mental step back from the
equipment. look first at the overall system, that is, the master
system or at least the major subsystem in which the events are
taking place ( Fig. 2 -4). For example, if you were called about
a problem in a recording booth, it is highly unlikely that you
need to consider the station's overall master system. Consider
only the recording -booth system. This initial wide view
indicates that many parts of the master system are
functioning properly.
Next, proceed to a narrower view of the system, scanning
that section. With each step, make the view smaller in scope as
data and facts indicate portions are working properly, until
43
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TAKE A
STEP
BACK
a
COLLECT
DATA
t
Fig. 2-3. One troubleshooting method,
in condensed form.
MAKE
REASONED
JUDGEMENT
i
MAKE
DECISION
you finally arrive at a very narrow view, which is the
particular fault or circuit in trouble itself. If the engineer has
divided his station system in various sections or subsystems as
described in the previous chapter, this scanning of the system
can be done in a very short time.
This technique of starting with a wide view and narrowing
the view progressively is similar to a TV cameraman covering
a baseball game. When action is about to begin, since he
doesn't know what will happen, he opens his lens to a
wide -angle shot to cover a large area. As soon as the action
breaks and he can determine the direction it is taking, he then
begins zooming in with a progressively narrower view until he
ends up with a very tight closeup of the outfielder catching the
ball.
The Senses
brought to bear on the inthis should also be done in a
systematic manner. For example, suppose a defective
All the senses should be
vestigation of a problem, and
44
- - --1
LOOK AT WHOLE SYSTEM OR MAJOR AREA
(A)
r
I
1
1
LIN
P
CONSOLE
UTS
TRANSMITTER'
AGC
-I
r- - - - - - - - i
(B) NARROW THE VIEW
(WIDE
VIEW)
I
LCONSOLE
AGC
-J
r --1
(C)
CONSOLE
I
I
1
)
L
J
ACG
ZERO IN ON THE TROUBLE SPOT
Fig. 2-4. Proceed from the wide view to a very narrow view.
amplifier has been pulled out of the system, and troubleshooting
of the unit begins.
First, visually scan both sides of the chassis or circuit
boards, but do this systematically (Fig. 2 -5). Read it as you
would a book or printed page. That is, start top left, read
across, then move down a section on the left side, and again
left to right, until the whole board is scanned. Continue this
process until both sides of the chassis have been scanned.
During the scanning, be on the lookout for obvious faults, such
as discolored or charred resistors and other components,
leaky capacitors, arc -over points, broken wires, etc. But even
if you find something of this nature, continue the scan until the
whole chassis is scanned. There can also be other defective
parts on the chassis, but at some other place or on the other
side.
Smell -Use your sense of smell at the same time you are
scanning the chassis. Overheated components, burned or
charred wiring and components, all give off distinctive odors.
Although the component is not used as much today as it was a
45
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PRINTED -CIRCUIT BOARD
,1-2---
2
FLIP SIDE
Fig. 2 -5. Scan the chassis or printed- circuit board as you would a printed
page.
few years ago. one needed only to step into the room where a
selenium rectifier went defective to know what it was. The
odor was very distinctive -rotten eggs. With a little practice
and experience, the engineer can soon learn to distinguish
these different odors, and they can be definite clues in locating
the problem.
Touch-While the visual scan is taking place and you are
on the alert for distinctive odors, the sense of touch can also be
used. But be careful if the power is on the unit, and also be
careful not to get burned.
Overheated components often radiate heat that can be felt
without actually touching them. Do not touch large power
resistors, as those normally run at a temperature that give a
very bad burn. Other components, such as electrolytic
capacitors. should run cool. If an electrolytic is leaking
internally, it will run warm or hot. Many small transistors can
normally run very hot. In all cases, be very careful.
Aside from heat, the sense of touch can also check for
loose connections by gently wiggling components, cables, and
other wiring. Do be gentle! Overworking and bending wires,
terminals. etc. can create open circuits. If the connection is
loose, gently pulling on it will show it up, or you may need to
use a probing action. When power is applied to the chassis, it
is wise to use some insulated tool,such as an RF alignment tool.
46
Hearing-Many defective components create noise; this
may be either electronic, acoustical, or both. It is acoustical
when it vibrates, buzzes, or gives off noise directly from the
component or via the chassis. It is electrical when the noise
becomes part of the electronic signal, for example, as hum in
the program audio. Listen for noise, both from the chassis and
from earphones or a loudspeaker. Microphonic components
and noisy resistors often show up as noise in the speakers when
the parts are probed or tapped with the alignment tool.
There are other noise sources that can be very misleading.
These are the low- background /mechanical noises in a
recording booth or studio. These noises are picked up by
microphones. By the time they are processed along with other
program material, their level may be much higher than was
originally heard in the studio. Such noises come from
air -conditioning units, heat ducts, turntables, tape machines,
fluorescent -lamp ballasts, and elsewhere. When troubleshooting noise problems in the program material, listen carefully for this type of background noise. But listen in the studio
or recording booth without any program running and with the
speakers quiet. In other words, listen to the studio itself. You
can turn tape machines into the"run "condition, then turn on
the mike to mute the speaker. This will show up mechanical
noise from the machine itself.
A systematic investigation making use of all the senses
detects many of the areas in which the fault lies. But don't
expect to find all problems this way.Thereare many problems
that do not give off external clues, except through erratic
operation of the equipment or the poor quality of the program
sound. In those other cases, it is necessary to resort to making
measurements of the signal, stage parameters, voltages, and
HOT
NORMAL
SIZE
SWOLLEN
I
CHASSIS
I
1
,
4
`-------LEAK
Fig. 2 -6. Defective electrolytics can leak, swell, and get hot.
47
even checking out suspected components. Still, the habit of
using all the senses during the troubleshooting process should
be developed.
Brain Power
Systematic use of the senses may have uncovered a defective component, but it would be a mistake to assume that
the component is the real fault or entire problem. On the
contrary. it may only be one of the results of the real fault.
Naturally. defective components must be replaced, but we
must investigate why the part failed or burned up. If we simply
replace the defective parts without correcting the real cause,
the new parts will only burn up also. For example, upon
investigation, suppose a burned resistor is found on a circuit
board (Fig. 2 -7). We assume this is the problem and replace
the resistor with a new one. But if this resistor is in the
SHORTED TRANSISTOR IS REAL CAUSE
I
i
.-,
BURNT
AND
OPEN
RESISTORS
i,
/
i
1
/
Fig. 2-7. Defective parts are not always the real cause. In this case, the
shorted transistor burned out the resistors.
collector of a shorted transistor, it won't take long to burn up
the new resistor. Go ahead and replace the defective parts; but
before turning on the equipment, investigate the reason the
part failed in the first place and correct that also. There can be
many reasons for component failures. The part may have been
underrated for its use in the circuit or may have simply worn
out, but more than likely it was overloaded by abnormal
circuit conditions or failure of another part. Reasoning must
be used in making many of these determinations.
48
Reasoning
As discussed earlier, the investigation gathers facts about
the problem and a reasoned judgement must be made. The
reasoning applied should be cause -and -effect reasoning. This
simply means that, when certain conditions exist
cause- certain results are obtained-effect. The same reasoning can be used in reverse order; that is. when certain
effects are present. they have given causes. This is the usual
reasoning in a troubleshooting situation, since the effects have
already occurred. and now you must find the cause.
-
Knowledge
One cannot effectively bring reason to bear upon a
problem unless one has some knowledge upon which to base
the investigation. And this knowledge is of three types ( Fig.
2-8). The first is general knowledge of electronic fundamentals that is acquired through schooling, home study
-SCHOOLING
-BOOKS
-READING
WORK
EXPERIENCE
GENERAL ELECTRONIC
KNOWLEDGE
PRACTICAL
KNOWLEDGE
WORK ON
A SPECIFIC
SYSTEM
SPECIFIC
KNOWLEDGE
ENGINEERS
TECHNICAL
KNOWLEDGE
Fig. 2 -8. Sources of technical knowledge.
courses, and books on electronics. The second is the engineer's
practical knowledge, gained through a variety of working
experiences with equipment and under a variety of situations.
And third is specific knowledge of the system or equipment
that now has a problem needing correction. The engineer can
be weak in one or more of those areas and still do an
acceptable job, but it takes longer to correct the problem.
If the engineer is weak in his knowledge of electronic
fundamentals, there is really no excuse, since a wealth of
49
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technical knowledge is available through many good technical
books. including many by TAB BOOKS, technical trade
magazines, and information from equipment and component
manufacturers.
As for specific knowledge of the system and its components
in a particular station, any station with a reasonable amount of
technical equipment in use has a wealth of technical
information available in the equipment instruction manuals.
And in the majority of troubleshooting cases. it is the practical
information and knowledge that is used to find solutions and
make corrections. The engineer does not need to know how to
design the circuits in use -he need only have a fair
understanding of how they work. As a matter of fact, the
engineer who has only theoretical design knowledge. not
tempered by practical knowledge. may have difficulty in
troubleshooting problems because he always looks for the
worst in any situation.
I know a broadcast engineer who once worked in
equipment design for an equipment manufacturer. This
fellow's hangup is capacitors. In his design work on critical
circuits, he well knew the variations and caprices of
capacitors in these circuits. Whenever he had a broadcast
problem to solve at a station, he made a beeline to the
capacitors in the unit at fault. He eventually got the real
problems solved, but his equipment unnecessarily sported
many new capacitors.
PRACTICAL PROBLEMS
Most modern equipment is stable and reasonably trouble
free. During the first few weeks of shakedown, the weak or
borderline components and transistors are weeded out. There
can also be marginal or faulty designs that begin to show up
with problems, or local situations may push some equipment
into marginal operation -for example, high or low power line
voltages on a regular basis put marginal components to the
test. After the first few weeks, the equipment usually settles in
and "purrs" along.
Still there are daily technical problems for the engineer to
contend with. But a very high percentage of those day -to -day
problems are usually minor in themselves, although it may not
sound that way on the air. Wrong keys or switches thrown, a
dirty jack, a sluggish relay or one with dirty contacts, a patch
50
cord left out or plugged into the wrong jack, a transmitter left
on the dummy load after nighttime maintenance -these are
but a few of the more common problems the engineer is called
upon to correct during the broadcast day.
Because so many of these problems are simple in nature
and simple to correct, the engineer who looks for the worst or
panics can cause a considerable loss of air time. The engineer
who expects to find the worst situation imaginable, most often
completely overlooks the fact that the problem could be
simple! In a very short time, he has the system so completely
disassembled that a considerable amount of time is required
simply to put it back together again-and perhaps he still
hadn't found the patch cord plugged into the wrong jack.
On the other hand, the engineer who panics starts twisting
every knob in sight. twittering screwdriver setup adjustments,
and really getting the system fouled up. sounding worse than it
did before he leaped into action.
Major problems develop from time to time, but the wise
and efficient troubleshooter expects and looks for the simple
causes first.
Illustration
A hypothetical troubleshooting situation should help
illustrate many of the principles that have been discussed.
Consider this situation: A station has its transmitter on remote
control, located several miles from the studios. The only
engineer on duty is also working the control room as an
announcer. He spins a record, cues up the next one, and leans
back -going to take it easy for a couple of numbers, simply
segueing from one turntable to the next. Suddenly his monitor
goes silent! Now anything in the system could be at
fault-from the turntable itself to the antenna collapsed on the
An
ground.
He should apply first the technique of a mental step back
from the equipment,
then observe the system -wide
view-scanning the system in a systematic manner, gathering
and sorting facts, and making judgements. First, he observes
the console VU meter showing audio levels, and the meter is
dancing along merrily. From his knowledge of the system, he
knows the meter is at the output of the console, so he reasons
that everything is okay to this point. He also knows there isn't
much between the console and transmitter but a line am51
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plifier telephone lines. He checks the transmitter readings and
modulation monitor indications. All of these are normal. Since
most of the system is operating properly, he reasons it must be
the monitor amplifier itself, which is the last thing in the chain.
There is a small portable radio in the control room, so he tunes
in the station and in comes his music loud and clear. But now a
decision is called for: what to do about the monitor. He makes
a decision: He uses the small receiver with an earphone for
monitoring the on- the-air audio and tries to see what is wrong
with the amplifier when he gets free.
Thus, by the use of the method, he quickly runs down to
isolate the problem and makes alternative corrections -all
before the record runs out -and the listening audience never
even knows he has a problem.
But consider what can happen if he panics. He may start
flipping switches on and off, causing many disruptions to the
program on the air. Worse, he may leave a switch off and then
no program would be on the air. He could switch the
transmitter on and off several times in quick succession,
causing something to blow up and trip a circuit breaker. This
necessitates a trip several miles out to the transmitter
site-for the station is now really off the air.
Mark the Manuals
Another important aspect of troubleshooting is the matter
of accurate circuit diagrams and equipment manuals. Since
time is critical in many situations, the engineer must have aid
from the prints and manuals and not confusion.
Most broadcast electronic equipment today has other
applications or may be sold outside the U. S. Consequently,
there must be some different circuitry for other applications
and standards. There is a variety of instructions in manuals
telling how to set up under these other conditions. And besides
all this, the particular unit may have other possible
configurations, for example, a tape machine may be either
monaural, stereo, or multitrack.
An instruction manual, then, may have several different
diagrams, different sets of explanations, and a variety of
instructions to hook it up. From the manufacturer's point of
view, this necessitates only one instruction manual to cover all
these other usages and standards for the unit. But from the
troubleshooters point of view, all this extra information
52
and becomes a briar patch that he must wade
through to find the current print or instructions he needs. It
can be extremely frustrating. especially if the station is off
the air and the fault must be corrected with as little time lost
as possible. Under these circumstances, the engineer simply
doesn't have time to sort out all these prints for the correct
one.
When a new unit is installed and working, go through the
instruction manual to line out all the information that does not
apply. The information does not have to obliterated, simply
lined out (Fig. 2 -9). If it is an arrangement that may later be
used, then the information can be restored.
LINE OUT UNUSED
INFORMATION
OUR
MODEL
INSTRUCTION
MANUAL PAGE
Fig. 2 -9. Line out unused instructions.
On unused circuit diagrams, if they are detachable,
remove them from the manual. Store them in a safe place. If
not, then mark a large X across the diagram. On the correct
print, out in the margin or in some other conspicuous place,
write OUR UNIT in large letters. Any similar words can be
used, but they should quickly identify the print.
In those sections which show different terminal connections, use a red pencil or some other color that stands out to
show which arrangement is actually in use.
When a troubleshooting situation exists where time is at a
premium, these markings help the engineer find the correct
prints faster and save both time and frustration.
And one further word -don't use a pen to mark prints. Use
a pencil or something that is easily erased. You may want to
change the method of operation or hookup at some later time.
If you have made changes in pen, you end up with a scribbled
mess on the prints.
SIGNAL TRACING
dynamic method for isolating faults and defective
components is signal tracing. This method can be used only in
A
53
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those cases where the equipment is still operational.
Naturally, if the equipment is smoking or operating in some
other very abnormal manner, it must be shut down, or
additional damage can be done. Signal tracing is not confined
to a single unit. It can be used on the master system just as
well, or on any subsystem. In the method of troubleshooting
previously described, signal tracing is an important part. It is
used in the area of collecting facts and data about the system
or unit that is malfunctioning.
Signal tracing is rather simple in principle. The unit is
placed in an operational mode, either in its normal location or
on the workbench. An input signal of correct amplitude is
applied, then some type of signal detector is used to follow the
signal through various circuits of the unit until the output is
reached. At the place where the signal fails to appear,
becomes distorted, or seriously abnormal, the faulty circuit
has been isolated.Although the technique is simple in principle,
results depend upon near -correct operation of the unit in a test
setup, correct signals, and interpretation of the results
observed by the troubleshooter. Proper interpretation is very
important, otherwise, the engineer may spend much fruitless
time and effort running down false trails.
Signal tracing may or may not lead to the defective
component itself. But it ordinarily leads to the defective
circuit. How much further the signal tracing can go depends
upon the circuit and the test equipment being used. When the
signal tracing can go no further, then other methods are
necessary to isolate the actual component at fault. For
example, signal tracing may lead to a stage whose gain is far
below normal. In a stage, as opposed to other nonspecific
circuitry, there are many components which set its operating
parameters. An oscilloscope can further the signal tracing, but
this may not isolate the real fault. It is necessary to measure
the DC operating voltages and perhaps check for shorts or
component values that have changed. Although signal tracing
does not always isolate the faulty component, it helps isolate
the faulty stage-this is half the battle.
Test Signal
Signal tracing can be performed more efficiently if a test
signal of steady amplitude is used, such as the sine wave from
a signal generator. The steady sine wave, with its known
54
amplitude and shape, helps in detecting variations through the
unit under test by making comparisons easier. And as a
practical matter, if the unit is on the workbench, connecting up
the signal generator is easier than trying to "haywire" a
connection to the program channel for a test signal.
Sine waves can also be used when signal tracing parts of
the system itself. But for the program channels that have
failed during programming, the more practical signal is the
program itself. The sine wave does not have to come directly
from a signal generator; it can be recorded on a tape or test
record, then played by a tape machine or turntable.
The difficulty with a program signal is that its composition
is constantly changing. This makes it difficult to geta correct
level setting for comparisons and to compare input /output
signals. For example, you set the correct input levels to start
signal tracing. You suddenly find the signal very low. The
program may be music that has hit a period of low -level
passages, which can be embarrassingly misleading.
Signal Detector
Some detector is needed that gives an indication of what
the signal is doing along the circuit path. The detectors in Fig.
2 -10 all work well in audio circuits. If working with RF or
digital circuits, different detectors as well as techniques must
be used.
SYSTEM
MONITOR
01 SPEAKER
PATCH PLUG ADAPTOR
EARPHONES
O
OSCILLOSCOPE
ALLIGATOR CLIPS
ADAPTOR
0
Fig. 2 -10. Some audio detectors.
55
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On the Bench
When signal tracing is done to a unit on the workbench
there are several precautions to take in the test setup. The unit
is always hooked up during the tests, even though it is faulty
now, in its normal manner. Thus, the AC power to the unit, its
input /output impedances, and signal levels should be those
that are normal to its use in the system. And don't forget the
grounding of the unit and the shields of the cabling.
Input impedances and levels should simulate those found
in the amplifier's normal habitat in the system. For example,
the bus which feeds the amplifier may normally run at +8 dB,
but the level into the amplifier is lowered with a 20 dB pad, so
that the input to the amplifier actually receives -12 dB. In
determining what input level to feed the amplifier, find out
where the pad is located. If it is external to the amplifier, then
feed the amplifier at -12 dB. But if the pad is located inside
the amplifier, then feed +8 dB to the unit. Also, during the test,
make sure the amplifier is terminated in its proper load
impedance.
Signal-Tracing Illustration
Assume that a self- contained audio amplifier which needs
only AC power and input signals and output load ( Fig. 2 -11) is
on the bench. A signal generator is used for the input signal and
an oscilloscope is used for the detector.
First, set the input to the correct level, with the output
impedance of the generator matching the input impedance of
the amplifier. Place the oscilloscope across the input signal at
the generator terminals.Observe the amplitude and waveform,
calibrating the oscilloscope to some arbitrary setting. Next,
move the oscilloscope to the output of the amplifier. This is
important, for you may find that there is nothing wrong with
the amplifier itself! The problem may be in the plugs and
interconnecting wiring. This input -to-output look can then save
signal tracing all through the amplifier only to find the fault is
elsewhere.
But for our example, assume the output signal was low in
level but not distorted. Proceed then to check stage by stage.
The option is yours whether you move front to back or vice
versa. But whichever method you select, be consistent and go
stage to stage, that is, don't jump around.
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Inside the amplifier, levels are high or low at different
stages, but this does not necessarily mean that a stage is
defective. This may be part of the amplifier's design. When
you come across a spot where the levels vary considerably,
move on to the next stage. If it is a design parameter, the
levels probably come back to somewhat normal after the
following stage. But, if the signal does not recover, then you
have the faulty stage. If you can proceed into the stage itself,
go ahead. But remember, even a normal stage can have a
variety of different signal levels across components that can
be misleading. For example, you may place the oscilloscope
across a resistor in the emitter of the stage, finding the signal
very low -lower than expected-or nonexistent. The resistor
may be bypassed to eliminate any AC signal across it, or it
may be used as part of an equalizing or shaping factor of the
stage. All these things should be considered before taking
components out and replacing them. Ordinarily, voltage
measurements and checking components for shorts or value
change do more for isolating the actual faulty component ( Fig.
2 -12).
Signal Tracing the System
Signal tracing the system is not difficult when the station
has the foresight to install adequate monitoring and metering
points during the installation.
When a fault occurs in the regular program channels, use
the metering and monitoring positions throughout the system.
If the problem is distortion, monitoring the audio is usually
more effective, although metering indicates if levels are too
high. overloading something and causing the distortion.
The procedure is the same as signal tracing a unit on the
workbench, except that now the unit is spread out over a
considerable area, and it is not always easy to see the results
as easily as when everything is within arm's reach. Use the
system's normal metering points ( Fig. 2 -13) the VU meter at
the output of the console, meters on AGC amplifiers and peak
limiters, and the modulation meter. There may also be a VU
meter panel where several points of the system are tied up into
a selector switch so that meter can quickly check levels
:
throughout the system.
Use the system's aural monitoring arrangement. This is
available at least at the console output. There may also be
58
DEFECTIVE STAGE
MULTIMETER
Fig. 2-12. When signal tracing has Isolated a defective stage, use
methods to check components in the stage.
other
other parts of the system switched into a master-monitoring
arrangement. A pair of headphones can be used in the circuit
path at the patch panel.
Use care when patching the earphones into the patch
panel, so that you don't take the program off the
air-assuming the program is still on the air. Look for those
positions which have a multiple jack wired to them. Tap into
this multiple for the monitoring. If there aren't any multiples
at the points you wish to monitor, here is a trick you can use:
Take a patch cord, insert the plugs into both the input and
output jacks, but don't push them in far enough to break the
circuit. Next, with one motion, shove both plugs in very
quickly. at the same time. There should be no break in the air
signal. and if there is, it is so brief that no one notices it. This,
of course, assumes the circuit is balanced. If it is not, make
sure you have the correct polarity on both plugs or you will
short out the circuit! Now that the patch cord is in place, it is
taking the place of the "normals" (normally closed contacts)
on the pair of jacks. Use the headphone either with the
patch -type plug or alligator clips and pick up the signal at the
pair of screws on either of the plugs of the patch cord (Fig.
2 -14). By the way, this little trick with the patch cord does, in
itself, locate many a problem with the normals on a jack! The
59
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ALLIGATOR
CLIP ADAPTOR
EARPHONES
Fig. 2 -14. When there is no multiple on a jack, use a patch cord to substitute for the jack "normals." Use earphones with alligator clips and attach to screws at rear of patch plug.
patch plugs either clean up the contacts or give a solid
circuit that is lacking previously.
ROUTINE MAINTENANCE
Routine maintenance can be defined as the cleaning,
oiling, testing. and adjustments made to the system on a
regular scheduled basis, without being triggered by some fault
or failure in the system.
In effect, routine maintenance looks ahead to eventual
problems that can arise, and doing that which is necessary to
forestall many problems. For this reason, routine
maintenance is also called preventive maintenance. These
routine procedures often detect small problems that are cured
in their infancy, long before they develop into large, serious
problems.
Fireman's Approach
There is another "method" of doing maintenance which is
unfortunately, practiced at some stations. This can be called
the fireman's approach to maintenance. In effect, no
maintenance is done until something fails! Then, of course, the
engineer goes to work correcting the problem. This is not the
recommended way. But, unfortunately, many stations in
smaller markets do not have adequate engineering personnel.
There may be an engineer who takes care of the station on a
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contract basis, or there may be one full -time engineer who
must devote the majority of his time to other matters at the
station. such as announcing and programming. Even the
station that does have adequate engineering personnel can find
itself so covered up with problems or major construction that
the routine maintenance suffers for periods of time.
When the fireman's method is practiced at a station,
whether this be due to an engineer's own attitudes or as a
result of station management. the results can often be costly,
not only in replacement costs, but in station down time.
Although routine maintenance does not detect and cure all
early stages of problems brewing it can catch many of them,
minimizing headaches and ulcers .
Scheduling
To be effective, routine procedures must be done on some
regular basis and not in a haphazard manner; otherwise, some
will get done and others may not get done at all. Memory can
be a poor substitute for schedules and records. The station
should decide which procedures to do on a regular basis, then
set up schedules. Be realistic and practical in deciding what is
to be done and how often. When setting up schedules, do not
make them hard- and -fast rules that must be observed no
matter what. Be practical and allow a certain amount of
flexibility. What is necessary is the realization that schedules
are important and things should be accomplished according to
schedule whenever possible.
One scheduling means is the desk-type calendar. When a
new calendar is obtained at the beginning of a year. go through
the whole calendar and mark the dates in each month of the
year when certain maintenance procedures are to take place.
For example. if certain measurements should be made on the
second Monday of each month, go through the calendar to
mark each one. As the calendar page is turned for each month
during the year, the entry will already be there to act as a
reminder. But sometimes when the event is scheduled, some
other important job must be done instead. This is where
flexibility enters the picture. The scheduled procedure may be
shifted to another day of that week. or may be even skipped
until the following week. A week off the normal scheduling is
still valid as far as routine scheduling is concerned.
62
Keeping Records
Scheduling ahead is one thing, but records also need to be
kept which show that the maintenance was actually done. In
the calendar, a small check alongside the scheduled entry can
serve to indicate it has been done (Fig. 2 -15). If the routine has
to be done later in the week, a mark on the original entry is
still adequate. But if the routine was bypassed for the month
because of other things, then do not add the mark. The lack of
the mark indicates the routine hasn't been done. If the date is
important, then show the date it was actually performed even
though that was not its regular scheduled date.
SUN.
MON.
TUE.
MINI PROOF ON
SYSTEM
15
16
CALIBRATE
FM POWER
OUTPUT
METER
22
23
17
CAL PWR
METER
//
24
REGULAR CALENDAR
Fig. 2-15. Use a calendar for scheduling. In some cases, show date it was
actually done, as here, on the 24th.
Reports
Whenever maintenance is done which is either routine or
in correction of a catastrophic failure, a daily maintenance
report is used (Fig. 2 -16). These reports can be saved for
several months or longer and can be a valuable reference
when certain problems seem to come up often. For example, a
certain capacitor has failed in a particular unit. But something
tells the engineer that this was replaced only recently. With
the report sheets, he can check previous reports. Checking
further, he realizes the same part has been replaced twice this
year already. This makes him take a harder look at the rating
of the capacitor. and he may decide to use one with a higher
voltage rating this time. Or perhaps there is an identical unit
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DAY
TIME
9 AM
TECHNICAL REPORT
MONDAY
DATE
7 7 -77
TROUBLE
MODULATION MONITOR
COMPLETELY DEAD.
ENG.
S.V.
REPAIRS
9:10
AM
FUSE BLOWN IN MOD. MONITOR.
COULD FIND NO OTHER FAULT.
REPLACED FUSE. MON. BACK IN
SERVICE.
11:00
AM
MODIFIED JACK2E8ADDED
A MULTIPLE 2E9.
P.F.
J.J.
Fig. 2-16. Use some type of maintenance report daily.
that gets as much operation as this unit without failure. This
leads the engineer to look for more subtle reasons why this unit
has been failing so often.
There are a variety of records devised which cover
maintenance situations or routines. The calendar and report
sheet are but two. Other records can also be made, including
those for transmitter power tubes and memory joggers for
events that happen infrequently, such as a quarterly
tower- lighting- equipment inspection (Fig. 2 -17). Even though
many of these inspections require maintenance log entries,
there are separate records kept that consolidate those dates
for easy review.
WHAT TO DO
What maintenance procedures that a station should set up
depends a lot upon how much equipment is in use and how
64
FM POWER METER CALIBRATION
1976
1977
1978
FEB. 3
JUNE 10
AUG. 25
OCT.
1
Fig. 2 -17. Consolidate widely spaced entries made in maintenance log on
a separate record for easy location.
sophisticated the equipment is. The station has to make its own
decisions in this area.
Gradual Wear
Aside from catastrophic equipment failures that must be
corrected, equipment that is in use for many hours a day
simply wears out. This daily wear is not perceptible, because
the increments are too small. It is only after many of these
small increments add up that we begin to notice the fall in
performance or mechanical wear. Because of this small daily
drop in performance, different sets of measurements should
be scheduled at intervals so that performance drops can be
measured and replacement or correction can be made before
the performance becomes bad or fails altogether.
Spot Checking
What is needed, then , is some method of spot checking the
equipment performance from time to time (Fig. 2 -18). For
example, if response, distortion, and noise measurements are
to be made on the system once a month, those need be only at a
few audio frequencies that outline the system bandpass, rather
than a full set of measurements as required in a proof of
performance. In this way, the checks don't become
cumbersome-but they give useful information.
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FM SYSTEM TEST RUN
RESPONSE DISTORTION
REF.
100 Hz
-1.0DB
1.0%
400 Hz
-0.4 DB
o.6'/c
0
0.3%
10 kHz
+ 12DB
0.5%
15 kHz
+ 16DB
0.7%
1
kHz
NOISE
-63 DB
Fig. 2-18. Use a few frequencies to spot -check the system. This cuts down
the time to make measurements and outlines the system's behavior.
Areas for Routine Maintenance
As mentioned earlier, what specific routines to set up
depends upon the equipment and how much there is of it, plus
the usage it gets. Usage is a very important factor. Wear is
directly related to how much use the equipment gets. For
example. a small station with two tape machines that play a
few spot announcements or other programs occasionally from
the machines during the day doesn't have the same procedures
as a station with several tapes machines -many of which may
be in a large automation system where everything is played
from tape all day long.
A few areas where routines can be advantageous are:
periodic measurements on the master system; each tape
machine; system level checks; individual equipment level
checks (such as tape machine playbacks that feed a console);
mobile radio transmitters, for power, frequency, and
modulation; carrier frequency measurements; and power
meter calibrations. Some of these are required by the FCC,
others are required according to usage.
Other Maintenance
Besides making sets of measurements to keep tabs on
performance, other maintenance should be done on a regular
basis: Tape machine heads and pinch rollers need cleaning on
a regular basis, meters in various machines and exhaust fans
should be oiled or greased at intervals, air filters should be
inspected. Some of these are important maintenance items.
66
For example, the bearings in tape machine drive motors can
run dry, causing noise on the reproduced audio, and they will
soon become totally defective having to be replaced. Without
proper oiling, the bearings may run dry, freeze up, and stall
the motor, burning it out also. Tape machine drive meters are
expensive.
Fan motors can gradually become defective or ineffective
in moving air (as can clogged air filters). Dirt may build up on
the blades or squirrel cage so that the actual amount of air is
reduced considerably. Or the bearings may dry up, get noisy,
and put a drag on the motor. The lack of cooling air has many
side effects in solid -state equipment. In transmitters, air flow
interlock switches shut the transmitter down when the air
pressure drops below its present control level.
All of these type measurements and routines should be
scheduled in some manner, keeping records for reference.
UPDATING
With technology advancing so fast these days, a particular
piece of equipment may have several modifications done to it
within a year. Weaknesses of the original circuit design show
up. bringing improvements by the factory. Updated parts,
circuits, and similar information arrive in the mail from time
to time.
As far as circuit weaknesses are concerned, the problem
may never occur in the particular unit in your station. If the
unit is performing satisfactorily there may seem to be no need
to update the unit until the problem happens. The
modifications, however, should be performed. When the
modification is completed, be sure to correct the circuit
diagrams and instructions where needed. There is nothing
more exasperating in troubleshooting than to discover that the
circuit diagrams do not match or agree with the actual circuit.
Spare Parts
Equipment updating through modification can change the
stock of spare parts. Of course, good judgement must be used
in this area.
When modifications require new specialized components,
such as ICs or transistors, those on the shelf can become
orphans unless they are used elsewhere in the system. And if
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the part is very specialized and costly, the dealer may not
want it back.
There is usually another effect on spare parts-tubes,
transistors, and ICs-by equipment modification. If the
modification calls for a new type, the present stock may
already cover it. But as it often happens, this usually is an
altogether new type and you will want to stock some spares in
the inventory. And if your luck runs like mine, it is a stock
number that falls somewhere in between numbers in the parts
drawers. So. all the drawers have to be shuffled down the line
to make room for it. When you are setting up your initial
inventory, leave a few open drawers at different places to take
care of the new numbers. Then, many drawers do not have to
be moved.
UPDATING YOUR KNOWLEDGE
Advancing technology has its effect on equipment obsolescence. It can have the same effect on the engineer's
technical knowledge. The engineer must keep constantly
abreast of changes or he will wake up some morning to find the
field has passed him by, and he is now in a different technical
world. True, many of the old principles are still in practice, but
there are a great many new ones on the scene also.
The broadcast engineer must keep abreast of not only the
new technologies, but also of components and equipments.
There are ways he can do this:
First, subscribe to technical magazines relating to
broadcasting, such as Broadcast Engineering. These
magazines provide articles on operation, maintenance, new
theories, and different types of broadcast equipment
available.
Second. obtain specification sheets and literature on new
broadcast equipment whenever possible. Write to equipment
manufacturers for information and application notes on new
equipment in your field of interest. Don't try to cover
everything or you will be inundated. If you are in radio, get
data on audio, radio transmitting, and test equipment; don't
worry too much about the television aspect. Of course, you
may desire to advance into another aspect and want to keep
informed.
Third. obtain data sheets and application notes on new
components whenever you can. These can be found in catalogs
68
from parts dealers and manufacturers. There are also several
trade papers available on a subscription basis that deal with
components of all types. These are usually directed to
purchasing agents of factories and other large component
users, but read them whenever you can.
There may be times when the station's economic picture is
not good so the station makes do with its old equipment until
things get better. But don't allow this to discourage you from
keeping up to date on what is available should the station
decide to purchase. You should keep up to date so that when a.
purchasing decision is going to be made, you can make
knowledgeable recommendations on what equipment should
be obtained. Even if your recommendations are not
considered, you will at least know what is going on in the
industry. And should you move to a job at another station that
is equipped with state -of- the-art equipment, the transition
will not be difficult to make.
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Chapter 3
Planning and
Installation
Whenever new equipment is to be installed, changes made in
the old, a complete remodeling, or a completely new station is
installed, a certain amount of planning should come first.
PLANNING
Installation planning could be described as follows:
carefully fitting together many equipment items so that they
physically and electronically intermesh into a single master
system that is efficient in its ease of operation and can be
easily maintained.
We must distinguish between general planning and
installation planning, as they are not the same thing. General
planning concerns itself with such questions as what model to
purchase, price ranges, station objectives, etc. Installation
planning concerns itself with how the particular equipment
that has been purchased will actually be wired into the
system: What are the terminal numbers? Where must you
drill the holes? And so on.
The amount of planning which must be done is dictated by
the project at hand. Thus, there are both stages and degrees of
planning. Major projects, for example, should receive much
overall planning; and the multitude parts of the project must
also have individual planning.
Small projects should receive their share of planning.
There is a natural tendency to give small projects only cursory
planning-with most of the planning actually occurring after
the installation has taken place and failed! As the saying goes,
70
"Back to the drawing board." In fact, most of these small
projects never get on the drawing board in the first place.
Any project, regardless of its size, should be considered in
relationship to the master system and what effects there may
be when this project is completed. Even small system
modifications can create large system problems. For
example, an engineer designs an alarm device which contains
several internal relays and incandescent lamps. No provision
is made for power. It is expected to simply attach to the system power supply.But what is not known is that the power supply is loaded almost to its maximum capability. Any additional
load will push the power supply into a near-overload condition.
Consequently, the new alarm device loads the power supply so
that parts of the master system become unstable, with some
intermittent relay operation at various places. An engineer
may spend much of his time searching for other problems in
the system which are really caused by the overloaded power
supply. Proper planning of this small project would have
investigated the power source. Having found it loaded without
spare capacity, the engineer could have provided another
source of power for the alarm.
DRAWINGS
Drawings are the maps of a broadcast system. These
drawings have many uses that, under different circumstances,
provide different benefits. That is, they are used for
troubleshooting problems, making system changes, and
providing general information about the system. There are at
least three types of drawings: block diagrams, schematic
diagrams, and pictorial diagrams.
Block Diagram
The diagram of the overall system should be a single -line
block diagram (Fig. 3 -1). Block diagrams should also be made
for each of the major subsystems and even minor subsystems,
showing direction of the action flow, signal levels, and other
pertinent data. Try to balance utility against clutter. It is
possible the diagram can have so much information that it
is difficult to use. Shoot for a happy medium.
Schematic Diagrams
The single -line block diagram is the best tool to obtain a
wide view of the system. But for all the specialized areas of the
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CONSOLE
PAD
1D1
-10dB
41
1C4
lui
Fader
-10dB
4
1C7
`Ly
TAPE
1
1D4
1D7
-10dB
Fig. 3-1. Typical one -line block diagram (partial). Note the amount of in-
formation supplied in a simplified form.
system, it is necessary to employ the schematic for all the
detailed information that is required.
All the individual projects within a large overall
installation project, must have schematics drawn of the area
involved (Fig. 3 -2). These drawings provide the necessary
detailed information required to complete the installation.
Typical information must show terminal numbers, wire color
coding, shield treatment, and similar information needed to
get the job done properly.
When equipment units are to be modified through change
of circuitry, or if additional components are to be added,
schematics are a necessary requirement. For example, the
console may contain one or two unwired lever switches so that
the user may wire according to his own needs. If one or both of
these switches are to be wired for station use, a schematic
must be available-draw it now. And during the wiring
operation, if any changes take place they should be entered on
the schematic as they happen. Such specialized schematics
should be preserved for future reference.
Regardless of how well the system installation may have
been planned, the final results invariably contain changes
from the original plans. No matter how much planning is done,
it just isn't possible to foresee all contingencies-or a change
of mind. Changes are to be expected. But when changes do
72
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Q
m
.1
Ú
'YYY`
)
(D
)u)
(
73
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take place, it is important that the diagrams are altered at the
same time, not at some later date. Making changes to the
drawings as the work progresses does tend to make the
drawings look a bit shop -worn, but they are accurate!
Soon after the new system has been put into operation and
is functioning as desired, make up a completely new set of
drawings and records from the working set. If equipment
modifications have taken place, correct the diagrams in the
instrument's manual also. And while correcting the manual
diagrams, also correct the other instructions, such as terminal
numbers, etc. A good set of accurate drawings and correct
circuit diagrams are far better for future use than a hazy
memory.
Pictorial Diagrams
There are many occasions when simple sketches are very
helpful during the installation. These need not be works of art
but should at least be accurate enough that they convey the
necessary information. Such diagrams can be very helpful
when it is necessary to wire a multiple -leaf switch, since the
terminals at the rear of the switch appear opposite to that of
the actual switch arrangement from the front that is
indicated on the schematic diagram. These diagrams are
almost always used in conjunction with a schematic diagram.
The schematic shows the actual circuit, while the pictorial
helps with the physical arrangement.
WORK PATTERNS
We humans tend to develop patterns of doing things and
then follow those patterns as a matter of habit. Many industry
practices in regard to equipment layout have developed
partly through utility and partly to agree with the way many
people naturally do things. For example, we read a printed
page left to right, line by line, from the top of the page to the
bottom. Thus, most terminal boards are laid out in a
left -to -right fashion. Everything, of course, can't follow such
patterns and somethings must be designed for the specific
usage. But you can expect to find many functions which follow
natural human practices.
When laying out a new station, we must develop a number
of patterns that are helpful for that station. Use standard
industrial practices where they can be applied. But in the
74
many other cases where it is purely a local situation, try to
develop a practice that follows natural human practices when
possible. But work at these beforehand. give them some
thought. then be consistent when applying them throughout
the construction.
OPERATOR'S VIEWPOINT
An example of a natural pattern translated into equipment
is a row of leaf switches on a panel. These must be assigned
their numbers from the operator's view standing in front of the
rack, console, or whatever is holding the switches. The
engineer wiring the switches from the rear must learn that his
wiring pattern is always the reverse -right to left from the
rear of the rack. While this may be a bit awkward for the
engineer, the operator's position must take priority.
AUDIO CONNECTIONS
The audio throughout the system should be kept phased,
just as a matter of good operating practice (Fig. 3 -3). In a
monaural system, there is no real burning issue why this
should be done. As a matter of fact, the system works just as
TERMINALS
TERMINALS
(HIGH)
RED
D1
2D
(LOW)
BLK
2
Fig. 3 -3. It takes very little extra effort to keep the monaural system
phased. Just be consistent in the terminal numbers and the color coding.
well regardless of phase. But it is just as easy to have a phased
system as one that is haphazard by simply following a set
pattern when wiring up the audio cables. It is simply a matter
of assigning terminal numbers and maintaining color -coded
wiring throughout.
Multiconductor Cables
Stations have many uses for multiconductor cables. These
are not cables which come already wired with a particular
piece of equipment, but rather those which the station wires.
When possible, try to use regular industry color coding for the
circuits. But when these are not suitable, assign colors -then
75
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stick to the pattern -especially when several cables perform
the same functions on a number of different units, for
example, remote -control panels.
The real danger in the use of the same cable for different
circuits and functions without being consistent in the coding is
that an engineer on a troubleshooting mission may follow the
natural pattern. When arriving at the nonstandard cable, he
starts checking it as per the previous cables. If +5V DC is on
blue and -5V DC is on white, in the original scheme, he may
want to pick up this IC supply voltage and use it as a pulse to
send a trigger into the circuit. But, if the same pattern wasn't
carried into the second cable, there may be +24V DC on the
blue and -24V DC on the white. Feeding this into the IC could
wreck it-be aware!
Jack Fields
The jack fields should be planned carefully. They should
be assigned areas before the actual installation begins. This
little exercise in planning often points out the need for more
jacks than had been anticipated. Without this planning and
assignment, installation can fill out the jack field long before
all the necessary circuits have been completed, leaving no
convenient space left in the rack to add additional jacks.
Jacks are operational units that must be viewed from the
front of the rack. So, when making the assignments,
remember this fact. From the front, they should read from left
to right as far as their number assignment is concerned. As far
as the actual circuit they carry, this must conform to other
considerations.
Audio Terminal Blocks
As with the jacks, the main audio terminal blocks in each
rack or major component should be assigned ahead of time.
The main block in the console already has its own assignments
since it came partially wired with the console.
When assignments are made on each block, give thought to
the separation of various circuits according to the signal levels
and functions the circuits carry. And give thought to the main
circuits that need to pass along on these blocks. It is surprising
the number of terminals that can be used when simple changes
in the wiring are desired -for example, adding a jack to the
output of an amplifier circuit. What may have been two
terminals before the decision is now doubled to four terminals.
76
Planning and assigning ahead of the installation points out
the need for more terminals in many instances, and it is handy
to know this before actual installation begins. It is far easier
mounting new blocks in an empty or unwired rack than to add
new ones once there are bundles of cables in place. And it is
especially important if holes must be drilled in the rack or
frame to hold the additional terminal blocks. One thing more,
always be sure to include many more terminals than are
actually needed for the initial installation. There is always
need for some spare terminals in future times.
Block Sheets
Assignment of the circuits on the terminal blocks can be
done most efficiently if block sheets are used. There should be
one sheet for each block. This form can be easily designed by
the station to suit its own way of doing things. The important
thing to remember is that the information must be readily
determined from the sheet at some later time when a problem
arises-the troubleshooter needs information in a hurry, or
when there are to be circuit modifications or additions.
A simple way to design a form is to list the terminal block
numbers from top to bottom of the page ( Fig. 3-4), or perhaps
to slit the numbers into two columns from top to bottom of the
page. On the left side of the numbers, leave space to write in
the name of the circuit or other pertinent identifying data. On
the other side of the numbers, leave space for circuit and
identifying numbers. Head the spaces to the left of the
numbers as external and the spaces to the right of the numbers
as internal. A normal block has 80 terminals, so leave space
for all of these. At the top of the sheet, number or name the
block and show its position, rack, and use. Use your own
judgement. What is needed here is information that helps the
troubleshooter find the correct block in a hurry when the
information is needed. Always remember that in
troubleshooting major problems there is no time for shuffling
papers to find information. The information must always be in
such a form and stored a place that the information can be
readily found.
When wiring up the blocks and making the assignments,
be consistent whenever possible. This is just another of these
patterns we discussed earlier. If the external wiring is
supposed to enter the block on the left side, then make sure
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wxxx
BLOCK
3
RACK
1
EXTERNAL
LEFT
POSITION
USE
INTERNAL
"A"
HI
1
MIKE
LO
2 E5
JACK
KEY
3 JACK
No.3IN
4 F5
LOW LEVEL AUDIO
INTERNAL
EXTERNAL
41
42
39
79
40
80
Fig. 3 -4. Typical audio terminal block sheet. List all the terminals even
though they may be spares at the moment.
that this is done in all cases where possible. Of course, this can't
be done in every case, but try to be consistent. When the
engineer comes along at a later date looking for circuits or
chasing problems, he assumes that the input side of the
terminals is on the left. If it is necessary to lift a terminal, he
can do so with the expectation he is getting the correct one.
Consistent wiring patterns are always a help to the
troubleshooter (which may be yourself). This is particularly
the case on terminal blocks because once all the wiring is in
place and laced up, it is almost impossible to determine the
actual physical direction individual cables are going. Of
course, we are visualizing several blocks at the base of a rack
and many cables. A single block and a few cables do not
present so much of a problem. However, being consistent and
following patterns always proves helpful later. So even though
you are at the moment working in a unit with only a few
circuits, stick to your established pattern. It is no more
78
additional work to do it according to a standard pattern than to
simply wire it without thought to a pattern.
EQUIPMENT PLACEMENT
The actual physical placement of the equipment is usually
very different from that shown on diagrams. Diagrams are
used to show circuits and similar information that is
necessary: they can't always show physical placement.
Sometimes units which are placed at a distance or in some
other unit are shown on the block diagram by enclosing them
in dashed lines ( Fig. 3 -5). This is very helpful if a unit is
placed in another room and interwired.
CONSOLE
1A1
1C1
1C2
1D2
r
Al
1B1
1D1
A2
- - - -J
L
CONTROL ROOM
Fig. 3-5. Equipment location can sometimes be shown on the block diagram with a dashed -in box.
Careful thought should be given to the physical placement
of equipment because thoughtless placement causes considerably more wiring, work, and maintenance difficulty. That is,
the actual placement of a unit has a direct bearing on the
number of jacks, terminals, circuits, and length of cables
necessary to interconnect the unit.
To show how the placement of a unit causes considerably
more circuitry and installation work, consider one of those
optional -placement situations. A peak- limiting amplifier is to
be used at the output of the console in a recording booth.
Should the limiter be placed in the booth or in an equipment
rack in another room? There may be no convenient place in
the booth for mounting. but it could be arranged if desired.
Consider first the booth location: Input and output wiring
should be direct from the console to the recorder selector
switch. This places the operating controls in easy reach so that
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operation can be observed more easily. But on the other hand,
there is no way to get into the input or output of the unit
without actually disconnecting wires. Plus, it may not be such
a good idea to have the unit within easy reach of the booth
operators. who may twiddle the controls as an operational
procedure.
This arrangement requires longwiring circuits from the
booth to the rack and then bask again. At the rack, at least four
sets of block terminals are needed, and four sets of jacks. Four
sets of terminals means eight terminals that must be
accommodated on the terminal block. Of course, you could
wire directly to and from the limiter in the rack without jacks
or terminal blocks, but this is poor practice. If the unit is to be
mounted in the rack, good engineering practice calls for test
entry into and out of the unit.
As you can see from this example, adequate planning
should be given to equipment physical location and to
supplying the additional circuits, jacks, etc. , that are needed
to get the job done.
RACK SPACE
Adequate rack space seems to be a perennial problem, not
only during an installation, but at later dates as well. It is wise,
therefore, to carefully plan the efficient use of every inch of
rack space carefully, then allow for additional space to take
care of some of the future needs. Unless this careful planning
is done. the engineer can soon discover that he has filled all the
available space, leaving several more important units that
must be mounted. This problem is really compounded if the
empty racks are beautifully mounted or built into a wall during
remodeling. The only way now to get additional space is to
knock out some of those new walls. In the worst situation, a
space is selected for racks that absolutely doesn't allow for
additional racks unless outside building walls are knocked out.
So. figure the space carefully ahead of time and then make
sure there is additional space for future use.
Figuring the Space
As a first step in computing absolute rack space needed,
total up all the panel heights of all equipment to be mounted in
the racks. These figures can be found from equipment
specification sheets, equipment manuals, or by actually
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measuring the panels if the equipment is on hand and
uncrated. Figure the total space in inches. Figure all the
heights, including all those fractions of inches. If the panel is
51/4 inches, you may call it a 5 -inch panel, but in the
measurement make sure it is figured at 5.25 inches. This gives
the total vertical mounting space absolutely needed.
Next, measure the vertical panel space the particular
racks permit. This may be taken again from the rack
specification sheets. Be careful here. If the specification sheet
says it can only mount so much, use that figure even though it
may appear you can get in more equipment. Don't gamble on
appearances. Now total all the available space for all the
racks. This is the mounting space you have available. If the
required absolute mounting space is more than the available
absolute space-you are in trouble already! If these two
figures are equal, then you are still in trouble unless you
simply want a display of equipment. There are other factors
which reduce the usable allocated space.
Most Efficient Use of Room
The rack space must be planned for its most efficient and
practical use. As mentioned, other factors reduce the
theoretical available space. The first is spacing of the bolt
holes predrilled into the rack. These holes are drilled at
intervals called rack units, each being 1.75 inches. The panel
heights are in multiples of these rack units. For example, the
5.25 -inch panel is three rack units, etc. By the use of standard
racks and standard panels, all the panels fit snugly together,
filling up the mounting space without gaps. If you start
mounting equipment in the middle of the rack rather than at
the top or bottom. make sure you line up the edge of the panel
at the right location between the holes. You should notice that
these are drilled in an uneven pattern. Place the edge of the
panel so that it falls between a pair of the closely spaced
holes. Then the holes in the panel line up with bolt holes. The
opposite edge of the panel then should be done in a similar
position. Be careful. as it is easy to start wrong. Then all the
panel holes won't line up and you end up with gaps.
There are also racks and panels today that use different
patterns. These have the holes at the edge of the panels. They
don't fit the standard rack. Although you can mount them, you
end up with many gaps between panels. If this type panel is
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used, the rack should be one that accommodates such a panel.
Or you can drill holes to match, either in the rack or in the
panel itself. It is best not to intermix too much, or there is too
much waste space and a poor front appearance.
Another important factor which can affect the use of
front -panel space is what is mounted behind the panels,
particularly what is mounted at the base of the rack ( Fig. 3-6).
There must be room for such things as audio terminal blocks,
transformers, AC power wiring, cables, and sundry other
RACK
FRONT
USABLE
PANEL
SPACE
TERMINAL
BLOCKS,
CABLES,
ETC.
UNUSABLE PANEL
SPACE
NOT ENOUGH DEPTH
TO MOUNT EQUIPMENT
Fig. 3-6. Equipment, terminal blocks, cabling, etc. mounted In the rear of
the rack will limit the usable panel- mounting space.
things mounted at the base of the rack in the rear. Ali these
things make the front -panel space of little use for equipment
mounting, as there is little depth left for the equipment to
project back into the rack. In reality, those bottom panels are
mere dress panels, covering up the front opening, hiding all
the wiring at the base of the rack.
Thus, to be practical. the actual rack space provided must
be reduced to a figure something like 75% of what is available.
That is. if the rack has 72 inches of panel- mounting space, only
about 54 inches of this can be used for equipment mounting. Of
course, this figure can vary somewhat, depending upon what is
limiting the bottom space.
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Heat -Producing Equipment
There is still another factor which affects the practical use
of rack space-the heat -producing equipment. Solid -state
equipment is sensitive to heat. Care must be exercised when
intermixing with heat -producing equipment such as tube
equipment or power supplies. The solid -state units should be
placed as far from the heat as possible; if they are in the same
rack, they should be mounted lower in the rack. Heated air
rises and accumulates more heat from other equipment as it
does. The air at the top of the rack is hotter than that at the
bottom of the rack. Everything that helps air circulation
should be used, such as louvered doors, vented tops of the
racks, an exhaust fan to remove the rack heat quickly, etc.
When you must intermix such equipment, give careful
thought to its placement in the racks, and adjust your required
rack space requirements accordingly.
Future Racks
After you have determined the practical amount of rack
space required, then make the addition of one or two racks to
the lineup for future expansion, even though these racks
remain empty. Broadcast stations seem to be in a state of
constant expansion, so it seems wise to provide additional
space now when the racks are being built into a wall or similar
arrangement. There is the tendency to use space right up to
the amount provided. You have planned your present needs
very carefully, now go ahead and use that planned space as if
it is the only space available. Use it wisely and sparingly. As
far as the spare racks are concerned, leave them empty.
Simply close up the front with rack panels. A less- expensive
method is done with regular pegboard material that is cut to
fill in the full length of the rack. The pegboard can be painted
to blend in with the rest of the surroundings.
WIRING
The air is filled with a great many signals and noise
pulses. All these electromagnetic radiations are potential
interference and noise problems for the broadcast system.
And to compound the problem, all the solid -state devices used
now are more susceptible to these interfering signals than
old -style tube equipment. Careful selection of good shielded
wire for use throughout the system helps immunize the system
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against interference. Equipment manufacturers realize the
problems with the broadcast systems and are building
protection circuits and components into the equipment so that
they are less sensitive to RFI. But unfortunately, the
interconnecting wiring often defeats what has been built into
the equipment.
Shielded Cable
Select a basic audio cable that can be used in many
applications throughout the station and can be bought in bulk
at quantity discounts. The cable can be used for other
applications such as low-voltage control circuits, pilot lamps,
and many other low- voltage applications-these can all be
shielded.
Of course, the cable should mainly be used for audio
circuits. When selecting the wire, look over the specification
sheets. Try to find one that provides 100% shield coverage and
also a ground drain wire. Many of the audio cables today are
available in this style. They cost a little more than those with a
little less protection, but it will be well worth the extra cost to
keep RFI out of the system.
Buy in bulk when you can. This is far easier than buying
different varieties of cables. Stocking is simpler, and some can
be kept on hand even after the installation has been completed.
Try to use the audio cable wherever possible in control and
other low -voltage circuits. By the use of shielded cable, there
is less chance of RFI entering the system through these back
doors. Besides the RFI and noise problems, the shielded cables
also reduces the possibility of crosstalk from these other
circuits. Audio may be picked up and carried along a control
cable, which does not affect the relay or similar control
circuit, but it can carry the audio into another audio circuit,
causing crosstalk or even oscillations.
Microphone cables must be flexible in use and still be
shielded. So select a good microphone cable. This should be a
three -wire shielded cable, using one of the wires as a solid
ground.
Although shielding is an important specification in cable
selection, another important factor is the capacitance within
the cable. This capacitance is directly in parallel with the load
impedance. It affects the high- frequency signals passing
through the cable. In the specifications, try to select a cable
84
with a low- capacitance -per -foot value. Remember that this
capacitance is in parallel, which adds up as the length of the
cable increases. At the end of a very long cable run, this
capacitance is considerable. And there can be many long cable
runs, even though the particular apparent run is short. For
example, a cable may run from a console about 50 feet to a
rack, then through several feet in the rack, then back 50 feet to
the console output, and on to connecting circuitry. By the time
the signal gets to its destination, it may have run through 200
feet of cable in what appears to be a 50 -foot run.
Insulated Cable
Although shielding is important in keeping out unwanted
signals from the circuit paths, these shields can themselves
carry the unwanted signals. When selecting a basic cable,
choose a type that has a plastic outer sheathing. This prevents
individual cable shields from intermittently touching each
other. When shields are bare, this intermittent contact of
separate shields can create varying potential differences at
various places on the shields problems. Bare shielded cable
should be tied together tightly in cables, so that the shields
make a definite contact all along the way. But the better cable
has an insulated outer sheath. This insulated sheath allows you
to control over the points where the shields are grounded. The
basic cable is used for purposes other than audio and can be
routed into areas of low -voltage circuits; a bare shield could
short out these circuits. The insulated shield prevents these
problems.
-
Cable Separation
When many cables of different signal types and levels are
run close together over long spans, there are all sorts of
potentials for crosstalk, hum, noise, and other types of
interference created. Shielding is a must, but in many cases
additional techniques are also required. The cables should be
segregated into types and levels. By types is meant control,
power. and content.
Cables need physical isolation as well as signal separation.
This can best be accomplished through the use of conduit or
ducts. The conduits must be large enough to accommodate
a number of cables of similar groupings. In a few cases,
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separate conduit for a single circuit is used, but for the main
cable runs, several, large conduits are necessary.
When ducts are used, all the cabling run in the same duct,
but they should be separated as much as possible. Some ducts
have metal separators running lengthwise. If not, the cables
can be anchored at various places to keep them apart.
Since broadcast stations have a propensity for quickly
outgrowing their facilities, when making the basic decisions on
conduit or ducts, be generous in the number provided. That is,
try to look ahead for future needs. You can be almost sure your
guess is wrong, but at least try to provide some extra capacity
now. Ducts and conduits are some of the first items installed
during construction of a new building or in a remodeling
project. So try to get the added capacity now because it is very
difficult and expensive to add at a later date.
Signal Levels
Separate cables according to signal levels ( Fig. 3 -7). Don't
get carried away. What is important is that the principle of
separation be kept in mind and applied within practical limits,
not only during the initial installation, but at any time cable
runs are made, now or in the future.
The usual cable groupings that I use are these: (1) microphone or similar low -level signals below -20 B, (2) midlevel signals in the range -20 dB to +18 dB, (3) high -level
signals above +18 dB and speaker runs, (4) power and control
circuits. These are relative groupings, so a few decibels one
way or the other won't upset the pattern. Each of these
groupings can be further subdivided if the situation seems to
call for it. But as with all patterns, once you decide to use one,
be consistent and stick with it whenever possible.
Fan -In
Although you maintain cable separation on the trunk lines,
cables must eventually telescope back together as they arrive
at their destinations, either in the rack or the console. Even so,
the principle of separation can be maintained to some
practical degree. For example, one rack contains high -level.
circuits and equipment, while the other rack contains low -level
preamplifiers and similar circuits. Or a single rack may be
divided, with high -level units at the top and low -level units at
the bottom. Other similar separation can be done, such as one
86
LOW -LEVEL CIRCUITS
LARGE CONDUIT
MID -LEVEL CIRCUITS
HIGH -LEVEL AUDIO
CIRCUITS
POWER AND CONTROL
CIRCUITS
Fig. 3-7. Segregate cables by signal levels.
row of jacks for microphones, and another row, as far away as
possible, for speakers; one audio terminal block at the
base of a rack for low level, and another block for high
level -or a single block with low level on one end and high level
on the other end. The principle is simply keeping them apart
whenever possible and practicable.
Internal Wiring
Cabling within a rack must also have separation.
High -level cables must be laced together in one large cable, all
midlevel cables into one large cable, and low -level cables into
another large cable. Even these larger cables can be run
separated. For example, dress the high -level and midlevel
cables up one side of the rack, and the low -level cables up the
other side of the rack.
GROUNDING
Throughout the station, a definite plan of grounding should
be established, not only during the construction and
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installation stages, but at any time in the future when changes
are made. This grounding can be divided into at least three
main aspects: cable shields, common ground system, and the
relationship of the grounds to lightning surges.
Controlled Shield Ground
The most successful method of handling the cable shields
in the system is through the controlled ground. This method
provides a positive control over the normal noise, crosstalk,
hum, and similar interfering signals that crop up. The system
is most effective when it uses a balanced system of wiring for
all signal circuits. By balanced, is meant that both sides of the
signal circuit are above ground potential.
The underlying principle is this: The shield of each cable is
connected to the main ground at only one place on its run ( Fig.
3-8). The shield must be insulated from all other shields except
at that one grounding point. This means that jacketed cable
(A)
(B)
OPEN
Fig. 3-8. Principle of the controlled shield ground. In A, when the cable is
grounded at both ends, circulating currents can be set up in the shields. In
B, the shield is grounded at one end only. Any currents picked up are
routed to ground and can't circulate.
should be used throughout. By connecting the shield at only
one place, there is an incomplete circuit path to any unwanted
signal that may be induced in the cable shield. And any
currents that are induced are carried to ground at the single
grounding point rather than being allowed to circulate
throughout the shield system. And these grounding points are
only at selected points, not in some haphazard method. The
88
majority of audio cables available today are jacketed and
work with this method.
Ground Points
Actually, there are many grounding points within the
overall system. But these are definite, selected points of
contact with the main ground. Each major unit -such as rack,
console, and transmitter -have a connection to the main
ground. The control of the shield grounding must be planned
ahead of time. A decision should be made ahead of time as to
where the individual cables should be connected to the main
ground. There are often questions during the installation as to
which end of the cable gets grounded. So these should be
anticipated and designated ahead of time. Consistency is
important. or some cables may end up grounded at both ends.
This allows a complete circuit path for unwanted signals
carried on the shield, and circulating currents set up.
Circulating currents must be avoided.
Make a Plan
To plan the shield system, draw a single -line diagram of
the entire system. On this diagram (Fig. 3 -9), show the points
that shields terminate. A common grid can be set up when the
RACK
JACKS
MICROPHONE
CONSOLE
TERMINALS
FRAME
FRAME
BUILDING
GROUND
BUILDING
GROUND
Fig. 3-9. Plan the control of shield grounding ahead of time.
overall system is laid out this way. For example, circuits that
run to a rack ground at the rack. Circuits to a console, ground
at the console. In other words, the grounding is always at the
end of the cable run. A microphone circuit, from a studio that
runs into a rack, through a jack field, and then back out to a
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console must have its shield grounded first at the terminal
block as it enters the rack. The next grounding point is at the
console terminal block. The shield of this cable leaving the
rack has its shield cut off and insulated from other shields. In
effect, this cable run has two grounding points: first at the
rack receiving point, and second at the console receiving point.
Individual Projects
When schematics are drawn for the individual wiring
projects and interface points, always add the shield
information to that drawing. This answers any questions that
arise during the actual wiring as to what to do with the shields.
Except for the major wiring projects, such as the rack or a
console, it is all these small interface areas where problems
arise. So draw in the information to help the installer keep the
system in order. Two installers may be working on the same
circuit but at the different ends. If both are confused and tie
down the shield, there is a ground loop provided, resulting in
possible problems at some later date. Remember to retain this
installation information for later reference when the system is
in operation.
The controlled- shield -grounding method is only one of the
methods in use. There are others that work as well in some
situations, but the controlled method works the best when it is
maintained.
Any grounding system is not a guarantee that there won't
be problems in the system. By taking precautions with the
grounding in the first place, you are at least providing a
greater amount of protection against all these problems; and
for most ordinary situations, you do, in fact, come up with a
very "clean" system. But outside changes can alter the
situation, and greater measures may be called for. For
example, I installed a complete studio system that measured
under 0.5% distortion and noise well below 80 dB. It sounded
beautiful until we increased our F M power to 50 kW and added
vertical polarization to the antenna by moving the antenna
directly overhead. This subjected the audio system to a
tremendous RF field. That blasted FM signal popped out of the
least crack in the system!
Common Building Ground
So that a common reference for all signals and equipment
in the station is obtained, a common building ground must be
90
provided. This is the station's main ground. If this common
ground is not provided, many possibilities exist for differences
of potentials developing across any intermittent contact or
irregular ground point that can produce problems of hum,
crosstalk, transient noises, signal intermodulations, and
system instability. At the same time, searching out and
correcting problems that do develop is made more difficult.
The building ground should be a heavy copper strap run
throughout the building and connected to ground at one or
more places by copper rods driven several feet into the earth.
Everything within the station is grounded to this strap.
There are many ground currents flowing through this
strap and from many sources, so it must be a heavy strap and
electrically one continuous piece. It must provide a very low
resistance to all these currents. It should be at least two inches
in width and wider if the expected currents are to be higher
than average. Some stations with high -power transmitters
nearby use a width of six inches or more for the strap.
Remember that RF signals travel on the surface due to skin
effect, so a wider strap is recommended with large RF signals
close by.
Naturally, it is difficult to roll out a single strap to route
throughout the station. It is normally several pieces connected
together. But when connections or splices are made in the
strap. make sure they are very low- resistance joints. Use a
hard solder, such as silver solder, to make the electrical
connection. Any resistance produces voltage drops. If the joint
is a nonlinear resistance because of corrosion or other
chemical action, intermodulation of signals occurs.
Each major unit of the system must connect to this main
ground by another heavy copper strap. The strap at the unit
need not be as wide as the main ground strap itself, unless the
piece of equipment has heavy ground currents. Thus, each
rack, console, transmitter, conduit, and power circuits must
connect to this ground bus. At each unit, make sure the strap is
bolted or soldered to the frame of each unit. On a rack or any
other unit that is painted, scrape away the paint so that a good
metal -to-metal contact is made.
Lightning and Grounds
Lightning generates enormous potentials and currents.
Currents must flow to earth ground by the shortest route and
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as quickly as possible; they must not be allowed to circulate
through the system ground. If these currents do circulate
through any part of the ground system, there can be many
unexpected results at the most unexpected places.
A surge or transient increases to a very high -potential
peak value, then trails off to a lesser value that may oscillate
into one or two overshoots or rings at the trailing edge of the
transient. The duration of the transient is very short, in the
order of milliseconds. with the rise time in microseconds.
This sudden change has AC characteristics and can be
translated into megahertz from a frequency standpoint. Since
this is an AC signal. the length of the ground conductors takes
on importance. since they exhibit inductance components at
the high frequency of the transient (Fig. 3 -10). And since this
surge current is in the order of 20 kA. tremendous inductive
voltages build up between the ground conductors and other
ARRESTER
EQUIPMENT
.'
L
.'
1
j.'
/
LONG
GROUND
LEAD
INDUCTANCE
FLASHOVER
VERY
HIGH
INDUCTIVE
VOLTAGE
Fig. 3 -10. The inductance of the long ground lead at the transient frequency generates dangerously high inductive voltages that arc over to ad-
jacent units.
nearby objects such as racks or equipment. These potentials
are dangerous; they create tremendous arcs just like the ones
in science fiction movies. In our previous considerations of
the building group strap. we were concerned with its
low -ohmic values and the skin-effect RF resistance. But the
92
length, in this case, should also be given full consideration
when designing the ground.
When any length of the building ground bus must be more
than 25 feet in length, try to break it up in some way. This may
be done by running parallel conductors or some sort of grid
system that breaks up all the runs into small segments. That
is, run several crisscross members to the ground bus and
make sure these are all soldered together. Take the ground bus
to earth ground at as many points as you can. What you are
trying to do here is break up the length so that is does not offer
a suitable inductance to the current that may get into the
system from the surge transient.
SURGE PROTECTION
The deadliest interference problem in the age of
solid -state is the transient signal problem. Solid -state
equipment is very suceptible to transients and most vulnerable to damage. The station must take adequate precautions to
protect against this problem, or it will find its solid -state
equipment on the bench for repairs more often than it is in the
racks operating.
The transient can come from many sources, both within
the station and outside the station. From inside, transients can
be caused by heavy motors turning on and off, such as large
air conditioners; switching of heavy current loads, such as in
transmitters; and similar switching operations that cause an
abrupt change in current flow. Studios have shown that the
normal 120V AC power system can carry transient spikes of
over 5000V for a very brief period. These spikes are so brief
that they don't trip a circuit breaker or blow a fuse, but they
can blow out solid -state equipment quickly.
From outside the station, transients can be generated by
manufacturing plants and their heavy equipment or power
lines swinging together in a windstorm, but the most common
cause is lightning. The lightning does not need to strike the
line. Nearby strikes induce large transients into the power
lines.
Once a transient begins to build up, it needs to be routed to
earth ground quickly. Let's consider the lightning surge over
the power lines as our main problem. The best place to route it
to ground is at the power entrance to the building. Some type of
arrester or suppressor must be mounted directly to this
primary entrance with a direct path to earth ground provided.
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The alternative method is protection of each individual
equipment item (Fig. 3 -11). However, it is better to stop that
surge at the door, rather than let it circulate, raining lightning
bolts through the building. But if the incoming arrester is not
totally effective, it won't be a bad idea to also protect some of
the more sensitive equipment.
EQUIPMENT
/
TRANSIENT
A
120V AC
GE-MOV
SHORTS OUT
TRANSIENT
Fig. 3-11. Individual units can be protected from transients coming in on
the AC power circuit by the use of devices such as the General Electric
metal oxide varistor.
Lightning Arrester
There are several firms supplying arresters. While these
different models try for the same end results, they are far
from the same in their principles of operation. Each of the
manufacturers has a different way of doing the job. But
regardless of the method, all fall into the category of a
"crowbar" circuit. That is, once the transient comes on the
line, it turns on the arrester. The arrester shorts the power line
to ground for the period the transient is in operation.
Ordinarily, this is only for a few microseconds or perhaps
milliseconds. At any rate, it is always less than a half -cycle of
the power line frequency. But during the transient turnon, the
power line itself is also shorted to ground. And it is important
that the arrester shut itself off once the transient has passed. If
it doesn't, the power line remains shorted to ground. The flow
of current from the power line to ground is called power follow.
Fusing
Tremendous currents are flowing during a transient, but
only for a very brief period of time. However, the circuit
breaker or fuse of the power line itself must be such that it
doesn't trip during this flow of current. If the arrester is wired
94
into the primary ahead of the main fuse, the arrester itself
must be fused. There must always be a fuse or circuit breaker
between any load and the primary power. But the particular
fusing must be of a special slow -blow type, otherwise the fuse
or circuit breaker trips every time a transient comes on the
line.
Grounds
As discussed earlier, very high currents flow through
these grounds when a strong transient is present. The ground
lead from the arrester to earth must be very short and direct.
Besides that, this ground lead should also be insulated from
nearby objects so that any potentials that build up will not also
flashover. Avoid tying it into the building ground system.
Power Line Circuit
Power distribution throughout the country is not done in a
uniform manner. It often depends upon practices in various
parts of the country as well as the individual power company.
Selection of an arrester for your station requires
information on the method of power distribution at your
station. Unless this information is already on hand, a call to
the local power company will probably obtain it for you. But
stations which have been in operation for many years may
have had many changes over the years, so the power company
records may but be up to date. However, they wish to have
their records up to date, so they usually send out an engineer to
survey the system to give you the desired information.
A station can do its own investigation and come up with the
information. This requires a visual inspection of the
transformer banks as the power lines arrive at the station.
Take a paper and pencil and sketch the number of
transformers in the bank. Take particular note of the
secondary connections. It won't be difficult distinguishing
which are primaries and which are secondaries. Observe how
many wires go into a weatherhead (the down conduit to the
station). There are several transformers in the bank, but each
grouping feeds a power entrance to the building. There can
also be more than one entrance from the same bank because
very heavy equipment may have its own special feed
directly from the bank. These additional feeds are easy to spot
since they tie to the same place on the secondaries as do other
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(CLOSED DELTA USES
3RD TRANSFORMER)
*) yi
240V
--1-
10
120V
-t-
208V(l-IIGH
Nt120V
f»
24OV
10
PHASE)
240V
Fig. 3-12. Typical open-delta secondary connections for a 3 -phase system.
One transformer provides a 120V single phase. Voltages shown are the de-
sign parameters.
weatherheads. But observe carefully-one of the transformers
may be a part of a three -phase bank but also providing a
single -phase entrance. Also note the main power panels where
these enter.
Measure the Voltages
Now that you have drawn a sketch of the transformer
banks and the wiring of them, go to the power entrance panels
and measure the actual voltages (Figs. 3 -12 and 3 -13). When
three -phase and single -phase power are obtained from the
same bank, the secondaries may be in either an open- or
closed -delta configuration. If it is an open delta, there are only
two transformers in use; one of these has a center -tapped
secondary. This tapped transformer provides 120V AC
single -phase power on each side of the tap. On a closed delta,
208V
-11-1
208V
208V
1
!
I
120 10
0- NEUTRAL
(120V FROM ANY
ARM TO GROUND.)
Fig. 3-13. Typical Y- connected secondaries of a 3 -phase system. Voltages
shown are design parameters
96
the third transformer is used for the third phase, and the
single -phase power is obtained from one of the transformers as
is done in the open delta.
In either case, you should be able to measure 120V AC
from center tap to either end of the transformer. From the end
of the delta opposite the center tap to the center tap, the
voltage measures 208V AC. End -to-end voltage measurement
across each transformer in the bank provides 240V AC. That
208V is called the high phase, even though its voltage is lower
than the end- to-end measurements.
These voltages are the design norms so they may or may
not be the exact values that you measure at your installation.
Actual voltages depend upon the voltage of the primaries
themselves, but the ratios remain the same.
Selecting an Arrester
When selecting an arrester, the normal values of voltages
you measured are important, as well as the fluctuations.
However, the high excursion is the more important than the
low one. This is because the arrester selected may be working
near its maximum design limit. When a transient comes on
and turns the arrester on, it may not be able to turn itself back
off when the transient passes. When requested, the power
company usually installs a recording device on the line to
measure its high and low excursions during a 24 -hour period.
Installation
This job is best left to an electrician, and in many areas of
the country. only a licensed electrician can do the job anyway.
The arrester is normally connected directly to the
incoming power line at the building entrance and ahead of the
main circuit breaker ( Fig. 3 -14). If it cannot be mounted ahead
of the breaker, you may have to change the breaker to a
slow -blow type. If the arrester is connected ahead of the
breaker, it must have a disconnect switch and its own fusing.
When possible, add small neon lights to the input side of the
arrester, so that these can be an indication that the fuse is
okay and that the unit is connected to the power line. Without
some indication, it is possible the arrester is open, the fuses
are open, or the switch has been left open. In all these cases,
the protection has been removed from the circuit.
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C
FUSES
n n n n
EARTH
GROUND
/47
ARRESTER
Fig. 3-14. Typical surge arrester installation on a 3-phase system.
The ground lead from the arrester should be as short as
possible and should go directly to earth ground. This should
also be an insulated wire to prevent flashovers to adjacent
panels if high voltage builds up on the ground lead.
Maintenance
Actually, there is little to do in the way of maintenace of an
arrester. One should check from time to time to see that it is
actually in the circuit and that the fuses are still intact or
circuit breakers not tripped. If there is dust collecting in the
box, blow this out occasionally. But be very careful when doing
anything around these circuits. A short across a pair of the
buses creates a blinding flash that can damage the eyesight
and cause serious burns.
INSTALLATION
After all the planning has been done, we must sooner or
later get about the task of making the installation, and there
q8
are easy as well as hard ways of doing things. The following
are few of the techniques I have developed and used with
reasonable success:
Pulling Cable
On a large project, when many cables are run through
conduit between two locations, it is difficult determining what
length to cut the cable or cables.
First, run a "snake" through the conduit. Then tie onto the
end of the spool to pull in one cable. Make sure it is adequate
length to reach the desired location on each end of the conduit.
If the distances vary, then make them all reach the farthest
point. To accomplish this, pull the first cable back out of the
conduit. Use that measured cable as the yardstick for the rest
of the cables. Tie or tape them together every couple of feet, if
desired, so that you don't end up with a tangled mess on the
floor before they all get pulled into the conduit.
Now that you have made a large cable out of all the small
cables, tie these onto the end of the snake. Make sure there is a
good tie. The end of the snake should be bent over into a hook.
There is tremendous pull on that joint; make sure it is tied
down good and tight. Next, tape over the hook on the snake so
that it can't hook a cable already in the conduit.
Of course, you should have decided which end of the
conduit you will work from. It takes some elbow room for the
cable when the pulling action starts; be sure to find the best
way to pull. It doesn't matter as far as the wiring itself is
concerned. Have someone feed the cable into the end of the
conduit. He can keep any kinks straight and help keep the
cable from binding on the edge of the conduit opening.
Identifying Cables
There are a couple of ways the cables can be identified.
One method calls for tagging each cable before it is bundled
into the large cable. But this can be time consuming, and if the
tags are not the small roll -a-round type, they can become so
mangled after the pulling operation that they are unreadable
anyway.
The next method is the ringout. In this method, all the ends
of all the cables are bared back so that you can get at the
wires. Simply skin back the insulation a bit and keep the wires
apart so they don't short to each other. You can use an
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(A)
0
OSCILLATOR
(B)
HEADSET
0
0
0
>4
OHMMETER
SHORT
Fig. 3-15. Two different ways to ring out the cables. In A, an audio oscillator and a pair of headphones are used. In B, shorting one end of the
cable and then using an ohmmeter at the other end is the method.
oscillator with a pair of headphones or use an ohmmeter. With
the oscillator method. attach the oscillator to one pair of
cables. then go to the other end and listen across various
cable pairs until you find the pair with the tone. With the
ohmmeter method. short one pair of wires; go to the other end
with an ohmmeter and look for the short. If there is any doubt,
make and break the short while watching the meter. Either of
these methods works better when you have a helper. At least
there is a lot less walking between the ends. If you have some
intercom arrangement. use one of the cables for a
communications pair until you finally wire all the others into
the circuit. then do that one last. It is best to find a pair, wire
in both ends to the terminal blocks, find another, and so on.
There is still one more way: Wire in all the cables to the
terminal block at one end of the run; then ring them out. But
be careful that you aren't reading the input resistance of a
transformer or stage.
If you have many cables to check, another little trick can
speed the process. Fan out all the cables on the floor, holding
100
them with your foot. Then take the ohmmeter and quickly
cross each pair until the shorted one is found
73933
Jack Fields
Wiring up jack fields is a tedious process; there are many
connections to be made. Whenever there are many repetitious
actions to make, always try to employ production -line
techniques.
Many jacks are constructed so that a single piece of bare
No. 14 wire will lay right across the ground terminal of each
jack in the field. This makes a neat installation, but keep the
wire straight. If it is bent in many places, it can be
straightened by placing it between a couple of boards and
giving a few taps with a hammer. Lay this bare wire across
the ground terminals, but make it long enough so that it
reaches to the main rack ground. Solder each of the jacks, then
solder the end of the wire to the main ground. This is the only
ground necessary at the jacks because all the audio cables are
grounded at the terminal blocks at the base of the rack. The
shield at the jack ends are cut off (on the cable).
Jumper Connections
There are many small jumpers to be made and wired in
for the "normal" connections on the jack field. Do this: First
deside on the length one of the jumpers should be. Don't install
it, but instead use it as the measure. Then measure and cut off
all the necessary jumpers for all the jacks. Install the jumpers
on all jacks mechanically first. Next, solder all of them. It is
less time -consuming to wire in all the normals than to wait and
decide which will have normaled jacks and which will not.
When the normal is not desired, it can easily be snipped off.
Wire Dress
Whenever there are hundreds of connections to wire and
dress, practice motion economy. That is. use as few motions as
necessary to do the operation. For example, first decide the
length the outer covering must be stripped. Mark off the
sample length near the spot you are working. You may be able
to mark the length on the strippers themselves. Then it won't
be necessary to actually measure each time-just use the
marks each time to measure (without laying down the
stripper). Do the same with the individual wires. Thus your
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operation should be simple motions-measure the outer shell,
strip, measure the individual wires, and strip -all this without
putting down the stripper or changing position in your hand.
Practice the motion economy that works best for you; make it
a habit. If you think this is a small point, observe other
engineers in the way they dress the end of a wire cable. It can
be amazing the number of motions some individuals make in
this simple operation!
Fan-In
When dressing the cable into a jack strip, terminal block,
or whatever connector, the cable must usually approach from
one end of the terminal. A neat job can be done if the wires are
measured and fanned into the connections.
Make sure all the cables are long enough to reach the
greatest length, then tie the cable at the entrance to the
terminal unit. Make sure it is tight so that individual cables
cannot be pulled out. Again, the stripper may be used for a
measure; decide the length of the first one, strip, and so on.
This process can be measured for the first few, but it won't be
long before the engineer can eyeball the length and come very
close. Start with the shortest cable and work each one until the
end one is reached. Work the length so that it is directly behind
the jack before measuring the length into the jack.
Lacing
After the wires have been carefully dressed into position,
lacing will make the finished product. You can either use a
lacing twine or some of the cable ties. The twine makes a
better looking job, but it takes a little longer to use. Use a good
waxed lacing twine. Cut off a few feet; don't make it too long
or it will be difficult to work. As each piece runs out, simply tie
another piece to it. Make the loops so that each one pulls down
tight and stays in place if let go. That is, lace over and under so
that the tight straight piece is holding the loop in place. At the
end, make three or four knots; snip off the excess. Another
hint: Use a golfing glove or simply use a couple of Band -Aids
over the little- finger joint where you naturally pull the string
tight. After a couple of ties, you will be able to determine
where this spot occurs. If you don't do this, the string
eventually cuts through the skin and you will have some very
sore fingers.
102
Chapter 4
Audio Characteristics
and Problems
There are several important audio characteristics that affect
the operation of a station's audio equipment. Many of the
problems that arise and require maintenance can often be
traced to these characteristics, which have either been
ignored or inadvertently disturbed. Only a few of the main
characteristics are discussed in the chapter, along with some
problems and their sources.
LEVEL SETTING
There are two different aspects of signal levels that should
be distinguished in these discussions. Program levels are
those which occur during normal programming, caused by the
program itself. Standard system levels are those around which
the system has been designed, that is, the setup levels.
Program Levels
During operation, program levels vary according to the
particular program, but the peaks should be maintained as
read on the VU meter. When signal levels are very high, they
should be adjusted back within limits by the gain control. The
same can be said of low signal levels: Adjust upwara to get
them within limits. This is simply old -fashioned gain riding,
which is a lost art today. This is so because too much reliance
is placed on AGC (automatic gain control) and limiter
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amplifiers to take care of all gain problems. And secondly
many operators are basically announcers doing their owr
switching. Actually, AGC amplifiers do a fine job of riding
gain-so long as signal levels are within their range. They
cannot do the entire job; but when signals are within their
range. they can often do a better job than an operator.
Meter Pads
The signal levels read on the VU meter are not necessarily
the true signal levels of the bus they are monitoring. The VU
meter itself has a definite full-scale range, just as any other
meter. When the bus levels must be higher than the normal
range of the meter, then pads must be inserted to bring these
levels back into range. The pads, however, must only control
the signal level to the meter, and must not affect the actual
signals on the bus itself.
The range of the meter is approximately -20 dB to +3 VU,
full scale. The meter can read bus levels that are within this
range, but in many cases they are higher than this. A pad must
be inserted to bring the bus level back within range of the
meter. Consequently, the meter may be peaking at 0 VU, but
the bus level may actually be +8 dB on the peaks.
The VU meter almost always has pads in its path (Fig.
4-1). The internal impedance of the meter is 3900 ohms, and if
PROGRAM
LEVEL
BUS
f +4 dB
y/ METER
INTERNAL
IMPEDANCE
IS
390011
PAD
Fig. 4-1. VU meters always have resistance pads added, so they present at
least a 7500 -ohm bridging to the program bus. With a total of 7500 ohms
for the pad and meter, the meter will indicate 0 VU when bus is +4 VU.
this is placed directly across a loaded 600 -ohm bus, it will load
the bus down a little (about 0.6 dB). Usually, there is enough
resistance added so that the meter circuit appears to be
104
approximately 7500 ohms. The meter will read lower, of
course. It will be lower by approximately 4 dB, or to put it
another way, it takes +4 dB on the bus to make the meter read
0 VU.
Standard Levels
The system is arranged to operate at certain levels, for
example +8 dB. The VU meters are then padded to read 0 VU
when these levels are present. That is, the system levels are
+8 dB, and not the 0 VU that the meter -and -pad combination
indicates. Every station should have its system set up around
standard levels. Industry standards may be used, and this is
preferable; but any standard can be set. Most broadcast
equipment, however, is designed to operate at the nominal
industry standard levels. Industry standards are essentially
these: low-level microphones -55 dB; midlevel, -12 dB,
program bus levels, +8 dB; other high -level monitor circuits
+24 dB. The station standards need not be these exact values,
but should be in the neighborhood.
Consumer Items
Many times, certain equipment items designed for the
consumer or hi-fi market find their way into broadcast
stations. This happens quite often in smaller stations, and even
in some of the larger ones. This is not to say that such
equipment should not be used, but the methods of designating
input and output levels and impedances may be different.
When mixing such equipment with regular broadcast
equipment, look at the specifications carefully to determine
what they mean. You don't want to compare apples and
oranges. The broadcast standard is based on power and a
definite impedance (Fig. 4 -2). Program buses are at 1 mW in
600 ohms, representing a 0 VU level. This is also a voltage level
of 0.775V across a 600 -ohm impedance.
Sine and Complex Waves
The program signals are made up of complex waveforms
and are far different from the sine -wave signal obtained from
a signal generator. It is a paradox that our broadcast system
must use the complex wave for all its program operations, but
the majority of our test and setup adjustments must be made
with the sine wave. This is necessary since many
measurements require the signal to "stand still" long enough
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4
60012
LOAD
IMPEDANCE
PROGRAM
BUS
Ip
RMS POWER =1mW
(.001 WATT)
VU METER READS 0 dB
RMS VOLTAGE = 0.775V
Fig. 4-2. The broadcast standard reference level is a measurement of
power in a program circuit, with a specific 600 -ohm load impedance. This
is designated as 0 dBm (the small m means milliwatt reference).
part of this is that the
equipment often behaves differently when amplifying the
complex wave than it does with the sine wave. Consequently, it
is important that the engineer understands the differences
between the two waveforms, and the different equipment
reactions, so that more meaningful measurements and
adjustments can be made to the system.
The sine wave obtained from a signal generator will
provide those nice, symmetrical "text book" waveforms. The
positive and negative halves of each cycle are identical in
amplitude and beautifully shaped, and each cycle repeats
itself until the generator is shut off. There are many things
that can be said for the use of this test signal. It does meet all
so we can measure it. The unfortunate
SINE WAVE
COMPLEX WAVE
Fig. 4 -3. The sine and complex waves.
106
the standard formulas for AC signals-peak -to -peak value.
averages. etc. -regardless of the audio frequency that is used.
The complex wave is another story altogether. In the first
place, very few of the cycles repeat themselves. Actually,
there are many cycles mixed together as the sounds the wave
represents occur. As a matter of fact, they bear little
resemblance to cycles in the sense of a sine wave. Besides
that, positive and negative portions are not always identical in
amplitude. The term complex wave adequately describes this
signal. Besides being so irregular in shape, the average, RMS,
and peak values are constantly changing in relationship to
each other, depending upon the program content at the
moment. These relationships are further changed when the
program signal is run through signal processors and
equalizers.
Peaks and Averages
The average value of an unprocessed complex wave in
relation to its peak value is almost always far lower than in a
sine wave Fig. 4-4). This change in relationship is due to the
waveform itself. To get a higher average value, each cycle or
time period must have more "body "; that is, cycle width in
relation to peak amplitude. In a given time, say one second,
the complex wave may run through a dozen or more cycles,
each with a different width-to-peak ratio. Anyone familiar with
pulse measurement, and particularly trying to rectify a pulse
signal to obtain a DC voltage, knows what a disappointingly
low value of DC voltage will be recovered from a
high- amplitude, narrow -width pulse train.
The peak amplitudes of the complex wave can be 8 to 12 dB
(or more) higher than the average value of the signal. And it
takes a special meter to read these peaks with some degree of
accuracy. This meter is the VU meter, and it is especially
designed for reading the peak values of a complex wave. It has
special damping to prevent overshoots and vibration of the
meter hand, and other ballistics so that it can reasonably
follow the wave. When it is measuring a complex waveform, it
is indicating in volume units (VU). and not decibels. The
standard AC voltmeter, as found on regular test sets or
multimeters, does not have these characteristics and will not
read correctly.
Any meter, including the VU meter. is an electromechanical device. so there is inertia present. Consequently,
I
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SINE WAVE
- PEAK
-RMS
i
THIGH AVERAGE
(A)
L,
/2
VALUE
CYCLE
COMPLEX WAVE
r
- -PEAK
(B)
-RMS
V
LOW AVERAGE
VALUE
L1/2
CYCLE
PULSE
(C)
_i
L
-j
PEAK
- - RMS
VERY LOW
AVERAGE
VALUE
Fig. 4-4. Comparison of sine wave, complex wave, and pulse.
fast, narrow peaks arrive and go by before the meter can
react. A sine wave, however, can be measured with the VU
meter (Fig. 4 -5). With symmetrical, repetitive sine waves,
both the AC voltmeter and the VU meter will read the same.
They will also both indicate in decibels. But even with all its
special characteristics, the VU meter, on a complex wave, will
still indicate lower than the true peaks of the signal. And these
peaks will be far different than the decibels read on the VU
meter with a sine wave.
An interesting test can be set up to verify the foregoing
discussions on complex and sine waves (see Fig. 4 -6). Use the
console. and a remote amplifier or similar device that has a
VU meter across its output. Make sure the output is properly
terminated in 600 ohms and the input signal is not the type that
has been processed. Use a microphone for best results, or use
108
a record without any other processors. (The peak -to- average
values will be somewhat distorted because of the processing of
the program before recordings.)
Next, feed a signal generator into one input, making sure
to use the correct impedance matching, and set the VU meter
to read 0 VU. Select a frequency that will not fall within some
equalizer range. Now, place an oscilloscope across the output
resistor and set the signal display to some arbitrary value.
However, make sure you leave plenty of space on both top and
bottom of the trace. Do not change the scope input after this
calibration.
(A)
0dB
VU METER
INDICATION
PEAK
(SINE WAVE)
1/2 CYCLE SHOWN
(TRUE
PEAKS
VU METER
INDICATION
(COMPLEX WAVE)
Fig. 4-5. When sine wave signal is used to set up the program level, (A) VU
meter will read 0 on sine wave peaks but program peaks will be much
higher. (B) The meter will indicate peak values as 0 VU, but the true peaks
will be 8 to 12 dB higher.
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rJ
AMPLIFIER OR
CONSOLE
SIGNAL
GENERATOR
ai60052
TERMINATION
PROGRAM
OVU
VU METER OSCILLOSCOPE
1.
CALIBRATE METER & SCOPE WITH SINE WAVE
METER
2. SWITCH OFF SINE
SCOPE
WAVE & SET PROGRAM PEAKS TO
0 VU ON METER.
OBSERVE: PEAKS ARE MORE THAN DOUBLE THE
AMPLITUDE OF THE SINE WAVE CALIBRATION ON SCOPE.
3.
SINE WAVE
CALIBRATION
SCOPE
Fig. 4-6. A test setup to compare sine wave with complex program wave.
110
Do not change any gain controls, but switch off the signal
generator. Feed the mike or program source into another
fader and adjust the gain control of the channel so that the VU
meter is reading 0 VU on the peaks. Observe the oscilloscope.
The program peaks will be much more than double the peaks
obtained with the sine wave. The scope, by the way, is
indicating peak -to-peak values, but it will be the same for both
signals. so the indications are still valid. The scope also reads
voltage. Since the program peaks are more than double the
sine wave peaks. they are more than 6 dB higher. Doubling the
voltage is 6 dB, but since these peaks are more than double,
the figure is more than 6 dB. This is only a relative test to
visualize what is happening in the circuit. Compressors and
peak limiters, of course, lower these peaks considerably.
Headroom
A very important aspect of level setting and control, which
is often overlooked, is headroom (Fig. 4 -7). The amplifier
designers do give consideration to headroom; how much will
AMPLIFIER
STATURATION POINT
TRUE PEAKS
HEADROOM
O
PEAK IS
CLIPPED
[14
VU
VU METER
INDICATION
INPUT SIGNAL
OUTPUT SIGNAL
Fig. 4-7. Headroom is the space between the peaks as read on the meter
and the amplifier saturation or overload point. Any peak past the saturation point will be clipped.
reflect the quality of the amplifier. In the previous discussions
and demonstration, it was shown that the peaks of the program
material are far more than what shows on the VU meter. The
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amplifier must be able to pass these peaks without going into
nonlinear operation or clipping. The region between the peaks
indicated on the VU meter and the point the amplifier goes into
distortion is the headroom.
Lack of headroom can be due to poor amplifier design, or
it can be due to an improperly chosen system standard level or
improperly adjusted levels. For example, if +8 dB is chosen,
make sure all VU meter pads are set to indicate 0 VU at this
level. If they're set wrong, levels can be higher than indicated.
Check for headroom after the levels have been set up.
Feed the signal generator at 10 dB higher to the system.
Measure distortion first at the standard level, and then at the
10 dB higher level. If distortion increases at the higher level,
then headroom is not adequate and the program peaks will be
distorted. It will be necessary to choose a lower standard
system level. In this case, perhaps 0 dB should be used as a
standard system level.
DISTORTION
The signal going into a system should come out the other
end unaltered in any way, except for an overall increase or
decrease in signal amplitudes across the pass band. Whenever
the signal is altered in some way, it is distorted. In practice,
however, there may be many deliberate, controlled alterations
of the signal. Although in a strict sense these are distortions,
they are not necessarily bad. We don't usually think of these
controlled alterations in terms of distortion, but those which
are uncontrolled and detract from the signal are considered
distortions. They fall into four categories: phase, frequency.
amplitude, and intermodulation distortions. Whenever any one
of these forms is present, other forms can also be present.
Phase Distortion
This type of distortion occurs when all signals in the pass
band do not pass through the system in an equal time
reference. That is, some frequencies may lead or lag other
frequencies. Faulty components and stage overload can cause
shifts in the phase. Small amounts of phase shift may not be
detectable in a monaural system nor give any real problems.
FM stereo is a two -channel system that relies on the
matrix action in the stereo generator and in the receiver for
proper operation. Phase is very important, and varying
112
amounts of incorrect phasing will cause deterioration of
channel separation. Out-of-phase (180 °) signals cause channel
reversal and cancellation in a recovered monaural signal Fig.
4 -8). Each channel should be identical in the stereo system,
and the patch length of each should be electrically the same.
(
(A)
LEFT
-
RIGHT
(RIGHT)
LEFT
h
RIGHT
LEFT
STEREO
PROCESS (LEFT)
RIGHT
(CABLES REVERSED)
(B)
LEFT
RIGHT
LEFT
RIGHT
MONAURAL
COMBINING
PROCESS
f
MONO OUTPUT
OUTPUT
SIGNAL
CANCELLED
(ONE CABLE
REVERSED)
Fig. 4-8. When 180-degree phase shift occurs: In A, the cables in the stereo
system are actually reversed, which interchanges left and right channels.
In B. when one of the stereo cables is reversed and stereo is fed into a mix-
ing process to produce a monaural signal, the out -of -phase signals
cancel.
Both of these phasing problems can be caused in the
installation as well as the operation. Path length problems
often develop when telephone company lines are used for the
connecting link to the transmitter. Both of these lines are
equalized, and this in itself can cause phase shifts. The
physical length of the individual line may vary considerably.
There are other forms of phasing problems, and from
different sources, although we may think of these in terms of
feedback. That is, output signals get back into the input stages
in various ways, such as circuit capacitance, wiring
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capacitance between cables, input /output connectors too close
together, or connections close together in jack fields, terminal
blocks or elsewhere. If the output signals are in a phase
direction to increase signal amplitude, this is positive
feedback; and if they're in a direction to reduce or cancel
amplitude, this is negative feedback. In positive feedback,
oscillations can occur. However, if the feedback is not quite
enough to cause the circuit to go into oscillation, circuit gain
near certain frequencies can rapidly increase Fig. 4 -9). This
is similar to the condition used in the superregenerative
receiver. The Q of the circuit near the resonant frequency,
however. may not be high enough, so the resonant curve will
(
IN -PHASE
FEEDBACK
SIGNAL
PEAKED OUTPUT
WITH FEEDBACK
/NORMAL SIGNAL
(A)
OUTPUT
AMPLIFIER
IA
NORMAL
INPUT
SIGNAL
(B)
100
AMPLIFIER
BANDPASS
CU
CURVE
1K
5K 10K 20K
HIGH O
IN CIRCUIT
100
1K! 5K:10K 20K
LOW O
IN CIRCUIT
Fig. 4-9. (A) Positive feedback can create a signal boost or oscillation. (B)
How wide the affected frequency area of the bandpass depends upon the
0 of the audio circuits involved.
be broad. Only a few frequencies are affected, and this creates
a peaking effect. In negative feedback, there is an opposite
effect. That is, there is a cancellation of a few frequencies. or a
notch effect. What actually occurs in each case will depend
upon circuit conditions and the nature of the feedback signal
itself.
114
RFI
Another source of feedback can occur when RF
interference (RFI) is present from the transmitter being
modulated with the same audio signal. The RF signal is picked
up and demodulated by circuit components, and the resulting
audio fed into the offending stage. The same conditions can
exist as were discussed for regular audio feedback. But there
can also be another type situation. In this case, there may be
considerable delay (Fig. 4 -10) The resulting signal then has an
echo effect, and the greater this delay, the worse the effect.
This is similar to the effect of the input /output signals from a
tape recorder being fed into two open channels on the console.
.
RFI FEEDBACK
AUDIO
NORMAL OUTPUT SIGNAL
ECHO SIGNAL
V
I
\
1
I
DELAY
NORMAL
AUDIO
AMPLIFIER
CANCELLATION
IN THIS AREA
Fig. 4-10. RF feedback to the audio that is delayed can cause an echo
effect.
Strong RF signals can be rectified by circuit components
or the transistor itself. This can be converted to DC voltages
by circuit elements filtering the rectified RF. This voltage
may then be applied to a stage's bias, for example, and thus
shift its operating mode into a distortion zone. The stage may
be able to handle most of this, or the pickup may not be strong
enough; but the shift can be to the point that the signal is being
clipped on the peaks. This has its own peculiar sound in the
audio, that of a severely overloaded stage.
Whether from shifting operating points or a strong audio
input signal, the positive or negative peaks, or both, may drive
a stage into saturation. This means outright clipping of the
signal. There will be distortion because the output is not a
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perfect reproduction of the input, and a clipped signal will be
very rich in harmonic content.
Frequency Distortion
This type of distortion occurs when all the audio signal
frequencies are not amplified equally across the pass band. In
other words, the response curve of the pass band is poor. This
frequency discrimination can occur in any part of the pass
band (Fig. 4 -11). For example, if the discrimination occurs at
high frequencies, there will be a rolloff or cutoff of the
(A) RESONANT CONDITION CAN CAUSE PEAKING
n
100
(B)
1r1(
5.10K
15K
I
SHUNT CAPACITY CAN CAUSE
HIGH -FREQUENCY ROLLOFF
I
100
1K
5K
10K
15K
(C) LOW -VALUE SERIES CAPACITANCE CAN CAUSE
LOW- FREQUENCY ROLLOFF
\
100
1K
1K
1
10K
15K
Fig. 4-11. Different ways frequency distortion can occur in the pass band
of the unit or system.
response curve at the high end. And when it occurs at the low
frequencies, the low end of the respone curve suffers in the
same manner. If there is a sharp rise of a few frequencies at
116
any place in the pass band, this is described as peaking or
boost, and the opposite of this is a notch or cut. Naturally,
equalizers do all of these things to a response curve; that is
their purpose. It becomes distortion when the amplifier or
circuit does it all by itself.
Reactance
Two factors which cause frequency distortion are the
capacitance and inductance of circuits or components. Both of
these have a reactive value that is frequency dependent.
Perhaps the capacitive reactance is the biggest offender in
audio.
Series capacitance will affect low -frequency audio. This is
because the reactance increases at lower frequencies. For
example. a large -value coupling capacitor with a
several -microfarad value dries up or opens up, so that its
effective value is now only a few picofarads. Only a few of the
higher audio frequencies will get through, and none of the
lows. This is, in effect, a differentiator circuit. The actual
effect, of course, depends upon the actual values present.
High- frequency rolloff is usually caused by capacitance
across a circuit. That is, the reactive element is parallel to the
load. When the capacitance goes to ground, this is usually
considered a bypass. Since the reactance of a capacitor
becomes less as the frequency goes higher, the capacitor will
provide a low- resistance path for the high frequencies; and if
the values are such as to produce a very low-resistance path, it
will short out the highs. One source of the problem is
capacitance across the audio cable. This is also one of the
reasons for low -impedance circuits, since this reactive path is
parallel with the impedance.
Impedance Matching
Mismatching of impedances can also cause the frequency
response to be affected. The mismatch will present a different
impedance value to different frequencies. And unless the
voltage developed across the impedance is the same for all
frequencies, there is frequency distortion. A source of the
problem can be the T -pad on a balanced circuit. The pad is
intended for unbalanced circuits but is often used on balanced
circuits because it is less expensive than an H -pad. ( Here we
are talking about the variable type used as a gain control.)
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This pad will work all right, so long as it is kept in midrange.
Toward the extreme sides, there can be serious response
problems.
Defective Components
When circuit components become defective, they can
change their characteristics drastically. These changes
contribute to many distortion problems. Something may. for
example. move a stage into a position of near oscillation. This
could create a peaking condition in the audio. The same type of
conditions that are caused by feedback can occur when
components become defective.
Amplitude Distortion
One of the more common forms of distortion is amplitude
distortion. This is usually caused by misadjustment or misoperation of the system. Whenever some stage does not
amplify the audio signal in a linear fashion, the output is
distorted, and there are usually second- and third -harmonic
components present. Amplitude distortion is often called
harmonic distortion.
Aside from simply overloading amplifiers by not adjusting
gain as needed. there can be several other factors which can
cause this type of distortion (Fig. 4 -12). A stage's operating
parameters may shift when components fail or change value.
For example. a voltage-dropping resistor in series with the
power load may go down in value, thus allowing more voltage
to the stage than design permits.
Temperature Effects
Solid -state units are sensitive to temperature changes.
Temperature effects can cause a transistor to shift to a
different operating mode, and this can cause distortion. It is
important that heatsinks be kept clear of air obstructions and
that they are in place. It is easy to forget to replace one when
changing a transistor. Another cause of heat buildup is the
small ventilating fans often used and their filters. Filters can
become clogged and shut off the air circulation of a unit, or the
small motor can fail.
Push -Pull Stages
Stages in push -pull operation must be kept balanced or the
output signal will be nonlinear. This is due to the nature of the
118
(A)
NORMAL
OPERATING POINT
HAS SHIFTED
OUTPUT
SIGNAL
NEGATIVE HALF CYCLE COMPRESSED
INPUT SIGNAL
(B)
OUTPUT
BOTH POSITIVE
AND NEGATIVE
PEAKS CLIPPED
HIGH
INPUT
INPUT SIGNAL
Fig. 4-12. Two ways amplitude distortion can occur: In A, stage operating
point has changed on its input- output curve, causing output to be nonlinear. In B, input signal is too high, causing stage to be driven into clipping.
stage operation, as one side of the stage amplifies only one half
of the cycle, the other half cycle being amplified by the
opposite side of the stage. Both of the amplified halves of the
signal are reunited in the output of the stage. Should
components fail or change value, that would cause the two
sides of the stage to unbalance and the output waveform to be
distorted (Fig. 4 -13). For example, both sides of a push -pull
stage normally operate as a class B stage, but the bias on one
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CLASS B
..__C.,f--\___
COMBINED OUTPUT
IS NONLINEAR
INPUTS FED
OUT OF PHASE
\MORE LIKE
CLASS C
BECAUSE
OF BIAS
SHIFT
Fig. 4 -13. Class B push -pull stages must be kept balanced. If bias should
shift one half of the push -pull circuit so that one stage acts more like Class
C, the combined output will be nonlinear.
side has increased so that side begins to act more like class C
than B. The output waveform from this side is less than the
other side. so the combined waveform is nonsymmetrical and
distorted. Many complex waveforms are nonsymmetrical.
That is. the positive side may be higher in amplitude than the
negative, or the reverse of this. The human voice often
generates such waveforms, and since these are natural, they
are not considered distorted. The amplifier should reproduce
them the same way they are received. Of course, shaping
circuits may be used to make these waveforms symmetrical.
At any rate, as long as the stage is faithfully amplifying what
is at its input (even a distorted signal), that stage is not
causing distortion.
Intermodulating Distortion
Whenever a stage is operated in a nonlinear manner or
clips the signal and there are two or more signals present of
different amplitude, intermodulation distortion will occur. A
single tone passing through an amplifier and driving the stage
120
into severe clipping will result in a very distorted output
signal, but there will be no intermodulation distortion present.
The conditions are right, but there can't be intermodulation
distortion without a second signal present. When the conditions
are such that distortion can occur, and the second signal is
added. one signal will modulate the other. Note that this is a
modulation process, and not simply a mixing process ( Fig.
4 -14). Signals can be mixed without modulation taking place;
in fact, this is what the program material is-many mixed
(A) MIXING
LOW
FREQ.
SIGNAL
liI
HIGH
FREQ.
SIGNAL
AMPLIFIER
MIXED
SIGNAL
OUTPUT
(B) MODULATION
SGNAL
.
SIGNAL
AMPLIFIER
MODULATED
SIGNAL
OUTPUT
SIGN FREQ.
SIGNAL
Fig. 4 -14. Intermodulation distortion is a modulation process-not mixing.
sounds, all blended together. When a stage operates in a
nonlinear fashion, however, and especially if clipping is taking
place, then modulation will take place. This is similar to the
process which takes place in the first detector or mixer stage
of a superheterodyne receiver. The same results are obtained:
Both original signals are present, plus sum and difference
frequencies of both, and many harmonics are present. But the
program signal has many different frequencies present at the
same time. Many of these can be modulating each other and
producit1g all the exults, and all the additional harmonics are
1
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also tossed into the stew. The harmonics will be outside the
system pass band, but the sum and difference frequencies will
be in the pass band. They can't be removed. The sound of this
defect on the air will depend upon the degree of distortion
taking place. Small amounts may go unnoticed, but they tend
to "muddy" the signal, or dull its clarity and brilliance.
Stereo Signal
Stereo signals that have been recorded with narrowly
restricted bandwidth in the original material may be
accompanied by high -frequency noise or other unfiltered
signals. When this is fed through a limiter that also clips the
peaks above a preemphasis curve, unexpected results can
occur. Severe popping and cracking sounds can be heard in a
monitor speaker causing the modulation meters to intermittently peg. In one situation where this occurred, news
people had done some interviews with a small cassette
recorder, dubbed this to a regular monaural cartridge, and
then dubbed the monaural cartridge to a stereo cartridge. All
of this dubbing certainly didn't enhance the quality of the
signal. but on top of this, there was a very high- frequency hiss
(probably tape hiss) in the background. Although barely
audible, it did have a high amplitude, which kept the limiter
continuously in a clipping condition. The pops on the air
monitor speakers were not gentle pops, but more like the
crack of a bullwhip.
GENERAL CAUSES OF DISTORTION
It is important to remember that the master system of the
station must work as one unit, and no part of the system should
contribute an inordinate share of the total distortion. Besides
audio amplifiers, other parts of the system can contribute to
distortion. Faulty components in a unit may cause distortion,
as can power supplies. equalizers, signal processors. transmitter modulators, and RF tuned circuits that are of insufficient bandwidth. In sections of the book where individual
units or parts of the system are covered. distortion possibilities
with the unit and its operation will be brought out. For now.
only a few of these general aspects are discussed.
Power Supplies
A power supply sets the operational parameters of all the
stages in a single unit or units that it supplies. These may be
122
regular or bipolar supplies, regulated or nonregulated. Even
the regulated supplies can drift and need to be measured and
adjusted from time to time. Most regulated power supplies
today are solid -state units themselves, and the regulators are
large power transistors or ICs. Temperature changes can
affect these regulators just as much as they can affect signal
transistors in the amplifier stages, and will cause the output
voltages to drift or the supply to go out of regulation.
A power supply may have several different output
voltages that are derived through resistor networks. Some
fault in the load can cause higher than normal current through
the current -limiting and dropping resistors for a given tap
(Fig. 4 -15). Two things can happen: The higher current will
cause a greater voltage drop, so the bus voltage will be lower
than normal. Second, the higher current may exceed the
power rating of the resistor changing its value and eventually
opening it up. During this time Iwhich may be several days or
weeks) that voltage bus is low. Consequently, other units or
stages relying on the voltage bus to set their operating
parameters will shift operation. This new mode of operation
can be susceptible to overloads or other forms of distortion.
With the stage operating with lowered voltages, even normal
input signals may now be too high and cause it to overload.
Jacks
Either loose connections or dirty jacks can cause a form of
distortion that is difficult to find. The difficulty in finding the
culprit comes from our natural tendency to suspect an
operating unit rather than a passive unit. As heard on a
monitor speaker, this form of distortion sounds as though peak
clipping were taking place in an amplifier. There can also be a
loss of low frequencies because of the low value of capacitance
that couples the signal across the contact.
In jacks, the "normal" contacts are usually at fault. The
springs may not have enough tension. and oxidation may build
up on the contacts, forming a high- resistance path for the
signal ( Fig. 4 -16). The same situation can occur on a loose
connection at a terminal board or any spot on a printed circuit
board that didn't get soldered. When wiring up a new station or
new jack field, there are many, many connections to be made.
It is easy to overlook soldering one or two of these. In all cases,
the oxiclatiCa will Lc fe ,istive awl will uibe. initiate Lgd.i. t all
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124
frequencies; and when the resistive value gets high enough, it
becomes an open circuit to the signal. But the two conducting
surfaces separated by a thin insulator will form a small
capacitor. And only the very high audio frequencies get
through.
Transistor Stage Design
Design problems are not ordinarily a problem for the
engineer in the field, unless he is building his own circuits or
remodeling other circuits. But a little knowledge of some of the
factors important in circuit design are helpful. That is, the
engineer can be on the lookout for those things which are
important to the design and most likely upset stage operation
When components change. And this is usually what causes
distortion-a small shift in stage parameters.
Distortion is one of those subjective qualities about a
signal. The hearer may not understand or recognize that he is
hearing distortion, but instead, the distortion may be sensed or
felt. (Serious amounts of distortion are something else and will
require immediate correction.) Thus, a stage parameter may
drift, resulting in a small increase in distortion, perhaps 1%.
INPUT
1
1
1
-
OUTPUTp
r
INPUT
C
OUTPUT
Fig. 4-16. Oxidation on jack "normal" contacts can create high series resistance and form a small capacitor. This is equivalent to an open circuit
with a small capacitor across it. Only the very high audio frequencies will
get through.
125
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This additional 1% will add to the system's total distortion, and
this may push the system out of FCC tolerances, even though
that 1% would not be discernible by itself.
A transistor stage has two sets of conditions: DC, AC. The
resistors around the stage and the DC power supplies set the
DC parameters. In a sense, these values are noncritical. They
are selected so that the transistor stage will operate within the
range of the transistor itself and the stage gain desired. What
is important about the resistors is that the ratios of these
resistance values remain the same. All of these values can
change a long way, just so long as they all change in the same
direction and the ratio remains constant.
The change. however, must not move the operation out of
the transistor's range. Under these conditions, there will be a
change in the impedance and gain of the stage, and this could
cause distortion in a following stage if the gain goes too high.
What is important, from a distortion- and -troubleshooting
viewpoint, is that one of the resistor values may have changed
radically in a stage. This would definitely upset the stage
parameters, perhaps its bias. So when troubleshooting resistor
values around a stage, do not be concerned if all resistor
values are off 10% or 20%, as long as they are all off in the
same direction, either low or high. If one resistor has changed
by 50% to 75% of its stated value, it should be replaced. The
replacement should be as near the stated value as possible
(assuming the other resistors are so). Thus, if the circuit calls
for a 1000 -ohm resistor and the closest value in the parts box
measures 950 -ohms, add a 50 -ohm resistor in series with it to
make 1000-ohms. The combination may not look as nice as a
single resistor, but the stage will work as it should.
The DC operating mode is the static mode of the stage. The
other mode is the AC, or dynamic mode. Control of the AC
characteristics is just as important as the DC characteristics.
The capacitor values around the stage are important, for if
they change value, the AC characteristic will change, and on a
frequency -dependent basis. Impedance and the AC gain will
also change. and distortion may result. When a capacitor must
be replaced, the replacement should be as near the stated
value as possible. That is, the handiest value in the parts
cabinet should not be used, simply on the theory that there
ought to be some capacitance in the stage at this point and any
value will do. If the correct value is not available, add
126
capacitors in parallel to get more capacitance, or in series to
reduce the value or get a higher voltage rating.
Equalizers
The excessive use of equalizers in a system can often lead
to distortion. There may be too much boost to certain
frequencies. which can overload following stages. There is no
problem in this regard if the frequencies are lacking in the
original signal (there could be noise problems) and the boost
is required to bring the response back to normal. But when the
original signal is not lacking in these frequencies, and the
boost is done for effect. the boosted frequencies can overdrive
other stages following the equalizer and create distortion.
IMPEDANCE MATCHING
A variety of equipment can be interfaced electrically and
the most efficient transfer of signal power from one unit to the
other is obtained through impedance matching. Impedances
have been standardized in the broadcast industry.
Impedance is a term for an electrical value containing
both resistance and reactance and is expressed in ohms. The
amplitudes of signal levels in and out of an amplifier are
directly related to the impedance value. To obtain the most
efficient transfer of power from one circuit to another,
impedances should be matched, and this match must hold
across the pass band.
Impedance Mismatch
Mismatching always results in some penalties in terms of
poor system levels, amplitude and phase distortion, and poor
response curves across the pass band.
How severe the penalties will be depends upon the degree
of mismatch and how critical the application. A source and its
load should be matched. The source is called the driving
impedance. This is the output impedance of the amplifier
feeding a bus or other load. The input of the next stage, at the
receiving end of the bus, is called the load impedance. The
following are a few examples of how mismatching can become
an operational problem.
When the load impedance is mismatched towards the low
side of what it should be, the signal amplitude across the lower
impedance will also be lower. To overcome this lower level,
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the operator may increase the driving -amplifier gain. This will
cause the driving amplifier to work harder than normal. But
the increased gain may affect the headroom designed into the
amplifier so that it has all but disappeared, and clipping may
take place. causing distortion. At the same time, the output
stage of the driving amplifier is also working harder to deliver
more signal current. This increase may be more than its
design permits. This could damage the output transistor, or it
may drive the stage into distortion.
When the input impedance of an amplifier is low in
relation to the driving- amplifier impedance, a mismatch again
occurs. (This is the same situation as in the previous example,
except now we are discussing the other amplifier.) The
mismatched impedance will provide low input voltages to the
amplifier. And with a low input signal, the operator may
increase the gain setting of this amplifier to make up for the
signal loss. Now, however, the signal-to -noise ratio will suffer.
At the same time, an amplifier running at high gain is
susceptible to interference signals, especially RFI.
Reactive Components
If the reactive components of the driving and load
impedances are not properly compensated (as during a
mismatch situation). there are frequency- related effects. The
type of frequency- related effects depends upon the type of
reactance and how much is present. If there is a resonant
condition. peaking of one or more frequencies will occur.
Should there be shunt capacitive reactance, a high- frequency
rolloff will occur. A typical example is a long cable run
terminated in a high impedance rather than a normal low
impedance. The capacitive reactance will be in parallel with
this load impedance and the response curve will show high
frequency rolloff Fig. 4 -17).
(
The Ideal and the Practical
Most theory and calculations are based on ideal
conditions, but practice often falls somewhat short of the ideal.
There are many cases where a mismatch does occur, and the
theoretical penalties also occur; but in any particular situation
they may have no real effect and may go unnoticed. Although
mismatch will occur in some situations, how much penalty can
be tolerated will depend upon the particular case. For
128
500 FT OF CABLE
600f1
PROGRAM
BUS
=1_0.0275 uF
600 f2
LOAD
TOTAL SHUNT CAPACITY OF
CABLE
CABLE CAPACITY 55 pF/FOOT x 500 FT= 0.0275µF
XC OF 0.0275 uC AT 10 kHz= 60011 (APPROX.)
EQUIVALENT LOAD
500 FT
60011
30011
AT 10 kHz
PROGRAM
600
il
+
xc
30011
AT
AT
10 kHz
10 kHz
Fig. 4-17. Cable capacitance will shunt the high- frequency response. In
the example, even a properly terminated cable will have a rolloff at the upper frequencies when the cable is very long.
example, a schedule E telephone company remote line has a
very poor response curve. Even a loaded line has a pass band
of approximately 200 Hz to 3 kHz. Now when the system
receives a program over such a line, but mismatch occurred in
the patchup that caused the audio to roll off above 10 kHz, the
effect would be unnoticeable, since there isn't any signal
frequencies left in this range after passing through the line. As
a matter of good engineering practice, it is well to strive for
the ideal, but also realize that there may be situations in which
the ideal is not achieved, yet the results are acceptable.
129
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(A) ISOLATION TRANSFORMER
PRI. 600
SEC. 600
0
0
(B) MATCHING TRANSFORMER
1500
600 0
`
600/150 0
{j
600/1500
(C) BRIDGING TRANSFORMER
20kO
600/150 CI OUT
STRAP: SERIES FOR 600 0
PARALLEL FOR 150
are strapped in series
Fig. 4-19. Useful transformers for audio. Windings
or parallel to get the desired impedance.
these have series resistors built into them and enclosed inside
20K and
the case. The input side of this bridging transformer is
used to
the secondary is 600 ohms. When this transformer is
the
bridge a circuit, there will be a 20 dB loss across
up
transformer, so the following circuit must be able to make
what is
this loss. Often, however, the lower signal level is
desired.
Resistor Pads
Pads used around the station are mainly of two types,
is
bridging and matching. There is also the loss pad, which
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often used. Resistors are always loss devices; even the
minimum-loss matching pads have an insertion loss. So,
whenever a resistor pad is to be used for matching, always
consider this insertion loss and its effect on the system. There
are many styles of precision commercial pads available in
either balanced or unbalanced types.
Homemade Pads
For many situations around the station, homemade pads
may be used successfully. They don't look as nice as the
commercial models and aren't as precise in either loss value
or the impedance. The lack of precision is because the exact
value of resistors calculated will not be available in standard
resistor values. Besides that, the standard values may be 10%
or 20% of the stated value.
Building a Pad
When building a pad, consider whether the circuit is
balanced or unbalanced. The input /output resistances of the
pad will be the same in either case. However, if the circuit is
balanced, the input resistance will be divided equally into two
parts, that is, half on each side of the circuit (Fig. 4 -20). The,
(A) UNBALANCED
IN
-
"T" MATCHING PAD
Ri
R2
--a-
R3
OUT
(B) BALANCED "H" MATCHING PAD
IN
-
I/2(R1)
1/2(R2)
R3
1/2(R1)
-a-
OUT
1/2(R2)
Fig. 4-20. To make a balanced pad from a T -pad calculated resistor values,
divide R1 and R2 in half for each side of the circuit (R3 remains the same).
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output resistance will be divided the same way. For example,
suppose calculations show that the input resistance must be
100 ohms. If this is to be used on a balanced circuit, use a
50 -ohm resistor on each of the input legs of the pad. In effect,
this is converting a T -pad to an H -pad.
Don't accept the indicated resistor values on the resistors,
that is, the markings. Actually measure each resistor for its
true value and then select resistors which are as close as
possible to the calculated values. Even though values found
will not be exactly the calculated values, select identical
values for each side of a balanced pad.
Once the pad has been constructed, make a test setup and
actually measure the loss of the pad. This can be done by
connecting a signal generator to the input side of the pad and
measuring the level of the signal at the output of the pad Fig.
4 -21). Make sure the generator is driving with the correct
impedance and that the pad is terminated with the correct
(
OUTPUTZ=INPUTZ
Z
OUTPUTZ=TERM
Z
Z
j
SIGNAL
GENERATOR
MEASURE INPUT
LEVEL FOR
REFERENCE
PAD
ofTERMINATION
RESISTOR
MEASURE OUTPUT
LEVEL FOR
LOSS OF PAD
Fig. 4 -21. Test setup to measure loss value of homemade pad.
impedance. Unless the generator has a calibrated output
meter /pad arrangement. the signal into the homemade pad
must also be measured so that you have a reference. If the pad
is removed to measure the output of the generator, make sure
the generator is properly terminated. With this arrangement,
the loss of the pad can easily be determined.
Because the standard resistance values didn't come too
close to the calculated values, the loss of the pad will be off a
few decibels from the calculated value, but it will work
satisfactorily in most cases. However, when circuit values are
critical, use the precision commercial pad.
134
Transformer Strapping
When different windings must be strapped in various
manners to obtain the desired impedance of the input or output
of the transformer, the values don't figure out in such a
straightforward manner as with resistors. This is because of
the construction of the transformer; whether the windings are
separate or tapped, the turns ratio of primary to secondary,
and the magnetic flux lines that affect all the windings. Thus if
two windings on the primary are tied in series to obtain 600
ohms, when tied in parallel the result will be 150 ohms, and not
300 ohms (Fig. 4 -22). But if two separate transformers have
(A)
6000
(B)
60011
PROGRAM BUS
600f1
Fig. 4 -22. When windings on a transformer are strapped in parallel, they
usually provide one -fourth the impedance value of the series strapping.
When two separate transformers are strapped in parallel, the impedance
is halved.
the primaries tied in parallel, the two 600 -ohm primaries will
result in 300 ohms net impedance. With two separate
transformers, the flux lines do not connect the windings of the
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transformers. Actually, this is what happens when two
amplifiers with 600 -ohm input impedances are placed across a
600 -ohm bus; the resulting load is 300 ohms. It is the same
situation when the inputs of the left and right audio channels
are strapped together for test purposes in stereo. The resulting
load on the signal generator is 300 ohms.
Making a Bridge
mentioned earlier, bridging transformers are
available. These usually have resistors built into the case and
wired to the input winding to provide 20K input impedance.
A homemade variety can be created with a couple of
resistors and an isolation or matching transformer. (This also
has the advantage of converting back to a matching arrangement when bridging is no longer desired.) Simply insert
a 5K resistor in each leg of the input of a 600 -ohm transformer.
This will provide a 20K bridging impedance and the loss across
the transformer /resistor combination will be approximately
As
20 dB.
Ohm's Law
For operational purposes, impedances can be treated as if
they were resistances. For practical purposes, this will be
accurate enough, as long as the presence of the reactive
component is recognized. Whenever several outlets are tapped
off a circuit, remember to calculate the total value of
impedance placed across the circuit (as was done with the
earphones in an earlier example). There is often a tendency
not to consider the values when the taps are bridging. Usually
this is all right, as long as the standard 20K bridging units are
in use. In that case, it would take approximately 33 of these
units across a circuit to represent a 600 -ohm load. This is not a
likely situation. But when making use of homemade varieties,
the resistor values are not always as high as the standard
values of a bridge. In some cases, the circuit following the
bridge cannot tolerate the 20 dB signal loss, so a smaller
bridging value is used. Actually. such a bridge can be made
with as little as 2K. and the effect on the bus will be less than 1
dB drop in the bus level. But care must be taken not to add any
more such bridges to that bus. This 2K load is about the same
as for a pair of headphones.
136
RF INTERFERENCE
There has always been a problem of RF signals getting
into the audio system, but the problem in the past was not as
severe as it is today. Tube -type equipment is not as susceptible
to this type interference. and FM causes more trouble than
AM. With certain precautions in wiring and a few corrective
tactics, the problem could once be solved easily. But this is not
the case today. In recent years, the industry has converted to
solid -state audio equipment. And there has been a sharp
increase in the number of stations operating, not only in
broadcasting, but in all other services. So now a station must
not only be concerned with RF signals of its own, but must also
be on the lookout for a variety of other transmitters that may
be nearby. In many cases, several stations share space on the
same tower for FM. TV, two-way communications, etc., as well
as AM. Interference, however, comes from very strong signals
nearby, and not the usual clutter of signals that are picked up
when tuning across a receiver band.
There are many ways that RF signals can get inside an
amplifier. This can be direct radiation to an open chassis,
carried on signal lines, shields, DC control circuits, and AC
power circuits. Once the signal enters the amplifier, the basic
reaction takes place at one or more transistors, and the results
are amplified.
When a strong RF signal is presented at its terminals, a
transistor can act as a diode and demodulate the signal
through rectification. This is easily understandable, since the
transistor is essentially two diodes back -to-back in the same
case, sharing a common base. The most sensitive part is the
base -to- emitter diode, and when detection takes place here,
the results are amplified by the same transistor. In other
situations, the bias shifts so that the stage acts like a class B
detector; or slope detection may take place on an FM signal.
Detection of the RF signal's audio is not the only way RF
can be a problem. Situations can exist where only rectif ication
of the RF is taking place, but the audio is not recovered or is
filtered out, and only a DC voltage remains. This DC voltage
can shift the stage parameters so that the stage operates in a
distorted manner. This is the same as operating the stage with
improper adjustments. Although no audio is recovered from
the RF signal, hum or other noise on the carrier may be added
to the audio signal. The engineer troubleshooting the problem
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may be led down false trails attempting to find other circuit
defects.
Another false trail can occur when the RF is demodulated
and fed back into the audio. This is the same audio that is
modulating the transmitter. If it is coming from another
transmitter. this injected RF audio will appear as crosstalk.
When it comes from the transmitter being modulated and the
delay isn't so great as to create an echo, the audio heard from
speakers may sound as if a stage is being overdriven slightly
and is distorting on the peaks.
General Remedies
There are no surefire solutions to RFI, but there are a
number of defensive measures that can be taken to reduce the
possibilities of problems. Always keep in mind that the RF
signal doesn't need wiring to enter a unit. An RF signal has
both a magnetic and electrostatic field around it. A circuit
carrying RF energy-whether this is a sampling -loop coaxial
cable, open-wire transmission line, or even the terminal -type
coaxial line end connectors-can radiate a signal.
When the transmitter is located adjacent to the audio
equipment. there can be direct radiation from the cabinet of
the transmitter unless it is kept tight. The actual amount of
radiation from the cabinet must be kept very low so the
transmitter will meet FCC type approval requirements. But
this is for its manufacture. When it is in operation at the site,
care must be taken that all the antiradiation devices built into
the transmitter remain functioning. As a transmitter design
engineer once told me during a discussion of this problem, the
FCC type acceptance measurements are those of the
electrostatic field only, although a transmitter cabinet may
have a strong magnetic field radiated from it. And it is this
magnetic field which is the biggest offender in RFI problems
in the station.
Aside from the transmitter itself and the transmission
line, there is, of course, the antenna. The very purpose of the
antenna is to radiate a signal. Adjacent to an antenna there are
very strong RF fields. The FM antenna that is only horizontally
polarized provides less problems when the studios are located
near it than does the same antenna with vertical polarization
added. With vertical polarization, there is a strong field
directly below the antenna.
138
A Clean Environment
The best defense against very strong RF fields is tightly
screening the entire building, but this is also very expensive
and many stations could not afford to do it. The next best
defense is screening of the operational areas of the station.
This method is also expensive unless the station is undergoing
complete remodeling or it is a new station.
There is still another way of providing a clean
environment for the audio circuits themselves, and this is
through the use of conduits, metal ducts, enclosed equipment
racks and equipment cabinets, and good shielding of the wiring
throughout.
A Tight System
Whenever plans call for a complete or partial updating of
the station. good shielding should be kept uppermost in these
plans. A first step is a carefully controlled ground system
throughout. This ground system should be consistent all
through the system.
Very tight shielding is necessary for all wiring, racks,
consoles, and other cabinets, and all of these should be given
careful treatment throughout the installation. The audio wire
itself should have a shield with 100% coverage; this information about a cable will be described in its spec sheets.
Enclosed equipment racks should be used rather than
open -frame racks. The steel panels and doors of an enclosed
rack will divert quite a bit of the RFI.
Ground Leads
Ground leads from equipment to the main building ground
should be kept as short as possible. Whenever this lead must be
longer than a few feet, the ground lead should be shielded Fig.
4-23). Now this may sound a little odd, shielding the ground
lead, but RF at FM frequencies has short wavelength. The
ground lead can actually become an antenna at the FM
frequency. At 100 mHz, a quarter -wave antenna is about 2.34
feet long. At the upper FM channels, the antenna length
becomes shorter yet. See Fig. 4 -24.
(
Cabinets
Often, there is a single unit mounted in a cabinet by itself,
rather than a rack. This cabinet must be kept tight, with all the
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AMPLIFIER
GROUND TERMINAL
LEAVE
SHIELD
UNGROUNDED
GROUND
SHIELD AT
THIS END
BUILDING
GROUND
Fig. 4-23. Long ground leads at amplifiers should be shielded.
panels and screws in place. The cabinet itself must connect to
the building ground, but paint can often insulate the panels
(this can also be the case with anodizing processes). This
makes an attractive cabinet, but is a poor shield since all the
panels are insulated from each other. Remove paint,
anodizing. or similar dress features so a good metal -to -metal
contact is achieved.
Equipment manufacturers are aware of the problems with
RFI in broadcast stations and are taking steps to make their
equipment less susceptible to RFI by adding internal
protection circuitry to and components. Although this
RFI -proofing does do the job in most cases, it does not
guarantee that a particular application or installation will not
have RFI problems. Equipment made by other manufacturers
who do not ordinarily make equipment for the broadcast
market, but for consumer use, may not have any of this
protection built in. If a station makes use of consumer -type
equipment, caution should be observed in its specific
application so that it doesn't introduce RFI to the rest of the
broadcast system.
Brute Force Remedies
In spite of the best installation efforts, RF will find its way
into equipment at some locations in the station. When this
140
occurs, brute force methods may be necessary to eliminate the
specific problem. But when such methods are used, care must
be observed that the audio signal of the program does not
suffer deterioration from the treatment. At the same time, do
not forget that those old -fashioned problems of hum, shield
buzz, crosstalk, and other interferences are waiting just
outside the door.
Identification of RFI
The first step to take when a problem occurs is to identify
both the offending RF signal and the affected stage. This is
easier said than done! RF can usually be identified by its
programming. Use any of the various signal-tracing methods
and different test instruments to isolate the affected stage, but
whatever instrument is used, make sure the instrument test
leads themselves do not also pick up RF and feed it into the
system. This will only contribute to confusion, rather than a
cure. An oscilloscope is a good instrument to use, but it may
not be sensitive enough, because the RF may be a very low
level signal getting into a low -level, high -gain stage.
Bypassing
Once the offending stage has been located, try bypassing
the base -to- emitter junction of the transistor Fig. 4 -25) . Place
(
(A)
1
/4À
ANTENNA =
234
FINMHz
234
=
100 MHz
2.34 FT.
(B)
HIGH RF
1
GROUND
TERM.
CAN BECOME
1/4A ANTENNA
AT FM
CARRIER
VOLTAGE
AT THIS
END
1/4
VOLTAGE LOOP ON
YX ANTENNA
Fig. 4-24. Ground leads can become antennas at FM carrier frequencies.
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AUDIO STAGE
OUTPUT
INPUT
ADD BYPASS
CAPACITOR
Fig. 4-25. Bypass the emitter to base at the transistor if this won't upset
stage.
the capacitor directly on the transistor terminals. The value of
this capacitor must be chosen so that it doesn't upset the stage
operation or cause a rolloff of the higher audio frequencies.
Bypassing. when used as a defensive technique in any
circuit. places a capacitive reactance across the circuit.
= 1200 fl (APPROX.)
1200 kHz = 12 S2 ( APPROX.)
15 kHz
(A) Xc OF 0.01uF AT:
(B) CIRCUIT IMPEDANCE CAN BE AFFECTED AT 15 kHz
NORMAL
60011
Z
.4
+
f
l
12000
í
t-----'
4O0û
AT 15 kHz
=
.11
(C) EFFECT ON LOW- IMPEDANCE CIRCUIT WOULD BE LESS
NORMAL
Xc
.1h
50R
ti
1200(1 t,
_
48f2
AT15kHz
a
Fig. 4-26. Figure the value of the reactance of the bypass at both the offending frequency and at the highest audio frequency.
142
Before deciding upon the final value of this capacitor, always
compute its reactance, not only at the offending radio
frequency, but also at the highest audio frequency you want to
preserve. And the capacitor should be one of the better
ceramic or silver /mica ones. When bypassing to ground on a
balanced circuit, use equal -value capacitors on each side of
the circuit to ground. There is always the danger of rolling off
the high audio frequencies when bypassing, and it is for this
reason that the technique cannot always be used (See Fig. 4-26.)
Inductance
The pickup of RF may be due to a tuned situation in the
input circuit of a transistor stage. That is, all the elements
form the proper components of a resonant circuit at the
offending RF signal and thus develop an efficient antenna.
Small ferrite beads can be added to a circuit lead and cause a
detuning effect on the RF signal. These beads have a small
hole in the center and can be slipped over the wire lead and
soldered in place. These are the same ferrite beads often used
in video circuits to prevent parasitics.
T-Pads
On balanced circuits that use a T -pad type of gain control
located on the line side of the input transformer, there can be
RFI problems ( Fig. 4 -27). Grounding one side of the circuit can
eliminate the RFI; but grounding one side makes the circuit
an unbalanced circuit, so other problems can be created.
GAIN
CONTROI.
INPUT TRANSFORMER
BALANCED
CIRCUIT
STAGE
INPUT
i
SHIELD
GROUND ONE
SIDE OF CIRCUIT
Fig. 4-27. When a T-pad gain control is used ahead of the transformer on a
balanced circuit, it may be necessary to ground one side and run unbalanced.
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There can also be problems if this circuit is patched into other
balanced circuits. However, it works sometimes and is worth a
try.
Power Circuits
Another port of entry for RFI is the AC power wiring to a
chassis (Fig. 4-28). Each side of the power circuit should be
bypassed immediately after it enters the amplifier. Additionally. series RF chokes may also be inserted in each side
of the line. If chokes are used, be sure they can carry the
AMPLIFIER
CHASSIS
POWER
PANEL
120V AC
NEUTRAL
BYPASS
BOTH SIDES
OF LINE
MAY ADD
SERIES RF CHOKES
Fig. 4 -28. Even though neutral is grounded at the power panel, treat AC
power circuits as balanced circuits for RFI.
current drawn by the amplifier. The 120V AC power circuit is
unbalanced, with the neutral grounded back at the power
panel. When treating a power circuit, consider it as though it
were a balanced circuit as far as RF is concerned. Although
the neutral is grounded at the power panel, a long stretch of
line is ungrounded as far as RF is concerned.
RFI is one of those phenomena that can't be pinned down
to a simple theory. A theory can be developed for a particular
case if all the elements of that case are known and can be
accounted for. And that, perhaps, is the nub of the whole
problem- identifying the elements that are peculiar to a
specific case. If those are known, solutions can be derived. But
the elements are not easy to identify. Consequently, after all
the typical solutions are tried and fail, it then becomes a
cut -and -try process until a set of techniques will eventually
work. It can be a long frustrating operation.
144
Chapter 5
The Control Room
Most of a station's local programming will either originate in
or pass through its studio area. There are many items of
equipment working together in this major subsystem. The
heart of the studio area is the main control room, and the piece
of equipment of greatest importance in the control room is the
console.
CONSOLE
The selection, mixing, controlling, and blending of all the
program sources into the station's finished program product is,
the main purpose of the console ( Fig. 5 -1) . To carry out these
functions, the console must be able to select quickly a variety
of program sources, mix them at will, amplify the mixture,
and route it to some output receiving equipment. During the
process, it must be able to monitor not only what is taking
place within the console, but must be able to monitor other
sources that are getting ready for the process. And at the same
time, it must be able to send out warnings that certain
functions are taking place and be able to mute selected
speakers so that feedback does not occur with open microphones.
Although the basic console concepts are relatively simple,
a console can become a rather complex unit with many
interlocking switches, faders, relays, amplifiers and meters
( Fig. 5-2) .
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146
Input Selector
A console has a number of input selector switches, which
may be either the lever type or push switches. In the usual
arrangement, with a lever switch, the selector can select one
of three sources to feed to the fader. The push switch can select
perhaps five sources. The contacts of these switches perform
many functions at the same time. They switch the audio from
the desired source into the fader, back -load the unused
sources, select and interlock the DC relay voltage for speaker
muting, etc. Any additional functions taking place depend upon
the particular console design.
The lever-type switch is a 3- position switch with all three
positions active. That is, there is no OFF position. Each of the
positions selects a source. The push switch is really several
switches, but they are interlocked so that only one can be
operated at a time. That is, when one is pushed "on," the
previous switch will pop out and turn off.
The selector switch connects directly to the input sources.
There are no transformers or pads between them (unless the
station has added one). The purpose of the switch is to select
one of those sources to feed into the console and maintain the
load impedance on the unused sources.
Preamplification
Most consoles provide preamplification for low -level
sources, while some provide preamplification on all the fader
inputs. In the usual arrangement, only the faders designated
for microphones have built -in preamplifiers. The other fader
inputs have a plug -in assembly (usually a dummy cord) that
directly connects the input /output terminals, or they may
include an isolation transformer. Should the station desire
preamplification in any of these other paths, they can plug in a
preamplifier instead of the dummy cord. The output of the
preamplifiers are at about -10 dB, which is compatible with
the design of all faders. That is, each fader will need about -10
dB, whether from a preamplifier or from the source itself.
Mixer Bus
Once the source has been selected and routed through the
preamplifier or dummy cord, it feeds directly to the fader
input terminals. Each of the faders on the console is a part of
its mixer system. The output of each fader is switched to a
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OFF
PGMs +-+AUD
(PAIR)
N
HI1
MIC
1
(ORNI)
MIC 2
M IC 3
, X
{
X
X
MIC
1
(OR HI)
MIC2
HI2
oo-
MIC6
0-'1(
M1
MIC6
HI 2
MIC 2 (OR HI)
MIC 5
D-x
TO MON
SEL 1M
MIC 2 (OR HI)
MIC 5
k
M2
M3
M4
-
TO MON
SEL 2M
11
>
MIXERZ
.I
1R
2L
MIXER 2
MUTING CIRCUIT
I
>
2R
X
H13
HI
L
HI5
X
H16
X
HL
HL
HL 5
X
D--X
HL6
X
HL7
-iC
3L
MIXER 3
R
X
3R
111.8
L
HL9
HL10
R
X
4L
X
MIXER 4
H17
X
HI8
X
H19
X
HI10
X
4R
Fig. 5 -2. Block diagram of the RCA BC -15 console.
148
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m
x
X
1L
MUTING CIRCUIT
M4 13--X
?
MIC 3
R
MI
M2
M3-X
HI
R
j
K
O
CO
CL.
MASTER GAIN
PROGRAM
LINE
OUTPUTS
RECORDER
OUTPUT
x
x
PGM
MON
r-----x
-4/VV
-NAM
AUD
BUS
OUT
O
L
V
o-
PHONES
OUT
-,
R
O L1
O
R
UNMUTED
OUTPUT
A
Q
1L
LEFT PGM AUD AUX
x
*
2L
3L
4
O
4L
k
x
I
I
A
I
MONITOR SELECTOR SW.
MONITOR
GAIN
i
RIGHT PGM AUD AUX
1R
AUD
BUS
OUT
PGM AUD
M UTE
b
M
2R
O
4R
3R
b
2M
MONITOR
SPEAKER
(EXTERNAL)
?
R
o +DC
(Illustration: Courtesy of RCA)
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common mixing bus, and this is where the mixing action takes
place. The fader maintains correct impedance with the
selected source or preamplifier and attenuates the signal
according to the setting of its control. At the same time, it
maintains the impedance of the mixing bus.
Faders
A mixer system must be well designed if it is to control a
variety of program sources and at the same time not interact
with all the faders connected to the bus. That is, program
levels should only change according to the setting of the
individual faders, and not because impedances are shifting
and affecting all the bus levels. The better consoles use
precision faders in the form of ladder -type attenuators or
similar precision units. Some of the less expensive consoles
designed for the production booth often use an inexpensive
potentiometer or wirewound control. The results are not
always too predictable in these units.
Cueing
Another feature used on the precision faders is a CUE
position. By turning the control all the way to OFF and then
beyond, the fader switches into a CUE position. This position
allows the output of the selected source be directed into a cue
bus so that the source may be monitored prior to airing. This is
a very handy feature, especially in the case of turntables or
incoming remote lines and network lines.
Cueing can take place only from the one selected source,
and the only time that cueing can be done is when that fader or
source is not on the air or being routed through the console in
the program mode. If you desired to cue any of the sources at
any time, this would require a different type of cueing
arrangement. However, this arrangement in the standard
console is a very workable system.
Other Inputs
Besides the regular program inputs on a console, there are
usually one or two faders designated for remote and network
lines. The selector switches are usually somewhat different in
the contact arrangement since they need to perform actions
that are not necessary on the regular inputs. Some of the
actions of the regular inputs are not needed here, such as, on
the air lights and speaker muting.
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Ordinarily, these circuits contain an isolation transformer
between the selector and the fader, and there may be pads also
since these are intended for high -level input signals.
Besides selecting one of the available sources for airing,
the switches also have either cor.tacts or poaitions which allow
sending cue signals back out the line. They also permit
talkback to the operator at the remote site before program
airing.
Designation
Different terms are used to describe consoles, and I
suppose the one used depends upon the way the operator thinks
of his console -that is, as either a mixer or a switcher.
Actually, it performs both of these functions and can be
described either way.
Mixer-When the console is described by its faders, one is
thinking in terms of the console as a mixer. The number of
faders the console contains is a good indication of its capacity
to handle a variety of program sources at one time. For
example, if the console has eight faders, it can control eight
program sources at the same time. Each one of the faders can
be feeding program material onto the mixing bus at the same
time. Naturally, if it has six faders, it has less capacity, and if
it has ten faders it has a greater capacity.
Switcher -When described by inputs, one is thinking in
terms of the console as a switcher. For example, the RCA BC -8
console has a capacity of 24 sources, or inputs. This does not
mean that all 24 of those inputs can be used at the same time.
What it does mean is that 24 sources may be wired to the
console at one time, but only 8 of these 24 sources can be used
at any one time.
Although the console does perform many switching
functions, it is not truly a switcher as this term is used in other
fields. The true switcher is merely a routing device that
switches many inputs to a number of outputs. The console only
has one output: All the inputs selected route to a mixer bus so
that there is only one output of all the combined sources that
have been selected.
Channel Selection
There are always two mixing buses in a console, although
one may not be called such. There is a channel selector
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switch immediately following the fader which allows the
operator to switch that fader to one of these two buses. In the
normal single -channel console, one bus is designated the
program bus and the other one is designated the audition bus. If
it is a dual -channel console, there are two mixing buses plus an
audition bus.
The channel selector switch is a 2- or 3- position switch,
depending upon the console model. The switch routes the audio
from the fader to the selected mixing bus, but there are other
contacts which also control the DC relay voltages for speaker
muting or on the air light relays. This voltage is interlocked and
routed through the input selector switch so that only the
affected studio is muted, not all of them.
Amplification
Mixers are loss circuits even though all the attenuation is
adjusted out of each fader. There is an insertion loss, and of
course, there will be the adjusted attenuation. Levels following
a mixer are down in the low -level category. This requires
amplification.
Immediately following the mixer bus, there are one or two
amplifiers, again depending upon model. If there are two
amplifiers, the first one is called the mixer follower. The next
amplifier is called the program amplifier. If it is only a single,
high -gain amplifier, it is called a program amplifier.
Each mixer bus must have amplification. In the single- channel console, there is only one program amplifier, and
the audition bus may be either a similar amplifier or it may be
the console's regular monitor amplifier. A dual- channel
console has two separate program amplifiers, one for each
channel. There may also be an audition amplifier, or at least
the bus can be amplified or switched to the monitor circuit.
Master Gain
The individual faders are designed to control the level
from the input source and adjust it to the correct ratio to other
signals being mixed on the bus. But the overall levels must be
adjusted also. Consequently, there is a MASTER GAIN control
on the console. This control works in conjunction with the
program amplifier or the mixer follower. It either works on
the audio directly or on the parameters of the amplifier. In
either case, it controls the overall console gain.
152
In the dual-channel console, there are two separate
MASTER GAIN controls, one on each channel. Adjustment of
either one has no interaction on the other; they are on
independent channels.
Metering
Some means is necessary to measure the signal levels
through the console, and this is done with a VU meter. The
meter is connected into the circuit directly after the program
amplifier. There will be meter pads so that the meter can be
adjusted to hit 0 VU on the peaks, regardless of how much
higher the overall level is. The normal output of the console is
at +8 dB. There is usually a pad following the amplifier so the
amplifier output itself is higher than +8 dB. The meter,
however, is connected ahead of the pad on the channel, and it
has its own pad to bring that level into range of the meter.
On the dual -channel console, there may be one or two
meters. If there is only one supplied, there is some switching
arrangement so that the one meter can be switched to meter
either channel. Usually, there is an optional meter that can be
purchased and added to the console. This is the preferred
method, rather than switching the single meter for both
channels.
Monitoring
Besides the VU meter, there are at least two other
provisions for monitoring provided in the console itself. There
are headphone and loudspeaker monitors.
Headphone jacks are supplied, and these normally have a
series resistance for bridging. There is a selector switch so
that many of the internal circuits can be monitored.
A monitor amplifier is provided to drive loudspeakers in
the studios associated with the console and the control room
itself. The output of this amplifier is routed through muting
relay contacts that are controlled by the microphone selector
switches. Whenever the channel key is thrown for the studio
selected, the relay opens and prevents the speaker from
operating. The contacts back -load the amplifier so as to
maintain the proper impedance on it.
Air Cue
One of the selector switch positions on the monitor will
provide for picking up audio from an outside source, such as
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one or the other mixer buses and program channels. So there
are simply two program channels.
The stereo console, however, is far different. It must have
two identical channels all the way from the selector switches
to the outputs; and these two channels must operate
simultaneously. These two channels are for handling the left
and the right audio channels of the stereo system. The input
selector switch will select the left and right audio from the
source and feed it to a dual fader. Two faders are ganged
together so that one knob operates both. Then the signal goes
on to the left and right buses, program amplifiers, and output.
The left and right channels must be identical to
maintain stereo channel separation. The audio response curve
through each channel, the noise, the distortion, and the phase
shift should be identical.
CONSOLE INSTALLATION
The console is the focal point of the control room and the
main operating position. Clustered around it are several
equipment items that supply program signals which are mixed
together into the final program. These must be within reach of
the operator or announcer. The physical placement and
location of the console should be given some very serious
thought before the final position is decided on. When this
consideration is taking place, give adequate thought to the
maintenance of the console after the control room is in normal
operation. Also think about the initial installation, when there
will be many, many connections made to the console. And
when it is necessary to troubleshoot problems, the engineer
often needs to get to those terminals to signal- trace.
If at all possible, try to place the console out in the middle
of the room. This will allow easy access to all sides of the
console for maintenance. Unfortunately, the physical
placement is often dictated by the room size and what other
equipment must be placed in the room besides the console. If
you are fortunate enough to get in on the planning of the
building or remodeling, try to get a large enough control room.
If you can't manage to come up with a decent physical size, at
least try to get enough space to allow the console to be placed
away from the wall. It is extremely difficult doing
maintenance to the wiring side when the unit is against the
wall.
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Maintenance is done on the console in the majority of
cases during programming time; something has failed or is
failing and needs correction, but the console must continue in
operation. We don't have the luxury of a spare console too
often in the regular radio station. Of course there is some
maintenance done after signoff when all is quiet, but expect
most of the problems to occur during programming.
Grounds
The console should be the main grounding point for all the
cable shields of circuits going to the console (Fig. 5-4). Make
sure to get a good connection to the main building ground from
INCOMING
CIRCUITS
SHIELD
GROUND
1
"GROUND
STRAP
BUILDING GROUND
Fig. 5-4. Get good ground from the console to the building ground.
the console. Use at least a 1 -inch strap for this connection, and
solder to the building ground. Ground the console frame by
bolting to the strap, and clean off paint to get a good
metal -to -metal contact.
Jacks
Route all the audio circuits both in and out of the console
through a jack field. This requires many jacks, but it increases
the flexibility of the installation considerably and is a great
help for troubleshooting. This flexibility extends to the
operation itself. When all the circuits are on jacks, equipment
may be substituted at will by the use of a patch cord. That is,
157
you can bring the regular units in on different faders if the
particular program calls for this arrangement; and if
something fails, other units can be patched into the console in
the defective unit's place. The substitute units may be left in
their regular positions, such as, in a recording booth. Then,
too, should it be necessary to have another tape machine-for
example. during a special recording session in the booth -one
of the control room machines can be patched in for the
occasion. A jack field pays for itself many times during the
lifetime of the equipment in a station.
MODIFICATIONS TO CONSOLE
Seldom do standard stock consoles exactly fit the needs of a
station. Consequently, each station makes modifications which
customize the console for that station. But when making
modifications, be careful not to alter the normal performance
of the console. The following are a few suggestions to follow
when making modifications.
Power Supplies
Whenever some additional power- consuming modification
is added to the console and it draws power from the console
power supply. investigate the capacity of the supply and how
heavy it is now loaded. There are two things to consider: is
there reserve capacity, and what is its maximum capacity?
The power supply may have its own instruction manal, or
at least there may be a section in the console manual that
gives some specifications on the supply. Determine from this
information what maximum load current the unit can supply
(Fig. 5-5M. This is the starting point, but the figure may not be
given in the manual, or at least not in a form that can be
readily determined. There is a method that can be used to
determine this maximum figure, but it does not give absolute
figures. However, they are close enough to be practical. Look
for a fuse on the DC load side. If there is one, this fuse will
indicate the maximum current that can be supplied. The fuse
is rated somewhat under the maximum value, but the load
current should not exceed the fuse value. Do not increase the
size of the fuse! If you do. the power supply may be damaged.
If there is no secondary fuse, then look to the primary fuse.
Compute the primary power, using the primary fuse as the
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current value. Consider the power supply to be perhaps 70%
efficient and multiply the primary power by 0.7 to give the
secondary power. Then determine the maximum load current
from the power formula, using the power figure just computed
and the secondary DC voltage (I = P /E)
Next, open the output circuit of the power supply and
insert an ammeter in series with the load. Turn the supply
back on. and turn on as many functions as can be expected in
normal use during operation, plus a couple more as a safety
margin. Subtract this current reading from the maximum
current that you computed. The difference in the reading is the
reserve capacity. This assumes that the measured value was
less than the calculated value. If it is not, that power supply is
working at maximum now and you must go elsewhere for
.
power.
Relays
When making modifications that require relay switching
actions, look for relays or switches that operate in the way the
modification requires and have some spare contacts. Make
sure they are "dry" contacts-ones with no other connections
to them IFig. 5 -5B). Also make sure that there isn't a contact
with voltage that the spare contacts will make when the relay
relaxes. This could place your modification across unwanted
voltage, which can cause damage to the modification or the
regular circuit. It is best to have the contacts completely clear
of any connection to the console circuits. In this way, you use
the action only but none of the circuits.
However, there may be cases in which you wish to pick up
the switched voltage from the circuit; then it is okay to go
directly to the desired contacts that have that voltage. Another
source is across the relay coil. Whenever you tap into a circuit
in this manner, always make sure the modification will not
upset the regular circuits. There can be too much current
drawn at that point through series resistors, and these may
burn out. While we have been discussing relays, the same
holds for switches as well.
Cabling
Modifications often call for running some outboard cabling
into the console. There are some precautions to take in this
regard.
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3A
+ 24V DC
POWER SUPPLY
AND FILTER
120V AC
- 24V DC
PRIMARY = 360W
020X3=360)
SECONDARY = 252W
252W
PSEC
70% EFF
(360 X 0.7
=
ISEO
252)
ESEC
24V
-
10.5A
A
REGULAR
RELAY
y
e
24V
CONTROL
f
REGULAR
CIRCUITS
SPARE "DRY" CONTACTS
B
Fig. 5-5. Modifications to console. (A) Method of roughly calculating the
maximum lead current of a power supply. (B) "Dry" contacts have no
other circuits connected to them.
First of all, make sure the new cabling follows the regular
console cabling, regardless of how irregular the path it may
take. Remember that when the front panel is down, attached
cables must bend back out of the way for the panel to close. So
attach the new cabling to the regular cabling.
Also be careful of the path of the cabling. The new cabling
may be carrying relay switching transients, hum, or other
noise, so be careful to avoid low -level microphone or similar
circuits. Remember, the circuits immediately following the
mixer bus can be at a very low level also. Just because the new
cables are carrying DC relay voltages does not mean that they
can't carry other unwanted signals as well.
Switched Voltage
Another source of voltage that can be of use in
modifications is the voltage that is switched to operate
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speaker- muting relays. This is often brought out to a handy
connection board for selecting the keys to have the mute. At
this point, the voltage can be picked up and used to operate
external relays, such as a relay to start a turntable. This
voltage will switch on and stay on when the channel key for
that fader is thrown, so it works well for this use. But use an
external relay. The contacts of that relay can carry the 120V
AC to switch on the turntable motor. Use of an external relay
avoids carrying the 120V AC into the console where it can
introduce hum into various circuits.
When using external relays to be powered by the console,
try to select those which have a low current demand for the
coil. This makes less of a load on the power supply.
Add -Ons
There are often occasions when it is desirable to add
switches to the console panel. for example. remote start
switches for tape machines. This calls for drilling the front
panel of the console.
First, the location: When a likely position is selected for
the switches, open the panel and check the rear side. Quite
often the reason the panel is clear is that there are components
or cabling on the reverse side. Make sure there is clearance
before deciding on the position.
Before drilling, measure off the panel space and use
masking tape across the area to be drilled. Use a centerpunch.
a small pilot drill and then a larger drill for the final -size hole.
A large drill is difficult to start and can skip out across the
panel. causing scratches that can't be removed. Remove the
VU meter before hammering on the panel to set the
centerpunch: a good jar can damage the meter. Before
actually starting to drill, make sure the drill will not break
through the panel and bore into a bundle of cables. If there are
cables that can be reached with the drill, use a board as a
shield between the drill and cables.
The drill spews filings over a wide area, so care must be
taken to prevent these from getting into switches, circuit
boards. etc. Tape some paper underneath the drilling area to
catch the filings. If a vacuum is available, have it sucking in
the filings right off the drill bit. This will save a lot of cleanup.
When all is done, vacuum out any filings that have gotten into
the console in spite of the precautions.
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CONSOLE SETUP
When a new console has been installed. it needs the levels
set. and measurements made to determine that it is working
within specifications.
Levels should be adjusted so that the fader knobs and the
MASTER GAIN control will normally operate at 12 o'clock,
that is. half open. Use program material or tones through each
of the input sources and set the output gain on each of the units
so that the console is reading 0 VU on peaks. Try to adjust each
of the sources that will be selected for a fader so that they
balance well. In this manner, switching from one source to the
other requires very little adjustment of the fader.
Make a set of measurements through the console, jacks,
and wiring. There should be a set of response, distortion, and
noise measurements. Also be on the lookout for RFI. In the
area of noise and RFI. here is a little trick: With the sources
attached to the console. but with no programming, turn the
faders and MASTER GAIN control wide open (you should do
the same with the monitor amplifier GAIN control) : listen for
background noises or RFI. In good installation you will not
hear much of anything. The system is running at very high
gain, and there is a possibility of feedback oscillations, so stay
under this point.
CONSOLE MAINTENANCE
Consoles are generally trouble -free units, but there are
problems from time to time. Amplifiers and components may
fail occasionally. The points of greatest wear are the switches
and faders. and here is where the majority of maintenance
centers.
Switch Problems
The first problem area is usually switches not making
proper contact. Even though these are self- cleaning contacts,
oxidation. dust, or wear eventually makes the contacts
intermittent. Clean these with a burnishing tool or coarse piece
of paper. Place the tool between an open set of contacts, then
close the switch. This will put pressure on the tool and clean
both sides of the contact at one time. Rub the tool across the
contacts several times, and then do the same to the other
contacts. Use caution, as some of the switches also carry DC
relay voltage on some of the contacts. If the power can be shut
off. this is the best method. But if not, use paper to clean those
162
contacts. Otherwise, the burnishing tool may short out the
relay power and cause damage. Also, be careful not to bend
springs on the lever-type switches.
Straightening Contacts
If the contact springs on a leaf -type switch have been bent
or have lost some of their tension, be cautious in bending the
contacts. It is best to use special adjustment tools, but it can be
done with a pair of long -nose pliers. The best type has the nose
bent at an angle. Bend the springs in small amounts only.
Overbending can cause a change in pressure distribution so
that adjacent contacts become intermittent.
Wiring Problems
When a switch or other component works loose, the
component can have slight movement during operation, and
this causes the wiring connected to it to flex. After a period of
time. one or more wires break off. Unless the wire is sticking
out in the open where it can be easily seen. this can be difficult
to find. You must signal -trace the circuit. Follow it through all
the interlocking switches. The troubleshooter may suspect a
bad switch contact. but trace the signal-don't start bending
contacts. If it is an audio circuit. feed program into that path
and use a pair of headphones. Or you can use the monitor
amplifier and a small jumper to complete the circuit from
point to point. If it is open between the points of the jumper
connection. you have the problem isolated. The same trick can
be used by shorting across the contacts on a leaf switch with a
screwdriver. But make sure you are on the correct contacts.
and not on the relay voltage.
Fader Problems
Problems with step-type faders are usually noise or
erratic operation. This is caused by the contacts becoming dry
and oxidized. Of course. they may be wearing also.
Take the cover off the rear of the fader. Use the special
fader cleaning oil. Apply a few drops of the fluid and operate
the control several times through its full travel. Then, with a
clean cloth, wipe off this fluid. Next, add a couple more drops
of lubrication and then run the control through its travel to
distribute the oil. Be stingy here and don't overoil. Replace the
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cover and send audio through the fader. It should now be clean
and smooth in level adjustment.
Be careful of what type of material you use for a cleaner.
Some solvents will leave the contacts very dry, and they will
wear and become noisy again. Others may leave a gummy
residue. When possible. obtain the special fader cleaning fluid
from either the console manufacturer or the fader manu-
facturer.
Cleaning
The console needs cleaning from time to time. Clean the
front panel with Glass Wax or a similar solvent. This will not
only clean the panel of fingerprints etc., but will leave a nice
polished look. But be careful of cleaning the face of the VU
meter. This may be plastic and a strong solvent can fog the
plastic so that it is difficult to read. Just use plain water or
one of the regular window cleaners. Be careful not to get any
fluid inside the meter.
Use a vacuum on the inside of the console. It is surprising
the number of paper clips and odds and ends of audio tape that
accumulate inside the console. Clean these out. But don't give
in to the temptation of closing up the openings to prevent
debris from getting in. These are also for ventilation, and to
close them can cause the units inside to overheat and fail.
MICROPHONES
The microphone collects and directs sound waves in the
air to a diaphragm. This diaphragm may be attached to a
moving coil, part of a capacitor. or a ribbon suspended in a
magnetic field. The sound waves cause this diaphragm to
vibrate in a corresponding fashion, which generates an
electrical signal at its output terminals that is a replica of the
sound waves. The connections have been standardized so the
forward pressure on the diaphragm will cause the "high"
terminal on its output to go positive. This is a very simplified
description, but getting the few simple components to
reproduce the sound faithfully is another story.
Over the years, much progress has been made in the
development of microphones. Today, there are many, many
models available that have excellent response curves and fit
many specialized situations. But radio has also changed over
the years. so that very little production work is done today.
164
Most of this has moved over to recording studios. In broadcast
use. microphones are employed mostly for speech.
Patterns
An omnidirectional microphone picks up sounds equally
without any directional effects (Fig. 5 -6). But this holds true
for only some of the microphones, while others may have some
(A)
OMNIDIRECTIONAL-WILL PICK UP SOUNDS EQUALLY
IN ALL DIRECTIONS
(B) BIDIRECTIONAL -MIKE WILL PICK UP EQUALLY FP "M
FRONT AND REAR, LITTLE AT SIDES
(C)
CARDIOID -MIKE WILL PICK UP ONLY FROM THE FRONT,
NO PICKUP FROM REAR OR SIDES
Fig. 5-6. The three most common microphone sensitivity patterns.
directional characteristics. A bidirectional mike will pick up
equally from the front and back, and very little to its sides.
This produces a sensitivity pattern that looks like a figure
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eight. The cardioid microphone will be sensitive at the front
only. very little pickup from the sides and rear. The sensitivity
pattern is heart -shaped.
The specification sheets that come with the microphone
will show the patterns available for a particular microphone
as well as its response curve. Before selecting a microphone
for a specific use, look these over carefully.
Physical Structure
The physical structure has much to do with the performance of the microphone and its directivity. Internally,
there are chambers in which the sound is controlled and
directed. The sound enters the microphone not only through
the front opening, but through slots or ports. By allowing the
sound to enter these other openings, it is directed into various
phase patterns to mix with the direct sound. This is what
produces the various effects and patterns.
When the microphone case is to be refinished, care mast
be taken that these small ports are not covered over with
paint: the same is true of dirt or other matter. In use, do not
cover these ports with the hand. If the sound can't enter the
ports or slots, performance of the microphone will deteriorate
and the directional patterns will be lost.
Switched Patterns
Besides the fixed patterns, many microphones also have
adjustable or switchable patterns and response curves (Fig.
FLAT POSITION
OdB
- 5d8
."----- ROLLOFF POSITION
-10dB
50 Hz
100
400
1
i
kH
10
15 kHz
Fig. 5-7. Typical selective response curve of a cardioid microphone. When
switched to its rolloff or cut position, the low frequencies are rolled off.
This is useful in reducing unwanted, low- frequency room noises.
the response curve is usually done at the low
end. below 100 Hz. The rolloff is something in the order of 10
dB. This low- frequency rolloff can be a useful device when the
5-7). Control of
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room has much rumble, reverberation, or other low- frequency
acoustical noises.
In other locations, it is necessary for the announcer to be
very close to the mike. This causes a rise in the low- frequency
response. In effect, it makes the reproduced signal sound very
bassy and muffled. The rolloff helps in these situations. But
some microphones are designed for close talking, and these
have built into them the necessary chambers to cause the
response curve to remain normal during close talking.
Wind Screen
Close talking on a microphone also brings up other
problems. Certain sounds, such as a "p" sound, causes the
microphone to "pop" each time the sound is used. When the
microphone is used outdoors where there is wind, the sound of
this wind rushing past the front of the microphone creates
noises. All of these are objectionable sounds in the program.
To overcome these. a wind screen can be used. This is a
porous foam -plastic material that can be shaped to fit the
front of the mike. This can be purchased in form -fitting units
for a particular mike. or it can be purchased in flat sheets. In
sheet form. cut it to size and staple the ends together so as to
make a small sock to cover the mike. Microphones that have
been designed for close talking already have materials built
into them to prevent wind sounds or popping. If you are
caught outdoors on a windy day. a handkerchief wrapped over
the end of the mike will be somewhat effective.
Sports Microphones
These are always close -talking microphones. A regular
mike will give problems. In most of these locations, there is
much crowd noise, and unless the announcer works close to the
mike, he won't be heard over the crowd. There are special
sports microphones available that are also attached to a
headset. This arrangement keeps the mike at a constant
distance from the announcer's mouth and also provides for
headset monitoring. Besides that, it allows both hands to be
free. There is one drawback. If the announcer wants to do
interviews, this type of microphone is not appropriate. The
best thing to do is carry a regular hand -held microphone along
for such occasions.
167
Impedance
The output impedance of microphones is important. They
may be either high or low impedance. Almost all broadcast
stations use the low- impedance arrangement. The capacitance across long cables seriously rolls off the response of
a high- impedance mike, and it can be used only with a short
cable. Broadcast situations often make use of very long cables.
Various microphone manufacturers have settled on their
own impedance values. These may be 30, 50, 150, or 250 ohms.
All of these values work into the normal low -impedance circuit
with only a change of a few decibels in signal level.
Phasing
When only a single microphone is used at a location at one
time, the phasing is of little consequence. However, when
more than one microphone is used at the same location at the
same time, phasing does become important. The phases being
discussed are those which are 180 degrees in error, and not
minor phasing problems.
When two microphones are 180 degrees out of phase and
the same sounds are picked up and fed to the amplifier, these
sound signals cancel each other in the amplifier; it is no
different than connecting any other AC signals that are 180
degrees out of phase. The phasing is a matter of connection of
the microphone cables and plugs to the microphone and to the
amplifier or console. Even though all these factors are correct,
it is still possible to get the mikes out of phase when they are
patched in a jack field. by turning over one of the patch plugs.
The output of the microphone is a complex AC signal, and
because it is an AC signal, the operator might give little
consideration to the phasing. This is a mistake. Microphones
should be phased just as a matter of good engineering
practice. whether a multimike situation is expected or not.
There is no great problem keeping the mikes phased if a
standard wiring pattern is set up along with a color -coding
pattern for the cables. The microphones have been
standardized so that when the sound pushes the diaphragm
forward, one of the terminals goes positive: this is the high
terminal. Carry that through the wiring plan. If there is any
doubt, check the specification sheet for the microphone. It tells
what the numbers should be in the plug and the one which is
high. This will be the #2 pin of the plug. The low side is #3, and
the case of the microphone is #1.
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Test Setup
A simple test setup can be arranged to check out the
microphones for phasing IFig. 5 -8). This should be done when
microphones have been sent in for repair also. Select two
CONSOLE
EQUAL
VOICE
INPUT
OUTPUT
MIKES
SIDE
BY SIDE
1
IN PHASE
(ADDS)
2
OUT OF PHASE
(CANCELS)
Fig. 5-8. Setup to test the phasing of two microphones.
microphones. plug them in to two different faders on the
console or a remote amplifier. Hold the microphones side by
side or have them on stands side by side, but very close
together. What is needed is the same audio from approximately the same distance and direction hitting both of the
microphones. Now, with someone talking continuously and at a
steady level into both mikes mike 1 and mike 2). turn off mike
2. Set mike l's fader so that the peaks are 0 VU on the meter.
Now turn off mike 1. and turn mike 2 on. Set mike 2's fader so
that the peaks are 0 VU. Turn mike 1 back on. If these mikes
are out of phase, the level on the VU meter will drop
drastically. If they are in phase. the level should increase. In
the out -of -phase situation, the signals are cancelling; but
when in phase. the signals add together.
(
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Problem Areas
The majority of microphone problems occur in the cables
and plugs. In fixed locations. such as a control room. these do
not occur too often. but in studios or out on remote locations.
the cables and plugs get a lot of flexing and handling. Even in
the fixed locations. problems can occur with the jacks in a jack
field. These need to be cleaned from time to time.
Cables and Plugs -A regular routine maintenance program
should be set up and followed in checking out mike cables and
plugs. Flexing from remote locations often causes the shield
to break. This often appears at about six inches from the plug
itself. When a shield opens up, there is a buzz or sizzling
sound in the audio. It sounds a little more like hum when the
microphone is touched with the hand. The best repair is
cutting off the cable past the shield break and redoing the
connection. Once the cable has been repaired, check it out with
an ohmmeter for continuity and for shorts across the
connectors.
Plugs seem to have a penchant for losing those small
screws that hold them together. The bad part of this is that
they are not standard screws that can be picked up at a local
hardware store. They are hard to come by. The station should
have an assortment of these on hand. They can be purchased
from the plug manufacturer.
Internal -Problems that develop internally are almost
always caused by someone dropping the microphone on the
floor or a similar mishap. Unfortunately, those who use the
mikes often fail to report that the mike has been dropped. A
sharp jolt, as can happen in a fall, can break the diaphragm or
knock something out of place within the mike. When other
outside tests of the cables and plugs do not correct the
problem, look for internal problems. How far you can get into
the microphone depends upon the type. Some you can't get
very far into without special tools. But even if you can get it
opened up, you may not be able to correct the problem
anyway.
Most microphone manufacturers have repair stations, and
there are some independent repair stations. Quite often these
have a fixed fee for repair of the mike. It usually comes back
as good as the original, and the case will be refinished. It is
better to send in the mike for repair than to try to repair it
yourself, unless of course, the fault is minor.
170
Room Noises-Room noises can sometimes be a real
problem, expecially in a fixed location such as a recording
booth. These noises may come from air -conditioner units,
furnaces, motor vibration through the walls, and elsewhere.
Air ducts should be lined for several feet with a sound
insulating material Fig. 5 -9). There are special ducts already
prefabbed that include this soundproofing material. Even
(
i
INSULATED AIR DUCT
.
,
REMOVE LOUVER
CEILING
8"
-
I-
r ^!/
DEADENED AND
DIFFUSED AIR
STREAM
ADD BAFFLE WITH
INSULATION
DEAD SIDE OF MIKE
f
5._.CARDIOID MIKE
Fig. 5-9. Method of reducing noise from an air duct.
then, the actual movement of the air itself can cause noise as
the air blows across a louver. Try to move the mike out of the
direct path of the air blast or movement, as the sound carries
stronger in the air stream. When this can't be done, take off
the louver and add a baffle about six to eight inches away from
the opening of the duct. This makes the air stream shoot into
that baffle (which should be lined with soundproofing
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material) and thus absorb part of the rumble or other noises.
There is still another trick: Use a cardioid mike and place it so
that the dead side is towards the air sounds. This makes the
mike less sensitive to the noise. For an electronic technique,
use one of the mikes with low- frequency rolloff and insert a
high -pass filter in the mike line (this should cut off below 100
Hz).
TURNTABLES
With music as the staple in many radio stations today. the
turntable has become the station workhorse. The turntable is
actually a system composed of several integral parts. There is
the table itself, its drive motor and mechanism, and the
cabinet to house and support the assembly. The electronics
includes the stylus, the cartridge, the tone arm, the equalizer
and preamplifier.
Recording
A master recording is made with a disc recorder. This is
an amplifier system driving a cutter head and cutting stylus to
cut grooves into a blank disc. The audio signal is pre emphasized to overcome some of the inherent noise in the
system.
NAB standards describe the width and depth of the
grooves and the direction and angle of cutting ( Fig. 5 -10). On
the stereo disc, the left and right channels are recorded at
45- degree angles into the walls of the groove. The right channel
is on the wall towards the outer rim, and the left channel is on
the wall towards the spindle.
When the left and right audio channels are fed an equal
inphase signal. the cutter modulates the groove laterally: and
when fed an equal out -of-phase signal. the cutter modulates
vertically.
The width of the stereo groove at the top is 0.001 inch, and
at the bottom is 0.0002 inch. This is designed to take a
playback stylus with a diameter of 0.0005 or 0.0007 inch. The
monophonic groove is 0.0022 inch at the top and 0.00025 inch at
the bottom, designed to take a playback stylus of 0.001 inch.
Reproduction
Very few stations today cut their own records. There was a
time. before audiotape, when almost all stations had their own
172
(A)
(TOP)
INCH
r -- - -0 001
- - - -T
SPINDLE
45°
LEFT
CHANNEL
OUTER
RIM
45°
'RIGHT
s,
CHANNEL
i
41
0.0002 INCH
(BOTTOM)
(B) L +
R
(IN PHASE)
LATERAL MODULATION
OF GROOVE
(C) L
-
R
(OUT OF PHASE)
VERTICAL MODULATION
OF GROOVE
Fig. 5 -10. In (A), the NAB groove standards for stereo. In (B), the result of
in- or-out -of -phase signals to the cutter head.
disc- recording machines. But with audiotape so convenient,
cutting of discs has fallen by the wayside. Today, the station is
basically concerned with the playback of prerecorded records,
which are really pressings made from a master. Some of these
pressings are good, others not so good. Also, the majority of
records made today are in stereo, so most of the following
discussions will be directed toward stereo equipment, even
though a station may be programming in monaural. It is a
simple matter to combine stereo channels to obtain a
monaural system.
Rumble
Very low frequency noise can be generated by the
turntable and its drive mechanism. This is referred to as
rumble. Mechanical resonance of the table itself can set up an
often inaudible mechanical vibration. This vibration travels
from the outer rim to the center spindle and couples to the
stylus through vibration of the record.
The drive mechanism can become hard, develop flat spots.
or be adjusted too tight, and this can set up vibrations. Since
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the drive is often at the outer rim, these vibrations travel
across the table. Some models drive a special rim which has
been forged into the table near the spindle area and out of the
record playing area. Still other units make use of a bolt drive
to soften and isolate noise coupling to the table.
These rumble noises can be annoying to a listener if they
are audible. The broadcast audio system today has an
extended range that amplifies these very low frequencies. and
if they are high enough in amplitude, even though inaudible
they can cause intermodulation distortion. This distortion
degrades an otherwise good audio system. Special filtering is
often incorporated, besides design considerations, to keep the
rumble at a very low value. Measurements for rumble are
made after the filtering and preamplification. This should be
at least down 35 dB on each stereo channel, and preferably
lower than that.
Stylus
The modern stylus is a precision- engineered device and is
a far cry from the old needle of yesterday. The type of stylus to
use will be dictated by the particular cartridge which will use
it. Cartridge manufacturers will also make the stylus. but
there are other manufacturers who make only replacement
styli.
The diamond -tipped stylus is perferred over the sapphire.
The diamond is more expensive, but it wears better and has a
longer life. For stereo, use either the 0.5 mil or the 0.7 mil
stylus. depending upon the equipment and the records the
station plays. When only records are used, the 0.5 mil stylus
will treat the record grooves more kindly. But if 45 RPM
records are intermixed with LP's. then use the 0.7 mil stylus.
The grooves in the 45s are not always the best nor hold too
close to tolerance. Consequently. the 0.5 mil stylus will drop
too deep into the groove, allowing the cartridge to drag along
the top of the groove. A few plays in this manner and the
distorted top of the groove has a permanent noise problem.
Use the larger stylus for this application.
Cartridges
These are well designed units today. especially those for
broadcast use. They may be dynamic. variable -reluctance, or
ceramic. All of these types have somewhat different charac174
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teristics. but they all do an excellent job. All are designed
for easy replacement of the stylus.
Replacement of the stylus is usually a simple matter of
pulling out the old one and slipping in a new one...no screws or
other parts to take apart. But do use the correct stylus with the
correct cartridge; they don't interchange. Whenever possible,
the station should use the same type of cartridge in all of its
turntables, not only in the control room, but in the recording
booths as well. Then only one type of replacement stylus need
be kept in stock.
Output Connections
At the rear of the stereo cartridge there will be four pin
connectors. This allows for a 4 -wire output system, providing
for both a high and low side of the left and right channels. They
usually come with a strap across the low side, which grounds
to the cartridge frame: but this can be removed when a 4 -wire
system is desired. Otherwise, the output is run as a 3 -wire,
unbalanced system.
Small pushon connectors are provided to attach the tone
arm wires to the cartridge. These should be soldered to the
wires, but away from the cartridge. After soldering, push
them onto the cartridge pins. Heat from soldering can damage
the cartridge, so do not solder or heat the pins.
Monaural or Stereo
When a station is in monaural, such as an AM station, the
stereo pickup should still be used Fig. 5-11) . There are two
ways to combine the stereo to produce monaural. In the first
method, strap the output terminals of the left and right
channels at the cartridge and feed to a monaural preamplifier.
This places the two outputs in parallel and will affect the
output impedance of each one, but it can be done in most cases
without too much problem. The second method makes use of a
stereo preamplifier, and the outputs of the preamplifier are
strapped together in parallel. This method maintains the
correct impedance load on the cartridge.
(
Installation
Proper tracking and record wear depend upon several
things. and one of these is a level surface. Before making any
adjustments, the cabinet itself should be leveled and resting on
a solid base. Select a sturdy location or floor section on which
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(A)
MONAURAL PREAMP
CARTRIDGE
HIGH
LG
RG
LOW
1
OUTPUT
SHIELD
STEREO PREAMP
L
IN
R
IN
LEFT
OUT
RIGHT
OUT
MONAURAL
OUTPUT
STEREO INPUT
Fig. 5-11. Two methods of obtaining monaural from a stereo cartridge.
to place the cabinet. If the floor is not supported too well.
someone walking across it can cause the floor to vibrate and
the tone arm to bounce on the record. If necessary. add
additional bracing to the floor.
Cabinets designed for broadcast use have four individually
adjustable feet. Some level- measuring device is needed. A
small carpenter's level will do. but a better one is the type that
fits directly on the spindle and has a bubble in liquid, under a
window with two crosshairs. Adjust each of the feet until the
bubble is directly in the center, where the hairs cross, but
make certain that all four feet are solidly on the floor at the
same time this occurs. Whatever level device is used. always
measure from the top of the turntable rather than the cabinet
itself.
Tone Arms
Select a tone arm the same size as the table. These are
available in both 12 -inch and 16 -inch sizes. Most stations use
the 12 -inch record or less, so the 12 -inch arm and table will be
sufficient. The 12 -inch arm will be too short to work with a
16 -inch table.
Before making weight adjustments to the tone arm, install
the cartridge in final form on the arm. Use the special small
scales designed for this purpose. Adjust the tone arm pressure
176
as stated in the instructions for the stylus and cartridge in use.
This will be somewhere between 1.5 and 4 grams. Try to set the
weight at the lowest stated value that will track the disc.
Load Impedance and Wiring
Most cartridges are designed to work into a 47K load
impedance. On the stereo cartridge, this means that the left
and the right channel will each be loaded with a 47K load. The
cabling to the preamplifier input should be as short as
possible. as the cable capacitance is in parallel with this high
impedance, and a long cable will roll off the high- frequency
response. Mount the preamplifier in the cabinet itself, but keep
it away from the magnetic field of the meter, which will
produce hum in the amplifier.
The shield of the leads from the tone arm should be
grounded at the preamplifier input only. There should also be a
ground lead run from this point to the motor frame. It takes
some experimentation to get the best grounding arrangement.
and this is especially true when RFI is a problem. Without the
proper grounding. there will be a sizzle or hum in the audio.
Signal Levels
Use a good test record as a signal source, such as the NAB
or CBS test records. Those records provide a variety of test
signals for both monaural and stereo.
Use the cut with the standard level and adjust the
preamplifier GAIN control so that the feed to the console
allows the console fader to be at 12 o'clock. This usually places
the preamplifier GAIN control at about the same location.
Ordinarily, there is ample gain in the preamplifier.
Run a set of measurements for response, distortion, and
noise, using the test record as a signal source, and measure at
the output of the console. The tests then indicates the system
performance rather than the turntable performance only. You
will also have a good indication of how well the interface to the
console was accomplished.
Maintenance of Turntables
Most of the turntable problems are with the stylus. The
stylus is a very delicate part of the system, and it is also the
greatest wear point. It is subject to the most accidents and
abuse. The tip is easily damaged if the tone arm is dropped on
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the table or cabinet, and it often loses the tip if scooted across
the record. The operator needs a delicate touch, or accidents
will happen.
Distortion
Inspect the stylus often, especially when all records played
on that unit sound distorted (Fig. 5 -12). A small pocket
microscope is an excellent tool for this purpose. It has a very
(A) NORMAL TIP
(B) TIP MISSING
V
Fig. 5-12. Inspect the stylus with a microscope. If the tip of the stylus is
missing, it will cause distorted sound and damage the records.
high power, so it takes some knack getting the tip into focus.
This requires that the end of the microscope be almost
touching the object, and this shuts out the light. Lay out a white
sheet of paper on the desk to reflect plenty of light onto the
hand -held stylus. It also gives backlighting and makes the
point stand out.
Take a look at the tip. Often the tip is missing and the
stylus has been playing records with the blunt stump. This will
cause distortion and probable damage to the record grooves. If
not missing, the point may be worn down. How much wear can
easily be determined by making a comparison with a new
tip -under the microscope.
The size of the stylus may be wrong for the type of record.
Many of the 45 records, especially those with "top 40" music,
have the grooves far overmodulated. Not only are the lands
between grooves almost nonexistent in places, but they may be
very wavy. This makes it difficult for the tone arm to track
properly. The 0.7 mil stylus should be used on these records.
When poor tracking does occur, it may have many causes
besides poor grooves. The record may be warped, and this
causes the lightweight tone arm to bounce. Such records
should be discarded, but unfortunately the operator often
starts to tinker with the weight adjustment to make the arm
178
track. The adjustments always end up far too heavy. So check
the weight adjustment often to correct for this misoperation
before damage occurs.
Record Cleaning
Records will accumulate dust. oil from the hands, and
other foreign substances. This will build up on the stylus
during play and prevent it from performing properly.
Records can be cleaned with warm water and a mild
detergent. then rinsed with clean water and dried with a
lint -free cloth. Commercial cleaners are also used, but
experiment first, as some of these do not really do the job. and
also leave a residue.
There is a small record -cleaning machine that is not too
expensive and does a good job. The record is inserted in it. and
it automatically spins the record while internal brushes clean
out the grooves.
Speed
Speed of record rotation is very important to the
reproduced audio. The table should run neither slow nor fast.
and it should not waver. Small strobes are available that are
placed on the turntable to check its speed. These work best
under fluorescent lighting. The test is best done with a record
on the turntable and the tone arm down and playing. This will
put the normal load on the table. The marks on the strobe
should stand still.
The marks will move either forward or backward.
depending upon the incorrect speed. If you can't remember
which direction is slow or fast, here is a trick you can use to
make it definite: Place a finger along side of the turntable and
apply a slight pressure, dragging the speed down. Then you
can easily determine which way the marks move and whether
the table has been running fast or slow.
Poor speed is usually due to problems in the drive
mechanism or lubrication of the bearing at the base of the
spindle. The drive puck may have hardened or become oily, or
the table rim may be oily or gummy. Clean these off with
alcohol and make sure the bearing doesn't run dry.
AUDIOTAPE MACHINES
The tape recorder is one of the most basic equipment
items, in the control room, and in other areas of the broadcast
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station. Needless to say, when the tape equipment is
inoperative, a severe bottleneck is placed in the normal daily
operations.
Tape -Head Area
There are many electromechanical functions in a
recorder. and these can cause problems during operation. The
most critical section of the recorder, however, is the tape
head. All other recorder functions support the tape head
function. Many problems in this area not recognized as tape
head problems are often blamed on other sections of the
recorder. This area deteriorates and goes unnoticed for some
time because the loss in performance is gradual. An example
is a falloff in audio response. Proper tape head function is very
important in both the open -reel and cartridge tape equipment.
Head
The head is a specially designed. properly shielded
electromagnet. A coil of wire is wound on a magnetic iron
core. which is shaped so that the pole pieces are separated
from each other by a very fine gap. The signal current flows
through the coil, producing magnetic lines of flux that are
basically confined to the iron core. The lines of flux are
continuous loops that cross the gap in the pole pieces even
though the gap provides a greater resistance to their
movement. The flux lines follow the signal current in the coil
both in direction and intensity Fig. 5 -13).
(
Audiotape
The audiotape. with its iron oxide side held tightly across
the pole pieces of the head, is pulled along at a constant speed.
The iron oxide of the tape bridges the gap between the pole
pieces. and this iron has less magnetic resistance than the air
in the gap. Consequently. the flux takes the path through the
tape. In so doing. the flux magnetizes the iron particles in a
pattern that conforms to the flux lines in both direction and
strength at any instant. Thus. the iron particles on the tape are
magnetized in a pattern which is a replica of the signal current
flowing in the head. The pattern is laid down on the tape in a
track as the tape is pulled along.
Low Frequencies -For a given input signal, low
frequencies magnetize particles more deeply in the tape than
high frequencies. This is because the tape speed is constant
180
INPUT SIGNAL
HEAD
FLUX LINES
IN CORE
/!"
TAPE MOTION
IRON OXIDE
BASE
IRON PARTICLES
AT RANDOM
FLUX
LINES
IN TAPE
TAPE
PARTICLES MAGNETIZED
ACCORDING TO PATTERN
OF SIGNAL CURRENT
Fig. 5-13. The tape head system in an audiotape machine.
and a low-frequency portion of the tape is subjected to the
signal for a longer period of time than high frequencies, and
have a chance to penetrate deeper into the iron layer.
Signal Levels-Increasing or decreasing signal current
flowing through the head causes a corresponding increase or
decrease in magnetization depth. These factors affect the
playback level recovered from the tape, because the greater
the penetration of the flux into the tape. the stronger the
magnetization of the particles, and the greater the recovered
playback level.
Playback
During playback, the reverse of the recording process
takes place. As the tape is pulled across the pole pieces of the
head, the magnetic fields recorded on the tape induce currents
in the playback head. The output voltage of the head is a
replica of the signal that was recorded on the tape. The
stronger the magnetic track on the tape, the greater the output
voltage of the head, and thus the greater signal level.
Playback and Record Heads
The heads used for these two purposes have different
characteristics. The record head must carry higher currents,
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and has a lower impedance than the playback head. The
playback head usually works into a higher impedance.
although there are some machines that are designed to use the
same head in both positions. In the usual arrangement, they
are different, and when replacing heads, care must be taken
not to get the heads in the wrong positions. The results of
misplaced heads can be disappointing. A record head used for
playback produces a very low output signal.
Equalization
As discussed earlier. low frequencies record deeper into
the tape high frequencies. From this it follows that the output
of the playback head will vary in the same manner -the
high- frequency response falls off. To correct this situation. it is
necessary to preequalize the recording signal before it reaches
the record head. The high frequencies are preemphasized
according to a standard curve. The playback amplifiers use a
standard deemphasis curve.
Erasure
A tape that has been recorded can be reused by erasing the
recorded information. This is done by applying a strong AC
field across the tape. which rearranges the iron particles in a
random fashion on the tape. This field may be applied either
by an erase head. as used on open -reel machines, or by a bulk
eraser as used on cartridge machines. An erase head erases
only the information on the section of tape that passes over it.
while the bulk eraser erases the entire tape at one time.
How well the tape is erased depends upon how strongly the
particles are magnetized and the strength of the erase field.
Tapes that have been severely overloaded -that is, very
strongly magnetized -will take many passes over the erase
head to be completely erased. The portion that is not erased
remains on the tape and appears either as crosstalk or
background noise. With this in mind, caution should be taken in
the use of bulk erasers, which develop strong magnetic fields.
The tape should not be in the field at the moment of turnon or
turnoff. This causes a magnetizing of the tape by itself that is
difficult to erase. Instead, turn on the eraser, then bring the
tape into the field. When done take the tape out of the field
before turning the eraser off.
182
Residual Magnetism
There is another important problem in recording that
must be overcome. The head is an inductive device and will not
allow currents to flow through it in a linear manner. This is
due to residual magnetism and the fact that magnetic fields
are reluctant to change directions. Thus, as the current begins
to flow into the head until it reaches it's maximum in one
direction, the field will be established (Fig. 5 -14). Now the
current reverses itself and goes to the opposite maximum
MAGNETIC
CURVE
MAGNETIZATION
ON TAPE
DISTORTION IN
THIS AREA
AUDIO
INPUT
SIGNAL
Fig. 5 -14. Effects of the magnetic properties of the head and tape cause
distortion near the zero -crossing line.
polarity. However, the reluctance of the magnetic field to
change, along with the residual magnetism in the core and in
the iron oxide on the tape, produces the familiar magnetic
curve. This results in the signal current being distorted near
the zero -crossing line.
To overcome this distortion, a high- frequency bias is used
(Fig. 5 -15). The bias is about 75 kHz and several times the
amplitude of the signal current. This bias and the signal are
mixed together before going to the recording head. The audio
signal appears as twin curves riding the peaks of the bias
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MAGNETIZATION
ON TAPE
../
BIAS
OUT-UNDISTORTED
AUDIO
MIXED
AUDIO AND
BIAS
AUDIO
SIGNAL
BIAS SIGNAL
Fig. 5-15. Using bias to lift the signal Into the linear area.
signal when viewed on an oscilloscope. During playback, the
bias signal is filtered out, leaving the audio curve undistorted
since the audio signal was lifted into the linear part of the
magnetic curve.
Head Alignment
Head alignment is most important to assure that tapes are
compatible from one machine to another, and output levels
and frequency response are maintained. The face of the head
and the tape must be aligned in their proper relationship.
There are at least five requirements that must be met to
obtain good alignment. These five positions are shown in Fig.
5-16. These requirements may not be met 100% and the results
may still be passable: but the more accurate the settings, the
more consistent the quality of the results. Head alignment is
always necessary when replacing a head.
184
1
TAPE
TOP VIEW (ROTATION)
1`
2
90°
TAPE
I
1
I
DECK
FRONT VIEW (AZIMUTH)
3
2 TRACK POLE PIECES
O
O
DECK
FRONT (HEIGHT)
4
fl'
i
\
TAPE
i
SIDE VIEW (TILT)
90°
1
1
DECK
5
TAPE
TOP VIEW (PENETRATION)
Fig. 5-16. The five conditions of head positioning which must be met in
head alignment.
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Removal and Replacement
Before removing the old head, note the markings and the
positioning of the present head. Try to get the new head into
the old position as closely as possible. Then the new head may
require only a minimum of adjustment to bring it into full
alignment. Some of the manufacturers have gauges and plates
available to assist in the correct positioning of the heads. This
is particularily true for cartridge equipment. These gauges
can put the head into nearly the exact position. But there is
always a need for final adjustment to bring the head into full
alignment.
Playback Head -Always align the playback head first,
while playing a standard NAB head alignment tape. (There is
also a standard NAB cartridge-type alignment tape available.)
Standard tapes should be handled carefully and stored
separately. To prevent stretching, avoid using the fast -winding
capabilities, and especially fast stops. Once the playback head
is properly aligned with the standard tape, the playback will
serve as the standard for the record section and head.
Record Head-Single tones are applied to the record
amplifier input while adjusting the record head. Be careful to
get the standard alignment tape off the machine before switching into a record mode, or the tape will be ruined. Another
precaution concerns the signal levels into the recorder. These
should be at least 10 dB below standard program- recording
levels, and 15 dB lower is better if noise pickup is low. These
lower levels are required because of the equalizer in the
recorder, which boosts the high frequencies. If levels are too
high going in, the boost causes the head to saturate at high
frequencies, and this appears as a flat response-which it is
not. In reality, the true response may be far from this
indication. The reference level to use is between 200 Hz and 1
kHz. To be consistent, use the same tones that were used on the
standard tape. There will be less confusion when this is done,
and a better comparison can be made.
Cartridge Alignment
Head alignment in cartridge machines must meet most of
the requirements as on open -reel machines, but there are a
number of other requirements that must also be met. This is
because the tape, guides, and pressure pads are within the
cartridge itself. In cartridge alignment, consider these
186
conditions: Pressure pads should be in good shape and press
the tape against the head firmly; tape should fall easily and
directly into the guides that may be on the head assembly; the
cartridge should allow the head to penetrate into the tape
deeply enough (check any depth adjustment that may be
provided); the keyhole in the base of the cartridge should
allow the pinch roller shaft to pop through without catching or
pulling the cartridge to one side or holding the roller off the
drive shaft (the cartridge should not touch the drive shaft);
and side guides must be set to allow the cartridge to move
directly into the head assembly. All of these conditions can be
seen by careful observation and adjustments made
accordingly.
On the Bench
It is often more convenient to take the machine to the
workbench to make these alignments, provided that it's
unnecessary to make many haywire hookups. Make up cables
ahead of time that will fit the input and output plugs of your
machines. These cables should also provide for the correct
impedance matching, or at least allow for simply adding
terminating resistors. Keep these cables along with other
maintenance cables for future use.
False Indications
High input levels to the recorder can "ring" equalizers so
that they oscillate, giving rise to spurious frequencies. The
output meter can indicate these spurious signals rather than
the input signal. Quite often these spurious tones will be lower
in frequency than the input signal tone. If you have the output
up on earphones or a loudspeaker, you can hear that the tone is
wrong. Reset the input levels below the point where this can
happen.
Worn Heads
The iron oxide on the tape is very abrasive and wears
down the heads, and after much use, a groove or rut will be
worn on the face of the head. These create ridges at the edge of
the groove. Tape may fall so that its edge is riding atop this
ridge and out of contact with the pole pieces of the head. The
result is a pressure problem, and the output from that side of
the tape can be low or nonexistent. On a full -track machine,
this may not show up as a serious problem; but on a multitrack
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head. it produces serious problems. For example, on a
stereo -plus- cue -track tape, the cue track is at the bottom edge
of the tape. If it rides on top of a groove and has low output, the
cue and stop tones on cartridge tapes are low and the machine
will not operate properly.
Besides the erratic behavior of the signal from a worn
head, the amount of wear can often be detected by either
observation or feel. Shine light across the head. If it is worn,
the light shows up the edges of the wear spot. Another method
is to run your fingernail lightly across the face of the head. You
can feel a groove on a worn head.
Clogging
When the head wears, it opens the head gap. Besides
causing a falloff in high -frequency response, this also causes
oxide from the tape to lodge in the gap and short it out
magnetically. Shorting out the gap causes the output voltage to
drop off to practically nothing. Heads in this condition should
be replaced, but until they are, they must be cleaned often. If
the output drops off while the machine is in operation, an
emergency measure can be used. Squirt a head -cleaning
solvent or alcohol onto the head while it is running. The
movement of the tape and the fluid cleans out the gap.
Pressure Pads
Pressure pads must be kept in good condition or the
machine will operate erratically, usually with weak output
signals. All machines do not require pressure pads -it depends
upon the head and the way in which the machine wraps the
tape onto the face of the head. Cartridge machines have pads,
and these should be kept in good condition. The pad maintains
a constant pressure of the tape on the face of the head; but if it
is worn or missing, the tape will flutter past the head or change
pressure intermittently. All of these produce poor output
results.
Capstan and Pinch Roller
Tape speed across the heads is determined by the capstan
speed and the pressure of the pinch roller. The rubber in the
rollers can become glazed from lubricant, or hardened and
glazed with age. In all of these conditions, the roller can slip
and cause speed variations. These show up as `wows" in the
program material. The rubber roller can be cleaned with
188
alcohol or other solvent. If it becomes hardened, it must be
replaced.
Capstan Motor
The motor often develops dry or bad bearings, and defects
in the windings. With the power shut off, the motor or flywheel
can be spun by hand. This should spin freely and coast for
some time, and there should be no noise. One that is stiff or
stops immediately after spinning needs to be taken out and
reoiled or have the bearings replaced, if possible. If not, then
the motor itself needs to be replaced. Many motor
manufacturers have an exchange arrangement for capstan
drive motors.
Bias Adjustment
The bias affects the output level and high- frequency
response. The best way to adjust this, if metering is possible, is
to record a high- frequency tone and then adjust the bias for the
highest output level on playback. Playback must be observed
while recording. Improper bias can produce some curious
results, even showing up irregularities in brand new tape.
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Chapter 6
Peripherals
Although the control room generates the bulk of the station's
programming, there are many program segments gathered
and prepared outside the control room. Besides the
programming sources. there are auxiliary functions
performed which either contribute to programming indirectly
or facilitate the station's operation. In this chapter. we
concentrate on newsrooms. recording booths. teleprinters. and
house monitoring.
NEWSROOM
News -gathering operations in today's radio station
are
often hectic. fast moving, and require many electronic aids.
The reporter out on the beat carries along an array of
electronic aids. The advent of solid -state equipment. its
corresponding reduction in size and weight, and its
adaptability to battery operation, have enhanced portability to
a high degree. For the first time, perhaps, since the early
beginnings of radio, news gathering is able to make a greater
use of broadcasting's own tools of the trade -electronic gear.
Whenever called upon to design or arrange equipment for
use by the news department. always follow this basic
concept-keep it simple, as simple as possible while still
getting the job done. Remember that nontechnical people may
operate the equipment. Complicated equipment arrangements
190
that only engineers can operate will, when placed in the news
department, probably fail, or at least fall far short of
expectations. This is not intended to downgrade newsmen. On
the contrary, news gathering is demanding work, and the
electronic equipment must assist it, not impede it. During an
interview, for example. the reporter must concentrate on
developing pointed, probing questions, and listening carefully
for shaded meanings in the answers given. He cannot also be
trying to operate complicated equipment at the same time.
What Is Simple?
In this case. simple means ease of operation. Design and
arrange equipment in this manner: cable plugs that can only
go into a jack one way (color coding is helpful); simple
switching that operates in a left -to -right fashion; switches that
have straight on -off operation; functions all labeled clearly;
on /off and nonlocking switches that must be pressed to
operate. Perhaps these are rudimentary conditions, but they
have a better chance of performing as intended. Remember
that the newsman is concentrating on his story, not the
operation.
Many recordings are made in the newsroom that end up on
open -reel and cartridge tape. Consequently, there should be
both open-reel and cartridge tape recorders in the newsroom.
Since the product from these machines end up on the station's
regular tape machines. the newsroom machines should be the
same quality as the regular machines. and completely
compatible.
Switching
Many tapes from cassettes and portable recorders are
brought in for editing and dubbing onto larger machines. and
other news sources feed into the newsroom. It is necessary to
provide some simple switching arrangement so that all these
sources can be edited and blended together into the final form
desired. A small production console can be used, and some
stations provide this. But a simple switching arrangement
Fig. 6-1) can be built that allow these sources to be wired in
permanently, and only switching is necessary to pick up the
desired source. In effect, this switcher will customize the
arrangement for that operation.
When designing a switcher, keep in mind the output
impedances of the various sources and try to maintain these as
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much as possible. Remember that when you simply parallel a
couple of sources, you will affect the impedances and signal
levels. If necessary, use bridging connections, and even use
back -loading resistors on some of the switch terminals. Also,
use pads to keep levels within reason.
Input Selection -Allow for the selection of several
program sources, which can be permanently wired to the
selector switch; for example, the telephone, remote pickup
receiver, output of the cartridge tape machine, and output of
the open -reel tape machine. Have one position that wires to a
jack in the regular station jack field. This allows using any
source in the station to feed to the newsroom. Also add an
auxiliary jack so that small, portable equipment can be
plugged into the switcher.
Recorder Selector -The output switch should be able to
select the record input of either the open -reel recorder or the
cartridge recorder. This allows for recording on either one.
Between the switch and the recorders, an AGC amplifier can
be used to good advantage in maintaining levels during
recording.
Monitoring
There should be both earphone and loudspeaker
monitoring. Add a small speaker amplifier and GAIN control
for this purpose. During times that live recording is being
done. the speaker can't be turned up, so earphones must be
used. The earphone can be connected directly to the input
selector bus or after the AGC. The earphones should be ahead
of the monitor amplifier. A switch can be used to turn off the
amplifier, or the GAIN control can turn it down. The earphone
switch can be arranged so that it will automatically switch off
the speaker when the plug is inserted into the jack.
Telephone Recording
Many interviews can be made over the telephone and
recorded for use on the air. There are different ways this can
be accomplished. In the direct method Fig. 6 -2). the recorder
is connected directly to the phone line, but there should be 0.1
µF series capacitors in each side of the line for isolation of DC
voltages. Use a 600 -ohm transformer for other isolation.
Telephone recordings are not always the best, particularly
when there is a poor line with low levels. When gains are
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STATION
TELEPHONE
4
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RECORDER INPUT
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Fig. 6-2. A simple, direct connection can be made to regular telephone
circuit, but use isolation.
turned up background noises increase, and the recording is
very poor. An AGC amplifier can be of some value in leveling
out the signals. In almost all cases, the local voice will be
much stronger than that from the other end of the line.
The telephone company can lease a connection device that
has an amplifier plus an AGC unit in it. I have had poor
experience with these. They have regular AGC amplifiers that
cause serious "breathing" or "pumping" effects on some
recordings.
Whenever a telephone conversation is to be recorded or
broadcast live. the person on the other end must be informed
of this, and permission must be given. The rules are very strict
about this. Anytime a phone conversation is to be recorded or
broadcast live. first inform the party and get an okay-then
turn on the recorder or put him on the air. Stations have been
fined for airing the initial "hello" of a conversation without
first having informed the party he would be on the air.) There
are exceptions. of course, such as your own reporter calling in
a news story that he knows is supposed to be recorded or put on
the air live. But don't become careless in this matter. or the
station can get into hot water with the FCC and may face a
lawsuit from the other party.
(
Other Monitoring
To keep in touch with activities occurring in the
community. most stations monitor the police and fire
194
communications channels. This may be done with a
scanner -type receiver, or each channel may be monitored with
a single crystal -tuned receiver. The scanner can only lock onto
and monitor one channel at a time, but individual receivers
can monitor all channels all the time. Individual receivers
provide more information, as there may be events occurring
on several channels at the same time.
Speakers
Live recording may be going on in the newsroom, so
speakers can't be blaring. With a single- scanner, it can be
turned down; but with several individual receivers, it is a
chore to turn down all the speakers at one time. And if they are
turned down, they may not get turned back up again.
A simple switching arrangement can be made that will
switch off all the speakers at one time ( Fig. 6 -3). Run each
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receiver speaker lead through a multicontact ganged switch
on the speaker box. One turn of the switch turns off all the
speakers at the same time. To make sure the speakers get
turned back on. use a set of contacts on the switch to turn on a
red pilot lamp to warn that the speakers are off.
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Police and fire department channels are private
communication channels. The Communications Act forbids
use of their information in any way. When a station listens in
on these channels for news events or news tips, it is making
use of the information. This is not permitted unless the station
has permission to do so. Therefore, the station must obtain
permission from someone in the particular agency who has
authority to grant permission, for example. the chief of police.
Get these permissions in writing and keep them on file at the
station. And if the individual changes, get the permission
renewed by his replacement.
Maintenance
The majority of problems with news equipment are
operator errors. Problems are also caused by dirty equipment,
damaged plugs. cords, batteries. etc. There can be major
problems in the equipment from time to time, but look for the
simple problems first.
Operator Errors
Operator error is the first thing to look for when called in
to correct technical problems in news equipment. Try to
determine if the operation was done correctly. If the problem
is an operator error and it is obvious the newsman does not
know how to operate the particular equipment, explain its
operation in simple. nontechnical terms.
Dirty Equipment
A lot of news work is done on tape, so the heads, pinch
rollers. and drive shafts need cleaning often to remove oxide
buildup and the bits and pieces of tape that accumulate. Clean
the head with alcohol on a cotton -tipped swab. Clean out any
bits of tape. Sometimes these wrap around the capstan drive
shaft or the pinch roller causing a change in tape speed. Also
look over any pressure pads that might need replacing.
A regular routine should be set up to clean the equipment.
If the news people do this, fine; but even then, check from time
to time that it is getting done. And don't forget heads on those
cassette machines -they need cleaning too.
Cables and Plugs
Cables and plugs get a lot of use and abuse. Many times
cables will fall on the floor and get stepped on. Look for broken
196
leads and shields in the cables, bent plugs, or plugs that are
falling apart. Some of these are molded types, and unless a
replacement plug can be obtained, the whole cable will have to
be replaced.
Microphones
Microphones for outside use must be rugged, for they take
a lot of abuse. Unfortunately, much of this small equipment is
intended for home use and not commercial use. These are
often dropped on the floor or the concrete pavement. In many
cases, it is cheaper to replace the whole mike rather than try
to get it repaired. The fault may be minor, and if it can be fixed
locally, go ahead. Small capacitor mikes used with cassette
recorders have a separate battery that must be replaced
occasionally.
Batteries
One of the big problems with portable equipment are the
batteries. Instruction booklets that come with the instrument
give the estimated life of batteries. This should be a
reasonable estimate. Unfortunately, unless some type of log of
the hours in use has been kept (hours add up quickly), the
batteries may go dead right in the middle of an interview.
Batteries should always be checked out before use. When
possible, try to use rechargeable batteries.
Memory
Certain rechargeable batteries, such as nickel- cadmium,
develop a "memory" if it is not used to full capacity at some
regular interval. If only a small part of the capacity is used
each day and then it's recharged, the battery will
"remember" this small capacity, and if called upon for full
design capacity, it my run down when it reaches the capacity it
has become accustomed to. This can't be reversed, and the
battery must be replaced.
Batteries should always be recharged after use, but set up
some schedule to run the equipment on test. Run it until the
battery is all the way down: then recharge it. Caution: Never
leave fully discharged batteries on the shelf for very long. Only
put fully charged batteries on the shelf.
Chargers
Keeping batteries charged depends upon the use of
chargers. Small chargers are as delicate as some portable
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equipment. That is, the plugs and cables are easily damaged
or pulled apart. When batteries fail to charge, check out the
charger by measuring the voltage at the output plug. If the
voltage is there in the correct amount, there are problems in
the equipment or battery. If the voltage is missing, check the
fuse if it has one. (Some small fuses are hard to come by and
must be ordered.) If you have problems. change the fuse
holder to take one of the larger, standard fuses. If it won't fit
inside the case, use one of the line holders. Inside the case,
short across the terminals so as to complete the circuit.
If the batteries still don't charge, check the socket pins,
and especially the battery terminals and contact terminals
inside the unit. These often corrode and develop high
resistance.
Bulk Eraser
Most of the small open -reel portable recorders are
half -track machines. Tapes are brought back into the
newsroom for editing, and the tape is placed on the large
machine. Of course, small machines can be used as a
playback.
The tape may go directly to the control room for play.
Most AM stations use full -track machines. So the other track of
this tape must be clean. If there is recorded material on it, the
full -track will play both tracks at the same time, and the tape
can't be used.
Before going out on assignment, bulk -erase the tapes. In
this way, the track will be clean, but the reporter must only
record on the tape in one direction.
RECORDING BOOTH
Another area that contributes much to the station's
programming is the recording booth. With so much program
material on audio tape, a recording booth becomes an asset.
Not only will the booth free the control room equipment, but it
will allow the announcer to record announcements and other
program material in a more relaxed atmosphere. If the AM
station also operates a sister FM station that is fully
automated, the recording booth becomes a necessity. For tape
recording the booth should become the quality control center
of the station.
198
Monaural or Stereo
The equipment used in the booth must match the type of
service the station is supplying. If monaural, then only
monaural equipment is needed, and if stereo, then stereo
equipment must be used. A common combination is the
monaural AM station and a sister station that is stereo. In this
case. the booth should be made to do double duty. That is, use
stereo equipment, but also have a monaural cartridge tape
recorder.
More than one booth may be required if there is a great
amount of recording. When additional booths are set up, they
should all be made identical: but this is optional. One can be a
straight monaural booth and the other stereo if desired.
Identical Booths
There are advantages to making all booths identical in
equipment. layout, and functioning. This allows any recording
in any of the booths and thus makes scheduling easier.
Announcers can work any of the booths, as all the controls and
equipment will be in the same configuration and in the same
places.
There are also maintenance advantages. When one booth
is down, it is easy to compare the faulty equipment with that
which is working properly. This makes troubleshooting easier
and bypasses some of the procedures. If one booth must be
taken out of service, then either monaural or stereo recording
can be done in the other booth. And there will be fewer spare
parts required.
Basic Equipment
There are certain basic equipment items any booth should
have i Fig. 6r4). and any additional needs can be met by adding
specific equipment to fill those needs.
Cartridge Recorders -There should be one master
cartridge tape recorder, and in a double-duty booth one should
be monaural and one stereo. If the station uses the auxiliary
switching tones, then the recorders should be equipped for
these additional features.
Open -Reel Recorders -There should be at least one
open -reel recorder, and if stereo recordings are done, then this
should be double -track stereo. It would be well if this could
also play back 4 -track stereo. This recorder can be used for
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monaural. When doing so. feed both the left and right channels
together. The resulting tape can play back on the control room
full -track machines. If there is much dubbing done, then a
second open-reel machine can be advantageous. These
machines should be set up to act both as recorders and
playback machines.
Consolette-There may be a need to do switching,
mixing, and blending in the production of spots and certain
other programming. A small production -type consolette
should be provided. One of the larger control room -type
consoles can be used, but these provide far more capacity
than is really needed in the booth. Most of their features will
not be used and are thus wasted.
Some production consolettes come as a package unit, that
is. with a desk or table, integral monitor amplifiers, and
turntables. These usually serve the purpose well and can save
buying all the components and putting it all together.
Don't expect to get the same rugged quality in these small
units as in the large consoles. Technically they are okay, but
mechanically they leave much to be desired. The quality of
components is not as good nor as lasting. For example, faders
are often simple carbon or wirewound pots rather than step
attenuators. And the equipment doesn't hold up as well.
Expect to spend more time correcting problems here than in
the large consoles.
Turntables-There should be at least one turntable in the
booth. The output may be either stereo or monaural, but the
cartridge should be stereo. The stylus size depends upon the
records played-if only LP's, then use the 0.5 mil diamond.
But don't skimp on the turntable. The turntable should be of
the same quality as the regular units in the control room.
Cartridge Playback-There should be at least one
monaural and one stereo playback machine for dubbing
purposes. This. too. should be of the same quality as the
master machines and preferably of the same manufacture.
AGC Amplifier-Following the consolette, an AGC
amplifier or a combination AGC -peak limiter should be
used. This will be of advantage in controlling signal levels
and will prevent overmodulating the tape. In stereo, two
identical units should be used for the left and right channels.
Strap the control voltages together so that the units work as a
201
single unit. By operating them in this manner. the original
ratios of the left and right channels will be maintained.
Installation
Treat the booth installation just as you did the control
room. Get a good, heavy ground connection to the building
ground, and run a controlled shield ground on the cables.
When wiring up small consolettes. expect to run into some
other arrangements than with big equipment.
Terminal Boards-One of the first problems will be the
audio terminal boards. These may be simple strip units with
screws. and they may be mounted far inside the unit so that
the outside cabling must run all the way into the equipment,
rather than at a terminal block at the rear. When running the
cabling inside the unit, try to follow the regular cabling
routes. Avoid running high -level cables near low -level
circuits, or low -level cables near high -level circuits.
Jacks-When possible. bring the outputs of the consolette
to a pair of jacks on the station's regular jack field. If AGC
amplifiers are used, bring their wiring to the jack field also.
Should the amplifiers fail, they can easily be bypassed with a
pair of patch cords. Jacks also provide an outlet for the booth
to the rest of the station equipment. There may be times
when the booth can operate as a subcontrol room, or even as
the control room in an emergency. And in cases of an automated FM station, the booth can be used to go live if the automation fails or it is desired to remove the booth from
programming for major maintenance.
Input Jack-Try to provide at least one of the inputs on
the consolette as an external input by running this to the jack
field. This will allow patching of any of the station's other
equipment into the booth for use when needed, and it can
serve such uses as recording from the telephone. A single
jack used in this manner can expand the booth's flexibility
considerably.
Standards
The master recorders should be the quality control
standards for the station tape- recording facilities. The
standard should be NAB standards, so that your standards will
be compatible with industry standards.
202
For open -reel tape. use a NAB standard alignment tape
to optimize the machine. Make sure this is the latest standard.
as there have been changes in the past. These are full -track
tapes.
Standard cartridge alignment tapes are available from
NAB. There is only a monaural test tape. While work has been
done toward developing a stereo test tape. as of now there is
not one available. This monaural test tape has the same track
configuration as regular cartridges. That is. it is not full- track.
The monaural test tapes can align the stereo machines. On
the open -reel machine, since it is full -track, both tracks will
play: but there is no way to actually check out the right
channel head in the stereo cartridge machine. The nearest you
can come is to feed the left head into the right audio channel.
This allows tweaking up the equalizer, but it is being done
against the left head, not the right head. When a monaural test
tape is played on a stereo machine, there is an apparent rise in
the low- frequency response (below 100 Hz) of 3 to 4 dB. This is
a normal condition, so don't try to level out that tilt in the
response. When a stereo tape is played. the response will be
normal.
Some equipment manufacturers have developed their own
stereo cartridge test tapes. These can be used if desired. In
effect. your stereo tape standards are set to those tapes. but
this is not necessarily an industry standard.
Local Standards
Once the master recorders have been optimized, make
a number of different tapes that will be used for a variety of
station tape standards, the first of which should be a level set
tape.
Use the same tone as was used for reference on the NAB
cartridge, or select your own somewhere in the range of 400 Hz
to 1 kHz. Feed the tone into both channels of the consolette and
set the fader so both meters read 0 dB. At this point you may
discover the two meters on the consolette do not agree. Check
with your standard test meter at the output of each channel
properly terminated) to see if they actually are different, or if
it is the meters. In most cases, it will be the meters.
Adjust the recorder levels so that each one reads 0 dB. and
then lock these controls or remove the knobs. These should not
be regular operational controls. Any other adjustments for
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production should be done on the faders. Select a new blank
cartridge of possible) and run it through one time to settle the
tape. Erase the tape to make sure it is clean. Then make the
recording. First feed the left channel for awhile, then the right
channel, and then both channels. Add announcements if
desired, but don't stop the tape. The cartridge should be at
least 2 to 3 minutes' running time.
When the recording is finished, play it back on the master
recorder. Set its output meters to read 0 dB and lock these
controls. You now have a basis upon which to check other
cartridges in the future.
Make up a monaural tape in the same way, except use the
monaural recorder. Be sure to properly label these tapes and
store them with other test tapes.
Other Test Tapes
Once the equipment has been set up properly and the level
tapes have been made, go ahead and make other test tapes
that will help in the setup and test of station equipment. For
example. an auxiliary tone test tape can be made at this time.
Leave the audio channels blank and use only the auxiliary
tones of 150 Hz and 8 kHz. If using two tones, set up some
pattern in the way they are applied. Space the test bursts about
five seconds apart. This will allow the circuits in the unit under
test to settle down after being activated. If the equipment is set
up for a variable -length auxiliary tone pulse. record several
different lengths -those that will have particular application
at the station. Label the tape; if special information is
necessary, attach this to the top of the cartridge.
If you use a logging system that makes use of tone pulses
on the cue track of the tape. make up a test of these. Try to get
a variety of combinations that will put the logger through its
paces. And make sure to place a label on top of the cartridge
showing the numbers and the sequence that was recorded.
These tapes. by the way. not only will help in setting up
adjustments on cartridge machines. but they can be used to
measure the actual signal levels the machines put out.
Playback Comparison
When there are problems in station recording and some
question is raised about a tape, the tape should be taken back
to the master recorder and played on that machine. If the tape
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plays all right, there is something wrong with the station
machine. If it doesn't, then it is probably the tape that's bad.
There is a possibility, however, that something has happened
to the master machine and it isn't recording properly. If there
are identical booths, play the tape on the other master for a
check. If there is no other booth, make another short recording
on the master and try it. If it plays okay. the original tape is
definitely defective.
Heads
When head replacement is necessary on the master
machines, do this very carefully. Remember, these are the
masters. Be careful not to get the record and playback heads
interchanged, and replace them with the same quality and
type of head that was taken out. Use premium heads. Be
careful that the wiring to the heads isn't interchanged. If the
wires are interchanged, phasing problems may occur. Always
be conscious of the fact that these are the machines that set
station standards, and make all the adjustments accordingly.
Track Identification
For heads used on broadcast recorders, the track
placement is as shown in Fig. 6 -5.
Head for Monaural Cartridge Tape -The program track is
the one farthest away from the deck; the cue track is next to
the deck.
Three-Track Stereo Cartridge Tape-The left channel is
farthest away from the deck, the right channel is the center
one, and the cue track is next to the deck.
Two-Track Open -Reel Stereo -The left channel is farthest
away from the deck; the right channel is next to the deck.
Phasing
When changing stereo heads, there are two ways in which
the phasing can be affected (Fig. 6 -6). In the first case, the
wiring is interchanged to the left and right channels so that the
channels are reversed. In the second case, the high and low
side of one of the heads is reversed. This will place that
channel out of phase with the other channel, and when
combined in a monaural system, the signals will cause
cancellation and low levels.
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(A)
MONAURAL CARTRIDGE
\\
PROGRAM
HEAD
CUE
DECK
(B) STEREO CARTRIDGE 3 TRACK
HEAD
CO
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-
LEFT CHANNEL
RIGHT CHANNEL
CUE
\`
DECK
(C) STEREO TWO TRACK -OPEN REEL
LEFT CHANNEL
HEAD
\-
\
RIGHT CHANNEL
DECK
Fig. 6-5. Tape track Identification on the heads.
Phasing is very important. Before taking the wiring off the
heads. make notations of the wiring and color coding. Also,
make sure the high and the low side remain as they were
before removing, even though they may have been wrong in
the first place-that is, both wires on each track were
reversed. This should have been caught on initial setup, but to
change after many tapes have been made could create
problems. This is particularily true of the cue head. If the
shape of the pulse is such that the leading edge is very steep
(auxiliary tone) the cue circuit in some machines can
interpret this as a cue tone and stop. So replace the wires on
the heads just the way they came off.
206
Cartridges
Many poor recordings or failures in the system can be
traced to negligence on the part of the announcer in not
recognizing defective cartridges before they are recorded or
failure to listen to the cartridge after the recording was made.
A simple visual inspection of the cartridge will often detect
faults such as wrinkled or worn tape, missing or defective
pressure pads, a broken case, or missing parts. When a
defective cartridge is recorded and fed to the regular system,
problems are brewing. Expect to get a maintenance call that
something is wrong with some of the program machines.
Check the cartridge as a first step. When it is obviously
defective-for example, in case of missing pressure pads -have
(A) CHANNELS REVERSED
LEFT
RIGHT
CUE
RIGHT
(B) ONE CHANNEL REVERSED WILL CAUSE CANCELLATION
WHEN COMBINED FOR MONAURAL
LEFT
±
RIGHT +
Fig. 6-6. Two different ways phasing can be Incorrect in stereo.
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a new spot recorded and take that cartridge out for repairs.
Even adding pads will not help the poor recording that is
already on the tape.
If the cartridge doesn't have any obvious defects, try it on
the master machine or another regular machine. If it plays
poorly on more than one machine, there is more than likely a
cartridge problem or a bad recording. At least it is not the
regular machine at fault. Make a new, short recording on the
same master machine for comparison. Many problems could
be eliminated, fewer spots lost, and tempers saved if the
announcer would audition the tape after it is recorded. This
should detect distorted audio, low signal levels, etc., before.
the cartridge is sent into regular program channels.
TELEPRINTERS
In the newsroom of every radio station, there are one or
two teleprinters in operation throughout the day. One of these
receives news from one of the news wire services, and if there
is a second one, it receives weather information from the U. S.
Weather Service. Just how involved the engineer becomes
with these machines depends upon the station and what
arrangements have been made. The news machine is leased
and the service most likely done on it by some outside service
company or the telephone company. The weather machine
may be station owned and serviced, or it may be leased and
serviced by an outside concern. If you do get to work with
these units, you are involved with a different technology than
broadcasting. This technology has its own way of doing things,
as well as its own language.
Printer
Generally found at the station are receive -only units (R0),
which do not have a keyboard. If a unit needs to send
information, it must have a keyboard. Keyboards on sending
units are not the same in character and symbol lineup as
ordinary typewriters, nor are they the same on all
teleprinters. The keyboard will correspond to the code in use,
and there are several codes. The receive printer must also be
arranged for the code in use on the circuit. A printer can work
on all the usual codes but only works on the code it is adjusted
for. That is, the unit must be adjusted for one of the regular
208
codes: this is ordinarily done at the factory, although it can be
done in the field.
Mechanical and Solid -State
Today there are many models with a varying degree of
solid -state electronics incorporated in them. There are still
those that are largely mechanical, and on the other end are
those that are very high in electronics. The Extel Corporation
printers are 80% electronic, and this is solid-state.
Input Circuitry
The printers normally found in the broadcast station for
news and weather use have a DC input circuit. The DC must be
supplied from the signal circuit itself. Look for a tag or sticker
on the machine which will give this input current limit. The
current should not exceed this limit, or the machine will be
overloaded and develop problems. including burnout of some
of the input circuitry.
Each printer is preset for a particular code. and this is
ordinarily done at the factory: but the factory cannot set a
printer to a code unless it has the information. If you are
involved in ordering a machine, determine the code that will
be used and supply the information to the factory.
Signal
The signal is a DC on- and -off type or pulse train. This is
what the printer itself accepts. When the pulse is on positive),
this is called a mark: and when the pulse goes off zero) , this is
called a space. The width of the pulses do not vary, but
remain constant. There is no gap between pulses; these pulses
line up side by side. Any gaps that appear are spaces and have
a definite meaning in the train. For example. there may be
several mark pulses in tandem. This would appear as a single
wide pulse whose width is equivalent to the number of mark
pulses involved. The number is what is important, not the
apparent width.
(
1
Codes
The present codes are either updates or outgrowths of the
old telegraph codes, and there are several of them. One of the
more common codes used for communications is the ASCII.
These letters are abbreviations for American Standard Code
for Information Interchange. This code can accommodate 128
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characters and is thus more flexible than the Baudot code.
ASCII is an 8-level code, which means that there are seven
pulses which define the character in the pulse train. Actually,
there are 11 pulses in a group for each character sent. There
will always be a space pulse to set up the machine to receive
the pulse group, followed by a combination of 7 mark and
space pulses, then a mark or space pulse for parity checking,
and then two marking pulses Fig. 6 -7) The first pulse and the
last two pulses are synchronization pulses. A group of seven
(
.
FINISH-
START
1
i
2
3
4
5
CHARACTER
6
7_
8
9
1t
SPACE
PARITY
(SYNC
PULSE)
PULSE
10
11
Y
MARKS
(SYNC
PULSES)
Fig. 6-7. ASCII 8 -level code. The full 11 -pulse group is used to define one
character.
pulses in a variety of combinations defines the character or
symbol. The pulse for parity is a method of checking the
system for errors.
The news wire services use this code. and in some areas.
the U. S. Weather Service is now using it.
Baudot Code
This is a 5 -level code and is basically an old telegraph
code. The 5 -level means that there are five pulses which define
the character, plus the space and mark pulses for
synchronization. Seven pulses in each group are sent. This
code is limited in the number of characters and control
functions it can accommodate and requires much shifting.
This shifting is the same as on a regular typewriter when you
need capital letters or symbols. These limitations are why the
newer codes were developed and also why they contain more
pulses in the group. More pulses simply allow more
combinations of pulses and thus more functions. But for all its
210
limitations, this code is still very much in use today in most
parts of the country. The U. S. Weather Service uses this code.
Transmission
Codes can be sent over a variety of circuits, including
radio channels. The teleprinter signal must modulate some
carrier, and this is usually a midrange audio signal of 2 to 4
kHz. At the receiving end, the signal is demodulated back to its
DC character and fed to the machine. These units are called
modems. for modulator- demodulator.
For the weather service, this is accomplished at the
telephone company's switchboard. They send the signal to the
station over a local DC line. The news signal comes into the
station as a modulated signal and has an outboard modem to
convert the signal to the DC pulse input to the printer.
Ordering Weather Service
The services of the U. S. Weather Service are free since it
is a public agency. They supply weather information and
maintain a weather teleprinter network. There is a continuous
stream of weather information sent out over this network 24
hours a day. To obtain this service, the station must provide its
own printer and pay for a local line to the telephone company
switchboard. No formal permission is needed from the U. S.
Weather Service. Order a standard DC teleprinter circuit from
the telephone company and tell them you wish to connect to the
U. S. Weather Service network.
Weather Service
The U. S. Weather Service is divided into regions, states,
and districts. Your contact will be in the office of the service in
your state. You will need to obtain information from them
about the code, speed of sending. and the severe weather alert
signal used in your area. When ordering a teleprinter, the
factory must have this information. In most areas, this will be:
5 -level Baudot code, printing speed of 75 wpm, and the alerting
signal of two or three uppercase Hs followed by a lowercase w.
U. S.
Weather Alert
The printout for the alert will appear something like this:
# # #Aw. The symbol it is what prints out when the shift signal
is sent and the H key is pressed on the sending machine. The
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symbol has no significance in itself its pulse train has simply
been designated as the arming part of the alert signal. Shifting
is the same as you would do on your regular typewriter. If you
would press the carriage shift key and the key for the letter H,
you would get the capital letter H. On the keyboard setup for
the Baudot code, however, the uppercase is not a capital H but
the symbol #.
Following the symbols # # #, there may be other letters. In
the preceding example, the A in the state of Indiana) will
open up all the machines in the state that are connected to the
network. Other letters can follow, and these are used for
selective signaling or for verification. Then the w is sent.
When the w is sent, the alarm will sound. There must be some
other character sent after the w, or the alarms will continue to
sound. There is usually a space signal or something sent.
Actually, the message will follow. This is pointed out here in
case you are in a test setup and have the phone company
testboard send a test signal.
When the signal is sent for weather alerts, there is also a
10 -bell signal sent as is done with the EBS alert on the news
machine. This bell signal can also be used to set off alarms,
and in fact. that is the way the EBS alarm works.
;
Station Selector
To receive the special alerting signal from the Weather
Service, or to turn on the motors in some machines and other
special selective calling or signaling, a station selector is
required. This is an outboard device that is located at the
printer location. Servicemen call these stunt boxes. When the
selector is an all- electronic device, it must be programmed
exactly for the signal it is to accept and act upon. Take, for
example. the alerting signal. If the code is # #Aw and the
Weather Service sends # #AFw, the alarm will ignore the
signal and not fire.
Installation
The input signal level to the machine is important, and it
should not exceed the maximum current stated on the
machine. The weather line is adjusted at the testboard to a
fixed figure and maintained at that, but the level on the news
wire is adjustable at the output of the modem. Too high an
input signal will overload the machine, and the printout results
will be garbled.
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On the weather wire, if the standard current set by the
testboard is too high for the machine, shunt a 50-ohm 5 -watt
resistor across the input of the printer itself (Fig. 6-8). This
will provide a path for some of the current around the printer.
PRINTER
INPUT
+
-
a
5011
TELEPHONE
SW
COMPANY
Fig. 6-8. When there is no way to adjust the signal current, add a resistor
across the input of the printer to a provide a shunt path for part of the
signal current.
Selector
When a selector is used, wire the input of this in series with
the input of the printer (Fig. 6-9). This provides less of a load
on the (telephone) circuit, and also provides an interlocking
arrangement with the printer. The selector must be connected,
or the printer will not work.
SELECTOR
INPUT
+
-
+
TELEPHONE
COMPANY
INPUT
PRINTER
PRINTER
1
2
INPUT
+
-
Tr-
INPUT
+
-
1
e
Fig. 6-9. When other devices or units are used on the same circuit, wire
them all in series across the telephone line.
If it is necessary to shunt a resistor across the printer,
make sure this is not shunted across the selector also, or the
selector may not work properly. This would be the case if the
resistor were shunted directly across the line.
Measure the Current
At the time of installation and at any time when garbling
appears in the printout, measure the signal current on the
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incoming weather line or the input to the news machine Fig.
6-10). Open one side of the line at the terminal block and insert
a milliammeter in series with the line. You will not be able to
get a good measurement until the line is idle. When messages
are coming in, there are pulse trains on the circuit, and the
meter will read very low and vibrate, as it cannot follow the
pulses. Wait until the sending stops, then get the
measurement. The signal current must be adjusted below the
maximum value and should be set somewhere out in
midrange. where the printer will work reliably The
(
MILLIAMMETER
TELEPHONE
TO PRINTER INPUT
COMPANY
TERMINAL
BOARD
Fig. 6-10. Break one side of circuit and insert milliammeter in series with
line to measure signal current.
maximum for the Extel solid -state printers is 60 mA. If the
current is not there, the circuit is open. There will always be a
steady line current of approximately 60 mA (or local
standard) on the telephone circuit. Absence of current means
the line is open.
Alarms
What type of alarm to use depends upon what is supplied
with the machine and selector. For EBS on the news machine,
a relay is added to the bell circuit. This relay provides dry
contacts so that any external alarm device may be used. The
relay will close each time the bell sounds. An external device
must also sort out the five -bell bulletins from the ten -bell EBS
signal. or the alarm will go off every time the bulletins come
in. And there are many bulletin bells!
The ten -bell signal can also be used on the weather machine
for severe weather alerts in the same way as the EBS is on the
news machine. But if a selector is used for the special -alert
signal. then the output of the selector will fire off the alarm.
Some of the electronic selectors only provide an IC output of
5V DC at 3 mA. This requires an interface if something more
214
than a simple electronic alarm, such as the Mallory Sonalert
device, is to be used.
Maintenance
There may be times when the network is active, other
times when it may be idle. The weather network can
sometimes stand idle for perhaps an hour. The station must
know if there is an idle network or if the circuit has gone open.
There is a red circuit -alarm lamp on the machine. This is
normally extinguished, or it flickers on and off rapidly during
transmissions. These are normal conditions. If the circuit goes
open, this lamp will come on at full brilliance. First, check the
line for signal current. If the signal current is not there, the
line is open. For the weather wire, call the phone company
testboard. If the current is present and the lamp is lit, then
there is an open circuit in the wiring to the machine. Check the
connections and plugs.
Power
Of course, the machine can't print if the AC power is off or
the fuse blown. On each machine, there is a power pilot lamp
(usually green) which shows the power is okay. If there is no
power lamp or if it is burned out, a quick check for power is to
turn the machine power switch off and on. There will be a
single bell when the power comes back on. and if the print head
is out in midstream somewhere, there will be a carriage return
back to the left margin.
Polarity
The input signal to the printer is a DC signal, and this
means that the circuit must be polarized. If it is not, the
printout will be severely garbled. During setup. the circuit and
wiring is determined and polarized correctly. Check the tag on
the machine to determine which terminal is plus and which is
minus. At a later date, when maintenance is done or the wiring
taken off for some reason, it might be reversed. During initial
setup. mark the terminal boards for polarity and color coding
of the wiring. A quick check can determine if the circuit has
been properly restored.
Print Heads
There are a variety of print heads on various models of
teleprinters. Some machines use ribbons and others use
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ribbonless paper. Print heads need to be cleaned from time to
time as they may become clogged with paper, dust, and ink.
Clean these with a stiff-bristled brush.
On the electronic type, the print head provides a 5 x 7 dot
matrix to form the characters. A battery of small solenoids are
clustered together. A needle -like plunger pops out of the
individual solenoid to form its dot. These heads can become
clogged so that the plungers can't operate properly. Clean out
dust or debris from the face of the head with a brush or
airblast. There is a small hole at the rear of each solenoid. Use
a needle or similar small instrument to work the plunger
mechanically. This will usually clean it. Don't take the head
apart as it is difficult to put back together.
Ribbonless Paper
Ribbonless paper requires no ribbon on the teleprinter.
The ink is imbedded in the fibers of the paper itself, and
pressure from the print head releases the ink so that the
character is formed.
When installed in the printer, the paper must be inserted
so that it will unroll in the proper manner. If it does not, the
wrong side of the paper will face the print head. At first glance
it may seem difficult to tell which is the correct side. One side
will be much darker, however, than the opposite side. If the
paper is put in the wrong way, the printing will be faint and
difficult to read.
There are both heavy -duty and light-duty machines
available. When selecting a machine for station use, get a
heavy -duty machine as it will get a workout in the station, and
the light-duty machine might not hold up well.
AUDIO MONITOR SYSTEMS
All broadcast stations require audio monitoring for
checking the air product, for auditioning, and for maintenance
purposes. A good, reliable monitor system is similar to good
test equipment in producing accurate results.
Classes of Monitors
Monitoring can be divided into four general classes:
control room, house, special, and maintenance.
Control room monitoring includes the directly associated
studios and the console. This monitoring is for the benefit of
those involved in the production of the air product.
216
House monitoring is generally from the off -air signal, and
distribution is made throughout the station at comfortable
listening levels. Speakers are provided at many locations.
Special monitoring includes specific areas or circuits to be
monitored, such as network or remote lines, or perhaps the
signal from a sister station.
Maintenance monitoring encompasses all the special
techniques and check-points used to listen in on the signal
throughout the system.
Basic Ingredients of Monitors
Regardless of the size and complexity of the monitor
system in use, there are some basic ingredients common to all.
In all cases, except headphone monitoring, there is a
monitor amplifier. The power output of this amplifier is
dictated by the amount of audio to be distributed and the
levels. The control room console monitor is usually a part of
the console itself, and its power output is specified by the
console manufacturer. This amplifier is basically designed to
supply three or four speakers only. These are located in the
control room and the studios associated with the control room.
House monitor systems often make use of a high -power
amplifier and, in some cases, there is more than one amplifier.
Amplifiers for special-purpose monitoring, on the other hand,
may use low -power units to supply a single small speaker.
Speakers
Speakers are needed at many locations. The
power- handling ability of individual speakers depend upon how
much signal level is required at a given location. The quality of
the speaker and its enclosure will be subject to wide variation.
Many inexpensive speakers and cabinets are available, and
these have a reasonable quality. There are some locations,
such as a client auditioning room, where a high -quality
speaker is desired.
Level Control
It is most desirable to control the signal level at the
individual speaker location ( Fig. 6 -11) . These controls can
have a knob mounted on the cabinet, or a setup control located
inside the cabinet or on the rear, out of sight. Such controls
permit the volume to be adjusted at each speaker location
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BUS
SPEAKER
MATCH NG
TRANSFORMER
TOR
L
PAD
Fig. 6-11. Use an adjustable pad at each speaker to control volume.
without affecting the remainder of the system. In a house
monitor system, once the main amplifier gain control has been
set, there should be little reason for its readjustment.
The control room monitor will be used to monitor many
circuits and programs, so its levels will be adjusted quite
regularly. Even so, the speakers associated with the console
should have an individual control at the speaker so that it can
be adjusted to studio listening levels. A comfortable level in
the studio may be entirely too low for control room use.
Speaker controls of both the L and T types are
commercially available. The pad selected should match the
speaker impedance and should be wired between the matching
transformer and voice coil.
Impedance Matching
Impedance matching throughout the system is important.
A matched system will give an efficient transfer of power and
maintain correct frequency response. The amplifier output
transformer will provide a number of impedance taps. A
single system using one speaker would match or connect the
speaker voice coil impedance to that impedance tap on the
output transformer. As the system becomes more complex.
the impedances become more important.
The speakers are the load on the amplifier and can be
computed as any other parallel resistors. In the single- speaker
system using an 8-ohm voice coil connected to the 8 -ohm output
tap on the amplifier, the system is matched and an efficient
218
transfer of power will take place. Now, if a second 8 -ohm
speaker is connected in parallel with the first one, the
combined load impedance is now 4 ohms, and a mismatch has
occurred. Add two more speakers in parallel with these two
and the load impedance becomes 2 ohms instead of the
required 8 ohms. Power transfer is now very inefficient. This
situation can be corrected in either of two methods. Either
move the tap on the transformer to the appropriate tap (if such
is provided), or add a matching transformer at each speaker.
Console Systems
Packaged systems supplied with the console have
matching transformers in each speaker. The monitor output
impedance of the console may be listed at 16 ohms. This is
generally set up for four speakers. One might conclude from
these specifications that the tap on the monitor output
transformer is set for 16 ohms. but it is not. It is set for 4 ohms.
This anticipates that the amplifier will see four 16 -ohm
speakers in parallel, or 4 ohms. Since most speaker voice coils
are 6 -8 ohm impedances, it would require a matching
transformer to translate the 6 -8 ohm voice coil to this 16 -ohm
value.
Mismatch
Amplifiers designed for use in broadcast systems usually
have a large number of impedance taps available, while those
designed for public address work often have fewer taps. Small
amounts of mismatch are not noticeable, as the amplifiers
usually have enough reserve gain to make up for the loss
caused by mismatch. When the system becomes complex and
there is careless attention to matching, problems can become
more serious. If the mismatch is far off, the amplifier output
stages may be seriously overloaded, which results in distortion
and short life.
High- Current Circuits
The distribution system is very important in a complex
monitor system. These are all very low- impedance circuits,
that carry high currents. The resistance of the wire in the
distribution system should be low, or power will be lost in long
runs. As a matter of practice, these wires should be reasonably
large in diameter, and not the ordinary audio cable. An
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alternative method is to keep the current lower by going to a
higher impedance distribution system and matching
transformers at the speakers. The higher impedance reduces
the current. and the transformers will restore the impedance
match. The impedance can be as high as 600 ohms if you obtain
transformers to match the 600 ohms of the speaker voice coils.
Going higher than 600 ohms creates other problems with
high- frequency response due to capacitance in the cables.
CONSTANT- VOLTAGE SYSTEMS
The constant -voltage distribution used in public address
systems can be used in a house monitor system (Fig. 6 -12). In
recent years, the 25V system has also come into use.
Transformers in this system are marked in power taps rather
25W
AMP
70.7V TAP
70.7V BUS
OUTPUT
10W
2.5W
.AAA..
..A-lJ1..
.MA
I
1
I
10W
SPEAKERS
2.5W
I
MATCHING
TRANSFORMERS
TOTAL LOAD WATTAGE = AMPLIFIER MAXIMUM POWER RATING
Fig. 6-12. The 70.7V distribution system. Matching transformer primaries
are marked in power taps.
than impedance taps. These markings allow the system
designer a simpler method and reduce computation to a
minimum. The amplifier output transformer is marked for
either a 70.7V or 25V tap. The system is based on a constant
bus voltage at the maximum output power of the amplifier into
a matched load. The amplifier usually does not operate at its
maximum output power, as there should be some reserve in
the system. In this system design, the total speaker load must
equal the rated power output of the amplifier.
The design of a speaker distribution system is a simple
matter if one selects components marked for this type of
system. Each speaker transformer has its input taps marked
in power, while the secondary have taps to match various
220
speaker impedances. In system installation, simply connect
each primary tap at the level selected for the speaker. The
only requirement is this: The speaker power taps must add up
to the amplifier's maximum rated power output. Assume, for
example, that a 50W -rated amplifier and 10 speakers are used
in a system, and each speaker receives equal power. The rated
power of 50W divided by 10 gives 5W for each speaker. The tap
on each transformer is set to the 5W tap. Unequal distribution
of power can just as easily be made, as long as the total power
taps equal the amplifier's maximum output rating. Don't
worry about the power rating of the transformer itself, since
no tap available would exceed its rating.
"Unmarked" Parts
A constant- voltage system can be designed even though
components are not on hand or readily available which are
marked for the 70V system. It requires more computation,
some good-quality universal output -to- voice -coil transformers
of adequate power rating, and an amplifier with a suitable
impedance tap.
System Design
Figure 6-13 illustrates how to design a 70.7V system with
unmarked components. First is the amplifier output
transformer, and assuming that it has no tap marked 70.7V or
25V output, compute the imedance tap required by the use of
the power formula Z = E /P, where P is the amplifier's
maximum output rating, E is the bus voltage, and Z is the
desired impedance tap. For example, we have a 50W amplifier
and want a 70.7V line. Then,
Z
= (70.7 x
70.7) /50
= 5000/50 =
100 ohms
Find a tap as near to 100 ohms as possible and use that one. If
there is no tap near 100 ohms, we can try for a 25V system. The
impedance for a 25V system will be:
Z
=
(25
x 25)/50 = 625/50 =
12.5
ohms
The amplifier would most likely have a tap near this value, so
a 25V system could be set up.
Design Variables
We will take a slight detour here from the system design to
point out a few facts. You probably noticed that the impedance
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n
s
<
>
CV
a
H
N
222
value changes for the different systems. Actually, there is no
standard impedance for these systems. Since the system
voltage must remain constant, the impedance depends upon
the amplifier maximum power. For example, using the 70.7V
system, the impedance for a system with a 50W amplifier is
100 ohms (5000/50 = 100). Now if a 25W amplifier is used
instead, the impedance becomes 200 ohms 5000/25 = 200) .
Another fact is that the voltage on the bus will certainly
not remain constant, nor may it even reach the design figure.
That is exactly what it is, a design figure. just as the maximum
output of the amplifier is a design figure. For example, a 100W
amplifier and a 70.7V bus will have an impedance of 50 ohms.
In operation, the amplifier gain controls are set so that the
amplifier is actually delivering only 50W to the line. By the
power formula,
(
=Vj =50V
E_V1=
Even though the design is for 70.7V, the actual bus voltage is
only 50V. Also, in the computations, the figure 5000 is used as
the square of 70.7. This figure is easier to remember than
4998.49, which is the true figure. The error is small enough to
be ignored.
Back to Design
In our design example, we have a 50W amplifier and the
70.7V constant -voltage system. We have found the correct
output tap on the transformer to use for this. Next, we need
some good universal output -to- voice -coil transformers. We
have eight speakers in the system, and there is unequal power
distribution. Again, use the ordinary power formulas, except
the power figure in our formula is the power we desire the
particular speaker to have. The impedance factor is the
primary impedance of our matching transformer at the
speaker. Thus. Z = E2 /P, or P = E /Z. We want two 10W.
one 7.5W. one 2.5W. and four 5W speaker locations. The total
wattage is 50W. the maximum of our amplifier. Using this
formula, the primary impedance tap for the 10W speakers will
be:
2
Z =
E2/P =
5000/10
=
500
ohms
For the 7.5W speaker we calculate: 5000/7.5 = 667 ohms. Of
course, if we couldn't find an impedance value on the amplifier
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output transformer that would give us the 70.7V system and
decided on a 25V system, the 25V figure would be used in the
formulas.
Maintenance of Monitor System
Control Room Speaker System-There are three or four
speakers fed from the console amplifier. It is necessary that
studio and control room speakers are muted when a
microphone is turned on in the particular room. Relays
mounted in the console do this muting. When the relay
operates. it disconnects the speaker and substitutes a resistor
of equal resistance so as to keep the load on the amplifier
constant. Should it be necessary to replace one of these
resistors, the replacement should have a value equal to the
speaker impedance. This value, remember, is the value of the
input impedance of the matching transformer, and not the
voice coil of the speaker. Thus, if the transformer input
impedance is 16 ohms, this is the value to make the resistor.
Make sure it has the correct power rating.
Switch Mute -There are times when it is desirable to
shut the speaker completely off in the studio because
rehearsals are going on. Many of the T -and L -pad controls
used on the individual speakers will not shut off completely. A
switch on the speaker cabinet can be installed that substitutes
a load resistor on the amplifier when switched to remove the
speaker. (See Fig. 6-14.)
SPEAKER CABINET
MATCHING
TRANSFORMER
8í125W
MUTE SWITCH
Fig. 6-14. Add a switch to speaker cabinet to act as a manual mute.
Jacks-If individual speakers do not have a cutoff switch,
the speaker input leads should be routed through a jack field.
Should the speaker mute become defective for any reason, a
patch cord plugged into the jack will open the speaker leads.
224
plug with a terminating resistor attached will both open the
leads and terminate the amplifier.
A
Muting Relays- Contacts can become dirty and cause
problems. The speaker will sound intermittent or distorted.
Contacts should be cleaned with a small relay- burnishing tool
if the monitor amplifier can be turned off; otherwise, use a
piece of coarse paper run between the contacts. It is especially
important with transistor amplifiers that the output does not
become shorted. This will almost certainly blow the transistors.
Other Systems
There may be situations where a separate monitor is
desired for a specific service or purpose. For example, the
station may use a national network service and would desire to
have constant monitoring of the network line. The amplifier
should have a gain control near the operator, so the amplifier
gain can be raised to listen for cues or lowered to a background
for general monitoring. Unless the amplifier can be located at
the control position. this should be a remote control. The
control can be on the speaker itself, or the input, whichever is
more applicable to the situation.
Bridging
Most monitoring is tapped into regular program buses, so
monitoring should have a bridging arrangement so as not to
load the buses. Buses are also balanced, so if the amplifier
input is unbalanced, use an isolation transformer or a bridging
transformer.
60052 PROGRAM BUS
5K
SPEAKER
Fig. 6-15. Always use a bridging connection for the monitor amplifier.
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Fixed bridging may be done in some applications ( Fig.
6-15). If the amplifier is a 600 -ohm balanced input, use fixed
resistors in each leg. The value should be 5V to 10K in each leg.
If only a single bridge is used on the circuit, the resistors can
be as low as 2700 ohms in each leg. Don't go much lower than
this and don't add any more bridges to the circuit if possible.
226
Chapter 7
Remotes
On- the -spot, live,
remote broadcasts have been an integral
part of radio since its infancy. Going outside the studios to do
programming does present many engineering challenges. One
major problem is how to get the program back to the studios.
There are two ways to accomplish this: through the use of land
lines or by a radio link. We consider these two methods in this
chapter and also some of the equipment to take out to the
remote. And we delve into the studio-transmitter link, which,
although not a remote, does have many things in common with
remotes.
LAND LINES
wire circuit leased from the telephone company is a very
common method used for remote broadcasts. The telephone
company has a great many circuits, but only a few of these
ever see use as broadcast channels. When a circuit is assigned
for a broadcast loop, then it must meet the quality of the
service ordered. There are several grades of circuits, each
with a different bandpass. These are listed by the telephone
company and are filed with the FCC.
A
Quality
The actual quality of a particular circuit depends upon the
pair of wires assigned, and to some extent, upon the expertise
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personnel -their
the local telephone company
least
needs.
But
at
the bandpass of
understanding of broadcast
the various local channels are listed; there are several
grades. When ordering a circuit, the station must specify the
grade of circuit desired; otherwise, the lowest grade circuit is
assigned. Tariffs filed by Indiana Bell Telephone Company, for
example, list nine different categories for broadcast channels,
and these are divided into five grades. These range from the
top-grade circuit, a schedule AAA line, which has a bandpass
of 50 Hz to 15 kHz, to the lowest grade, a schedule E line, which
has a bandpass of 300 Hz to 2500 Hz. The reason for the nine
designations in only five grades is due to listing of each circuit
by two designations according to usage. For the top grade,
AAA is the designation given when this circuit is leased on a
monthly basis. But if the circuit is leased only occasionally,
this same grade of circuit is designated schedule BBB. A bit
confusing. perhaps, but probably necessary as far as rates and
tariffs are concerned. One thing to remember when ordering a
circuit: Specify the grade of circuit you want. Contact the local
telephone company and ask for the schedule of broadcast
channels they have available. Don't be afraid to ask about the
rates for the different grade of lines either.
Except for the individual drop from the pole, the circuit
ordered is only one pair in multipair trunk and feeder cables.
To keep the cable diameter within practical dimensions, the
size of each wire is small, generally a 26 -gauge wire. For the
same reasons, the insulation around each wire may be a thin
paper wrap or plastic. The small -gauge wire has high- series
resistance, and the close spacing of the conductors gives high
capacitance in shunt across the pair. Both of these factors add
up to high losses for the signal as it passes through the cable.
of
Losses
The yardstick for measurement in telephone work is
usually the mile. For an unloaded cable pair using 26 -gauge
wire, the signal loss for each mile of cable is approximately 2.9
dB at 1 kHz. The losses are worse at higher frequencies -about
9 dB at 15 kHz. Rates quoted are based on distance between the
studio and the remote site, but this is not necessarily the actual
cable routing. In most cases, the actual distance is less than
the cable distance. But the signal must pass through the cable,
wherever it meanders, and these losses add up. It is not
228
uncommon to have three or four miles of cable in the routing.
This means the losses are three or four times as great as they
are at a mile.
Overcoming the Losses
Both equalization and amplification are required to
overcome the cable losses. Just how much depends upon the
circuit in question.
For ordinary telephone voice channels, the phone
company adds loading coils at approximately each mile along
the circuit. Spacing of these coils depends upon the cable, so
the spacing may be at 3000. 6000, or 9000 feet. These coils are
simply an inductance placed in series with the line at that
spacing. cancelling the line capacitance to provide a degree of
equalization. The typical loaded voice circuit has a bandpass
of approximately 300 -3200 Hz.
The broadcast channel, however, is not supposed to have
any coils in the circuit. If there are loading coils, then the line
cannot be properly equalized. The coils tend to peak the line at
about 3200 Hz: there is a very rapid dropoff in response above
this frequency. As a matter of fact, the higher end of the
response curve drops into the cellar.
When a circuit is ordered, the phone company may not
have any unloaded cable pairs in the area of the remote. If not.
they assign one of the regular voice channels instead. This
happens quite often these days because of the heavy demand
for circuits. When a voice channel has been assigned, the
phone company should go out to disconnect the loading coils
along the way. They may forget to do this: if you try to
equalize the line, the effort will be in vain. For practical
purposes and when equalization is not going to be attempted,
schedule E line with loading coils left in place provides a
reasonably good circuit for many remotes where the
programming is all talk-such as a sporting event or similar
pickup.
Equalized Circuit
When music is part of the programming from the remote
site. or if a better circuit is desired or required, then either
equalize the circuit yourself or order a better grade line.
When a high -grade line is ordered, the phone company will
equalize it. They use a self -contained equalizer- amplifier unit
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at the studio location. This provides both the equalization and
high-gain amplification to make up the equalizer and line
losses. These are portable units brought to the station for the
occasion. If the station transmitter is at the same location, be
on the lookout for RFI in these units.
Do-It- Yourself Equalization
Local equalizers may be used that are either the passive
type or those more sophisticated units that allow for boosting
or cutting the response curve in frequency bands. To equalize
the circuit, someone must be at the remote location with a
signal generator or a remote amplifier that contains a tone
generator.
The passive equalizer does its job by reducing the low
frequencies down to the level of the higher frequency point.
This introduces a considerable amount of attenuation into the
circuit. The farther you stretch the bandpass, the greater the
loss becomes. So don't overdo it.
If you are going to equalize the line yourself, here is a little
trick that helps the process: Insert a matching transformer at
each end of the line. At the remote location, strap the
secondary for 150 ohms (Fig. 7 -1). Leave the primary at 600
ohms to match the amplifier output. At the studio end, strap
REMOTE
AMPLIFIER
6008 TELEPHONE
150S1
REMOTE SITE
15051
STUDIOS
Fig. 7-1. Strap transformers as if the line were 150 -ohms impedance. This
makes equalization easier.
the primary for 150 ohms, again leaving the secondary at 600
ohms to match the studio equipment. The shunt capacitance is
now across 150 ohms rather than 600 ohms. This has less effect
and the line will be easier to equalize.
On -Air Equalization
Equalization and experimentation must be done before the
program gets on the air. If there wasn't time to do it, some
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experimentation can be done while programming. However,
do this gingerly, being constantly aware of the attenuation the
equalizer adds to the circuit. Try to make adjustments in small
increments. Keep one hand on the fader, trying to smoothly
correct for the loss introduced. It really sounds bad on the air if
the levels are varying all over the place. Gradually add small
amounts and listen for the voice to crispen up.
Low -Level Output
Whenever equalization is done, whether with the
announcer's voice or with a signal generator, remember that
the equalization adds attenuation to the circuit. If there is
much equalization, the output of the equalizer can be a very
low -level signal, down at the microphone range or at least in
the preamplifier input range. This makes the output circuit
susceptible to noise pickup, hum, crosstalk from high -level
circuits, and RFI.
SIGNAL LEVEL
20 dB
-
TELEPHONE
30 dB
EQUALIZATION
EQUALIZER
- 50 dB
STUDIO
LOW-LEVEL
OUTPUT
Fig. 7 -2. When heavy equalization is done, the output of the equalizer may
be at a low level.
When equalization is done, listen for noise in the
background. A practical trick that gives a relative appraisal of
the signal -to-noise ratio is this: First, set the levels in the
remote system. Have the announcer talk into the microphone,
or use the signal generator. Try to use the spare channel on a
dual console or the console itself. Set the faders and master
gain so the peaks hit 0 VU. If the fader is wide open, there can
be problems, since the console gain is very high. Now take the
signal off the channel and listen to the channel. Turn the gain
on the console wide open and listen to the background noise;
note any reading on the VU meter. The operator must make a
judgement about how much noise is acceptable from the facts
presented and the program to be broadcast. If the program is a
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basketball game in a gymnasium with a screaming crowd, the
program can tolerate a higher noise level than a quiet pickup.
While only a relative method, this can be useful for the
occasion. For example, once you've calibrated for normal
operation. and if the noise peaks do not move the VU meter at
all, the noise is down at least 20 dB. You can estimate from
there. The faders are usually 2 dB per step. So if you now open
the fader 10 steps and the noise peaks start to jiggle the meter
a little, the noise is down about 40 dB.
Jacks
When a station does many remotes, there may be several
regular weekly remotes. such as churches. Rather than
disrupt these circuits to handle a special remote, have several
circuits from the jack field to the telephone terminal box.
These should terminate on the station terminal board. Before a
remote and after the line has been ordered, the telephone
company usually supplies the information on circuit number
and terminal numbers in the box. Then it is only necessary to
cross -connect between your terminal board and the terminals
on the telephone company board. Post the regular circuits that
are in service so that you don't take down one of the active
circuits. Post this right at the telephone company box.
Problems in Remotes
Many problems can happen to remote lines both before
and during a program. Some of these are human errors, others
are simply breakdowns in the circuits themselves.
At the Remote Site -One common problem is the
connection of the remote amplifier to the telephone line. In
many cases. there is a four -wire cable that connects to a
four-post terminal block ( Fig. 7 -3). So the installer ties his four
wires to the block even though only two actually are used. Now
RED
CONNECT TO RED & GREEN
PROGRAM GREEN
PAIR
YELLOW
DISREGARD THESE TWO
BLACK
CONNECTING
BLOCK
Fig. 7 -3. Be sure to connect the remote amplifier to the correct pair of
wires. Disregard any other wires.
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the announcer comes along and ties the remote amplifier to
the wrong pair of terminals. He simply isn't connected to the
remote line at all, only to the cable. Do this: Ask the telephone
personnel to instruct their installers to use only two wires, the
active two. Cut the other two off. Also, find out which color
code is used for the program pair. In the four -wire cable, this
is usually red and green. but find out what it is in your area.
When you find out the code, make a small tag. Tape it to the
remote amplifier so the announcer can make sure he connects
to the program pair.
Oscillators -In many cases, the phone company attachs a
small oscillator or buzzer across the line. The battery lasts a
couple of days. They can listen in on the line at the test board
to know the line is still intact. The station can also listen in
occasionally. If the tone disappears, call the telephone
company and inform them. Some test boards are attached to
an alarm at the downtown office; if the tone quits, an alarm
goes off. Sometimes the batteries play out, but in many
locations such as school gymnasiums, the little oscillators
disappear...!
When an oscillator is used, make sure the announcer
knows he is supposed to disconnect the oscillator before he
attaches the remote amplifier. If he does not, there will be a
low -level tone in the background of his program. It may not be
low either, which will certainly make the program sound bad.
In one case, the phone company thought they would play it
wise and hide the oscillator so the kids wouldn't steal it. The
installer mounted it at the terminal box located in a room
somewhere else in the gymnasium. He used the other two
wires in the cable to route the oscillator output back to the
remote terminal, attaching them in parallel with the program
pairs. This worked fine for line- continuity testing, but the
announcer did not know it was there and attached the
amplifier to the program pairs. The oscillator buzzed along for
a large part of the first quarter, until the station could get the
phone company (out-of-town game) down to the gym to cut the
oscillator off. The announcer was not a technical man; he
simply followed instructions. He saw the other two wires
attached, didn't know what they were, so he left them
connected. Watch out for the tricky installer.
Resistor Terminations -This is the most common method
used for checking line continuity. The installer attachs a 10K
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resistor across the program line at the remote location. The
test board personnel can check for line continuity by measuring across the line; the station can do the same thing. This
is important for those circuits which are on a permanent basis.
Simply measure the resistance across the line terminals at the
telephone box at the station. You should measure the 10K
resistor plus the line resistance. When the line is first set up
and checked out, take a set of measurements with the
ohmmeter to record these for future reference when there are
line problems with the circuit. The announcer should not take
this resistor off the terminals when he attaches his amplifier.
The high -value resistance does not affect the circuit in any
way. By the way, if the amplifier is attached to the line, you
will not read the 10K resistance, but the low resistance of the
output transformer. It still provides a continuity checkpoint,
but at first glance, you may think the line is shorted. If there is
any question. have the announcer remove the amplifier.
Naturally. none of this can be done if the program is in
progress.
Noise
Most problems found on the boradcast loop are noise, hum,
and crosstalk. These are balanced lines, and if anything
happens to upset the balance, noise problems occur. So when a
line is first set up, measure each side of the line to ground with
an ohmmeter. There should be an open ( x) reading. There is an
initial kick of the meter pointer as it charges up the line
capacitance, but this is normal.
When these problems show up, measure the line to ground
again. If there is a reading or a short, check the carbon blocks
that are mounted on each side of the line. Pull out the carbon;
if the reading disappears, the carbon has arced through. These
carbon blocks are protective devices against lightning or
similar high -voltage surges that appear on the line. If one of
the blocks has arced through, leave it out of the circuit unless a
new one is available. It should be replaced as soon as possible.
Hum is another common problem. It happens if the line is
unbalanced for any reason. There are other reasons for hum;
it can be caused by connecting the amplifier incorrectly at the
remote site; or it can be that one side of the studio equipment
has been accidentally grounded. In either case, hum comes up
as soon as the equipment is attached; but with the equipment
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off the circuit, the hum disappears. If the ground is in the
console itself, then an isolation transformer inserted in the line
corrects the problem for the moment ( Fig. 7-41: At the earliest
opportunity, the real problem or fault should be corrected.
When hum is strong on the circuit, contact the local test board
personnel to report the problem. Always check out your own
connections and circuits first. It can be embarrassing to have
telephone troubleshooters go out to the site or studios and find
that the station equipment is at fault.
BALANCED
TELEPHONE
CIRCUIT
60052/60052
ISOLATION
TRANSFORMER
STUDIO
60052
ACCIDENTALLY
GROUNDED
Fig. 7-4. If one side of equipment is accidentally grounded, insert an isolation transformer as a temporary measure.
In one case where hum was a real problem on a remote
pickup. an announcer went to cover his first ball game. He was
not accustomed to working with balanced circuits. He
connected the line to one of the amplifier's output terminals
and ground terminal, creating an unbalanced output. We lost a
good part of the game before the hum problem could be
corrected. The telephone man found that one.
Talk -back Hookup
In the ordinary loop. two -way communication is possible
between the studio and remote site. The remote amplifier at
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the site and the talk-back amplifier in the console are used.
Naturally, this can't be done during the program itself unless
the station is airing a commercial or other announcements.
This talk -back arrangement is very helpful in the
preliminaries before broadcast time in setting up the cues,
timing, instructions, and so on. But if the phone company has
equalized the line, there are amplifiers in the circuit. Two -way
communication is not possible on the equalized line. If
communication is important enough, then order a regular
business phone at the remote site. This is a little more costly,
but there are shows where this is important enough to justify
the extra expense.
QKT Circuit
The use of the regular telephone for remote broadcasts has
flourished in recent years. As with many developments, this
grew out of economic necessity.
Basically, the QKT, or voice coupler, is a convenient
device for connecting the remote equipment to a telephone.
Instead of using the phone instrument itself, regular audio
equipment is used. The telephone contains a jack or an
external box, which contains a transformer for isolation, and a
push -to-talk switch or exclusion key. To do a broadcast, the
announcer simply dials the station phone number. When the
connection is made, he uses the phone in the regular manner.
Without breaking the connection, he simply releases the switch
or turns on the exclusion key. This places the amplifier output
signal directly onto the phone circuit. The program goes over
regular voice channels just as any other phone call. The
station is charged for the phone call but there are no line
charges. There is, of course, the installation charge.
There is one important difference to remember about a
broadcast over a QKT and one over a regular broadcast loop:
The QKT is a regular or long- distance phone call. Therefore,
the announcer must dial the correct phone number at the
station. And at the station, there must be a connection to the
station equipment. Most stations have more than one phone
number. perhaps an unlisted number. Cross -connect the phone
circuits for those numbers in the telephone company's box to
the station jack field. Use the isolation circuit as described in
Chapter 6 for the news recording.
236
Long -Line Network Hookups
Whenever the station is connected to a national, regional,
or special network, the lines will be handled by the Long Lines
Division of American Telephone and Telegraph Company.
These circuits converge at and pass through the toll -test
section of the local phone company. They may have a different
name for it, but it is usually a separate part of the local phone
company with its own personnel.
Generally, broadcast stations and the telephone companies work together well and have very good relations. Good
relations with the local phone company pay off in the long run
and in tight spots.
RADIO LINKS
In many locations land lines are not available or even
practical, so the radio link is used. The remote pickup
transmitter offers a flexibility not possible with fixed land
lines. Use of remote pickup transmitters has been accelerating
over the past few years, as has the use of all mobile
communications systems. Some remote pickups operate in the
shortwave band. There is also a heavy concentration in the
VHF band along with an expanding use of the UHF band.
These small systems, although flexible, are not a cure -all.
They have their own problems and limitations and are not as
reliable as the land line.
Propagation
With radio link, the propagation of the signal is an
important consideration. Radio waves have different
characteristics according to the frequency band in use.
Shortwave signals travel a greater distance with less
transmitter power, but they are subject to more skip than the
higher frequencies. Atmospheric conditions skip the signal
hundreds of miles, causing interference to other signals on the
same channel. Theoretically, the VHF signal is line -of -sight
transmission, but it does spread beyond the horizon and is
subject to more skip than theory indicates. Signal attenuation
along the path requires more transmitter power to deliver a
strong signal where you want it to go. The higher the frequency
band, the higher the path losses. At UHF, the signal does
conform well to the line-of -sight theory, and propagation losses
are more severe.
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Transmitters and equipment operating in these different
bands require different components, antennas, operating
techniques and maintenance procedures. Since there is a very
high concentration of remote pickups in the VHF bands, most
of the following discussions applies equipment in this range.
Antennas
The transmitting and receiving antennas are the interface
with the radio path, thus an important part of the system.
These antennas must be as efficient as we can make them
since we have no control of the signal out on the radio path.
Once the signal leaves the transmitting antenna, it is subject to
the foibles of nature, its own characteristics, and man -made
interference. There are many elements which affect the
antenna and signal propagation that should be weighed against
the practical requirements of a particular system.
Height
In the VHF and UHF bands, height is a very important
element in overcoming the line -of-sight characteristics in
relation to the curvature of the earth. In fact, height is as
important as transmitter output power and, in some cases,
more so. Achieving ideal antenna height is not an easy matter.
The portable transmitters, such as vehicular or walkie- talkie
units, have antenna heights of only a very few feet above
ground. This is typical of most of the remote pickup locations.
To overcome this low height, the receiving antenna at the
studios must be as high as possible. However, there are
practical limitations to this also.
When the studio antenna is too high-several hundred
feet-it becomes susceptible to skip signals. Although the high
antenna enhances pickup from the remote locations, the skip
signals boom in strong enough at times to wipe out local usage
of the channel. Also, a point is reached in height where the
transmission-line losses are greater than the advantage
gained by height.
Gain
Different configurations of antennas collect more receive
signal, reinforcing the signal by proper phasing, so that a
power gain is realized. Actually this gain figure works in both
ways, that is, in receiving and transmitting. The quarter -wave
238
V4A
60W ERP (P x
1)
60W
TRANSMITTER
5/8A
54WERP (P x 1.78)
30W
TRANSMITTER
Fig. 7 -5. The gain of the antenna effectively increases the transmitter out-
put power.
whip anetnna is popular for vehicles and hand held units,
having a power gain of 1 dB. So high -gain antennas should be
used whenever possible. The %-wave length whip antenna has
also become popular. This antenna has a power gain of 2.5 dB,
which is almost the equivalent of doubling the transmitter
output power. For example, a 30W portable transmitter and
the %-wave length antenna radiate almost as much power as a
60W transmitter and quarter -wave antenna.
Higher gains than this can be achieved by stacking or
using special antennas such as the yagi or collinear array. The
yagi is also highly directional and can be used to advantage in
areas where there is much interference, but it is much larger
physically than the whip antennas, not lending themselves to
portable use. They can be used to advantage at the base
station.
Efficiency
Antenna systems are efficient or inefficient, just as any
other electronic devices. Efficiency, as used here, means the
input /output ratio. For practical purposes, the transmission
line is considered as part of the antenna system. Efficiency is
considered as the ratio of transmitter output power versus
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radiated power from the antenna. For example, if losses are
10% of the transmitter output power, the antenna system is
only 90% efficient.
There are many factors that consume power before it is
radiated: physical diameter of radiating elements (skin-effect
resistance) ; resistance in coaxial connectors corrosion, rust,
or loose connections); resonant frequency of the antenna in
relation to the carrier frequency (incorrect length);
transmission-line losses (high-loss dielectric material or
moisture in cable); standing waves (impedance mismatch);
and other conditions.
Polarization
Another important factor relating to efficiency
propagation is polarization of the antennas. Both the
transmitting and receiving antennas must be in the same
physical orientation, either vertical or horizontal. If one
antenna is vertical and the other horizontal, theoretically,
there is little or no reception. Practically, there are enough
reflections that distort the original field so that reception is
possible over a short distance, but the efficiency is far less
than if both are polarized the same way.
(
Estimating Coverage
When setting up a system, a rough calculation of the
coverage area can be made by use of FCC charts for either FM
or the upper TV channels. These charts do not truly depict
conditions of the 150 MHz band, but they can provide a rough
idea of what to expect. They also consider the receiving
antenna at a 30 -foot height, which is seldom realized in mobile
work.
You can expect at least coverage to the horizon or better,
so if you want to compute this distance, this formula can be
h.. where ht and hr are the antenna
271 +
used: D =
heights at the transmitter and receiver. For a base station
antenna height of 100 feet and a remote unit antenna height of 4
Nri= 14.14 + 2.83 = 16.97
feet, we calculate D =
miles. These computations are theoretical and can only be
used as estimates. They all consider the earth to be flat. Hills
and obstructions will alter these figures.
System Bandpass
The next important consideration of the radio link is its
bandpass. There are quite a few factors that affect the
V+
240
bandpass. Conversely, bandpass affects the quality of the
audio signal that passes through the system. Bandpass is as
much a design factor as an operational factor. Systems which
are designed for communications work are intended for
speech and provide the equivalent of a good telephone speech
channel. Those designed specifically for remote pickup work
have a bandpass equivalent to that of a class B (or better)
equalized line. See Fig. 7 -6.
SPECIAL REMOTE
PICKUP SYSTEM
r
30 Hz
300 Hz
3 kHz
7.5 kHz
INDUSTRIAL SYSTEM
Fig. 7 -6. There is a different system bandpass on narrowband industrial
transmitters than on wideband special remote pickup transmitter systems.
Primary factors of design which determine bandpass are
the audio and modulator section of the transmitter, and the
bandpass filter and audio system of the receiver. Operational
factors are the adjustments to these sections, as well as the
tuning of various RF and IF stages in both the transmitter and
receiver and the receiver's discriminator.
Industrial or Broadcast Equipment?
Transmitters that are used for remote pickup service may
be either those designed for industrial services or specifically
for remote pickup service. There are many manufacturers
who build equipment for the industrial services, but only a
very few who build equipment especially for remote pickup
service. There are differences!
First is duty cycle: Industrial units are intended for short
bursts of speech communication in which the transmitter is on
for only a short time. The remote pickup units are designed for
a continuous duty cycle; that is, they can transmit for long
periods of time. This is important when program is to be
transmitted. The industrial unit may overheat and deteriorate
rapidly because the cooling system is not intended for this type
of operation. For example, if the station is trying to broadcast
a ball game over an industrial unit, odds are the unit won't
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make it to the end of the game. But for shorter programs, such
as news interviews, they seem to work well.
Second is bandpass: Industrial units are narrowband,
while the special remote pickup units are wideband. (Standard
industrial units can be made wideband, but this is a
modification done at the factory.) The audio system in the
industrial unit is adequate to provide speech communications;
however, special remote pickups have better audio systems. In
the transmitter, there are provisions for two or more regular
broadcast -type microphones: plus a mixer, peak limiter, and
AGC amplifier built into the unit. It may also have a VU meter.
The receiver audio section has audio filters and pads that may
be switched in, and the output is a balanced 600 -ohm
transformer to match the studio equipment.
FCC Rules Regarding Remotes
Remote pickup systems come under the jurisdiction of the
FCC. The technical requirements, operation, and other
information about their use is found in Rules and Regulations
Part 74. Subpart D -Remote Pickup Broadcast Stations. The
remote pickup rules differ in many respects from the rules for
industrial services, but stick to the remote pickup rules when
the station has a system in operation; do not become overly
concerned with the industrial services. In many respects,
especially in bandwidth and frequency tolerance, the remote
pickup rules are more lenient. But in operation. applications,
and licensing. the remote pickup rules are more stringent.
At the time of this writing, the remote pickup rules are
under consideration for extensive revision. Likely changes
include a narrowing of the bandwidth, tighter frequency
tolerance, and licensing of the system rather than each
individual transmitter as is now done.
Maintenance of Remote Pickup System
Like any other electronic system, the remote pickup
system must have a regular maintenance program. There are
some required FCC checks, such as frequency and modulation,
to make periodically; but the system deteriorates over a
period of time, especially the antenna system, which is
constantly exposed to the weather.
There are some general suggestions to be made about
maintenance: First, become familiar with the particular units
242
the station has in use -read the instruction manuals. These
provide the specific tuning and adjustment information that is
recommended for the units. Second. always be conscious of the
system's bandpass when adjustments are made, whether to
the modulator, speech amplifiers, or RF tuning. Many things
affect the bandpass; unless major adjustments are required. it
is best to leave the tuning alone. This is particularly true of the
IF stages and discriminator in the receiver. If there is a
problem. correct it; but that doesn't necessarily mean the unit
needs a complete tuneup. If you farm out the work to some
shop that does communications work only, make sure they
don't narrow the discriminator bandpass.
Antennas
Many problems originate in the receiving and transmitting
antennas. Be especially on the lookout for loose connections at
or in the coaxial fittings of the transmission line. Also, look for
rust and corrosion where the antenna mounts to the vehicle.
Whenever poor signals are apparent, look over the antenna of
the particular units involved. If the problem is common to all
mobile units, then look at transmitting or base antenna. Mobile
antennas can be bent or broken off. When the vehicle must be
run through a car wash, try to remove the antenna whip if
possible.
Field Strength
A relative measurement of the field strength of the signal
can be made with one of the small kit -type field- strength
meters (Fig. 7 -7). These are not accurate or absolute figures,
but do give a relative indication of how much signal is coming
off the antenna. When a unit is first installed in a vehicle, set
up some arrangement to make a field -strength measurement.
USE SAME DISTANCE
EACH TIME
10'
VEHICLE
ANTENNA
T020'
FIELD STRENGTH
METER
Fig. 7 -7. Use a kit -type field- strength meter to measure radiated power. Do
the procedure the same way each time.
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Do this after the installation is complete and appears to be
putting out a good signal. Save this reading for future
reference. When there appears to be a poor signal output from
the unit at a later date, then make another measurement for
comparison, but use the same setup and procedure as in the
original test. If the transmitter tunes up to full power in a
dummy load, then take the coaxial fittings apart and check the
antenna mountings for corrosion, open connections, and other
problems.
Coaxial Line
When the coax line has fittings at both end, a simple check
can be made of the line. Disconnect the antenna and attach a
dummy load and wattmeter. Tune the transmitter directly into
the load first for a comparison reading. Then move the dummy
load to the end of the coaxial line. The readings should be
essentially the same if the line is in good shape. Cable that is
fished under carpets and seats of a vehicle can become
damaged by crushing or abrasion, or it can simply be cut. This
test will show it up.
Base Antenna
In most cases, the base antenna is mounted on a tall tower
or the station's regular tower. Consequently, it gets less
attention. If it is very high, then treat the antenna the same as
you do the regular FM transmitting antenna. The best device
for checking out an antenna system is the Bird Electronic
Corporation's Mini- Monitor directional wattmeter. This
BASE ANTENNA
ON HIGH TOWER
FORWARD
POWER
BASE
TRANSMITTER
LONG
COAX
LINE
DIRECTIONAL
WATTMETER
REFLECTED
POWER
Fig. 7 -8. On high base antennas use a directional wattmeter to measure
power and tune to line.
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instrument allows measuring the forward and reflected
power at the transmitter end of the coaxial line feeding the
antenna. If there is anything wrong with the line or antenna,
power is reflected back to the input of the line. This unit does
not read VSWR directly; it only gives you the two power
measurements. To obtain VSWR, it is necessary to compute
the value, using the measured powers. However, absolute
VSWR figures are not really needed; what is important is that
the reflected power is very small. When the station or antenna
system is first installed, measure these values and compute
the VSWR and save for future reference. At later dates, this
should be checked and comparisons made. If there is any
gradual increase in the reflected power, deterioration is taking
place in the line, antenna, or both. But if there is a sudden
increase in reflected power and the transmitter tunes oddly,
something has happened to the system, and it should be
checked out as soon as possible.
Transmitter Output
Getting a good match from the line to the antenna is
important. but so is the tuning of the output stage to the load.
The output tuning affects both the radiated power and the
efficiency of the output stage. Most manuals say that the
transmitter be tuned up into a dummy load and wattmeter,
then connected to the antenna without further tuning. This is
all right if the antenna and line have a good match and, in
effect, offer the same load as the dummy load. But this may
not be the case; the output stage can be operating detuned,
thus dissipating too much power. Tune the transmitter into a
dummy load and wattmeter, and load the transmitter to the
correct current values in the output stage. Then attach it to the
line and antenna. If these readings change appreciably, the
antenna is not well matched. Touch up the output stage tuning
in small increments, always keeping the stage current
"dipped," until the readings approach those of the dummy
load, or at least the maximum loading value.
Modulation Percentage
Even though a strong RF signal is received, the output
audio signal can still be low and the signal- to-noise ratio poor.
This can be the case if the transmitter modulation is not
properly adjusted. To properly adjust modulation, a
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modulation meter is needed. The manual may instruct that
this be set with tone modulation. This is okay as far as a
preliminary adjustment is concerned, but it should really be
set on voice peaks. First, adjust the speech clipper so that it is
not in a position to affect the audio. Then use the microphone
and talk into it at program level; that is, make the voice level
at about the same level an announcer would talk at when
giving a report. Set the level on the modulation meter to the
desired amount on the peaks of the voice. If the output of the
receiver (base station) audio can be observed with an
oscilloscope, adjust the level to the point where clipping starts
to occur on the peaks. Now, this may be much less than the
deviation is supposed to be, but in that case, the system cannot
handle the deviation. Back off below this clipping point, then
adjust the regular speech clipper to clip at this point. While the
level may be less than the system is designed for, the audio
will not be distorted, although the output level is less than
expected. If the transmitter cannot reach its required
modulation, then other maintenance is called for.
PORTABLE EQUIPMENT
Doing a remote today is a pleasant experience compared
to what it was years ago, when equipment was heavy, bulky,
and tied to an AC power source. But with the light weight,
solid -state equipment of today and its battery power, the whole
remote kit can be carried in a small bag. Besides that, there
need be no concern about how to find power at the remote site.
Portable Amplifiers
There are a variety of small, battery -powered remote
amplifiers available today, and their quality is as good as the
studio equipment. While all run on batteries, some can also be
run on AC power if desired. They are all light weight and easy
to carry.
There are some models which have a built -in telephone
voice coupler. This type of remote is handy if you would
rather not rent the equipment from the telephone company.
Aside from the desirability of the light weight and size, the
amplifier should have two or more microphone inputs ( Fig.
7-9). Although many broadcasts are single -mike situations,
there are other occasions when more mikes are needed. If the
station does many remotes, there should be more than one
246
remote amplifier available. When there are several
amplifiers, they should all be the same type. This makes it
easier to stock batteries and parts and for announcers to
operate the equipment. There can be a couple of the smaller
amplifiers for the simple pickup occasions, but several should
be of the multimike input variety.
MIKE
AUX
1
1
FADERS
BRIDGING
MIKE 2
AUX 2
OUTPUT
MIKE 3
AUX MIKE 4
REMOTE
AMPLIFIER
Fig. 7 -9. The remote amplifier should have several input capabilities
for
flexibility.
Inputs
Besides microphones, other inputs are desirable on the
amplifier. These other inputs may be on a switchable
arrangement. The additional inputs can be used to obtain a
feed from the local PA system, or portable tape recorders may
plug into the amplifier to play back special types, such as
interviews. By having a variety of input options on the
amplifier -it doesn't need to be too complicated -can have
more flexibility to cover more types of remotes.
Tone Generator
For test purposes on the line and for a rough equalization,
an internal tone generator is also desirable. It should provide
at least three audio tones: 100 Hz, 1 kHz, and 5 kH. By use of
these three signals, a rough equalization of the line can be done
before the program starts. There aren't enough tones to make
a complete response run, but if these three are reasonably
close in response, you can expect reasonably good results from
the line.
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Earphones
Many amplifiers provide a combination earphone jack and
power switch. When the earphone is plugged in, the battery
power is turned on. When the announcer has arrived very
early at the remote site and has set up, he should pull out the
earphones to conserve the batteries.
Many remote amplifiers only provide a single earphone
jack. But on many remotes, more than one announcer may be
involved, and each should have a headset. For example, a
sports announcer and the announcer to do "color." So if the
amplifiers do not have more than a single jack, add more
jacks. Check behind the panels to see that there is clearance.
Mount the additional jacks, and wire them in parallel. Make
sure to use insulated washers on these jacks. The earphones
are usually wired directly across the output of the amplifier,
and if a noninsulated jack is used, it will unbalance the
amplifier output and line.
Program Level
The program fed to the telephone line should be no higher
than +8 dB on the peaks. Higher levels can cause crosstalk
into other circuits in the cables. It the line is a clean line, then 0
dB on peaks will be sufficient. When the remote site is at a
great distance from the station, such as over the long lines of
AT & T, or if the line is equalized, there are amplifiers along
the circuit. If the program level is too high, these amplifiers
can be overloaded and distortion results.
Some remote amplifiers have a meter pad switch that
allows adjusting the meter to read 0 VU on the peaks even
though the actual output level may be +18 dB, for example.
The announcer must make sure this meter pad is set correctly,
or he will be feeding much higher levels into the line than is
desirable.
Multimike Remotes
Some special remote programs may require more mikes
than a particular amplifier has inputs, and these mikes must
be available for immediate switching on and off. Such a
program might be coverage of a special meeting of the city
council or some public hearing. Such remotes require far more
mikes than the normal multi -input remote amplifier can
provide.
248
To solve the problem, two amplifiers can be used IFig.
7-10). The outputs of the amplifiers may be mixed together in a
simple pad to feed the remote line if the line can stand the 6 dB
loss of the pad, or the two outputs may be simply strapped in
parallel and fed to the line. I have used this technique on
several occasions without apparent ill effect on the system
METHOD
(PAD)
MIKES
REMOTE
AMP.
1
60011
>.
MIKES
PAD
REMOTE
AMP.
TELEPHONE
COMPANY
600ít
2
METHOD 2
(PARALLEL)
-o-s-
0.
TELEPHONE
COMPANY
Fig. 7-10. When more microphones are required for the setup, use one or
more amplifiers.
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response effect except some level loss that could be made up.
These were voice broadcasts similar to those mentioned.
Checkout
Before taking equipment out on a remote, it is a wise
procedure to collect all the equipment, set it up, and check it
out for proper operation. Besides determining if all the
equipment works, the practice also determines that enough
equipment has been collected to do the remote. There is
nothing more disconcerting than to begin setup at a remote
location only to discover that some important connector or
adapter has been left behind. If there is no one at the studios to
bring the part, the remote man has to go back after it, leaving
all the equipment in an exposed, unprotected position. He
could return to find his microphones missing! So gather
everything up beforehand to check it out.
Batteries have a definite life span. Whether this happens
right in the middle of a special broadcast or at some more
convenient time depends upon how well someone keeps tabs on
the hours of use the batteries have had. Some remote units do
carry spare batteries internally, so if one set fails, the
announcer needs only to switch in the good set. The instruction
manual provides some estimate of the hours of battery life to
expect. In estimating hours of use, include the time spent in
the checkout of the equipment before the program began. On
very special broadcasts, install a fresh set of batteries before
the broadcast, especially if the program is a very long show.
Remote amplifiers should be ready to use and kept in some
regular storage cabinet. If there is a problem in one of the
units, send it to the shop. That is, keep the defective amplifiers
separated from the rest. When the defective amplifier is
repaired and checked, then it can be returned to the regular
storage cabinet. With this method, there is less danger of
someone using a defective amplifier on a remote.
Maintenance
Remote amplifiers and other components that have been
out on remotes-especially if they have had hard usage over a
period of time, for example, a week or two at the county
fair-should be brought back to the shop on their return to the
station. Here they should be checked out and any necessary
repairs made before returned to the storage cabinet.
250
Equipment gets rough handling on remotes. Small things
should not be allowed to accumulate. There are enough
potential problems at the remote site without also having the
knobs falling off the equipment.
Checking before and after use provides a double -check
system. It's a little extra work perhaps, but if you have driven
150 miles to cover a ball game, only to discover the amplifier
won't work or is missing, the precheck seems worth the extra
effort.
Measure the Current Drain
Although the estimate of battery life is be reasonably
accurate for equipment that is working well, battery life can be
much shorter if there are circuit faults which cause a heavier
current drain.
When the equipment is new and working well, install a new
set of batteries and measure the current from the batteries
with all the circuits in operation (Fig. 7 -11). Save this figure
for future reference. Either attach it to the unit's instruction
REMOTE AMPLIFIER
AMPLIFIER
CIRCUITS
+~
r
MILLIAMMETER
BATTERY
Fig. 7-11. Measure the current drain of a remote amplifier In
Save the figures for later reference.
full operation.
manual or keep it in the engineering files. When battery life
seems to be much shorter than usual, remeasure the current.
If the current drain is now much higher than the original
figure. there is some fault in the amplifier that needs to be
corrected.
Batteries
Batteries are composed of corrosive chemicals. Even
though sealed, there can be leakage or "breathing" by the
251
battery. These chemicals corrode the battery compartment
voltage terminals.
As a safety measure, remove the batteries before the
amplifier is placed in the storage cabinet, especially if it won't
be used for some time. The batteries themselves can be kept
alongside the amplifier or in a pocket of the carrying case.
When the amplifier must go out again, then the batteries can
be installed. By keeping the batteries out of the unit when in
storage. there is less chance for damage to the battery
compartment by corrosion.
Tag or label the batteries that have been removed from a
particular amplifier, so that they are returned to the same
unit. If the batteries of several units get mixed together then
the estimates of life span become confused.
Type and size of battery to use is dictated by the particular
unit. Although you may prefer to use the larger D -cell for its
extra capacity rather than a C -cell as called for, there may not
be enough space to handle the larger batteries. However, you
can sometimes go to a different kind of battery, although it has
the same physical shape and size. For example, instead of the
regular zinc flashlight battery called for, you can use the
alkaline battery. This is a heavier duty battery and lasts
longer, although it is more expensive than the zinc battery.
The manufacturer claims that this battery has ten times more
energy than a comparable zinc battery. You may be able to go
to the rechargeable battery. This must be recharged from
time to time. but will last a long time before it needs to be
replaced.
INTERCONNECTING LINK
Stations that must operate their transmitters at a great
distance from the studios have a far more complicated system
arrangement. There are considerably more initial engineering
problems and potential operational problems; system
maintenance is more difficult to perform. With the
interconnecting link, as with remote broadcasts, the station
has a choice of using land lines, microwave, or a combination
of the two.
The first need is a routing of the station's audio from studio
to transmitter. This circuit is an integral part of the station's
audio system that must perform in all respects with
characteristics comparable to any other part of the system.
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second important need is a control link the transmitter
from the studio. The flip side of the coin is the need to monitor
all the parameters of the transmitter from the control point.
These two functions require special equipment at both the
control point and the transmitter itself. The choice of this
equipment and how it operates has a bearing upon the
interconnecting circuits. There are many functions to
perform: to route each one of these separately over miles of
circuit path is uneconomic. With this in mind, the control units
allow for selection of the control command functions and a
sequential-scanning arrangement of the parameters so that all
the information can be channeled over a single wire pair. The
signal of the control unit may be DC, subaudible audio tones,
or frequency -shift keying (FSK) of audio carriers with digital
information.
A
FM Audio Line
The station needs a top -grade line for this circuit. This
should be a schedule AAA or class AAA. Remember that the
annual proof of performance must include everything from the
microphone input to antenna output. This includes the
interconnecting phone line. Since the system response must be
within 2 dB of the reference at 1 kHz, the line must have a flat
response. The telephone company equalizes the circuit, but a
long circuit is not easy to equalize to 15 kHz or down to 50 Hz.
Easy or not, do not accept a sloppy equalization job. It takes
work, but this circuit should have a flat response within at
least 1 dB of 1 kHz reference. It should not be "lumpy" nor roll
off at either end. Always keep the master system in mind. If
line response, for example, is down 2 dB at 15 kHz, the line
itself meets technical specifications. But this requires the rest
of the audio system to be flat at 15 kHz. Try to explain the
purpose of the line and the severe requirements to the
installers. Ordinarily, when they understand the need, most
telephone people bend over backwards trying to achieve the
desired results.
FM Stereo
The station that operates any part of its broadcast day in
stereo has more severe requirements. There must be two
identical circuits, one for the left and one for the right audio
channels. The equalization process must take this into
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consideration. Identical means the same all the way across the
bandpass. For example, if there is a 0.5 dB rise at 3 kHz on one
channel, there should be a 0.5 dB rise on the other channel at
the same frequency. If the response should drop at that point
on the other channel, this is even worse. Even through each
individual line is within specifications, the two together are not
so good where stereo is concerned. This is a part you may have
particular difficulty making the installer understand.
Path Length
The two lines should have equal path lengths, or there will
be phase problems. Here again, try to emphasize when
ordering the circuits that the two pairs should run side by side,
if possible, and should certainly run in the same cable ( Fig.
7-12). When the pairs run in the same cable and on the same
frame at the testboard, there is a better chance that the two
TRANSMITTER
STUDIO SITE
LEFT
5 MILES
LEFT AUDIO
RIGHT AUDIO
SITE
TELEPHONE LINE
_y
RIGHT
6 MILES
/
THIS
PAIR
IN DIFFERENT
CABLE
PHASE
PROBLEMS
AT THIS
END
Fig. 7 -12. If two audio circuits run In different cables, there can be different path lengths, which will mean phase problems in stereo.
circuits are nearly equal in length. In spite of all the efforts to
achieve this, it may still be necessary to add some electronic
device, such as the Garron phase corrector, to correct for path
length.
AM Audio
System audio response for the AM station is not as
stringent as for the FM station; if desired, a lesser grade line
may be used. To meet proof -of- performance standards, the
response need be only ±2 dB from 100 Hz to 5 kHz. A schedule
A or class A line can be used. Although the class A line is less
expensive than the class AAA line, the station may desire to
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keep the quality of its audio system intact and not allow the line
to restrict the bandpass; all the rest of the audio equipment
should have a response at least to 15 kHz or better.
Control Line
The type of line needed depends upon the type of remote
control the station uses. If the unit is simply used for DC
control and parameter measurements, the line must be a DC
metallic line (Fig. 7 -13). If transformers are in the line
somewhere, it is no longer a DC line, and the remote -control
(A) DC LINE
STUDIO
TELEPHONE LINE
TRANSMITTER
(B) AUDIO LINE
TRANSMITTER
STUDIO
TRANSFORMERS
Fig. 7-13. A DC line is a metallic wire pair. An audio line might have
transformers in it.
unit will not work. A line several miles long has considerable
DC resistance in it, and this must be considered in the control
unit design.
Audio Control Line
When the control signals can be sent in the form of audio
carriers, then the circuit can be a regular audio channel. The
more sophisticated control units do this by FSK audio carriers.
These carriers range from about 800 to 2700 Hz. Since this is
midrange audio, it passes through any transformers in the
line. The control signals sent to the transmitter are only half
the story.
Telemetry
Information about the transmitter must be sent back to the
station control point so that the operator knows the transmitter
is behaving properly and can log the meter readings. In units
255
with audio carriers, the signals are subaudible tones in the
neighborhood of 22 -28 Hz. The ordinary line with
transformers cannot pass this low- frequency audio very well,
if at all. These tones will pass over a DC line. So when an
audio -grade line is used for the control functions, these
telemetry signals for the transmitter parameters are
impressed as modulation on the main AM transmitter carrier
itself, or in the case of FM, on a channel. The return route then
is over the air from the main carriers to receivers at the
control point. When the main transmitter modulation is used
for telemetry, then consideration must be given to FCC
standards relating to the use of these signals.
Maintenance
Actually, the land lines are outside the jurisdiction of the
station personnel as far as maintenance goes. The
maintenance is performed by telephone technicians. But these
lines are very important to the broadcast operation, and station
personnel should make continuing checks on these circuits,
that is. on some periodic basis. A set of audio measurements
should be made on a regular basis. These need not be a full set
of response measurements, but only enough frequencies to
outline the response of the bandpass. Also check signal levels,
distortion, and noise. Cables may get moisture in them, or
grounds may occur which increase hum level or crosstalk.
There could also be problems with the telephone amplifiers,
and distortion may increase. Whenever your measurements
show a deterioration from the original measurements, notify
the telephone company immediately to get them on the
problem.
RFI
Modern line equalizers also contain a solid -state amplifier
that is located at the transmitter site, where it is subject to
strong RF fields. These amplifiers may also be susceptible to
RFI, especially from the AM carrier. Some are more
susceptible than others. Make sure the telephone company
debugs any RFI that shows up in the unit. If need be, have
them try other amplifiers. Debugging RFI may be a new
experience for the telephone technicians, so you may have to
pitch in and give them a hand. There can be problems in
debugging since this is a 15 kHz circuit, and the brute force
methods discussed earlier can't always be used.
256
Fig. 7 -14. The station may use a microwave unit instead of telephone lines
for the interconnecting link. Shown is the Marti STL -8F transmitter unit,
operating in the 950 MHz band.
Microwave
The station has the option of using a radio link instead of a
land line for the interconnecting link Figs. 7 -14 and 7 -15) This
is a common practice with many TV stations, both for their
remote sites and for the studio -to- transmitter link (STL).
Although a radio link, this is not the same as the remote pickup
transmitter used for remotes. Instead, it is a microwave RF
(
.
Fig. 7 -15. The receiving end of the microwave link. This is the Marti Model
R- 200/950 F receiver.
257
signal focused into a very narrow beam pointed directly at the
receiving location.
Microwave is a different technology that has its own very
special techniques. There are several microwave bands
allocated for different purposes. The frequencies for radio
station STL are 947 -952 MHz. Setting up a microwave link
requires FCC applications, construction permits, licenses, and
so forth, and the system must conform to the FCC standards
for this service, found in Part 74, subpart E -Aural Broadcast
STL and Intercity Relay Stations.
Unless the station engineers have a good working
knowledge of microwave techniques and requirements when the
station plans on switching from wire to microwave STL, have
your consulting engineer or the factory application engineers
design the system for your specific case. There are a number
of manufacturers in the microwave business, but only a few,
such as Marti Electronics, specialize in the radio station STL.
Signal Path
Microwave signals need a clear path to the receiving
antenna. This must not be blocked by tall buildings, trees, or
other structures. The beam should clear any obstruction by at
least 100 feet.
A profile is needed of the terrain between the transmitter
and receiver. This shows the contours of the ground and its
elevations. To the eye, the ground may appear to be flat, but in
reality there may be a gradual rise over a long distance. This
stands out very clearly when a profile is drawn.
Large buildings, water towers, or antenna towers can
cause real problems when they are directly in the beam path.
Unless the transmitting and receiving dishes can be mounted
high enough to clear these, then a multihop system must be
installed. For very long distances, a multihop system may be
required anyway.
Relays
When multihop systems are employed relay stations are
required at various points along the path. Even a short path
around a tall building may use a multihop system. In this case,
the beam is angled off to one side of the building, to a relay
station. The relay then shoots the beam to the receiving
antenna. Of course, the relay may be installed on top of the tall
building if permission can be obtained from the owners.
258
relay is a receiver-transmitter unit, each with its own
dish. The incoming signal is received and routed to the
transmitter, which sends it out again from its own dish in
another direction (or on ahead in the same direction)
A
.
Diffusion
It is not easy to keep the signal in a sharply defined beam.
The signal always tends to spread out (Fig. 7-16). Some of the
signals leave the beam at angles to the forward thrust,
striking objects, water, or ground surfaces. Some of these stray
signals reflect back into the beam, but not necessarily in the
same phase.
TRANSMITTER
RECEIVER
MAIN BEAM
1
/
\
i
\
STRAY
SIGNAL
1'
/
\\
\\
\
\/
/
I
/
WATER, EARTH, OR OTHER
SURFACE
Fig. 7 -16. Beam tends to spread out, and stray signal reflects off surfaces
and back into beam with the wrong phasing.
There is considerable attenuation of the microwave signal
along the path; the longer the path, the greater the
attenuation. Weather elements also cause attenuation of the
beam and signal fading. All of these factors must be
considered in the design of the system, and proper
compensations must be made to increase the reliability of the
system.
Remote Control and STL
When an STL is used as the connecting link, both the
station's audio program signal and the control commands for
remote control are sent over the microwave beam ( Figs. 7 -17
259
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260
V
and 7-18). At the receiving end, a receiver demodulates these
signals and routes them to their proper functions. For this
application, one of the more sophisticated remote -control units
is used. The command signals are FSK, an ultrasonic carrier
of 20 kHz to 27.5 kHz. This keeps it above the program audio.
For the telemetry of the transmitter parameters, the
station's main transmitters are again used for modulation and
sending the signals back to the control point over the station
carrier.
The station may or may not desire a backup when an STL
is used. This can be land lines or a dual system. In the latter
method, there are dual transmitters at the control point and
dual receivers at the receiving point. The transmitters are
combined to feed a single dish. The receiving -dish output is
divided into the two separate receivers. This dual system can
be switched over should the first system fail. With this method,
there is no need for a land line backup.
Stereo
There are two ways to handle stereo over the STL. In the
first method, the stereo generator is placed at the studio, and
the composite output of the generator is sent out over the STL
to the transmitter. In the second method, program audio of the
left and right channels is sent over a dual STL system. That is,
the left audio is sent on one channel, and the right audio on the
second channel. In effect, this is the same as is done with dual
audio land lines. This is the method recommended by some
manufacturers; however, there are proponents of the
composite method.
Which way to go is the station's choice. Economics and
reliability factors are involved, plus station expertise at
maintaining an STL. These are questions only the station itself
can answer, but here are a few points:
The reliability factor is important. When the link fails, the
station is out of business until it can be restored. The land line
does seem to have reliability factors in its favor, but telephone
technicians can accidentally pull down patches, cable splicers
may get into the wrong circuits, storms and vehicles can knock
down poles and cables, and construction work can cut cables.
While these don't happen too often, they are possibilities. On
the other hand, Marti engineers claim 99.99% reliability in
their STL system. According to their calculations, this means
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period.
Economics is another factor. Land lines are less expensive
to set up unless special cables have to be strung adding a
construction charge, but the lease and rental goes on forever.
And as telephone rates continue to climb, so will the future
costs for the service. The STL is more expensive to set up, but
its cost can be amortized in a three- to five -year period,
depending upon how extensive the setup and equipment.
Maintenance expertise is simply a matter of learning. At
the outset, maintenance on the STL could be done on a contract
basis with an outside firm or with the factory service division.
As experience is gained, it could then be done by the station.
Chapter 8
Program Automation
number of radio stations, especially FM stations, use a
major program automation system. All stations use some
form or degree of automation. A major programmer is a
complete system made up of many individual parts, all
working together in a closely knit operation, which replaces
both the control room equipment and the operator. To perform
its functions as designed, it must meet two important
conditions: Programming material for use by the system must
be carefully prepared for that system, and the system's
machinery and electronics must be kept functioning efficiently
by adequate maintenance.
A
THE SYSTEM
program systems are not the same, either in concept or
in the equipment used. Methods also vary from one
manufacturer to another. Systems can range from the simple
sequential operation, wherein one event follows another
according to a pattern preset by a multiple- switching panel, to
the direct -access system with a multistep memory system that
plays any source at any time controlling thousands of events
before filling up the memory. In between these there are many
combinations of some parts of both, such as, direct random
access with sequential- subswitching arrangements or
memories. In concept, the simpler system is essentially a
All
265
single unit (although it will have several parts) that performs
certain functions of the station's programming at different
parts of the day. On the other hand, major memory systems
replace all control room equipment and the operator.
For maintenance and troubleshooting, it is very important
that the station engineer understand the particular system at
his station. It is very easy to become wrapped up in the
individual components of the system and become confused as
to their role in the overall system. Study the components and
their functions, but get a good overall view of them as a
system.
The automatic programmer is a major subsystem in the
station lineup, and it is made up of many minor subsystems
that are still further divided into lesser subsystems. Use the
method described in Chapter 1 to develop a good
understanding of the system and get the whole picture into
focus.
Planning
No station should dive headlong into a major automation
system without first doing much research and planning and
knowing exactly what the system is supposed to do for them.
This happens all too often and is generally the cause for
disenchantment with the system's performance. But once the
purchase decision is made for a particular system, there is
still much planning to be done concerning its actual
installation in the station. This planning is not much different
than the planning required before installation of anew studio or
a control room.
Space
In the installation planning, serious consideration must be
given to the actual physical placement of the system. There
must be adequate work space to the system. Consider the
normal traffic patterns at the station. If this isn't given a
thought. the equipment may be placed in a narrow area which
is in one of the main traffic patterns of the station. When
troubleshooting, the engineer and his test equipment may find
themselves right smack in midstream of the traffic pattern,
with people constantly crawling over them to get through. This
situation is not conducive to the mental concentration required
to isolate the problem.
266
Expansion
You can almost count on soon needing more rack space for
expansion of the system. So plan at least two additional racks
with the initial system to make sure there is room for further
expansion. Use the rack space wisely and efficiently.
Efficient use of the space considers both the operation and
maintenance. If all the racks are filled from top to bottom with
tape machines, then when tapes must be changed the operator
is down on hands and knees filling the floor -level machines.
When trouble develops, the troubleshooter finds himself in the
same awkward position.
Storage
There are many reels of tape and special program
material that apply to the automation itself, and these should
be stored in an area adjacent to the system. This allows for
finding the replacement tapes when needed and keeps all the
pertinent materials at hand. This storage area also separates
the tapes which are to be used or saved for the programming
away from tapes of a sister station, recording booths, and so
on. This is no different than storing the current records or
commercial tapes in the live control room or an adjacent
room.
Recording Booth
major automation system requires at least one
recording booth to make up materials and announcement
cartridges for the system. This booth should also be adjacent
to the programmer. The booth is a very important part of such
a station. By having it nearby, announcers can update
materials quickly and can get the new one back into the
system.
This clustering of the system and its peripherals into an
operational area (Fig. 8-1) is no different from that which is
done in clustering the peripherals around a live control room.
When a major system carries the full day's programming of
the station, this is in essence, replacing a live operation.
A
Turnkey or Station Installed
Most of the major system manufacturers install their
equipment as part of a package price, but stations can select
their own major components and develop their own automatic
267
AUTOMATION CENTER
AUTOMATION SYSTEM
RECORDING
BOOTH
PROGRAM TAPE STORAGE
HALLWAY
Fig. 8-1. Cluster the automation system and its peripherals together into
its own operational area.
programmer. There are several manufacturers who sell the
individual memories, switchers, and other components,
working with station engineers in designing a system.
When the turnkey installation is done, work closely with
the installer if you can. Have the installation conform to
station standards. You will soon discover that not only do
different companies do things in different ways, so do
individual installers. Also, when the installer is being
pressured by both the home office and the local station
management to get the system operational in as short a time
as possible, he may take the easy way out, doing a poor
installation job. If you are tied up with trying to get a new
transmitter on the air or technical problems with a sister
station, you may not be able to spend any time with the
installer. When things have all settled down, with everything
working, you may discover the most tangled rat's nest of
wiring you have ever seen. On the other hand, if the installer is
not pressured and you can work with him, you have an
excellent chance for getting an early insight into the workings
of the system.
Installation
Try to keep the main power feed to the system separate
from other power circuits, except perhaps, at the power panel.
In each rack, use a plug -in strip for all the separate units to
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obtain power, and run the power feed through rigid or flexible
conduit back to the power panel. If you have sufficient circuit
breakers on the one panel, you can run each rack separately
from a single breaker. It really isn't necessary with the plug-in
strips. A single power feed can be run to the racks, then
distributed from there.
Ground
A solid copper strap should tie all the racks together, or
each rack may have a ground strap to the building ground. In
either case. make solid connections to the building ground. The
racks should also be bolted together. Where the ground strap
connects to the rack, scrape off the paint to get a good
metal -to -metal connection. Individual straps should be
soldered to the building ground.
Surge Protection
If the station has not already installed surge protectors on
the incoming power circuits, it should do so when an
automation system is installed. At the least, these should be
added to the power feed to this system. Most of these systems
use solid -state electronics. Without surge suppressors. expect
to have problems with the system after every electrical storm.
Also remember that other sources of transients abound in a
station, although these may not be the intensity of those from
lightning, and these can cause many unexplained triggerings
of the system.
Cables and Color Codes
Most of the individual units are connected into the system
through multiconductor cables. It is important that color
coding be kept as consistent as possible. Unfortunately, this is
not always done by the manufacturer.
Cable sheets should be kept which not only give the
function of each wire in the cable, but its color coding. If the
color coding is not standardized, always keep this fact in mind
when installing future equipment and troubleshooting. In this
situation, cable sheets are very important. Standard practice
dictates consulting these sheets before action is taken that
involves the cables, rather than relying on memory.
System Phasing
Pay particular attention to the phasing of the left and right
channels in a stereo system. This is the same attention you
269
would give to phasing when installing a live control room or
studios. But there is this difference. In a live studio you run
your own audio cables, but with automation, most of the audio
is routed through the multiconductor cables. Color coding may
be different from your regular audio cables. Even worse, it
can be different in cables from various units within the
system. This allows more chances for errors in phasing.
Remember to keep the high and low side of each channel the
same and. of course, don't interchange the channels.
Setting Levels
Once everything is connected together and operational, set
the audio levels throughout the system. Use test tapes that
have been made up in the recording booth for a standard. Or
you may use the set level tone on the NAB test tape.
Setting system levels is not as convenient as in a live
studio arrangement, where all the console inputs are on jacks.
There are far fewer test points on the automation system. You
can expect to use some different techniques, but the
requirements are the same; that is, each audio channel in
stereo should have identical gain. If the initial setup is done
from a tape machine, the master amplifier gains may be
unbalanced. At each place along the route, channels should
have the same gain -not only at the output of the system.
There are at least two ways to set up the system levels.
Master Amplifier
If the input terminals of the master amplifier are available
on terminal boards and can be easily disconnected from the
system, feed a signal generator to the input of each channel
separately with the correct driving impedance (Fig. 8 -2). The
driving impedance is usually 600 ohms. Make sure the output
of the system is terminated at 600 ohms, and also connect your
test meter to the output. Your test meter verifies that the VU
meters are accurate.
Feed the input to the amplifier at 0 dB. Adjust the VU
meters to read O dB. Check the test meter. If all is accurate, it
too will read 0 dB.
Check for distortion. Now check for headroom. Change the
meter pad or disconnect one side of the meter on the system
(also the test meter). Without changing any amplifier gain
settings. increase the input level from the generator to +10 dB.
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SYSTEM
MASTER
AMPLIFIERS
LEFT
L
TO
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TRANSMITTER
RIGHT
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SIGNAL
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TAPE
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A
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L
SET
LEVEL
TEST
TAPE
L
R
TO
TRANSMITTER
R
L
R
SYSTEM
SWITCHER
0 dB
MASTER
AMPLIFIERS
OTHER
TAPE
MACHINES
B
Fig. 8-2. Set the levels of the master amplifiers first (A), and then (B) the in-
dividual tape machines.
Measure distortion at this level to compare with distortion
measured at normal levels. If there is an increase in distortion
at +10 dB, the amplifier does not have adequate
headroom-overloading is taking place. If that is the case, you
should use a standard level which is 5 -10 dB below the 0 dB
levels. But most modern amplifiers are designed for at least
+18 to +24 dB output, so that by setting the input to 0 dB or +8
dB, there is adequate headroom. Nevertheless, it is well to
check this out and know what the amplifier will do -not what
the specifications say it will do. If there is not adequate
headroom and it is necessary to use a lower standard level,
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then make sure a note of this is placed in the instruction
manual and the engineering files for future reference.
Playbacks
If you are able to adjust the master amplifier as
previously described, it is now a simple matter of adjusting
each playback unit. At this point, reconnect the master
amplifier inputs into the system. Meters on the system can be
used to set the levels; however, system meters may be at an
awkward angle to be seen properly. Leave the test meter
attached to the system output, and move it into a position more
easily viewed. Set the output of each channel on the tape
machine so the meters read 0 dB. Again check for distortion on
each channel. You may wish to check each machine for
headroom also. Do it the same as you did with the master
amplifier. Set the levels of all the tape machines in the system
to this standard.
System Response
Now that system levels have been set to a standard and
balanced. check the system audio response and distortion. Run
a standard NAB test tape through the system while using the
test meter at the system output as before. You may wish to
tweak the alignment of the tape machine heads at this time
also. They will probably be on the button, but a simple check
will tell. The response curve should be flat. If it is not, tweak
the playback equalizers on the offending channel.
Alternative Methods
If it is not possible to get into the master amplifier inputs
very easily-for example, if they are directly connected to a
master printed -circuit board-then you will have to set levels a
little differently. In this case work with the playback machines
first. Set your test meter and connect it to the output of one
machine. Disconnect the machine from the circuit and
terminate the output. Or let the system terminate the machine
and use your test meter as a bridging device. Adjust the output
level of the machine to read 0 dB on both channels. Reconnect
the tape machine. Move the test meter to the system output
and terminate at 600 ohms. Now run the test tape again on the
machine which has been calibrated to adjust the master
amplifier gains for correct levels, and check VU meters as
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before. You can also check for headroom by adjusting the tape
machine output 10 dB higher, checking for distortion.
Remember to adjust the level back to normal. Don't change
the master amplifier setting unless it is necessary to use a
lower standard because of lack of headroom. With master
amplifier now calibrated, you can use it or the test meter on
the system outputs; adjust levels of each machine while it is
playing a test tape. Also, the other measurements of head
alignment, system response, distortion, and noise can be
checked as discussed previously.
Control Tone Levels
The system usally relies on the control tones on the music
tapes or the auxiliary 150 Hz tone on cartridge equipment to
create the switching pulses. Unless these are set up
accurately. the system can switch erratically or not at all. On
cartridge equipment the tones are on a separate cue track of
the tape, but in music machines, the tone is a subaudible tone
in the left music track.
Music Machines
There are two different ways of handling the switching
tones in different systems Fig. 8-3). In one method, there is a
single sensor that picks up the tone from the common
left-channel audio bus from all the music machines. In the
second method, each music playback machine has its own
sensor. The single -sensor arrangement is more difficult to set
up. In operation it must have enough leeway to accommodate
all the machines, yet not trigger on music itself. It is a
cut -and -try method. Even with an optimum setup, there can be
an individual music number with many low -frequency tones
that trip the circuit. A preferable method uses a single sensor
at each machine. Then the adjustment of each sensor only
involves the peculiarities of one machine, and the adjustments
can be made accordingly. In either case, set the adjustment as
insensitive as it can be and still trip reliably. Increase the
sensitivity just a little beyond this point to allow for slight
variations on music tapes. If you can create a test tape with
these low- frequency tones of the correct lengths that
corresponds with those the music library supplier uses, go
ahead. But these companies often supply a test tape to adjust
the sensors, which is compatible with the tones and level they
use.
(
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LEFT
TAPE I
AUDIO RIGHT
TO SWITCHER
L
TAPE 2
AUDIO R
L
TAPE 3
AUDIO
R
TONE
SENSOR
A
B
SENSO
SWITCH PULSE
TO AUTOMATION
SWITCH
PULSE TO
AUTOMATION
LEFT
TAPE 1
AUDIO
RIGHT
11.
ENSO
.TO
TAPE 2 L
AUDIO
SWITCHER
R
SENSOR
TAPE 3 L
AUDIO
Fig. 8-3. Two different ways the control tones of music machines are
handled.ln A, one sensor handles all machines, and in B, each machine
has its own sensor.
274
Cartridge Machines
Each playback machine should be set for reliable
triggering on the auxiliary tone and for stopping on the cue
tone. Again, set the sensitivity a little beyond the point at
which the machines operate reliabily. Make up a test tape in
the master recording booth to be used as a standard. Avoid
running the sensitivity control wide open; there may be noise,
transients, or other data recorded on the track if automatic
logging is used. These spurious signals cause the machine to
switch. Some models of playback machines are more sensitive
than others in this regard. While you are doing this
adjustment, attach an oscilloscope to the individual machine
output to check the waveform of the switching pulses. Also
observe for switching transients that occur when the machine
starts up or shuts down. Any random transients cause erratic
switching.
Maintenance
As mentioned earlier, it is unfortunate that these major
systems do not have convenient test points throughout. There
are test points scattered throughout the system, but most of
these are on the individual units themselves, and they are not
always convenient to get to. Besides that, they may be around
the rear of the equipment, and it is necessary to be in front of
the equipment to trigger equipment on or observe the action
taking place. This makes troubleshooting and signal tracing a
little more difficult.
There can be normal problems with the system in the area
of audio -electronic and mechanical-but there will also be
problems in the control, switching and memory circuits. One
of the most confusing occurrences is when a number of
different machines erratically fire on at the same time. That
makes it difficult to get a good picture of what is really
happening. That is, many uncalled-for events may be taking
place that are seemingly unrelated to the actual fault.
Transients on the switching bus are a likely cause of many
unrelated events taking place all at the same time.
The system problems can be divided into two broad
categories: improperly prepared program material or
material in the wrong trays, and system electronic and
mechanical problems.
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Audio Problems
Audio problems are those associated with improper signal
levels, which can cause overloading, distortion, low levels,
noise, silence- sensor switching, or loss of one channel. For
general level problems check out the tapes in question. If it is a
cartridge, take it to the recording booth and play it on the
master recorder. This should quickly determine if it is that
tape or something in the system. If there is still some question,
play another known -good tape in that machine to check
performance. If the tape is at fault, have it redone. If it is a
music tape from an outside library, complain to these people
that the levels are not held consistently
If the poor levels appear to be the machine, check out the
heads for oxide buildup, head wear, etc. It requires the same
methods as for regular tape machines.
Head Cable
Sometimes the left or the right audio channel drops out
when music is playing. One source of the problem is the cable
plug that attaches to the tape head. Either the pins in the plug
lose tension, or pins on the head are bent so that proper contact
is not made. If the left channel is gone, the machine will not
stop because the cue tones are not present.
Try to spring the contacts in the plug. This can be done
successfully sometimes. Or straighten the head pins if bent. If
the trouble can't be corrected in this way, then it is necessary
to replace the whole cable because the plug is usually molded
onto the cable.
Photoresistors
Losing one or both channels can be caused by
photoresistor devices that are often used at various places in
the system to switch the channels on or off when routed
through some special unit, such as a network switcher. In the
dual stereo units, there is a pinhole at one end where the light
can be seen to glow when it is turned on. If the unit is
supposed to be on and the light is not evident, substitute
another unit. But if the unit is soldered onto the printed- circuit
board, then make some other checks before unsoldering. You
can use an oscilloscope to check for audio in and out of each
channel, or you can measure the DC voltage and note if it is
present or absent. If present but the light is still out, replace
276
the unit. If the light is on and audio is still not coming through,
check for input audio before changing the unit. In those
systems which use single units, one on each channel, they run
warm when the light is on for some time. Again, the checkout
is the same, except there is no pinhole to observe the light. On
both of these types, there is a terminal diagram on the body of
the unit which shows the internal connections to the pins.
Other Causes
There are other causes for low levels or loss of either or
both channels. Pressure pads on cartridge machines may be
defective or missing (in some cartridges). The pull -in
mechanism on a particular tray or on the single-head machine
may be out of adjustment, or small parts may be missing.
Unless the tape is held against the head with proper pressure,
the output levels will be low, not only from the program tracks,
but also from the cue track, causing erratic switching. Of
course, worn heads or oxide buildup can create the same
effects.
Failure to Switch
Anything along the switching-pulse route that is open can
cause the switching pulse to fail to do its job. This can be
caused by open circuits or loose plugs, a channel may not be
ready, a machine may have blown a fuse, or developed some
similar fault. It can also be caused by poor or no signal from
the tape heads or failure in the machine sensor circuits.
Check the machine sensor circuits first by measuring the
relay-contact output. Normally, the pulse itself does not do the
switching; it turns on a relay that routes DC voltage to act as
the actual control pulse. Make sure that there is no voltage on
the contacts that can damage an ohmmeter. Otherwise, use
the voltmeter on the output side of the contacts, checking to
see that it is at the input contact before the test ( Fig. 8 -4) Play
a tape, watch the meter, and listen for the relay to operate. If
the relay operates but no voltage is switched, the contacts are
defective. Replace the relay and try again. But if the relay
does not operate, then look to the machine or sensor for fault.
Check the actual tone pulse that is received from the tape
machine. If it is strong and normal looking, then the sensor
circuitry is at fault; but if the pulse is very weak, distorted, or
missing, check out the cue circuit to the tape head.
.
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+ 24V DC SUPPLY
2
FROM AUX TONE
CIRCUIT
DC SWITCH
PULSE TO
AUX TONE
RELAY
AUTOMATION
VOLTMETER
Fig. 8-4. If DC voltage is switched, measure the source voltage (1) first;
then measure after the relay contacts (2) when the relay operates.
If the voltage is not present at the input of the relay
contacts, look back into the circuit that is feeding the supply
voltage. If the relay switches the voltage, trace the pulse on
down the circuit path. Check for loose plugs or an open series
diode in the circuit.
False Switching
Static electricity can be a real problem, producing false
triggering of logic circuits. When carpeting is used, this
situation in dry winter months can be very troublesome. Nylon
indoor-outdoor carpeting is perhaps the worst for generating
body static. If a memory -type controller is used, the best thing
to do is go to special computer -room carpet. This has fine
wires woven into fabric, providing a ground terminal. It is
expensive, and perhaps the most economical and practical
route is to forego the carpeting altogether. Many of the large
industrial companies which have large computer systems
have given up on the carpeting and installed asbestos or
similar floor tile.
Static can be a real problem to either the memory unit
itself or to individual machines which use a positive logic
control. (By positive is meant that it takes a positive pulse to
turn it on or cause the desired action.) Most solid -state
equipment uses the negative logic, in that it takes a negative
pulse or a ground connection to make the unit act. With one
model of multitray cartridge machine using positive logic, an
announcer walked across the room to place a cartridge in a
tray of the machine. When he touched the machine, several
278
trays turned on and ran. The turnon was caused by body static.
Antistatic powders and other treatments for carpets either
failed to do the job or were very short lived.
Head Replacement
When heads must be replaced on either the music
machines or cartridge machines, always do this more
carefully than one might do with regular machines in a
standard live operation. Make sure the replacement head is
correct; that is. don't get a record head instead of a playback
head. Use the standard alignment tape. When removing the old
head, physically displace as little as possible, noting the
present mounting position of the head. The more precisely you
install the replacement head, the easier it is to make the
alignment.
Head Substitution
There may be a time that the station desires to change the
heads on the music machines to a different type, or perhaps
the factory might send a different head that is supposed to be a
direct replacement. When this is done, check out the results
very carefully. All heads do not have the same characteristics
or response. particularly at the very low frequencies where the
switching tones are carried. If the response below 50 Hz falls
off rapidly, then be alert for switching problems; but check it
out now, don't wait for switching failure. If you have a test
tape with 25 Hz tones on it. use this on the machine to read the
response on the system meters. It could be several decibels
lower than the previous heads. Now this doesn't rule out the
use of the heads, but the sensor will need some adjustment. In
most cases, the individual sensor can be adjusted to
accommodate the lower response, unless the response is
unreasonably low or nonexistent at 25 Hz. The real problem
can occur with the system that uses a single sensor for all the
music machines. In this case, the best procedure is to change
all the machines to the new type of head at the same time.
Then the sensor can be adjusted to these new conditions. If the
sensor is adjusted to accommodate this low -level machine, the
sensitivity will be too high for the other machines, and it will
trigger on music. For the single- sensor setup, all the machines
must be reasonably close in low-frequency response.
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PC Board Connectors
Another trouble spot can be the connections on
edge -connected boards. This is the type where all circuits run
to one edge of the board and the board plugs into a socket
connector. The cards can work loose, or oxidation can cause
resistance on the contacts. This causes intermittent switching,
circuit failures, loss of channels, and other problems.
Reseating the board if it is loose will correct this type of
problem. If it is not loose, the board should be taken out; trace
contacts on the card itself. Clean each contact with a pencil
eraser. This will usually do the trick, but make sure the unit
power is off before pulling out or replugging the card.
A similar situation can happen when a cable connects to
pins projecting out of a board. This may be a continuation of a
master board system from one chassis to another. In this case.
the cable has many circuits and hence will be a large cable.
The weight of this cable, plus occasional jarring, can cause the
plug to work loose, and of course, certain functions fail or
operate erratically. The best cure in this case is a good
anchoring of the cable so that the weight is off the plug. Use
some cable ties to anchor the cable down tightly, but make
sure there is no strain on the plug.
Cable Abrasion
similar situation can exist with units that roll in and out
of the racks that are connected by flexible cables. If these
units must have much movement during normal operating
routines, the cables can drag across surfaces. Enough of this
abrasion and the cable sheath and actual wire insulation wears
through. grounding out to the frame. Check for these wear
points. Also, make sure the cable is anchored well where it
enters the chassis. Keep an eye out for abrasion where it may
drag over rails.
A
Silence Sensing
Most systems provide a silence sensor that monitors the
audio. then fires off an alarm and steps the system ahead if the
audio fails. There is usually an adjustable time delay of three
to six seconds before the unit reacts. This is normally
connected to the monitor output of the system. In the usual
station setup, there are signal processors, phone lines,
transmitter, and much else beyond the automation itself. It is
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possible to have audio failures anywhere beyond the system
resulting in no program on the air, although the automation
may be purring along normally. The best monitoring point for
the silence sensor is at the audio output of the modulation
monitor or a receiver monitoring the station off the air Fig.
8-5). Then if anything fails beyond the system, the silence
sense alarm warns that there is no air program.
(
MODULATION
MONITOR
AUDIO
TO MONITOR AMPLIFIER
OUTPUT
STEP PULSE TO
AUTOMATION
AUDIO
OUTPUT OF
AUTOMATION
SYSTEM
SILENCE
SENSOR
SOURCE
SELECTOR
SWITCH
TO
ALARM
Fig. 8 -5. Monitor the audio after the modulation monitor with the silence
sensor.
Modifications
There are many modifications made to the automation
both during the original installation and at later dates when the
station trims the system to more specifically meet its needs.
Regardless of when the modifications are made, the system
diagrams should be updated with the new information. And if
there is a change in the operation of a particular unit, then
there should be modified instructions shown in the instruction
manual. If the added circuit or modification is one of station
design, then a complete schematic should be made along with
its notation on the diagrams where it interfaces. Keep a
regular file on these modifications along with the normal
prints and manuals of the system.
Cleaning
A system that is in operation for all of the broadcast day
runs miles of tape through the machines. Tape machine heads
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need regular cleaning to prevent oxide buildup. At the same
time check on head wear. When there are many heads to be
cleaned, these can be spread out over a period of a few days,
but music machines should be cleaned at the end of each day.
Those multihead cartridge machines do not have every head
used as much as single -head types. Such machines can have
the head cleaning spread out over a week without too much
difficulty in operation, but that depends upon the actual
operation of the machines in the system itself. The important
point is that a regular head-cleaning routine should be set up
and followed. At the same time, clean the pinch rollers and
other tape -carrying surfaces.
Racks and Units
The rest of the system and the racks should be also on a
regular cleaning routine. A vacuum should be used to clean out
the dust before it is allowed to accumulate. During this time,
check and clean any fans and air filters that may be used. Small
fans can accumulate heavy layers of dirt on the blades which
cuts down their efficiency, and the air filters can become
clogged and reduce cooling. Also, check that these small fans
are actually running.
Pilot Lamps
Major systems have many pilot lamps throughout. These
are not there for cosmetic purposes, but serve a most
functional end. When a number of these lamps are burned out,
it may be difficult to determine which machine is running as
programmed. which one is cueing, and what machines may be
running due to some faulty switching. So it pays, from an
operational viewpoint, to keep the lamps replaced and burning
properly.
Readouts
Readouts serve a still more necessary function than the
pilot lamps. Some larger readouts use small incandescent
lamps for each one of the seven segments. Access to these is at
the rear of the readout after some minor disassembly. But
getting to the rear of the readout itself can be another
problem. If some lamps are burned out, when an opportunity
presents itself to open the unit, change the lamps. Replace all
seven with new lamps. The ones still burning will not have too
many hours left, and you would soon need to do the job again.
282
The lamps that are still good need not be thrown away. They
can be used in other areas where it is much simpler to replace
the lamp. for example, with the lighted switches on the music
machines.
Transients
As mentioned earlier, transients can cause many
problems. The relays and motors can kick up some unhealthy
transients along the AC power circuits, and these can couple
into control circuits and even the audio. Capacitors across
relay contacts and diodes across relay coils reduce transients.
Another device is the small surge suppressor made by General
Electric Company for use on home equipment across the AC
input. If the system worked reliably prior to this and now has
transients, either the built -in suppressors have failed or
defective components are now present. Check for burned relay
contacts, as these can be a source.
MOS PROGRAM CONTROLLER
All major automation systems have a
program controller.
Smaller systems have mechanical switching matrices, and
these are limited in size and action because of the physical
limitations of the switches themselves. These are usually
sequential in operation. Once the designers learned how to
scale down computer technology and apply it to automation
systems, the systems began to expand in size and flexibility.
New devices have been also developed, such as the MOS
(metal oxide semiconductor) field -effect transistor. The
MOS -type program controller has become popular recently.
MOS
In the MOS field- effect transistor (MOSFET), the junction
area is a very thin layer of metal oxide, acting as an insulator.
The junction forms a small capacitor, which has the properties
of memory. To store any appreciable amount of information, a
large number of MOSFETs must be used. So the MOS IC was
developed. On each IC chip, manufacturers have been able to
squeeze in more and more memory cells so that present
memories can store well over 4000 events.
Lineup
To obtain any number of events, the individual MOS units
on each chip are arranged in series (Fig. 8 -6). To further
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CAPACITANCE IN EACH
FET JUNCTION
INPUT
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MEMORY IC
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BIT
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1
Fig.e -6. The MOS units are in series inside the IC, and ICs are wired In
series to increase number of steps a unit can store.
expand this storage capacity, many chips are mounted on
printed -circuit boards that are wired in series. How many
events that can be stored in the memory is limited by the
physical space required to handle the number of
printed -circuit boards. But there are practical limitations
because there are other peripheral circuits which process the
data into and out of the memory itself. While the memory is a
very important part of the controller, it is useless without the
peripheral circuits, which process the data, control external
equipment, and so on.
Binary -Coded Decimal
BCD stands for binary- coded -decimal and is a shorthand
method used in computer technology to conserve space and
components. There are several such codes possible, but the
most common code used is the 4 -line (8- 4 -2 -1) code. Any digit
from 0 to 9 can be formed by a combination of these four
numbers. For example, the digit 9 can be formed by the 8 and
1: a 3, by 2 and 1. By the use of BCD, then, only 4 parallel
circuits are required instead of 10. This conserves space and
components.
Memory Storage
Information is stored in the memory in BCD form. The
memory requires as many parallel paths as are necessary to
284
handle the BCD profile for the particular digit in question. For
example. a tape machine in the automation may be designated
source #7. The memory must have a special section for the
source information. The number of parallel paths required in
the memory is determined by the highest number of sources
the controller can handle. If, for example, it is a small unit that
can only control seven sources, then there need be only three
parallel paths, which could combine to make the number 7.
This would be BCD 4 -2 -1. A fourth path for 8 would not be
necessary since there is no number that high.
The storage and action of the memory can be compared to
columns of soldiers on parade IFig. 8 -7). Each long, single
column, marching forward in step, could represent a single
RECYCLE
OUTPUT
(BCD)
DATA BITS
JL
t_
SOURCE
44
D
(SHOWN
SOURCE 3
IN BCD)
w
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2
4b.
TRAY
(10'S)
B_
OUTPUT
TRAY
ETC
(UNITS)
MEMORY
DATA LINED UP
ABREAST FOR
ONE STEP
Fig. 8 -7. All data bits lockstep abreast from input to output and then recycle.
BCD line. The number of soldiers in that column could
represent the number of steps the memory can store. As the
reviewing officer, you would look across these many columns
and note how well each soldier from each column lined up
shoulder to shoulder with the men on both sides. In the
computer, there are as many "columns marching abreast" as
there are requirements for data storage. In the source
information we are discussing, the three parallel columns
"march past in lockstep," one event after another. For
example. if the source called for at this step in the memory is
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#3, then the memory parallel BCD lines would be: line 4, low;
lines 2 and 1, high.
The tray number for that step would also be lined up with
the source lines. This section must have its own parallel BCD
lines also. If there are code numbers or other data required at
the same time, each of these must have its parallel lines. All of
this informtion steps through the memory in parallel. The BCD
lines in the memory are MOS chips.
The MOS is a dynamic memory with the storage in the
capacitive element of each MOS unit. As with most capacitors,
there is leakage, and the charge will "fade out." There is
nothing to maintain the charge if power is removed, so the
power must be on at every instant. Even under continuous
power. the charge will fade, so the memory must be
"refreshed." To do this, an oscillator strobes the memory, and
each cycle of the oscillator frequency moves the columns ahead
one step at a time. The memory is cycled from the output back
into the input in a constant recycling process. The oscillator
will operate at 1 or 2 MHz, so the data is cycled through the
memory 1 or 2 million times each second.
When data is called for from the memory, the data that is
stored does not get "used up." When a call comes in for a
certain step, the output gating circuitry will wait until that
step cycles to the output. At that point, a charge is transferred
from all the columns to the output gating and sent on to the use
intended. The memory data continues on its normal cycle back
to the input of the memory. This is as if all the soldiers across
the marching columns, on reaching a turning point, reached
out and each pushed a start button simutaneously and then
continued on their normal march.
The oscillator, or strobe, synchronizes all this activity as
well as the recyling of the memory. You will find the oscillator
feeding many parts of the computer as well as the memory.
Maintenance
Before attempting maintenance on the computer, study
the workings of the model in use at your station. Some
instruction manuals are nothing more than a sheaf of circuit
diagrams with very little explanation of how the unit works. If
this is the situation in your case, you will need to obtain data
sheets on the various ICs used in the unit from the
manufacturer. Work with the block diagram of the overall
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unit. Sometimes the instruction manual will supply this data
also.
Troubleshooting
Use signal-tracing methods and keep the memory
operating all the time (unless it is burning up). Once you turn
the power off, the entire memory is lost and it will be necesary
to reprogram. Use the logic probe at the test points of each
board where they are provided. Some units may also include
some small test lamps at various critical circuits. These lamps
may be indicating in BCD, so take this into consideration when
trying to figure out their action.
When possible, run the automation manually or switch it
over to some other program source to play on the air. This will
allow you to operate the memory at will, and you won't have to
wait for long tapes to play through and cue up.
Cards
Follow the overall block diagram and check the
input- output test points of each card (PC board) with the
probe. (Refer to Fig. 8-8.) That is, determine if the input goes
high or low as it is supposed to. Then move the probe to the
LOGIC
PROBE
LOGIC
PROBE
I
I
INPUT
OUTPUT
PC BOARD
PC BOARD
OPERATING
PROGRAM CONTROLLER
Fig. 8-8. Check the Input test point and then the output test point of the
card with a logic probe to isolate problem to the PC board.
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output and check for the correct action. Two probes are better
because then you can observe the input and the output at the
same time. If the input indicates correct action but the output
does not, then shut down the power and put the card on an
extender board. Turn the power back on. Now the memory is
lost, but only program around the particular steps or actions
that were at fault rather than doing the whole memory at this
time. You may have the power on and off several times before
finding the fault. Now, with the card on an extender board, use a
logic clip for the in -line ICs. The clip allows viewing the action
of all the pins on the IC at the same time. Keep on tracing the
signal until the fault is tracked down. To signal trace on the
card, use the individual circuit or block diagram for the card
itself. When using the probe, be careful not to short two pins of
an IC together, or the IC may be damaged.
Memory
When problems occur, it may not be easy to determine if
they are in the memory or somewhere else. If the memory has
many identical cards on each side of it, the cards may be
swapped around in an effort to get the fault to follow the
position of the card (Fig. 8 -9). This can help isolate the fault to
MEMORY
INPUT
)
INPUT
MEMORY
-
)
)
MEMORY
OUTPUT
MEMORY
OUTPUT
L
MEMORY
Fig. 8-9. Swap identical cards from one side of the memory to the other
side in an attempt to isolate fault to memory itself or an individual PC
board.
288
an individual memory card. For example, assume that a fault
shows up as an incorrect number in the unit readout of the
trays. Looking at the print, you realize there are enough
circuits on the other side of the memory to swap some of the
cards. With power down, trade a single card (or all, if trying to
rule out the memory). Then reprogram some information. If
the wrong number now moves over into, say, the source
readout. the fault is in the memory. But if the fault remains in
the tray's units, it is elsewhere. Swapping cards can be done
anywhere there are identical cards on different circuits in the
computer. but make sure they are identical. Plugging a card
into a wrong slot can blow out all the units on the card. If the
problem is determined to be the memory, then go through the
process again; but this time, swap only one card at a time.
This will isolate the problem to a single card.
Power
These dynamic memories must have not only continuous
power, but well -regulated power supplies. Check the
regulators and battery- charging voltages, and keep them set
to the values shown in the instruction manual. Also check the
air filters and blower if these are used. Air filters can clog up
and fan motors can quit; and if the air is reduced, the power
supplies may go out of regulation, and all sorts of erratic
behavior can be expected from the memory.
Other Problems
Transients can be a real headache, both to the memory
and the automation operation. Besides damage which can
occur from strong transients, lesser transients can add
spurious information to the memory, causing false switching
of the automation by the controller. These transients can come
from many sources: the power line, switching of tape
machines, and static electricity from carpets.
Pushbuttons on the keyboard eventually wear out and
need to be replaced, but before they fail altogether. they may
become erratic or develop contact bounce. When this bounce is
happening, the contact may send out two or three extra
switching actions instead of the required one; this can add
spurious information to the memory or step the system
erratically.
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Last. but far from least, is human error. Many problems
will be the result of human error in programming the memory
with wrong information. For example, the finger may slip on
the keyboard and hit a 2 instead of a 3 for the source. So when
this step comes around, it calls up and plays source #2 on the
air instead of the #3 which was intended. Always remember,
the memory calls up machines -not programs or announce ments. It is up to the human operator to put the correct
information into the memory and place the correct tapes in the
correct trays of the tape machines.
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Chapter 9
The Transmitter
very important major subsystem in a station is its
transmitter. This is the unit which puts the word broadcast in
the term broadcast station. The transmitter must work
reliably, for without it, the station is out of business.
Reliability requires maintenance. When maintenance is
performed. somewhat different techniques are necessary.
This is because of the unit's RF nature, the dangerous high
voltages involved, and the FCC Rules. It is not enough to
simply make repairs and get the transmitter operating again.
Anytime that the transmitter is in operation, it must comply
with the Rules.
A
BASIC TRANSMITTERS
The AM and FM transmitters have many differences
because of the methods of modulation and the different carrier
frequencies. Even though they are different, they have very
much in common. Both the AM and FM transmitter can be
divided into at least six different systems: RF. the audio
modulation, power supplies, control, monitoring, and cooling.
Each of these systems are divided into a number of
subsystems.
Techniques
Your troubleshooting and maintenance techniques need
some modification when working with the transmitter.
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Remember that you are dealing with RF and high voltages, so
that some methods that work on studio equipment will not
work here. Learn how the particular transmitter works, divide
it up into its subsystems, fine -tune your techniques, and you
can handle transmitter problems as easily as audio equipment
problems.
AM Systems
The purpose of any transmitter is to convert the line power
to an RF carrier so that it can convey the intelligence
impressed upon it by modulation to all locations in the
station's coverage area.
The AM transmitter carrier originates in a crystal
oscillator stage and is amplified to the power output stage,
where modulation usually takes place in the plate circuit. See
Fig. 9 -1.) The modulated -carrier output signal then passes
through a matching low -pass filter network to the transmission
line and antenna.
The crystal itself usually operates at the carrier
frequency. and the stability of this crystal controls the carrier
within the required ±20 Hz tolerance set by the FCC rules.
The crystal is mounted in an oven or a vacuum. Solid-state
exciters use a higher frequency crystal, typically four times
higher than the carrier, and then divide this down to the
carrier frequency. The stages between the oscillator and PA
power amplifier) are straight amplifier stages, and they have
high gain and very narrow bandwidth.
(
1
Modulation
Audio at a standard +10 dB input level is fed to one or
more audio stages for amplification to the modulator stage.
Plate modulation in the PA has been used extensively,
although there are other methods in use. For plate modulation,
the modulators must supply the sideband power. which is
equal to 50% of the RF carrier's average power at 100%
modulation. and more than this when 125% positive
modulation is used. The modulator is simply a very high -power
audio amplifier that is used to control the RF signal in the PA
stage and produce modulation.
FM System
Although the carrier originates in the master oscillator, the
oscillator itself operates at some submulitiple of the carrier
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frequency. This is because the carrier is in the VHF range, and
oscillators at carrier frequency are impractical. The output of
the oscillator then is multiplied until it reaches carrier
frequency. This is done in several multiplier stages, and the
frequency swing is also multiplied. Therefore, these stages
must be more broadband than those in AM. for when a full
carrier is amplified, the stage must be broadband enough to
pass a carrier swing of at least 150 kHz (200 kHz would be
better) .
Oscillator Stability
The oscillator is far different from an AM oscillator. In
most cases, the oscillator itself is modulated. This presents
problems since a crystal cannot be used directly. There have
been several methods used to produce frequency modulation
but with the advent of stereo, most transmitters have gone to
the direct FM. reactance -modulated oscillator (Fig. 9 -2).
Because a crystal cannot be used and the stage is modulated,
its frequency stability is not very good. But indirect control is
used so that crystal stability can be realized. This indirect
control is called an automatic frequency control AFC) system.
The output of the main oscillator is sampled and mixed with a
signal from a reference crystal oscillator to provide an IF
(intermediate frequency) signal, as is done in a receiver. This
IF signal is well filtered to remove audio modulation, and then
converted to a DC control voltage that moves the master
oscillator back on frequency when it drifts. This control
system can maintain the carrier within the prescribed
tolerance of ± 2000 Hz.
(
Modulation
It requires very little audio signal power to provide 100%
modulation in FM. The audio signal will be fed to the exciter
input at the standard +10 dB for monaural. Or a composite
signal from a stereo generator or SCA (Subsidiary
Communication Authorization) signals. if they are used. may
be fed in. These may or may not pass through an audio
amplifier, and are routed directly to a pair of voltage -variable
capacitors. which inversely change in capacitance when the
DC voltage across them is changed. This varying capacitance
is across the oscillator tank circuit, so the audio signal will
modulate the carrier.
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Other Circuits
Power supplies convert the AC power line voltage into
various levels of DC voltage as required. High-power
transmitters require several thousand volts of DC for the
power stages. These may be tube -type supplies, but most are
now solid -state. Except for the voltages involved, they are
somewhat conventional.
Control circuits operate all the various time delays, power
contactors and other relays, and the blower system. This
whole arrangement is called the control ladder, because all
these various relays follow one another in the application of
power to different stages.
Monitoring within the transmitter is done by various
metering circuits. These usually measure only samples of the
actual circuit currents or voltages. Some samplers supply
low- voltage, low- current analog samples for external or
remote control metering.
Rules and Regulations
Many things can be done to the transmitter only after
application has been made to the FCC and approval received.
Any engineer who is required to do maintenance or operate the
transmitter should be well versed in the FCC technical
standards. Many rule changes have been made and it is well to
keep updated on these changes.
Some modifications require formal applications, others an
informal letter to the FCC. and others simply notification to
the radio inspector of the FCC district. Before deciding to
make any change in the transmitter which would alter its
operation in any way. consult the rules that apply in the case.
These will be found in various sections of Part 73 of the FCC
Rules and Regulations.
Installation
Prior to installing a new transmitter give much thought to
the actual physical location. Consider operational factors and
also what is required in peripherals. If the transmitter is a
replacement for one that is already operating, don't plan to
pull the old one out at signoff and have the new one installed in
the same spot and back in operation by signon. This works on
some occasions. but it is also possible that the station will be
off the air for several days. Yet there are managers and
owners who urge this approach to the installation.
296
Externals
Plan for the external connections that must be made.
There will be AC power. audio input equipment, monitoring
equipment. the transmission line, and perhaps a dummy load
and external exciter for FM.
Power
AC power for the transmitter is usually 230V, or higher for
larger transmitters. It is usually three-phase power, although in
some low -power units, it may be single phase. This power
should be run through conduit from the main power panel, and
the wire used should be heavy enough to carry the expected
current draw of the transmitter. The transmitter specification
sheets give the total power required from the AC line at
carrier only and at full modulation. Convert this power to
current demand and select power cable heavy enough to carry
this draw without overheating or producing a voltage drop.
FM has a constant power draw.)
Power Panel
The transmitter should have its own power panel, or at
least its own circuit breaker. The size of this circuit breaker
and panel must conform with the wiring codes for the size of
wire used. If the transmitter must run from a general panel
that supplies other units in the building, such as heaters, make
sure the main breaker on that panel is heavy enough to carry
the additional load of the transmitter. Don't overload the
power panel. or intermittent outages result. If the load of the
transmitter is too much for the main breaker, the transmitter
must have its own panel.
AC
(
Separate Power
For those transmitters (both AM and FM) which use a
crystal or oscillator in an oven, a separate 120V AC source is
required. This can be picked up from the main feed to the
transmitter, but it is better if this is fed directly from the
power panel and has its own circuit breaker. This allows
shutdown of the main power to the transmitter back at the
breaker panel and leaves the ovens operating on their separate
circuit.
Transmission Line
Location of the transmitter in relationship to the coaxial
transmission line is another important factor. High -power
297
transmitters require large- diameter lines. When a dummy
load external to the FM transmitter is used, mount a coaxial
switch so that the switch from line to load is an easy matter
and doesn't require disassembly of the normal line
connections. Positioning of this coaxial switch and all the other
coaxial plumbing should be given careful planning.
Phasors and Filters
When the AM station uses a directional antenna system, a
phasor cabinet is mounted next to the transmitter. More than
one coaxial line goes into this phasor from the antenna system.
The transmitter output itself feeds into the phasor, and the
phasor makes the distribution to the antenna system.
The FM transmitter has an outboard harmonic filter.
These are in different configurations. but many are a rigid
coaxial line section several feet in length. There must be
room for this filter to project from the transmitter.
Racks
Besides the phasor (if used). there is at least one rack to
hold the monitoring and audio input equipment. Such items as
phase monitors. the modulation monitor, audio monitors, and
audio -processing equipment are located here.
The FM transmitter generally has its exciter in an
external rack adjacent to the main transmitter. The reactance
modulator and especially the voltage- variable capacitors are
very susceptible to vibration, so the exciter is mounted in a
separate cabinet.
Grounds
Everything in the transmitter area should be bonded
securely to a heavy building ground. There are strong RF
fields from the antenna outside the building. so there are
possibilities of feedback, and problems in the audio from the
heavy AC drawn. All cabling is to be shielded for the same
reason. The AM transmitter has a remote meter fed from the
antenna base. If a directional antenna is used, a phase sample
of each of the towers is fed back to the control position
(transmitter location). All RF lines should be shielded to avoid
piping these signals into the system.
Cooling and Heating
Consider the transmitter location in relation to the cooling
and heating arrangements for the room or building. There can
298
air ducts, exhaust vents, and air inlets, and these ducts
have to intermingle with coax lines. They should be arranged
so that either can be taken apart for repairs without
be
disassembling the other.
Putting It All Together
During or upon the positioning and connecting of the
transmitter. check out the transmitter components and put all
the pieces back together. How much was removed for
shipping depends upon
the
transmitter and the
manufacturer. Check for relays that have been removed and
plug these back in. There may be power resistors either
removed or tied down. so look for wood or other blocking that
may have been inserted for shipping. Replace all tubes that
have been removed. and make sure they are properly seated
in their sockets.
Mount anything that has been been shipped separately,
such as power transformers, modulation transformers, and
cable harnesses or jumpers. On the power transformers, set
the primary taps of each one for the local line voltages
expected. Although the transmitter is set up and tuned up at
the factory. check to see that taps on RF coils are still in place
and tight. A tap may have popped loose during shipment.
Check for hardware that has been left loose at the factory
or has come off during shipment. Check closely on terminal
boards and across the terminals of voltage buses and
capacitors. Loose hardware may be lying across a couple of
voltage points waiting for you to turn on the transmitter and
blow the circuit breakers.
TUNEUP
There are some important actions that must be taken
before applying power to the antenna during the original
checkout.
FCC Notification
The FCC must be notified before you begin equipment
tests. Both the commission in Washington and the inspector in
charge of the district must be notified. This notification is
informal. Send a duplicate telegram to both offices, stating
that you will begin conducting equipment tests according to
construction permit number so and so, on a certain date. You
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may then proceed with the tests on that date without further
authorization from the FCC.
Experimental Period
The time between 12 midnight and 6 a.m. local time is the
experimental period. This period must be used by the AM
station for equipment tests and regular testing unless it has
special authorization to test in other periods. The FM station
may test during the experimental period without authorization, but it can test at any other time if it notifies the
commission in Washington and the inspector in charge of the
district. In many cases, the experimental period is the only
practical time to test, especially if the station is already
operating a lower power transmitter on the channel. However,
if you are operating on that channel and are going to use a
dummy load, be careful that there isn't enough radiation from
the load or other circuitry to cause interference with the
operating transmitter. Go out and listen on a receiver for beat
notes on the regular station carrier. If there is interference.
then do the testing after signoff. Unless testing is done with a
dummy load, the AM station must do its testing during the
experimental period.
Best Efficiency
All the RF stages should be trimmed up in tuning and the
antenna used as the load for the final testing. When you tune into
the antenna, adjust for the best match to the load and the best
efficiency of the PA stage. Adjusting for the best efficiency
encompasses setting the input drive to the PA. matching the
load, and tuning the plate circuit of the tube or transistors).
1
AM Power Amplifier
Start the tuning by setting the controls to the markings
shown on the factory checkout sheet. These are factory -tuned
into a dummy load, and your antenna system has a slightly
different load condition. So dip the plate current and adjust the
drive and loading to obtain the required antenna current for
the station's authorized power. Compare the readings obtained
with those of the checkout sheet, and if comparable, go ahead
and tune for best efficiency. This is called a unity power factor
condition and does not occur at the resonance dip in plate
current. Observe the antenna current meter and slowly adjust
300
the PA plate tuning toward the capacitive side of resonance
(more capacitance in the circuit). The antenna current will
continue to rise without a corresponding rise in plate current
for a short span of the tuning. Continue tuning in the same
direction until the antenna current ceases to increase and the
plate current begins to increase. This is the most efficient
point of operation. Compute the PA efficiency by calculating
output power divided by plate input power (voltage and
current), times 100. If your tuning does not result in the
efficiency as shown on the checkout sheet, there is something
wrong. Check the remote antenna meter calibration before
taking further action.
FM Power Amplifier
The output stage of the FM transmitter is different in its
nature and tuning. It is a broadband stage, and tuning
indications will not be as sharp as in AM. Standing waves on
the transmission line and antenna present a somewhat
different load than a dummy load. When first applying power
into the line, check the VSWR on the line. If the indicator
shows the standing waves to be low, then tune up. Set all the
controls at the factory checkout settings. Normally, the PA
input, plate tuning, and loading are tuned for a maximum
power output rather than for a dip in plate current. Power
output indication and efficiency depend upon some other
important factors. If the output indicator is a properly
calibrated reflectometer, then it will indicate true power; but
if it is simply a sampling loop in the transmission line, it only
indicates that RF is present. When the indirect power
measurement method is used for operation, then the plate
input power must be set for the prescribed efficiency, and not
what can be realized.
Efficiency
The type of tubes and the circuit configuration of the
output stage has much to do with the tuning, the indications,
and the efficiency. Check the manual or schematic to learn
about the transmitter you are to tune. When the output is a
tetrode in a normal configuration and the screen current is
metered. the screen current is the best indication of matching
of the load and overall efficiency. When the load is properly
matched. the plate circuit resonated, and the grid circuit
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peaked. the screen current will be at its lowest reading.
Under these conditions. when the grid current is lower than
required for a given plate input and output power. the stage
efficiency is better than when the grid current is high. This is
an indication of how much drive is required to get the power
output.
Grounded-Grid PA
Many FM transmitters use a triode in a grounded -grid
configuration for the output stage. This is a little trickier to
tune up and the indications are not as clearcut. In this case, the
IPA ( intermediate power amplifier) driver stage also supplies
part of the output power. The input tuning to the PA is actually
the IPA plate tuning, so both stages must be at their peak to
obtain the best output efficiency. Tune the PA LOADING and
TUNING controls and the IPA TUNING controls for
maximum power output. For best efficiency, consider not only
the PA plate input power (voltage and current), but also the
IPA plate current. If this is very high. the PA is actually
supplying less power than it should. Try to tune for best PA
efficiency at the same time the IPA efficiency looks good. This
will give the most efficient operation.
POWER MEASUREMENT
PA efficiency and transmitter power output depend upon
the load itself. How the power output is measured and the
accuracy of the instruments and the methods are the basic
factors in determining true power output of the transmitter.
True radiated power. however, must consider the efficiency of
the radiating system. and the only way that it can be
determined is through field strength measurements. For
logging purposes. the transmitter and antenna parameters are
used for indications.
AM POWER
The AM station is required by the FCC to measure
transmitter power output by the direct method Fig. 9 -3), and
this must be maintained within the prescribed power tolerance
of 90 percent to 105 percent of authorized power. Power
measurement is based on the regular power formula.
P = I2 R. This power is measured at the input to the antenna in
a single -tower station. or at the input to the common feed point
when a directional antenna is used. The current 1 is measured
(
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Fig. 9-3. Direct AM power measurement. Power is measured at base of
single -antenna system or at common feed point to a directional system.
across the resistance R of that point. To obtain the resistance
factor, a measurement must be made with an RF bridge. Both
the resistance of the antenna and the reactance are measured.
This requires special instruments and engineering knowledge,
and the measurements obtained must be filed with the FCC. If
there has been any change in the antenna -for example,
adding an FM antenna at the top of the AM tower -then a new
measurement of the base resistance must be made and filed.
Any reactance that is present must be neutralized by tuning
units to provide a good match, so that only the resistance
factor remains. This is the resistance for the power
measurement, and this figure along with the required current
will be shown on the station license. Incidentally, the latest set
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of measurements must be kept available for the FCC inspector
to look at during an inspection tour.
Meters
The meter at the base of the antenna (or common point) is
a thermocouple-type instrument. It operates on the principle
of heating by the RF signal, but since heat is the measured
factor. ambient temperature also affects the indication of the
meter. It would seem that in this day and age, a better way
would be designed to measure the AM power. At the same
time, it is worse than pulling teeth to get a meter
manufacturer come up with figures as to how much ambient
temperatures affect the indication. One manufacturer did
show this effect as a 0.9 percent decrease in the indication for
each 10 °F rise in ambient temperature, and the reverse of this
for temperature drops. Since when outdoor temperatures rise
the indication is lower on the meter, this tends to show the
transmitter efficiency as being lower in the summer than in
the winter.
Calibration Charts
The antenna meter must be calibrated by the factory or a
special meter shop and its accuracy certified. A calibration
curve is supplied with the meter (or should be). The accuracy
must be within two percent of full -scale reading. The station
should also develop a temperature chart unless it can control
air temperatures in the vicinity of the thermocouple) and post
this along with the calibration chart in the tuning house. When
calibrating the remote meter against this base meter each
week, or when reading the base meter itself, the indication
should be modified to take into consideration both the
calibration curve of the meter and the temperature chart to
arrive at the true current in the circuit.
(
Expanded -Scale Meter
A station which operates at two different powers should
use an expanded-scale meter so that both antenna current
values can be indicated on a single meter. This would be the
case for stations with a different day and night power.
However, it must be a special meter that has been certified by
the manufacturer to meet FCC Rules. Simply any
expanded -scale meter will not suffice; make sure the one you
use has FCC approval.
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FM POWER
There are two methods permitted for power measurement
in the FM station, indirect and direct. The power tolerance is
the same as in AM, 90 percent to 105 percent of authorized
power.
Indirect Method
This is the oldest method and is still common today. The
PA plate input power times an efficiency factor is used to
determine transmitter power output. The plate input power is
the product of the PA plate voltage and the PA plate current.
The efficiency factor which must be used is supplied by the
manufacturer, and is found in the instruction manual or a
chart supplied with the transmitter. This efficiency is not what
you may be able to realize by fine tuning, but a fixed factor.
This factor must be available to the FCC inspector upon
demand. Stations have been cited when this factor was not
available because the manufacturer did not supply it. Make
sure you have this on hand.
Power Tolerance
Remember that the station must stay within the
prescribed power tolerance. When line voltages change or
there is some change in the antenna which affects the output
stage tuning, a different plate input power will result and the
operator must compute the power to make sure the station is
within tolerance. To save a lot of later computation, do this:
Compute the plate input power for different values of plate
voltage change, perhaps 200V or 400V higher or lower than
normal plate voltage, and compute the corresponding plate
current required at each of these values. Also compute the
tolerances according to input power. This takes a lot of computation at one time but saves a lot later. Pot these values at
the operating position. A quick glance then shows what
current should be present for a given plate voltage and
whether the combination is within the power tolerance.
If these figures are not available and the transmitter goes
out of tolerance, the station is open to a citation from the FCC
either during an inspection or at license renewal time. People
at the FCC do check the logs submitted at license renewal
time, and if the figures show out -of-tolerance operation, there
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can be citations and even problems in getting the license
renewed.
Direct FM Power Measurement
The direct method requires more accurate output
measurements and calibrations Fig. 9 -4) . To accurately
measure the power. a directional coupler is used in the
transmission line. This device picks up only the forward wave
and feeds it back to a rectifier and then a meter (which must
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Fig. 9- 4.Direct power measurement
for FM. Power
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be calibrated). This directional coupler is far different than
the old loop pickup in the line, which has been common for
output meters. The loop will sample everything that is present
at that point -both forward and reflected waves. The
directional coupler, however, picks up only the forward or
reflected wave (according to insertion method), and this is
what makes it accurate for direct power measurement. The
meter arrangement and two couplers used to measure both
forward and reflected power. is called a ref lectometer. Power
output depends upon an accurate load of pure resistance
without reactance. The meter must be calibrated against an
outside standard, a wattmeter. which itself has been
calibrated at the factory.
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Power Meter Calibration
Terminate the transmitter with the dummy load and move
the wattmeter into a position where it can be read accurately.
Make sure the meter's mechanical zero is correct. Make sure
the cooling system of the dummy load is in operation, or the
dummy load will soon be damaged. Turn on the transmitter
and adjust to the authorized power output as read on the
wattmeter. If the authorized power is 10 kW, this is what the
wattmeter indicates, although the meter on the transmitter
may read anything at this point. Use the CALIBRATION
control on the transmitter and adjust the meter to read 100
percent. Write down the plate voltage and plate current of the
PA stage. Then raise the transmitter power to 105 percent as
shown on the wattmeter. For 10 kW, this is 10,500W. Check the
reading of the transmitter power meter; it should be 105
percent. If it is not, do not change the calibration. Instead,
mark the meter face to show where 105 percent appears.
Record the plate voltage and current at this power. Then do
the same for 90 percent power. This will be 9000W on the
wattmeter for 10 kW). Again record the plate input power.
Lock the CALIBRATION control so that it can't be accidentally
misadjusted. Now turn off the power and switch the
transmitter back to the antenna. Go through the same
procedure at 100 percent. 105 percent. and 90 percent, and
record the plate voltage and current for each. All of these
readings must be entered on the station's maintenance log.
When switching back to the antenna, there should ideally
be no change in the readings from what was obtained with the
dummy load. But we live in the real world and there is some
reactance in the antenna system. If the change is very great,
there is high VSWR on the line and other problems that need to
be corrected. Even a small VSWR changes the readings
slightly, since you are not as well matched to the dummy load,
but are matched to the line conditions.
(
Required Calibrations
When the direct method is used. the output meter must be
calibrated as discussed. This procedure must be done at least
once every six months, as required by the FCC, and the results
entered in the maintenance log. This is the minimum
calibration required, and the FCC man will want to see this
information on an inspection tour. How well the meter holds
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calibration is something that is learned by experience with the
particular unit. The calibration process should be done more
often than required, until experience has shown that it isn't
necessary. If someone should accidentally turn the
CALIBRATION control at some other time, the calibration
must be done over. Keep that control locked.
FREQUENCY MEASUREMENT
Anytime the transmitter is connected to the antenna
system. its carrier frequency must be within the prescribed
tolerances of the rules. This is ±20 Hz for AM, ±2000 Hz for
FM. Anytime the carrier is out of tolerance and an FCC
inspector or monitoring station measures the carrier in that
condition. the station will be cited for violation of the rules.
There is also a requirement that frequency measurements of
the carrier be made at least once each calendar month. The
interval between measurements may not be more than 40
days.
Just how the measurement is done is up to the station, but
it must produce accurate results and must relate to signals of
the National Bureau of Standards or to station WWV. In other
words, the measurement equipment must be calibrated
against WWV or Bureau standards. The old-style frequency
monitor that was required for years cannot be calibrated in
this manner and thus is not acceptable for the measurement.
There are two ways the measurements may be made that will
comply with the FCC: by an outside frequency- measuring
service on a contract basis, or by a station -owned frequency
counter that can be calibrated either directly or indirectly to
WWV.
Outside- Service Measurement
There are a number of frequency- measuring services that
will measure the station's frequency on a monthly basis. When
possible. there should be direct telephone communication with
the laboratory when the measurement is made. In some cases,
when distance is too great or there are interfering signals on
the channel, the measurement service will send out a portable
unit on a regular tour. In this way. they can get good signals
without interference, but in these portable setups, there may
be no telephone handy. In any case, the service should call the
station if they measure the carrier either out of tolerance or
308
drifting close to the edge. so that it can be adjusted to
authorized frequency.
Frequency Counter
A variety of accurate counters are now available at
reasonable cost. The counter must be calibrated either by the
station or an outside laboratory at sufficient intervals to insure
its accuracy. To be calibrated locally, the counter must have
an output signal from its internal oscillator for this purpose.
Many counters do not have this feature, or enough stability or
accuracy to meet FCC requirements. Those that do will
normally have a temperature- compensated oscillator, and the
spec sheets will state that it meets the FCC requirements.
Often, one of the regular counters is modified with this type of
oscillator so that it will meet the specs for FCC measurements.
This is done by the factory. as is calibration.
Calibration
You can check the calibration (or recalibrate) against the
signals of WWV with a communications receiver if the signal is
sufficiently strong. It isn't possible to measure the carrier of
WWV. because the local -oscillator signal in the receiver is too
strong. and the counter locks onto the strongest signal. You
can either beat the counter oscillator with the WWV signal or
use the receiver BFO (beat- frequency oscillator). In either
case. arrange the setup so that the output from the counter
is comparable to the WWV signal strength. If the counter signal
is too strong. it will be difficult to find the exact center of zero
beat. To use the receiver BFO. tune in WWV and use the
narrowest IF filtering the receiver will provide, and use the
BFO to get a good beat on that signal. Remove the antenna and
couple in a signal from the counter oscillator. If there is a beat
note. the oscillator is off calibration slightly, so adjust its
trimmer to bring it right into the center of zero beat. Check
back and forth a couple of times, as the BFO may drift on you.
To beat directly against WWV, tune in the signal as before.
but turn off the BFO and couple in a small amount of the
counter signal. Listen for a distinct zero beat between the two
signals. If there is a beat note, adjust the counter trimmer to
get right in the center of zero beat. Make that final trim during
the period that WWV is not sending voice or tone modulation.
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Logging
The frequency measurement that is made must be entered
in the station's maintenance log. The entry made must be of
the actual frequency that is measured. and not the deviation
from assigned frequency. Thus. if the FM center frequency
were 200 Hz below the assigned frequency of 104.1 MHz, the
counter would read 104099.9 kHz, and this is the figure that is
logged. If you wish, you may show the difference from
assigned frequency for your own information, but the required
entry is the actual measured frequency.
Both reading the counter and logging the frequency can
cause confusion. How many digits the counter displays
depends upon the time base selected and the number of
indicators provided. And at FM frequencies, to read or resolve
all the way down to one hertz requires more indicators or
readouts. What happens. however, is that the numbers which
cannot be displayed slide to the left into overflow positions,
and an indicator comes on to alert the operator to the fact. In
our example above, to indicate all those numbers requires
seven readouts on the counter. To resolve down to one hertz,
the counter would have to have at least two more readouts. But
with a seven-readout unit, when the time base is changed to
read in hertz. the display shifts to the left, and the first two
numbers on the left go into overflow, allowing two more
numbers on the right of the display to be seen. The numbers
would be displayed thus: 4099600 Hz. At first glance. this
number doesn't seem to have any relationship to the station's
carrier frequency of 104.1 MHz. but the leading digits 1 and 0
are implicit.
It is easier to determine that a carrier higher than its
assigned frequency is out of tolerance than if it is the same
number of hertz below. For example. when our 104.1 MHz
carrier is higher than assigned by 2150 Hz, the counter will
read 104102.1 kHz. This is obviously more than 2 kHz higher
than tolerance. But now assume it is in the other direction;
then the counter would display 104097.8 kHz. This is not quite
as easy to evaluate at first glance. And if the measurement
started out to measure with a time base for hertz resolution,
the reading would be 4097850 Hz. To avoid confusion, here is a
technique that can be used. Before taking the measurement,
write down the carrier frequency out to one hertz. For
example. 104.1 MHz would be 104100000 Hz. Then take the
310
measurement, starting first with the megahertz time base to
make sure the correct carrier is measured, and then change
the time base to read out in hertz. Write down the
measurement directly under that which you have written out
for the assigned frequency. Add or subtract as needed, and this
gives the difference from the assigned frequency and shows
whether the station is out of tolerance or not. Write down on
the log the actual measured frequency.
SIGNAL PROCESSING
Audio signals are always processed before they are fed to
the transmitter. Similar units are used in the control room, in
recording booths, and on remote lines. However at the
transmitter, the regular AGC amplifiers and peak limiters are
more sophisticated. This is because modulation processes
offer different problems and they are different in AM and FM.
Basic Processors
In the basic AGC amplifier (Fig. 9 -5). the audio signal
passes through a variable -gain stage which is controlled by the
audio itself. The audio signal farther down the line is rectified
and converted to a DC control voltage that changes the stage's
gain according to the signal levels. The time constant of the DC
filter circuit determines the rate of change in gain that will
occur. A long time constant causes slower action. This
controls the average level of the signal and is too slow to
follow peaks.
The regular AGC amplifier has many drawbacks. so a
gated unit is used. This allows expansion of signals to take
place only within certain ranges. and very low signals are not
expanded. By properly setting the operating position. the
same unit can act as a compressor also. This gives greater
control over the average signal levels.
Peak Limiters
The basic peak limiter is the same as the AGC amplifier,
with the exception of the time constant. This is made much
shorter so that the control voltage can follow signal peaks.
However, no expansion is used in a limiter, as only peak
compression is desired.
Both the AGC and limiting action are often combined in a
single unit. You will find modern units in both styles.
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Processing
Recall from an earlier chapter that the peak level can be
many decibels higher than the average level of a program
signal. It is the peaks that cause overmodulation. The highest
possible modulation that can be obtained on negative peaks is
100 percent. At this point, the audio modulation amplitude
equals the RF carrier amplitude. Any increase beyond this and
the carrier is actually shut off during that negative peak
period. This severely distorts the audio signal and creates
"splatter" outside the station's channel. Both are FCC
violations.) Since the signal peak is the controlling factor, the
average of the signal is low; and out in the fringe areas, the
signal will sound weak and not override receiver noise or
interference.
The AM station uses both a peak limiter and an AGC
amplifier. They may be separate or single units, but both
functions are employed. The AGC holds the average
modulation level high. and the peak limiter prevents
overmodulation. This combination prevents overmodulation and increases the sideband power to override noise and
interference in the fringe areas. This station's signal will
sound louder. There is a tradeoff in dynamic range for
increased sideband power.
AM
(
Asymmetrical Modulation
This term simply means that the positive and negative
modulation peaks are not the same amplitude Fig. 9 -6) It was
discovered long ago that the human voice does not produce
symmetrical peaks and these vary from one person to another.
Consequently. when the high peak is in the negative direction
at the modulator. the average signal is lower than normal. A
device that many stations use electronically equalizes these
peaks before they are fed to the transmitter. and many
stations use them. Modulator unbalance as well as audio
unbalances contributes to the same effect.
(
.
Forced Asymmetrical Modulation
Voice peaks are asymmetrical, and they arrive at the
modulator in a random fashion. But the highest peak can always
be shifted in the positive direction, and a consistently higher
modulation level is achieved in both the negative and positive
direction. Until recently. there was no limit set on positive
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ASYMMETRICAL AT 125% POSITIVE
MODULATION
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modulation peaks are unequal. Symmetrical modulation would have both
peaks equal.
modulation-only the negative. Now the FCC set a limit of
125 percent in the positive direction.
AM limiters for transmitter use have circuitry that senses
the highest audio peak and reverses its polarity if necessary,
so that the highest peaks are always positive. By adjusting the
negative peaks to no more than 100 percent, the positive peaks
can be higher than 100 percent modulation.
When the modulation on the positive peak goes past 100
percent the sideband power is increased also. Consequently
an additional processor may be used that deliberately creates
this asymmetrical condition. even by clipping if necessary.
This pushes the positive peak modulation to the 125 percent
maximum. The output connections of such a limiter are
polarized and must be connected correctly to the transmitter.
If they are reversed, the high positive signal will go negative
and severely overmodulate the transmitter.
FM Processors
The AGC amplifiers for FM are essentially the same as
those for AM. When used for stereo, however, the two units
should have identical characteristics. They are to be ordered
as a stereo pair and tested together at the factory for stereo.
This usually carries an additional fee, but it is worth it. The DC
control circuits are to be tied together so the two units are
synchronized. That is. the one with the highest amplitude
314
signal takes over and controls both channels. This maintains
the original balance between the two channels.
Limiters
The FM system presents a different modulation system
and also a problem because of the 75 sec preemphasis curve
(Fig. 9 -7). The 75 sec preemphasis is prescribed by the FCC
to overcome the energy distribution in the audio bandpass and
the noise problem in the receiver. A different approach must
be taken to the limiters. In some cases, the limiting is done in
the usual fashion, but the signal is also run through a 75 sec
preemphasis network. After clipping, the signal is run through
a complementary deemphasis network so that the effective
overall response of the unit is flat. The clipping causes some
distortion; how much depends upon the energy content in the
signal. Stations with a rock or similar program format often
use both limiters and AGC amplifiers in the same manner and
for the same reason as AM stations -to hit the modulation
hard. sound loud, and still not overmodulate. However, a large
part of the dynamic range is traded off for loudness.
The big problem is the 75 µsec preemphasis gives a
tremendous boost to the upper audio frequencies. Modern
programming contains a considerably greater high- frequency
content than when FM service was first approved years ago.
While the preemphasis boosts the high audio, the modulation
meter responds to a greater extent to the low- frequency,
high-energy audio. To modulate the transmitter at 100 percent
with these low- frequency peaks as read on the meter, the
high- frequency audio causes severe overmodulation that is not
indicated on the monitor. That modulation will show up on a
spectrum analyzer, however. To avoid this overmodulation, it
is necessary to modulate only 35 percent to 40 percent as
shown on the modulation meter (there are peaks, remember).
Out in the fringe areas, there are more problems from noise,
possibly a six to eight decible loss in signal /noise ratio.
Compared to a station using very heavy processing, this
station will sound rather weak. But many "good music" and
"classical" stations do go this route, as they desire the
dynamic range.
Dolby B System
For FM use, the Dolby B system provides an alternative to
the heavy -processing route ( Fig. 9 -8) This is now permitted on
.
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FM. This unit uses controlled expansion of the upper audio
frequencies. The audio is fed through a high -pass filter that
has a ramped lower skirt starting at 200 Hz. Only the very
lowest amplitude signals receive the expansion at its
maximum. There are varying amounts of expansion up to 10
dB. Those on the ramp receive correspondingly less, as do all
within the filter's bandpass. according to their amplitude. The
higher level signals receive none.
All this filtering and expansion takes place in a side
channel, and the expansion is controlled by the upper
frequencies themselves. The original signal in its entirety
passes right through the unit, but the processed part out of the
filter /expander is then added back to the main signal. This
combined, processed signal is then sent through a rolloff filter
that will produce an effective 25 sec preemphasis in the
overall modulation system. The transmitter time constant is
not changed; the processed signal is simply rolled off more, so
that when the two are combined, the net result is a 25 µsec
time constant.
When a station uses this system, it must be used in its
entirety; both the expander section and time- constantchanging section must be used together. The FCC does not
permit the station to change the time constant alone.
A station that does not use highly processed audio can now
modulate to 100 percent as shown on the monitor, without
overmodulating. But some peaks can still get through, so peak
limiters used with the unit prevent any from passing through.
On the receiving end, to gain the most advantage, the
receiver should be equipped with a decoder. This unit presents
both a 25 psec deemphasis and compression to restore the
original program balance. A receiver without a decoder has a
75 µsec deemphasis. and this provides greater rolloff of the
high- frequency audio, so the tone control will require some
adjustment. The higher audio tones also are expanded, does
not appear to bother the listener.
Setting Up Processors
Whatever the intended use, processors are set up with
tones as a starting point. Remember, there is a difference
between program peaks and the tone signal, so this must be
considered as only preliminary. Make the final adjustments
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with programming or program -type signals, and on the
transmitter itself. Another thing to remember is this: the
processing takes place inside the unit and presents a processed
output at its terminals. This output is adjustable. Unless this
output of the limiter is set in proper relationship to the
transmitter. improper modulation levels occur. So adjust the
output of the processor while observing the modulation
monitor. On the AM system, an oscilloscope monitor is best.
Asymmetrical Identification
The polarity of the signal into the transmitter must be
determined in AM when asymmetrical modulation is used.
This is somewhat difficult to do with programming, so use a
sine wave or other repetitive signal in which the two peaks can
be changed so they can be identified. Here is a little trick that
is used on a regular audio signal generator. Shunt a diode
across the output terminals of the signal generator. Add an
adjustable resistor in series so you can adjust the amount of
clipping. A single diode will clip one of the peaks. It doesn't
make any difference how you feed the output to the limiter, as
the sensor will switch the nonclipped peak to the positive. Use
an oscilloscope to monitor the amplitude -modulated carrier.
and note the position of the peaks. If the nonclipped peak is in
the positive direction, the connections are all right. But if it is
LEFT
AGC
CONTROL BUSES TIED TOGETHER'
RIGHT
AGC
Fig. 9-9. Stereo AGC amplifiers or peak limiters should have the pairs
synced together. The channel with the highest levels will control both
channels.
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in the negative direction. toward carrier cutoff, reverse the
output leads of the limiter. Mark the output leads and connect
to the transmitter input. If you desire to actually know which is
positive and which is negative. then use the scope and look at
the audio output of the limiter.
GENERAL MAINTENANCE
earlier, due to the RF technology and high
voltages in a transmitter, techniques must be developed so
that the troubleshooter can perform maintenance with safety
to himself and the equipment. The transmitter helps in this
matter, for it is usually well metered and conditions at various
points inside can be determined from the front of the
transmitter. With a knowledge of the transmitter's workings
and circuitry. these metering positions are very helpful in
isolating problems to certain stages in a section. Another
important thing to know are the limits, not only of operation
FCC tolerances). but also the limits of individual tubes and
circuits. Without a knowledge of individual circuit
parameters. the operator may be overworking stages until
suddenly they fail catastrophically. For example, if he is
concerned only with maintaining the output stage within its
parameters. and it is working inefficiently, the IPA stage may
be overworked. exceeding its ratings.
As mentioned
1
High- Voltage Precautions
Working around high voltages requires special techniques.
Capacitors can retain a lethal charge long after power has
been removed. Precautions must be taken in wiring and
insulation to prevent arcovers and ionization problems. But
the first order of business is personal safety. When you must
go inside the transmitter for maintenance, know what you are
doing. keep a cool head, and don't become careless. Develop
good safety habits around the high- voltage circuits.
Confidence and familiarity with the transmitter are one
thing. but when an engineer has worked around a particular
transmitter for some time he must be careful not to become
overconfident. There is an old saying "familiarity breeds
contempt." but contempt for high voltage should never be
allowed. These voltages are lethal, and one mistake can be the
last one.
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High-Current Precautions
Certain precautions should be maintained when working
around high -current circuits, for they have their own brand of
danger. As more transmitters go all solid- state, high current
becomes predominant in developing the RF power output. This
has been in trend in high-frequency RF circuits for the past
several years anyway: that is. tubes are using lower voltages
and higher currents.
Be especially careful with metal tools, rings, or other
metal objects on the person when the voltage is on. These
very- high -current circuits aren't much different from an arc
welder. If you should accidentally brush a ring across a
high- current circuit, it can instantly weld to the circuit and
turn white hot. You can't get away from it, and it can burn
clear to the bone. Further, high current generates much heat
in the components through which it flows, so be careful not to
touch them until they cool. Remember to make all circuit
connections having very low resistance, so you don't create a
heat hazard.
Basic Transmitter Divisions
As with the general functional divisions of a transmitter
can be divided transmitter maintenance itself transmitters
into four general categories: mechanical, electrical.
electronic. and monitoring. Mechanical includes the cabinet,
terminal boards, hardware, mountings, insulators, and
blowers. Electrical includes the control ladder, relays.
contactors. power supplies, and AC power source. Electronic
includes the audio, RF, and modulation circuits: RF and audio
filters: and all special circuits that generate the carrier and
impress audio intelligence upon it for modulation. Monitoring
includes the transmitter metering of parameters. samplers,
other external metering, and also signal monitoring, such as
modulation and audio monitoring.
Mechanical Maintenance
Blower motors and the amount of cooling air necessary
creates vibrations and noise. This is both acoustical and
electronic noise. Heating and cooling of parts will cause
expansion and contraction. The net effect of all this is that the
hardware tends to loosen up in the transmitter, both in the
cabinet and the connections and terminal boards.
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general routine should be set up, based upon experience
with a particular transmitter, to check out and tighten all the
hardware and connections. Experience soon show those places
or components that tend to loosen up more than others, so
more attention is given to the trouble spots before something
falls apart.
Loose hardware causes all sorts of problems. For
example. a door with an interlock works loose, allowing the
interlock to open and taking the transmitter off the air. If the
transmitter is several miles away from the control point,
much air time is lost.
Loose hardware in RF circuits causes arcing and burning
of the contacts. If the hardware is at the grounding point, then
parasitics. self -oscillation of a stage. feedback, or simply
detuning and inefficient operation develop.
Blowers -The air -cooling system is important and it should
be smooth running and quiet. The motor and fan bearings must
be oiled or greased regularly, unless they are sealed bearings.
Follow the instruction manual for the particular unit. Even
sealed bearings will eventually fail.
Belts must be inspected to ward off sudden failure. They
wear, and should be changed in due time. Also, the belts should
be adjusted properly: too tight. they cause excessive bearing
wear and vibration: and too loose, they slip and the air
movement will not be sufficient as it should be.
When adjusting belts, set them according to the instruction
manual. In general. without the blower running, you should be
able to push on the belt in the center between pulleys and
depress it about one -half inch. If you can't budge it. it is too
tight. and if you can press it down an inch or so. it is far too
A
loose.
Bearings -Bearings that are worn or dry are noisy and
produce vibrations. Now when there are a number of bearings
reasonably close together, it is difficult to detect which is the
faulty one. A technique that can be used is an old mechanic's
trick Fig. 9 -101. Take a long screwdriver and hold the blade
against the bearing in question. and put the handle to your ear
Be careful not to get tangled up in the rotating machinery or
fan blades. You can hear distinctly a bad bearing through this
mechanic's stethoscope.
Air Flow-Air acts in peculiar manners. It is easier to pull
air than to push it. Actually, a blower creates a
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FAN OR
MOTOR
PULLEY
BEARINGS
SQUIRREL CAGE
a
BELT
Fig. 9-10. Use a long screwdriver as a stethoscope to detect bad bearings.
low- pressure zone at its input and pushes
air at its output. but
back pressures can build up rapidly and reduce the flow. So it
is important to keep the path clear of obstructions. And keep
ducts tight. or the air will escape.
Air filters are directly in the air flow to filter out dust, and
these do offer resistance. As dust builds up, greater resistance
is presented. and the air flow drops. So filters should be kept
clean. If they can't be cleaned, they should be replaced. Some
filters are designed to be coated with oil. I can't vouch for their
effectiveness. because they will eventually have everything
inside the transmitter coated with a film of oil that collects
dust and cooks into a gummy residue. I do not recommend
these filters at all. If they are on the transmitter when you get
it. replace them with dry filters.
Fan blades and squirrel cages will coat up with a layer
of dirt after a while. and this reduces the blades' efficiency.
Clean these before the dirt can build up.
Air Interlocks -Most high -power transmitters and many
low -power ones) will have an air interlock on the air system
Fig. 9 -11). Anything that reduces the air flow and pressure
below the preset value causes the interlock to open and shut
the transmitter down. These are protective devices and should
not be bypassed. except in an emergency and you are sure the
air flow is high enough. Without adequate air flow, expensive
power tubes can be very quickly damaged.
Besides the air filters themselves. check for leaks in the
air system and that the air ports in power tube sockets or the
(
(
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LEVER CONTROLLED
BY AIR FLOW
AIR
DUCT
SWITCH
SERIES WITH
TRANSMITTER
INTERLOCKS
IN
AIR PRESSURE
Fig. 9-11. Air interlock operation depends upon enough airflow In the
duct.
cooling fins of power tubes are not clogged with dirt. In spite of
the best working system. all of these things happen over a
period of time. When there is reason to have a power tube out
of its socket. take the opportunity to clean out the socket and
tube fins. Use a vacuum to get what can be in this manner, and
then use a blast of air from an air compressor or a blower with
a small -port nozzle to blow the openings clear. Blow back
through the tube fins or the socket ports in the opposite
direction of the normal air flow.
Cleanliness -Even with air filters and tight cabinets, dust
gets inside and settles on terminal boards, contacts, and
insulators. High voltages attract dust particles like a magnet.
All high-voltage terminals and insulators must be cleaned and
kept that way to prevent arcovers. When an arcover does
occur, a carbon path is formed that is very difficult to remove.
In many cases, the insulator or other component must be
replaced. for if the carbon cannot be removed, it arcs over
again. Clean ceramic insulators with alcohol or clorothene.
Minor cleaning can be done with a clean cloth dampened in
water, but before applying the high voltage, let the blowers
and cabinet fans run some time to make sure they are dry. Be
careful about the cleaning fluid used when the insulator is
324
polystyrene or other plastic. Many solvents will melt or fog
them. Use only a mild cleaner, and in most cases, stick with
water alone.
Electrical Maintenance
The control ladder will usually operate on either 120 or
230V AC and comprises many interlocking relays and
contactors. The contacts of many of these relays carry heavy
currents, so they are subject to arcing, burning, or welding
together. For normal cleaning, use a burnishing tool or a fine
emery cloth. These just polish and clean the contacts. Avoid
using a point file, as this removes too much metal and changes
the shape of the contacts. When burning or pitting does occur,
(Fig. 9 -12). then stronger measures are required. Use a point
(A) NORMAL
MOVABLE ARM
CONTACTS
STATIONARY ARM
(B) BURNED
MOVABLE ARM
H.----CONTACTS
STATIONARY ARM
Fig. 9-12. (A) Normal contacts are shaped to make a good, low- resistance
contact. (B) When contacts have been burned, they make a poor contact.
file and hone off the damaged area, but try to reshape the
contact. That is. don't file it slat. as most contacts are somewhat crowned in the center. Contacts pitted so badly as to
require reshaping will not last long and should be replaced as
soon as possible. The majority of power contactors and large
relays are designed so the contacts can be replaced, and
spares should be kept on hand. The smaller relays do not
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usually have replaceable contacts, so these you have to coax
along, since replacement usually means the entire relay.
Primary AC- Power cable connections between the
transmitter and power panel should be tight. These carry high
currents. and a loose connection will heat up. These cables
should be heavy enough that they do not run warm with the
normal transmitter load. After signoff and immediately after
shutdown, feel the cable near the connections, both at the
transmitter and at the power panel. This can be done during
programming time if the cables are accessible. The objective
is testing the temperature of the cable before it has time to
cool off. Feel the cable within a foot of the connection. If there
is a loose connection and it is heating, the cable will be warm
or hot. Heat from the joint is conducted into the metal of the
cable, so it isn't necessary to actually feel the connection
itself. Also look for discoloration of the insulating material on
the cable at that point, as that is another indication of
overheating. If there is heating, take the connection apart with
the power off. If it still looks clean and bright, tighten
everything up. If the wire looks discolored or the connector is
discolored, you may save it by cleaning and tightening, but it
can require remaking the connection.
Balance -The load of the 230V single phase and the three
legs of 230V 3 -phase circuits should be balanced.
During the initial installation, care should be taken to
distribute the load equally. Although this may have been done,
at later dates additional equipment may have been added, all
on the same leg. When the power distribution becomes
unbalanced in this manner, there can be problems from hum,
because one side of the circuit is overloaded. To avoid this
measure the current in each leg of the system with a
clamparound AC ammeter. This shows the current draw in
each single wire. If you discover that the system has become
unbalanced, redistribute the load by shifting some of the units
over to other legs.
Circuit Breakers- Inspect the power panels and feel
the front of the breakers. If one is running warm, expect some
outages. Measure the current against the rating. If the rating
is adequate. the breaker itself may be going bad. It should be
replaced at the earliest opportunity, before it shuts down some
important item during broadcast time.
High -Voltage Supplies -A number of shorting devices are
usually supplied which will either short out the high voltage
326
when a door is accidentally opened or discharge capacitors
after the high voltage is turned off. Some transmitters also
supply a grounding stick, which is simply a wooden handle that
has a metal hook on the end attached to a heavy ground strap
(flexible cable). Whenever you work on high- voltage circuits,
don't simply rely on automatic devices to short out the supply.
Use the grounding stick, and touch it to all exposed
high- voltage points, such as the capacitor terminals Fig.
9 -13). Leave the stick on the high -voltage bus until you are
finished in the cabinet.
(
HIGH VOLTAGE
CAPACITOR
GROUNDING
STICK
Fig. 9-13. When working in the transmitter, hang the grounding stick on
high -voltage capacitors.
The grounding devices should be inspected, for remember
they are safety devices and should work all the time. With
power off check the grounding device on the doors or entries to
high -voltage compartments. This may be a device that falls by
gravity onto the bus, or it may a spring- loaded device that will
ground the bus when the door is opened. Physically inspect to
see that these do what they are supposed to. Something may
cause them to hang up, so work them by hand and see if they
operate properly. Also check the contact surface and make
sure it is clean. On any of these devices, measure the
high- voltage bus to ground with an ohmmeter with the safety
device operated. There should be a solid reading to ground on
the ohmmeter. If there is not, clean up the contact and try it
again. It may also need some adjustment.
In the power supply, there may be a relay that will ground
out the high- voltage capacitors after plate voltage is turned
off. Inspect the contacts for burning. This relay should not fall
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too fast. as there is a full charge on the bus immediately after
turnoff. If it hits immediately, the effect is the same as
shorting the bus during operation; the contacts burn badly.
Keep this relay's contacts clean and adjusted. Another check
of this relay's operation: Observe the plate voltmeter. When
the plate voltage is turned off, this meter indication should
drop to zero immediately. If it hangs up or does a slow dropoff,
the shorting relay is not working properly.
High- Voltage Capacitors -High voltage capacitors are
usually filled with a special mineral oil for insulation. When
this is beginning to break down or go bad, the capacitor may
leak oil, get hot, or swell, or all three ( Fig. 9 -14). Immediately
after turnoff, feel these capacitors. They may be warm from
H
11
LEAKS
OIL d
1
l
n
t
it
1
t
1
HIGH -VOLTAGE
CAPACITOR
e1¡
I
SWELLING
I
t
1
/
1
r
/
I
Fig. 9-14. Signs of high-voltage capacitors going bad. They may leak oil,
swell, get hot, or all three.
other components nearby, but if they are warmer than
expected. or hot, or if the other conditions are present. they
should be replaced at the earliest opportunity, as they are
going to fail soon.
A word of caution about measuring high- voltage
capacitors. The dielectric molecules have been under severe
stress from the high voltage and tend to take on a permanent
charge effect. So even when a capacitor has been shorted and
the short is removed, the stressed molecules will build a
charge right back up on the capacitor. So don't try measuring
these with an ohmmeter. The charge can become deadly after
a period of time. so when the capacitors are to be discarded or
placed on the shelf, wire the terminals together with a wire
328
that will not come off by itself. Twist the wire on or screw it
down with the capacitor terminal nuts.
Electronic Maintenance
Control of the RF energy is not an easy matter. Whenever
given the slightest chance. RF will leak out of compartments
and get into other circuits, causing crosstalk or feedback
oscillations. Maintenance calls for keeping all the cabinets
tight. all fasteners in place, and feedthrough capacitors
grounded tightly.
Neutralization -When feedback occurs within the tube
itself, the stage oscillates. Transmitters which require certain
stages be neutralized provide instructions on how it is to be
accomplished. Here is a general method you can use to check
for the neutralization of a conventional RF stage, providing
both the grid and plate currents are metered on separate
meters. Tune the plate through resonance. The grid current
should hit its peak at the same time the plate hits minimum
reading. This may occur a little off the plate tuning. but when
it occurs a long way from plate dip, the stage needs
neutralization.
A stage out of neutralization can oscillate, and in the
tuning process. the upper stages may be tuned to this rather
than the carrier from the oscillator. The frequency will not be
exactly correct, and of course, some tubes may burn out.
Whenever a tuneup has been done, to make sure a stage hasn't
"gone into business for itself." pull out the crystal. The RF
output indication should immediately drop to zero. If it does
not, an oscillating stage is supplying the RF. Shut the
transmitter off immediately or something will burn up ( and
the station will be out of channel).
Multipliers -The FM transmitter exciter has a number
of multiplier stages. These should be tuned carefully. In most
cases. tuning is only a touchup job, but if there have been
major components changed or a change of station frequency,
then a multiplier can be tuned to the wrong multiplication
factor; for example, tripling instead of doubling. It depends
upon the leeway in the component values whether there is
more than one possibility with the stage. If the tuneup is done
with a wattmeter at the output of the exciter, there is zero
power output when the wrong multiplication rate is used, since
the upper stages are not tuned to that frequency.
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PA Efficiency- Anything that adds resistance to the
circuit. lowers the Q of the resonant circuit, or causes a
mismatch to the load. causes a stage's efficiency to be poorer.
The higher the efficiency of a stage. the easier it works in
amplifying and passing on power to its load. The stage's
efficiency when it is properly tuned can be an indicator of
other problems either in the load or in the drive.
Contacts -The coils in the AM tuning box can be in a
shielded compartment. The clips and other connections should
be examined for looseness, signs of overheating, or arcing. A
loose contact arcs or overheats when it is carrying an RF
signal. If any are loose or burned, clean and tighten them.
Such contacts will lower the tuned -circuit Q by adding
resistance, and cause unstable operation.
Shorting Bars-In FM, the coils are somewhat different
than in AM. and may not look like a coil at all. The tuned
sections and their shorting bars provide the same action as the
AM coils. Always remember that RF travels on the surface of
the conductors. Keep the conductors clean and tight. When an
arc has occurred, there will be bits of metal protruding from
the surface of the conductor. Hone these off and polish up any
carbon or smoke residue.
Capacitors -The AM transmitter or its tuning units may
have air dielectric capacitors. Nothing should be allowed to
accumulate in these openings or it can cause an arc. If an arc
has occurred, clean up the surfaces so that there aren't any
protruding bits of metal. These can cause further arcing.
Check sealed capacitors for overheating. They may
become warm by conduction from power resistors or the
cabinet, but if a unit is overheating, it will be hotter than the
surrounding components. Check the current rating of the
capacitor. It may have been underrated for the present
conditions. There is a current rating on RF capacitors as
well as a voltage rating. Circuit conditions may now be
changed from the original, so that what was originally rated
correctly is not now correct. Tuning conditions in a resonant
circuit can produce much higher circulating currents than
those of the overall circuit itself. Thus, mistuning can cause
the capacitor to fail. If the tuning conditions haven't changed,
then replace with a capacitor with a higher current rating (if it
will physically fit. or a ratio combination of inductance to
capacitance may be used.
330
When the output stages efficiency appears to get worse but
the tuning and other parameters appear normal, this indicates
either a drive problem or a load problem.
The drive from the preceding stages is intended to amplify
the RF signal to the proper amount for the output stage
requirements, so adequate drive will be indicated by the PA
grid current. If this grid current is much higher than normal,
the problem is either in the PA tube itself or its load. If the grid
current is low, then look at the metering of the various stages
down the line for one that is not indicating normally. Trim up
the individual tunings, and if this doesn't do the trick, check
the stage that shows abnormal readings. The grid current will
be the most sensitive indicator of the workings of the previous
stage.
Grounded -Grid Output Stage-Many FM transmitters use
the grounded -grid configuration in the output stage. In this
arrangement. part of the output of the driver goes into the
load. Check the meter readings around this stage when the PA
REMOVE
PLATE CAP
-.10RF OUTPUT
PA
CATHODE CURRENT
LESS THAN 50%
Fig. 9-15. In a grounded -grid output stage, remove the plate cap and
check the PA cathode current. The PA should supply 50% or more of
normal current, the IPA less than 50 %.
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efficiency appears low. If the IPA is not efficient, this affects
the transmitter output and the apparent efficiency of the final
stage itself.
Inefficiency of the IPA can be caused by PA, drive, or load
problems. Here is one way to check out these stages. Remove
the plate cap from the PA stage. Tune up the IPA stage and
observe the PA cathode current. The normal cathode current
should be roughly divided 50 percent for IPA and 50% for PA.
If the ratio can be more for the PA than for the IPA. this is
better. But if the IPA is supplying more than 50 percent, the
PA tube is probably going bad. If this test is done when the
transmitter is first installed, there will be a reference for later
tests to judge the operation.
AM Loading-When the load changes, the stage must be
retuned and rematched. On a single -tower AM station for
example. there may be some changes in the system which
actually change the antenna resistance. but the same value of
current is being maintained as per the license. In effect. the
actual ouput power has changed, and this appears as a change
in efficiency of the PA stage. Remember that the power isgiven
by P = 12R. so if the current remains fixed but the
resistance changes. the power is different. If all other
parameters appear normal except the PA stage efficiency.
then look for a load problem. If the efficiency appears too
good the actual output power is less than normal, and if worse,
the output power is higher than normal. When these conditions
persist and no other reason can be found, have a new
measurement of the antenna resistance made.
FM Loading-FM load chenges affect the output stage,
but such changes can usually be detected by a change in the
VSWR reading of the line monitor. If the efficiency improves
considerably. then it was the tube. But without this change, the
tuning and loading positions will have changed, and this can be
an indicator of load changes. If the output stage is a tetrode
and the screen current is metered, this is a more sensitive
indicator of load changes. Changes in the load will produce
poorer plate efficiency. and this causes the screen to draw
more current.
Tube Life-The hours of a tube's use should be recorded.
This can be taken from the transmitter's running -time meter.
If there is none, one should be added. Tie it into the circuit that
will come on with the filaments. A record of total hours of life
332
is useful for warranty purposes and is an indicator of average
life expectancy of the system itself. It will take some time to
develop experience; but if tube records are kept. in a couple of
years or so. behavior can be established. Aside from
particular tube failures, when the life expectancy seems to be
shortening. look for leaks in the cooling system or changes in
the operating parameters. Also look for worn or poor
connections that cause burned contacts on the tubes, or
something in the load that causes the tube to dissipate more
power than the load (poor efficiency). These are long -term
indicators, but show trends.
AM Modulators-The stage used for plate modulation is
usually a push -pull arrangement. In such an arrangement, the
stages should be balanced and linear. It is easier to balance
two tubes that have about the same age than it is to balance
one with many hours of use and a new tube. If one tube should
blow in a pair having many hours, replace both of them. Save
good one for an emergency spare or use with another tube of
comparable hours.
Balance is usually done by adjustment of the bias on each
tube, but this also affects the audio input to the transmitter. So
go ahead and get a static balance on the plate current meters;
then apply audio tone modulation, and use an oscilloscope to
observe the envelope. Also. attach a distortion analyzer to the
modulation monitor output. Null the distortion and do a
dynamic balance for the minimum distortion. But observe the
envelope. If the bias increases modulation, it can cause
overmodulating and increase the distortion; or it can be too
low and not give a good modulation figure in that 95 to 100
percent area. Reset the limiter output to provide the correct
modulation values, and then run some programming material
through and make the final adjustment.
AM Carrier Shift -Carrier shift is actually a good
indication of the transmitter's ability to work under very
heavy modulation. This will show up poor PA tubes, poor PA
design, primary power source shortcomings, and undersized
primary wiring. This condition can become worse when 125
percent positive -peak modulation is used (or attempted).
Many of the older transmitters weren't designed for this type
of modulation and can't handle it. The carrier shift limit set by
the FCC is five percent maximum.
When carrier shift is present, first try trading out the PA
tubes if they have many hours on them. Often this corrects the
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problem. If it doesn't. then observe the AC power line meter to
see if the AC varies with modulation. If it does, the load is
"dragging down" the power line. This may be because the
primaries themselves are not adequate. but in most cases, it is
the wiring to the transmitter. Also check the DC plate voltage
for changes. Weak tubes in a power supply or defective series
dropping resistors can affect the voltage, and lower voltage on
the peaks will cause negative carrier shift. Still other checks to
make are the grid current of the PA for drive, and the screen
voltage. During peaks. and especially at 125 percent positive
modulation. a very heavy demand is placed on the drive and
screen circuits. Drive must be adequate or the peaks will
flatten off, and if the screen isn't well regulated, its current
MODULATION MONITOR
MODULATION
CARRIER
MODULATED
RF CARRIER
(CAL
ALTO100%
.i T-CONNECTOR
OSCILLOSCOPE
st
NEGATIVE
PEAKS SHOULD
JUST TOUCH
POS
NEG
j
?
NEG
POS
Fig 9-16. Use tone modulation on the carrier and oscilloscope to calibrate
the AM modulation monitor for 100% negative modulation.
will rise on negative peaks. There will be a weaker output
along with distortion.
Monitoring
constant check must be maintained on all functions that
affect the signal emanating from the station's broadcasting
facilities. This is imperative not only to comply with FCC
regulations. but also to present a clean signal to the listening
public. The paragraphs that follow give the problems that are
most likely to be encountered and their solutions.
AM Modulation Monitor -The AM monitor should be
calibrated with tone modulation and an oscilloscope Fig.
9-16). Use the same setup as when balancing the modulators
and observe the modulation envelope. How often to check this
calibration depends upon experience with the monitor and how
well it will hold calibration. When you have reached 100
percent negative modulation on the scope, then adjust the
meter to read 100 percent. This sets the accuracy of the meter.
Adjust the peak flasher at the same time. Some of the newer
monitors have a built -in calibrator, but always use the
transmitter and a scope as the final authority. You can obtain
the AM RF for the scope from the same input to the monitor.
Add a coax T- connector so the line can feed both the monitor
and the scope.
FM Frequency Problems-The carrier frequency can
become erratic or unstable in a direct FM system. There can
be many reasons for this, but generally there are problems
with the oscillator, the AFC loop, or overmodulation.
The master oscillator should be working reasonably close
to the carrier frequency without the AFC. When the basic
oscillator frequency drifts so far that it is at the outer reach of
the AFC, then it can be jumping in and out of control. Set it up
as follows. Turn off the AFC switch, but observe the AFC
meter. If this changes radically one way or the other, the
master oscillator is far from carrier frequency. Adjust the
master oscillator until it produces a reading on the AFC
meter close to Where it would be with AFC. What you are
reading here is the error voltage caused by the oscilator when
it is off frequency. When it is adjusted in this manner, switch
the AFC back on. The frequency is now well within the
capture range of the AFC.
Overmodulation -When the carrier frequency becomes
erratic with modulation, it is possibly an overmodulation
A
(
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problem. Unless there is close control of the average and
peaks in the audio overmodulation occurs, especially on the
high- audio -frequency content. If only modest signal processing
is used. pull the modulation peaks back to where they only
indicate 35 percent to 40 percent on the modulation monitor. If
overmodulation was a problem, the carrier frequency should
stabilize.
Audio Filtering- Another cause of erratic carrier
frequency in modulation occurs with the filtering in the AFC
loop. The control voltage to the master oscillator from the AFC
must be a pure DC voltage. If the modulation is not filtered
out. this causes the frequency to shift. So check DC control
bus with an oscilloscope. If there is modulation present. check
out the filtering circuit.
AFC Loop -There can be a problem with the crystal
reference oscillator in the AFC. It can be an oven problem,
noisy thermostat, or low output. Any of these things will affect
the ability of the AFC to control the carrier frequency. As a
quick check, switch off the AFC and observe the carrier
frequency. There will be some drifting around as the carrier is
not stable anyway, but if the erratic conditions stop, there is a
problem in the AFC. Check it out according to the instruction
manual. If it is a logic -type circuit, check for the correct
number and widths of pulses. Make sure the mixer is getting a
good signal from both the carrier sampler and the reference
oscillator chain. Look for a good, strong IF signal after the
mixer, and at the correct frequency. At this point, the IF will
be swinging with the carrier modulation, so if using a scope,
wait for a pause in the modulation to get a stable waveform.
The sample of the carrier and the point in the circuit
where it is obtained are important. In some exciters, this RF
sample is picked up at the output of the exciter itself. This
provides a good, strong signal, but it can be affected by the
load on the exciter, and especially if a long coax cable is used,
there may be standing waves on it. If there are erratic
frequency readings of the carrier, check the output of the
exciter. This will usually have a meter position. If anything on
the load has affected the exciter output stage, this reading will
be low. Consequently, there will be a weak sample to beat with
the AFC oscillator. Retune and check the input tuning of the
load stage. This tuning can affect the match and the VSWR,
and thus the exciter load.
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Calibration -Overmodulation
causes
problems of stability of the carrier frequency and also
problems with the FCC. The modulation monitor should be
calibrated so that it is indicating the correct percentage of
modulation. There are a couple of ways to do this, depending
upon the type of monitor that is in use.
Carrier Deviation-To actually measure the carrier
deviation itself, you will need a communications receiver with
a BFO and an accurate audio modulating frequency. With this
method, you tune in the carrier and listen for the carrier to null
as modulation is increased. Which null to use depends upon the
modulating audio signal frequency and its amplitude.
This method is based on mathematical functions which
predict where the carrier will go through nulls. The trick is to
detect these nulls. The ratio of the carrier deviation /u to the
audio frequency JA produces the modulation index M (Fig.
9-17). This factor will vary. At certain modulation indexes, the
carrier will null. By use of the specific modulation index for
FM
Monitor
(A) BASIC FORMULA:
M (MODULATION INDEX)
-
fr, (CARRIER DEVIATION)
fA
(AUDIO MODULATING FREQ)
(B) CARRIER WILL NULL AT THESE VALUES OF M:
M=2.405
M=5.520
M=8.654
M =11.972
M=14.931
1st NULL
2nd NULL
3rd NULL
4th NULL
5th NULL
(C) THE BASIC FORMULA CAN BE TRANSPOSED SO THE
VALUE OF MODULATING
BE DETERMINED.
FREQUENCY
CAN
fp (CARRIER DEVIATION)
fA
M (MODULATION NDEX AT DESIRED NULL)
EXAMPLE:
fA at
75,000 Hz
5th NULL =
14.931
- 5023 Hz
(NOTE: 75,000 Hz IS 100% MODULATION FOR FM BROADCASTING).
Fig. 9-17. Table and formulas for determining the modulating frequency to
use when calibrating the modulation monitor.
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the null points and by transposition of the formula, you can
compute what audio frequency is needed for a given null point.
Set up the receiver and tune in the carrier, preferably the
IF in the monitor if this is the range of the receiver. Most
communications receivers will not tune high enough to pick up
the carrier directly, but most monitors have an IF that is
suitable for use. If the monitor has an unusually low IF, the
manual often describes another method that can be used. A
frequency meter can sometimes be used if it is of a type that
will give a beat note on the carrier.
First, tune in the carrier (without modulation) and set the
receiver BFO to give a 400 to 500 Hz beat note. Decide which
null you will look for and use the correct audio modulation tone
to modulate he carrier. Apply the modulation very gradually
and listen for the carrier to null or disappear. Listen closely,
as these nulls are very sharp, and that first one comes up
rather fast. There will be many other beats from the
sidebands, but ignore these and listen for the carrier itself to
null. When you have reached the desired null, observe the
modulation meter. It should be reading 100 percent. If it is not,
then adjust the appropriate control on the monitor. The
accuracy of these readings depends upon the modulating tone,
so check it with your frequency counter. Here the term carrier
refers to the IF of the monitor.
Meter Calibrations- Remote meters that are logged for
normal transmitter parameters, such as remote controls or
extension meters, must be calibrated once each calendar
week. The FCC requires such meters to be within two percent
of the main meter they represent. Accurate calibrations
depend upon how well the engineers read the meters and upon
the samplers used. The remote meter and its calibration
should not affect the main meter. This is not always the case,
especially with extension meters. Regular remote control
units have more isolation, and there is usually little effect.
Much depends upon the unit and how it is wired and the
samplers.
Reading the Meters Accurately-Some meters have many
divisions on them, while others have relatively few. There are
many times the pointer falls between divisions, and then
interpolation is necessary. The remote meter, by the way,
should have the same divisions as the main meter.
If the meter is a base current meter at the antenna, the
engineer at the tower should take into consideration any
338
calibrating factors and temperature corrections that must be
applied. If these meters are slightly out of calibration, do this.
First, take the temperature and the calibration chart figures
and apply this to the true value of current that should be
indicated for that power. Have the engineer increase or
decrease the transmitter power to bring that meter into its
correct reading if it is not already there. Then, check the
remote meter reading and make the required correction.
Before the correction is made these readings must be logged.
If you desire to log what the base meter actually indicates
without the correction factors, do so, but also show what the
true current is. And, if using a temperature chart, show the
ambient temperature on the maintenance log.
Sampler Problems-Direct -wired remote
meters'
calibration controls can affect the main meter. especially if
the sampler is a series arrangement. This type of circuit is
often used on remote power meters or modulation meters
Fig. 9 -18). To calibrate the meters. first adjust the main unit
without the remote meter connected. Then, adjust the
(
MAIN METER'
REMOTE METER
CALIBRATE +
SAMPLE VOLTAGE
REMOVE REGULAR
RESISTOR TO ADD
REMOTE METER
Fig. 9-18. When the remote meter and calibrating resistor are in series with
the main meter, they are difficult to calibrate; the remote will affect the
main meter.
calibrating control. There should be two people doing this
so that both meters can be watched at the same time. The
remote meter must agree with the main meter since they are
measuring the same current. It takes much delicate juggling
to get the two to read together and correctly. If you are not
careful. the main meter will not read correctly. In the case of
an FM power meter. have the dummy load and wattmeter on
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the transmitter, and then calibrate both meters at the same
time. The wattmeter will show the correct value of power.
Dual -Power Operation -When an AM station operates
at two different powers for day and night, it is difficult to get a
remote meter to calibrate at both places with the same control
IFig. 9 -19). In this case, arrange to have a switch and two
calibrating controls at the remote meter. In this way, the
SEPARATE
CALIBRATING
CONTROLS
REMOTE ANTENNA
METER
HIGH
POWER
LOW
POWER
DC
SAMPLE
Fig. 9-19. When the remote antenna meter must read current for two dif-
ferent powers, add a switch and two different calibrating pots.
meter can be calibrated accurately for each power. In
operation. of course, the switch must be thrown to the correct
power so that the proper calibrating resistor is in the circuit.
Typical Meter Readings -A new transmitter Fig.
9-20) -with the gleam of the new parts. the aroma of fresh
paint and new insulation, and all -is a little like a new car.
That first checkout of the transmitter should be a real
pleasure. just like the first drive in a new car.
The instruction manual will show a complete set of typical
meter readings for the transmitter. These have been obtained
from a similar transmitter at the factory. Each transmitter is
slightly different, and the meter readings can vary from the
typical readings. Your transmitter will be checked out at the
factory on your channel before shipment, and all the readings
(
will be given.
When the new transmitter is installed, tuned up, and
working properly. go through and take a complete set of meter
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Fig. 9-20. Harris MW -1, solid -state AM transmitter with the front door and
access panels open, showing the simplicity of layout and easy access for
maintenance.
readings. The on-site operation will vary somewhat from the
factory checkout conditions. This is due partly to different
power line voltages. The load will be slightly different also,
since the factory's dummy load and your antenna will present
different situations. Thus, the on-site readings also vary. You
can write of all these initial readings either in the instruction
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manual or on the checkout sheet alongside of the factory
figures. This is a better reference than either the typical
readings or the checkout readings. This is where the
transmitter started at the station.
Reference Material
Drawings made of the wiring and jacks and any other
pertinent notes about the installation should be saved for
future reference. Note any special tuning done in the
instruction manual, and if any modifications are made, show
these on the prints. All this information will come in very
handy when problems arise.
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Chapter 10
Coax
Transmission Line
The RF carrier from the transmitter will be routed to the
antenna system over coaxial transmission lines. This is true
for all FM stations and the majority of AM stations. There are
many advantages in the use of this type of line, but it does have
its own characteristics, and there will be problems when the
operation is contrary to these characteristics. So when it is
necessary to select a new line, replace a presently operating
line, or troubleshoot problems that arise, a general understanding of the basic characteristics of coax line will help you
make better decisions or solve problems. Although we will
consider the larger diameter lines used for transmitting
power, the same characteristics hold true for all coax lines.
CHARACTERISTICS
The line's special physical structure and the rigid control
in maintaining the dimensions during manufacture are what
create the electrical properties peculiar to this line and what
makes it different from other lines or cables.
One conductor is mounted within the other conductor. The
inner conductor is mounted exactly in the center of the outer
conductor and is held in this position throughout the length of
the line by insulators placed at regular intervals. The
insulating medium may be dry air lalong with the spacers), a
solid material such as polyethylene, or a foamed polyethylene.
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Natural Impedance
A coax line will have a natural impedance value that is
called the surge impedance or characteristic impedance. This
value is not the same for all coax lines but is dependent upon
the diameter of each conductor. the spacing between the
conductors, and the insulating material. Thus, by making the
dimensions different, lines with different natural impedances
can be created to suit different needs. There is a special
formula for computing all this, but there is little need for it
unless you plan to manufacture coax line.
In a practical field situation, you need only know which
line is which. The important factor is the ratio of the diameters
of the two conductors to each other. So, if you need a small
section of line, and in the pile of line odds and ends you have,
you find a suitable length but wonder about its impedance,
then look at the inner conductor. The line with the larger inner
conductor will have the lower impedance. Such situations often
arise when the station may have several different coax lines
and cables in use. Or it may have changed lines in the
past-for example. from the old 51.5-ohm rigid line to the
newer 50 -ohm rigid line-and there are scrap pieces of both
lines in the pile. The 50 -ohm line will have a slightly larger
diameter inner conductor. A similar situation in cables occurs
with the RG-8 and RG -11 (50- and 75 -ohm lines). The RG -8 will
have a larger inner conductor.
Termination
The natural impedance of the line is its design impedance,
and it is the optimum impedance at which the line should be
operated. That is, the terminating load should be a pure
resistance equal in value to this natural impedance. All the
ratings in the spec sheets for any line are based on these ideal
conditions unless stated otherwise. Consequently, if the load
impedance has improper resistance value or if there is
reactance present. standing waves will be set up in the line.
When standing waves are present, all the design parameters
change and the line values must be rated downward.
Peak Power
An important rating of a line is its peak power- handling
ability. This is the ability of the line to withstand voltage stress
across the conductors, and it also indicates the amount of
344
voltage that can be present before an arcover occurs on the
peaks. The basic factors which determine this rating are the
spacing between the conductors and the insulating material
used. The rating is not frequency dependent; the rating is the
same for 60 Hz voltage as for a UHF signal. The stated rating
in the spec sheet is based on dry air for the dielectric material.
The rating can be increased by pressurization or if other gases
are used instead of dry air. But when standing waves appear
on the line, or there is moisture in the line, this rating will be
quickly lowered.
Average Power
The second important power rating of the line is its
average power rating. This rating is based on the temperature
rise of the inner conductor before it warps or the insulators
deteriorate or melt. Since RF power has a definite heating
effect that increases with frequency, this rating is frequency
related. The higher the frequency, the lower the rating.
Remember that this is basically a temperature effect, so
anything which can raise the temperature of the inner
conductor will affect this parameter. such as direct sunlight or
outside ambient air temperatures.
The heat from the inner conductor must be quickly
transfered to the outside air, so anything that can hasten this
transfer will improve the average power rating. Both
pressurization and the use of a gas which has a better heat
transfer characteristic than air will increase the average
power rating. Standing waves, on the other hand, cause a rapid
heat rise in the conductors, and so will quickly lower the
rating of the line.
Attenuation
There will be a loss of power as the signal travels along the
line. The determining factors for this loss are the resistance of
the conductors and the leakage across the dielectric. Both the
RF resistance and the leakage will increase as the frequency
is increased, so the rating is definitely related to frequency. As
a yardstick, the ratings are based on 100 -foot sections of line.
There isn't anything you can do to improve this rating
except to use a larger diameter line. The larger line will have a
greater conductor surface and less RF resistance, and the
wider spacing between conductors will reduce the leakage.
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Although you can't improve the rating, standing waves will
deteriorate the rating, by producing higher losses. Since the
rating is based on 100 -foot sections, the longer the line, the
greater the total losses.
Velocity of Propagation
This rating only has utility in certain applications, but in
those applications it is an important rating. The RF signal is
slowed down as it passes through a coax line, so its physical
wavelength is shorter than the same wavelength in free space
(Fig. 10-1). The insulator or dielectric material is the basic
cause of this. Lines with an air dielectric have only spacers to
ONE WAVELENGTH
LOF 100 MHz
SIGNAL IN
FREE SPACE
180°
K-K
270°
360°
T
3 METERS
2.7 METERS
-xi
i
1
180°
270' 3601
PHYSICAL
WAVELENGTH
OF SAME SIGNAL
IN COAX LINE
WITH VP RATING
OF 90%
FIg. 10-1. The wavelength of the RF signal Is shorter In the coax line than It
Is in free space. How much shorter Is a function of the velocity -ofpropagation (VP) factor of the line.
impede the wave and thus will have a higher rating. (The
ratings are expressed in percentages.) Coax, with a solid
dielectric, causes the wave to be delayed more, so it has a
346
lower rating. For example, air dielectric lines will have a
rating over 99 percent, while solid dielectric cables can have
ratings as low as 60 percent.
The percentage velocity -of- propagation is used as a
multiplying factor on the free-space wavelength of the RF
signal. For example, a 100 MHz signal will have a free -space
wavelength of three meters. But passing through a line with a
velocity factor of 93%, the wavelength in the coax will be 2.79
meters (3 meters x 0.93 = 2.79 meters)
.
Phasing
From the previous discussion, it is obvious that the line's
electrical length has a definite relationship to the RF wave it is
transmitting. This is a useful factor to know when
troubleshooting problems, cutting a line to avoid undesirable
wavelength relationships, or using special lengths of line for
phasing purposes. This latter is done in the feeder and
intertower connections in AM directional antenna systems,
and in joining the bays in a multibay FM antenna. For such
purposes, the line length is often expressed in degrees of the
RF wave rather than actual physical length. For example, a
quarter -wave section is 90 degrees and a half -wave section is
180 degrees.
POWER DISTRIBUTION
When the line is properly terminated in a load resistance
that equals the line's natural impedance, the power
distribution at any place along the line will be equal (except
for the gradual loss because of the line attenuation). The most
efficient transfer of power takes place under these conditions.
Standing Waves
When the line is not properly terminated in a load which is
a pure resistance equal to the natural line impedance, then all
the power is not absorbed in the load, and some of it is
reflected back towards the source end of the line. This
reflection sets up standing waves on the line. Just how much
power is reflected back to the source depends upon the degree
of mismatch and what reactance is present.
The reflected power will have a different phase
relationship to the forward power as it travels along the line.
At some places the two powers are in phase and add together,
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and at other places they are out of phase and subtract. When
the in -phase addition takes place, there will be a greater
voltage stress or higher current than normal, which can cause
arcovers from voltage or greater heating from the higher
current. These phasing conditions will affect the signal's
sidebands as well as the main carrier itself, so there can be
distortions in the final received signal at the receiver location.
Standing -Wave Ratio
This is a figure that describes the conditions on the line
and is a ratio between the forward and the reflected powers.
Measurement can be made of either the voltage or the current
components of the two waves, but most of the usual indicators
terms of the voltage
components. The term is then designated VSWR (voltage
standing -wave ratio). The voltage must be measured with
directional couplers that measure each wave separately, feed
the voltages into a resistor arrangement so that they can be
properly calibrated, and then read on an indicating meter.
in broadcast stations measure in
Tuned Sections
Coaxial line sections are often used for tuning or matching
sections, traps. and filters. In this application, very strong
standing waves are essential to the tuning action. To create the
desired effect, the line section is either shorted, left
unterminated, or equipped with adjustable capacitors or
shorting bars. When a line section is used for these purposes, it
is a short piece of the correct physical length.
When trying to visualize the conditions taking place in a
coax line, it is very easy to become confused with the
conditions that prevail when the line is correctly terminated
and when it is not (see Fig. 10-2). The two conditions are not
the same at all. When correctly terminated, the line is simply a
conductor for the signal, and except for slowing up the wave, it
operates in the manner in which it is designed to. The voltage
and current of the signal go through their normal rise and fall
just as any other AC signal does, and there is only one wave in
the line -the wave going forward to the load at the end of the
line. Now, when the load is improper and some of the signal is
reflected back to the source, there are two waves traveling in
different directions, and their basic amplitude and phase are
different. This is an altogether different ballgame, and all the
348
(A) CORRECT TERMINATION
FORWARD WAVE
LOAD=
Z OF LINE
SOURCE
(B) INCORRECTLY TERMINATED
\
ri
FORWARD WAVE
REFLECTED WAVE
SOURCE
ADDS
WHEN IN PHASE
CANCELLATION
WHEN OUT OF PHASE
ILOAD=
ZOF LINE
Fig. 10-2. Two entirely different conditions exist when the line is correctly
terminated and when it is not.
line conditions change. For power transmission purposes,
standing waves are very undesirable and should be kept as low
as possible. But if you want to use the properties of standing
waves to make a tuned section, then you create very strong
standing waves deliberately.
DERATING
As previously discussed, when conditions other than the
ideal occur in the line, just about all the characteristics of the
line are altered in some manner. Since the design
specifications are based on these ideal conditions, they must
be derated. Standing waves on the line require that most of the
parameters be derated by an amount related to the strength of
the standing waves. Also, the specifications are based on a
constant- amplitude signal, such as an FM carrier. When AM
carriers are used on the line, the specs must be derated
further.
Amplitude
When the station's carrier is amplitude modulated, the
peak power rating of the line must be reduced to a lower
figure. This is because the sideband power is transmitted
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along with the carrier itself. In amplitude modulation, the
peak power of the signal at 100 percent modulation is four
times the peak power of the carrier by itself. The peak power
rating as given on the specification sheet must be divided by
four to arrive at the rating for an AM carrier.
In the determination of the peak power rating of a line,
there are many factors involved; but you are not designing a
line, only trying to determine if a particular one can be used
for the service you intend. All these other parameters will
remain constant except the VSWR, so this simple formula will
give the peak power rating for AM use.
Derated peak power =
rated peak power
ll +
M)2
x (VSWR).
The modulation percentage M expressed as a decimal is
1.0. so (1 + 1) = 4. Thus, the rated peak power is divided by
4. Of course. the VSWR enters the picture, and that value is
also divided into the peak power to further derate the line. As
can be seen by this simple formula, the peak power rating of
the line quickly drops with AM.
2
"Super" AM
Now that AM broadcast stations are permitted to
modulated to 125% on the positive peaks, the peak power
rating of the line must be derated further. From the formula
given then. (1 + M12 = (1 + 1.25) = 5.0625, or about
one -fifth the original rating. And this can be a problem for the
AM station that can achieve that percentage of modulation.
Take, for example, a 1 kW AM station that is presently
using a 7/s -inch foam -filled coax line. This line has a
normal peak power rating of 44 kW. and since the line is solid
dielectric, there is no way to use gas or a pressure to increase
the power rating. On first glance, a 44 kW rating should be
more than adequate for a 1 kW transmitter. With normal 100
percent modulation, the peak power must be divided by 4, so
this brings the rating down to 11 kW. This is still plenty
2
of room.
But now suppose the modulation is increased to 125
percent. which gives us 8.8 kW. The transmitted power under
these conditions is 5 kW on the peaks, so the spread is getting
closer. This is still adequate room so long as there are no
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standing waves on the line. If there are standing waves, then
we must derate further. Let's assume the VSWR is 1.5; divide
the 8.8 kW by 1.5. and the rating becomes 5.86 kW. What at first
appeared as though we had the whole ball park to play in, now
looks more like the infield only.
Average Power
When VSWR is present. the average power must be
derated. This can either be done by computation or by a simple
rule of thumb. If you want to compute. use this formula:
Derating factor
-
VSWR2 +
2
1
x VSWR
+F
VSWR2
2
-
1
x VSWR
The factor F is taken from a chart in the manufacturer's
catalog sheets and is frequency dependent. For FM
frequencies. the factor is about 0.4 or 0.5, and for AM
frequencies. it is 1.
The rated average power of the line should be multipled by
the derating factor. This will give the amount of power to
subtract from the stated average power rating. Assume, for
example, a line with an average power rating of 15 kW and a
VSWR of 1.2. By use of the above formula, we find that
approximately 4.17 kW must be taken from the 15 kW figure. so
that the derated figure is now 10.83 kW. If the station had a 1
kW FM transmitter feeding this line, it is doubtful that that
amount of power would cause any damage to the line. But if
the transmitter were 10 kW. that is another story.
About the only time you might want to do all this
computation is in an installation where you can't get the VSWR
as low as you would like it. You must then derate the line for
regular operation. When some unusual situation arises, such
as ice on the antenna, simply divide the transmitter power
output by the VSWR.
Besides the line, you also want to protect the transmitter
output stages. In the previous example. if we divide the 10 kW
output of the transmitter by the 1.2 VSWR, this will give a
power figure of approximately 8.3 kW. Reduced transmitter
power will give added protection to both the line and
transmitter.
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SELECTION OF TRANSMISSION LINE
There may be a time when you must select a line for a new
station or a presently operating station that has been granted a
power increase. The main factors in this selection (aside from
economics) are the power losses and power -handling ability of
the line.
Size
The two main factors which affect both the losses and the
power- handling ability are the diameter of the line and the
type of dielectric that is used. The larger diameter line will
have less loss than a smaller diameter line of the same length.
Lines which use Teflon spacers and air dielectric have the
least loss for a given diameter. Use the charts supplied in the
manufacturer's catalog to determine the size according to the
amount of loss that can be tolerated and the required
power-handling ability.
Losses
To determine how much loss can be tolerated in the
system. use the transmitter power output and the power
required at the input to the antenna. If this is an FM station,
TOTAL LOSS = 6 x 0.1 dB = 0.6 dB
Fig. 10-3. Compute the loss for the total length of line, not just the 100 -foot
section.
352
divide the effective radiated power of the antenna by its power
gain to arrive at the input required at the antenna.
The loss figures in transmission line charts are given in
decibels per 100 feet, so you will need to convert power to
decibels. or the other way around. You will want to know the
total loss in the full line, not how much it will attenuate the
signal every 100 feet. So divide the total length of the line by 100
to get the multiplying factor.
Multiply the decibel loss from the chart by the multiplying
factor just derived. For example. a particular type of line has
0.1 dB loss per 100 feet on your channel. But your installation
requires 500 feet of line. Since 500 divided by 100 equals 5, and
since the loss is 0.1 dB per hundred feet. 0.1 dB x 5 = 0.5 dB.
The total line loss then will be 0.5 dB. This is a power ratio of
1.12. so if the transmitter output power is 10 kW. there will be
approximately 8929W at the end of the line to radiate from the
antenna (10000/1.12 = 8929). The loss in the line is
10000 -8929 = 1071W.
Power
Besides the loss in signal power. both the peak and
average power ratings of the line must enter into the
deliberations. Always try to select a line that will allow for
some reserve capabilities, particularly if there are
possibilities of the station increasing power at a later date.
Working a line very close to its maximum ratings may save a
little money on the initial investment, but it can sow the seeds
of many operational problems in the future.
When estimating the size of the line for power rating. don't
forget to derate the spec sheet figures for anticipated VSWR,
and for the modulation if it is an AM station. Although a unity
VSWR is very desirable, it is doubtful that this can be achieved
with practical antennas. and if it is achieved it may not stay
that way very long. Both the line and the antenna system arc.
exposed to the weather, and they will both age, causing
changes that increase the VSWR. But during unusual
conditions, such as ice on the antenna or the removal of a bay
of the FM antenna stack for repairs, the VSWR can rise. In
your computations. allow at least for a possible rise to 1.5
VSWR. This is an arbitrary figure, but it will leave a desirable
allowance and reserve in the line.
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Type of Line
There are many choices when it comes to coax line, but
basically these fall into two types, rigid or flexible. Each of
these has its own advantages, disadvantages, and electrical
characteristics. The rigid line will come in 20 -foot sections that
are bolted together at the flanged ends. This line requires
elbows to turn corners or change direction, it and takes more
installation time. But if repairs or changes must be made. they
are easier.
The flexible line will be one continuous piece, flanged at
both ends. This can make the installation go faster, and the
mounting is simpler. But you must make sure of the length to
order. And a large spool of this line is a bit clumsy to handle. If
you guess wrong about the length too long you will need to
cut off the factory -installed flange and redo it, unless you're
lucky enough to find a suitable place where the excess can be
coiled up out of the way. Also, repairs done on this line are far
more difficult.
(
1
Don't Skimp
During the deliberations and calculations in trying to
make the final selection, here is a thought to keep in mind. A
good line and installation will last for many. many years. and
with only a minimum of problems and very little maintenance.
But a poor installation of an underrated line will be nothing but
problems. So don't skimp on the line.
Select a good- quality line. allow for adequate reserve in
the power ratings. and then make a good installation. Note that
the statement said "adequate" reserve. You don't want to go
overboard either. Use good judgement. and if the budget is
tight. skimp on some other studio items and invest in a good
line. When a problem arises in the studio gear, at least it is
where you can get at it, and it probably would not affect the
station's ability to operate. But if the line burns up. the station
is off the air until it can be corrected. Correction can be costly,
and worse if it is a vertical run up the tower that also requires
tower riggers and their equipment.
INSTALLATION POINTERS
The installation should be planned well in advance of the
equipment arrival at the site. If this is an FM line, then tower
riggers and hoisting equipment are needed. In any case, the
354
horizontal run can be made by station personnel. In this
planning. give thought to storage of the line components before
they are installed, and where you will work to cut the line and
add flanges. Different techniques are required for rigid and
flexible line. And. of course, there is some difference between
AM and FM installations.
When the line is ordered. care should be taken to
anicipate all the components needed, including all the small
items. such as gas barriers, field flanges, etc. There is a
variety of small components that go into the line installation.
When all this arrives at the site, don't assume it is all
there-check it out against the original order. If there are any
shortages or wrong components. get replacements before
construction begins. Once the construction begins, everything
that will be needed should be on hand at the site.
INSTALLING RIGID LINE
In the FM installation, the antenna should already be in
place atop the tower. This may be a presently operating
antenna or a new antenna. By having the antenna in place, this
gives a fixed starting place to begin the line, and at the same
time, it will show up any problem in mating the antenna with
the line. In many cases, the tower structural members at the
top are close together, and routing the line can be a little
tricky.
Have the tower riggers build the FM line from the top
down. Some like to build from the bottom up to the antenna,
especially with larger lines. There are two practical
advantages to building from the top down. In the first case, the
open end of the line will always be down, so there is less
chance of getting rain or other materials in the line while it is
being constructed. Secondly, it is a rare case when the
distance between the antenna and the bottom of the run comes
out exactly in multiples of 20 -foot sections. There is always at
least one piece to be cut and a field flange to be added. So, by
constructing line downward, this cut piece will be at the base
of the tower and not at the antenna. At the ground level, station
personnel can get at it should later repairs be needed, but if it
is at the antenna location, then tower riggers and their
equipment will need to be called in to make the repairs. This is
simply a bit of foresight that can save additional expenses at
some later date.
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Mountings
The line will be suspended by hangers every ten feet, and
these will be either a spring hanger or some kind of movable
hanger that will allow the line to move during contraction and
expansion caused by temperatures. During the construction,
however, the full weight is not yet on the line, so these must be
compressed the correct amount when installed, according to
the manufacturer's directions. This information will be found
in the instructions that come with the hangers.
Rigid Mountings
At the top of the tower, the line should be anchored by at
least two rigid hangers. These will support the line and, at the
same time, prevent expansion from going past them. The
antenna should not support the line. Allowing the antenna to
support the line is poor practice, for all the tremendous
pressures built up by contraction and expansion will push
against the antenna. and it will be damaged. The FM antenna
may have a matching transformer mounted at its input, so the
rigid hangers should be below this; that is, right at the end
section of the line itself. Consider everything above that point
as the antenna.
Making Bends
It will be necessary to angle the line so that it can connect
properly with the antenna flange and with the horizontal run at
the base of the tower. There may be other problems on the
tower itself that require the line to change directions. The rigid
line cannot be bent, so both 90- and 45- degree elbows are
available. The 90- degree elbow will handle most of this
direction change if it is flanged at both ends with swivel
flanges.
At the antenna junction and at the base of the tower, use
two 90- degree elbows together (Fig. 10-4). This will allow for
all angles and should meet the antenna feeder at the angle
required. The use of two elbows also makes it easier to get the
line apart when needed. If the line should run directly into the
feeder, then it is necessary to loosen the rigid hangers and
drop the line far enough to get them apart. Once the line is
dropped. it will be impossible to get the line back together
without hoisting equipment or some jack arrangement at the
base of the line. Men could not lift the weight of the line by
356
IANTENNAINPUT
90' ELBOW
90° ELBOW
COAX LINE
Fig. 10-4. Use two 90-degree elbows at the base of the antenna.
hand. Double elbows at the base of the tower also provide this
facility at getting the line apart when needed.
Horizontal Run
Both the AM and FM station will have a horizontal run of
line. The AM line will terminate in the tuning equipment,
whereas the FM line will meet the vertical run. In this case,
the AM line should have a rigid hanger at the tuning house.
Both the AM and FM runs should have a rigid anchor
where the line enters the transmitting building. These rigid
hangers allow the line to expand and contract between them
but prevent the pressure from being applied to the equipment
on either end.
Between the rigid hangers the line must be allowed to
move. The hangers along the line are movable hangers that
both support the line and allow it to move. These are different
from the hangers for the tower but serve the same purpose.
Support and Shelter
The horizontal run must have support above the ground
level. It should not be laid on the ground itself, nor should rigid
line be buried. The supports may be steel pipes or fenceposts
driven into the ground and tall enough for protection against
mowing equipment, etc.
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In sections of the country where ice is a problem in the
winter months, a protective shield should be built over the line
to protect it from ice falling from the tower. This shield will
also protectthe line from tools or components dropped off the
tower when workmen are on the tower.
Cutting the Line
As mentioned earlier, it is rare when it is not necessary to
cut and flange at least one piece of line. There are always one
or two pieces that need to be shortened, or small sections may
be needed inside the transmitter building. When you make the
cuts. do it carefully and survey the situation before diving in
with a hacksaw.
First, make sure all the measurements are correct. This is
very important. If you don't allow for the flanges, and the
section is a snug fit anyway. you won't be able to get it in
place -or it may come up short if you measured the other way.
So get the measurements exact. The instruction sheet that
comes with the flange will show all the correct allowances to
make. including how much to undercut the line to allow for the
flange. Measure the full distance that is needed for the line
section-from flange face to flange face ( Fig. 10 -5). Use this as
the required length and then allow for the undercutting as
shown on the instruction sheet.
FINISHED LENGTH NEEDED
Fig. 10-5. Measure for the required finished length from flange face to
flange face.
When you must cut a section, try to use an end that will
already have one factory flange. In this way, you need only
add one flange. Of course, there can be situations when the
only piece of line left has no flanges on it at all. Then you must
add two flanges. But a suggestion here: Assuming the line
section is longer than needed and must be cut, go ahead and
mount one of the flanges before cutting the line. This will then
put the work in the situation as if the section already had a
358
factory flange. By doing this, it will be necessary to cut only
the one end of the line.
Outer Conductor
Before working with the outer conductor, pull out the inner
conductor and put it in some safe place for protection. Then
after careful measurement, mark the place you wish to cut on
the outer conductor. Don't rely on your eye to make a straight
cut: use a heavy sheet of paper that has at least two straight
edges. Wrap this around the pipe. line up the straight edges,
and mark with a pencil all the way around the pipe Fig. 10 -6).
If done carefully, this will give a straight cutting guide.
(
SQUARE UP EDGES
OVERLAP
-i
COAX
LINE
SECTION
i\
MARK LINE
ALL THE WAY
AROUND PIPE
Fig. 10-6. Mark the pipe all the way around. Use a stiff piece of paper as a
guide.
Now use a hacksaw with a good blade in it and cut through
the line. But don't try to cut clear through the pipe. Only cut
small sections. rotate the pipe. and cut some more. When
nearing the end of the cut, hold onto the end piece so that it
doesn't drop by its own weight and bend an edge at the cut
surface. These are difficult to straighten out. During this time.
keep the line section tilted at a slight angle so that the filings
will fall out the open end and not back into the pipe.
When the piece has been cut off, clean up the sawed edge
with a file. The inside rim of the cut can be cleaned up with a
heavy pocket knife. Next, clean out the pipe so that there are
no filings inside it. Take a clean rag and use something as a
ramrod to push it through. or tie a clothesline to it and pull it
through.
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Soldering
With the cut made, check it out for straightness. It is
important that the cut be straight, or the flange will fit at a
slight angle. This will make it difficult to connect the section to
the rest of the line, and if it is connected, there will be
pressures against the solder joint.
Clean up the copper with steel wool or similar material so
that it is polished bright. Then use a good hard solder such as
silver solder. Some of the field flanges already have a ring of
silver solder in them and need only be heated. If there is none,
you must supply your own.
Use a medium tip on the torch, as silver solder requires
more heat than soft solder. If there is a wind blowing and you
are outdoors, set up some sort of wind shield. Wear gloves to
keep from accidentally touching something with bare hands.
Once heating begins. "wipe" the pipe with the flame for
several inches from the flange. This will warm the pipe and cut
down heat flow away from the joint. Once the solder has
flowed evenly all around the seam, stop heating and let it cool.
During this time before the solder sets, don't move the work.
When it has cooled off, polish the joint with steel wool to clean
away any burnt flux or torch residue.
Inner Conductor
The inner conductor must be measured carefully. This will
be undercut more than the outer conductor. The inner
conductor must accommodate the bullet or connector. So
check the instruction sheet carefully for instructions on the
measurements.
There will be insulated spacers on this conductor, so watch
out that the cut does not come right on an insulator. If this is
the situation, move your measurements one way or the other
and then make a cut at both ends that will land between
insulators Fig. 10 -7).
(
Skin Effect
One characteristic of any RF signal is skin effect. The
wave will travel along the surface of the conductor and make
very shallow penetration into the metal itself. So keep this in
mind when making up flanges. Be concerned about the inner
surface of the flange and make this a nice smooth surface from
the pipe to the flange. Don't have gaps in the joint or large
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(A) PROBLEM
REQUIRED LENGTH
INNER CONDUCTOR
III
I
INSULATORS
(B) CUT BOTH ENDS
REQUIRED LENGTH
it
I
I
I
I
CUT
I;I-
I
CUT
NO .2
NO.1
Fig. 10-7. Avoid cutting at the spacers on the inner conductor. If
necessary, move the measurement down the line and make a cut at both
ends.
blobs of solder protruding inside the line. Those are directly in
the path of the RF signal and will increase the resistance and
create standing waves.
Also avoid making dents in the line. Should a line section
slam against the tower on its way up, have that section brought
back down and a new section sent up. The dented one should be
saved for making shorter sections.
INSTALLING FLEXIBLE LINE
The flexible line requires a little different handling than
the rigid line. When planning the original purchase, make
accurate measurements of the run for the whole line. The
factory will cut the length you order and apply flanges. If you
measure correctly, this will save putting on the flanges
yourself.
Hoisting
A cable hoist should be ordered if the line will run up the
tower to an FM antenna. This will attach to the end of the line
and is pulled up by a winch. Leave this attached to the line and
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use it as the top anchor on the line. Make sure the cable is free
to turn off its spool. Try to pick a day that is not windy.
If the tower is a "hot" AM tower, the hoisting should be
done after signoff when possible, or shut down the AM
transmitter long enough to get the line up the tower. If the AM
can't be turned off and the line must be pulled up anyway, be
prepared for many things. If it is a bare copper line, keep it
away from the tower, or arcing will occur, which will put noise
on the AM signal. The winch itself must use rope. not a steel
cable. Also, wear the leather gloves, as the line will get plenty
"hot" with RF as soon as it begins to stretch out.
In the transmitter room, look for much detuning of the
tower and erratic operation of the transmitter. I had to do an
installation in this manner one time, and the best
recommendation that can be made is this: Don't!
Bonding
Some of these cables are sheathed with an outer
polyethylene covering. This will insulate the line from the
tower, but it will also upset the tower's electrical
characteristics. So clean off the covering every so often and
bond the coax to the tower. Pick spots that are not electrically
related to the wavelength of the AM signal. That is, break up
the coax into random lengths. The bare copper line is perhaps
best for this type of installation, but it must be bonded tight to
the tower so that arcing does not take place between the line
and tower.
In either case, the line is bonded to a tower leg all the way
down the tower. No other anchors or hangers are usually
needed. But if the line must cross wide spaces on the tower,
install angle iron for a brace and then bond the line to this.
When a long stretch of line is not tied down, the wind will
constantly flex it. This will lead to early metal fatigue, and the
line will fail or fall apart.
Horizontal Run of Flexible Line
The best arrangement is the same as that for the rigid line,
except different suspension is needed. Hangers can be used,
but a piece of horizontal angle iron or a pipe is best and would
require fewer hangers. There will usually be a large conduit
that carries AC power for the tower lighting, and this can be
used to support the line. Strap it to the pipe every two feet with
362
banding. If the line is sheathed and long, again ground the
outer conductor at intervals that will break up its relationship
to the carrier wavelength (at an AM station).
There is no need to allow for contraction and expansion of
this line, as the flexible line will adapt to these conditions.
PRESSURIZING THE LINE
When an air dielectric line is used, whether a rigid or
flexible line, it should at least be pressurized with dry air.
Without pressure. any small leak will allow outside moisture
and dust to get into the line. Moisture will cause increased
leakage of the RF across the dielectric. This will change the
ratings. since they are based on dry air. Besides that, if there
are any low spots in a horizontal run. water will collect there
and build up until it shorts out the line. In the winter it will
freeze and cause additional problems.
Breathing
A line that is not pressurized will "breathe." If there are
any small openings. dust and moisture will be drawn into the
line. This breathing is caused by pressure changes inside the
line caused by the sun and air temperature changes. When the
line is heated. pressure will build up inside, and the flow will be
to the outside (Fig. 10 -8A). But when the line cools, the
pressure drops and the flow will be to the inside of the line
(Fig. 10 -8B).
If there are no leaks in the line, then there is no problem
except for any condensation that might take place from
moisture already inside the line. A line with even 1 psi inside
pressure will always have a flow to the outside, as long as the
inside pressure can be kept higher than the outside pressure,
nothing can get in the line. So consider some pressurization of
an air dielectric line.
Gaskets and Barriers
To be pressurized, a line must be sealed up. On the rigid
line, an O -ring gasket is used at each flange for this purpose.
The flexible line has no joints except at the ends, so it is
automatically sealed unless there is a hole in it somewhere. On
the rigid line, the gasket must be in its groove and not pinched,
and the bolts must be pulled down tight.
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I
(A)
/
INSIDE PRESSURE
BUILD UP
COAX
LINE
í1
+
1
PSI
)1,
LEAK
(B)
INSIDE PRESSURE
DROP
-
1
PSI
COAX
LINE
64 6
/0
CONDENSATION
)VLEAK
Fig. 10-8. An air dielectric line without pressurization and with a leak
breathe.
At the ends of the pressurized line, a gas
will
barrier is needed.
This is similar to a flange, except that the insulator is solid and
does not allow gas to pass by it. It will also have at least two
small ports with plugs screwed into them. These are for
pressure gauges if desired, and as entry ports for the gas or
dry air. They can also be used to bleed the line.
Pressurizing Equipment
Some method is required to pressurize the line after it is
sealed. This may done with a dry air pump or a cylinder of gas.
In either case, the pressurization equipment will have a
regulator on it to maintain the line pressure and act as a buffer
between the line and the source. In the dry air pump, air is
filtered through a chemical that absorbs the moisture. The
pump goes off at intervals, and a heater comes on to dry out
the chemical and ready it for another cycle.
364
The gas cylinder has high pressure. The gas will
automatically flow into the line until the cylinder pressure
drops below the pressure in the line. Because of the high
cylinder pressure, there must be a regulator between it and
the line, or the line will be damaged.
Bleeding the Line
When a new line is installed and sealed up, it should be
washed out with dry air or gas. Although dry air or gas will
absorb moisture that is in the line, it can't get rid of it. But if
you bleed the line (Fig. 10 -9), the dry air will force the
moisture to the outside, leaving the inside filled with dry air or
gas. This same procedure should be done if the line has to be
opened up for some reason, but it may not need as much
bleeding as a new line.
ti\
ANTENNA
GAS BARRIER
AIR EXHAUSTING
OUT OF GAS
BARRIER WITH
PLUG REMOVED
1-
GAS
BARRIER
TRANSMITTER
DRY AIR
PUMP
Fig. 10-9. Bleed the line by opening the plug at the end.
Pump up the line to at least five pounds pressure and let it
set for a few minutes to absorb as much of the inside line
moisture as it can. Then open the plug at the end of the line
and let all this air out. The petcock at the input of the line
should be closed. When the air quits blowing from the exhaust
hole. put the plug back in the pump it up again. Do this two or
three times.
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Another way: After the first pumping up and bleeding,
leave the plug open on the opposite end, and let the dry air
pump continuously bleed the line for about five minutes, then
seal it back up. If the end of the line is at the top of a tower.
someone will have to climb the tower to open the line. Have
one of the riggers do this since the job shouldn't be considered
finished until the line has been checked out for leaks anyway.
Give him a spare plug in case he drops the original. At other
times, the line can be bled from the source end if necessary.
This will do the job, but not as well as bleeding out of the
opposite end of the line.
Checking for Leaks
When the line is first pressurized, check it for leaks. This
can take place during the bleeding operation. Pump up the line
to final pressure or a higher value, shut off the petcock and
note the pressure gauge. Allow it a little time to settle down.
Leave the line that way for several minutes or a half-hour and
watch the pressure gauge. If it doesn't move, the line is tight;
but if it drops off rapidly. there is a bad leak in the line
somewhere.
Check all the flanges for a leak, especially the field flanges
that were put on at the site. On these, also check the solder
joint itself. Sometimes there is a pinhole in the solder and the
line will leak. You can check these with a soapy water solution
or one of the commercial leak- finding solutions. A leak will
blow a bubble. If the line pressure is high, you can hear the air
hiss out of the leak. By running a high pressure in the line (up
to 20 psi) a small leak will become a large leak and will be
easier to find.
There may be a pinched O -ring at one of the flanges. If
there is, the flange will have to be opened. The ring may need
to be replaced if it is flattened or cut.
CHECKOUT
A new line should have some preliminary checks before
power is actually applied to it (Fig. 10 -10). The first thing to
check is continuity. This is important as an inner connector
can be left out somewhere. Short -circuit the end of the line and
measure with an ohmmeter. If there is an infinity reading, the
line is open. If there is a short, the line is probably okay, but
there may be a short somewhere in the line besides the end. If
366
TO ANTENNA
ADD
SHORT
~-
TO TRANSMITTER
OUTER CONDUCTOR
INNER CONDUCTOR
OHMMETER
OR
RESISTANCE
BRIDGE
Fig. 10-10. Check line for continuity with an ohmmeter or resistance
bridge.
you have a DC resistance bridge that can measure very low
resistances (down in the hundredths or thousandths of an
ohm). this will provide a reliable check, providing the
resistance of the antenna is known.
The best procedure is to open the other end of the line. This
is easy to do if it is an AM line, but not as easy when it
terminated at an FM antenna. With the line open, the reading
should go open, and if someone on the other end shorts across
the conductor, then a low reading will be obtained. If using a
resistance bridge, make sure a good solid short is placed on
the other end, and measure the actual DC resistance. If it is an
FM antenna. reconnect, and again measure the DC resistance.
Save these readings for future reference. These readings
will typically be much less than an ohm, even on a long line.
Find the Missing Bullet
If your continuity check indicates the line is open, this
presents something of a problem. more so if it is an FM line.
There will be many sections of line and many bullets. Some
isolating techniques will be needed to save opening every joint
in the line.
367
First, open the line at the base of the tower and short the
conductors together. By the way, the line need only be opened
enough to slip some metal object in that will short the two. If
there is continuity, the fault is up the vertical run; but if it is
still open. the fault is in the horizontal run.
Now go open the horizontal run at midway and short the
conductors. If the line is still open, it is in the quarter towards
the source; but if a short occurs, then the opening is in the
second quarter toward the tower.
The same procedure can be used if the fault is on the
tower. And if a quarter- section of the run is long, you can use
the same isolation technique. You will find the missing bullet
much quicker this way than if you started opening each section
of line.
Impedance Sweep
An impedance sweep of the line should be made on FM
stations Fig. 10 -11) The station won't have the necessary
(
.
TERMINATE
FIRST IN
DUMMY LOAD
THEN
WITH ANTENNA
SIGNAL
GENERATOR
FREQUENCY
METER
,
DELAY LINE
COAX
LINE
DIODE
-
OSCILLOSCOPE
Fig. 10-11. Arrangement to sweep the line and the antenna across the
bandpass for VSWR and impedance match.
368
equipment, but the consulting engineer or one of the service
companies can do this for you. It will be a worthwhile
investment.
This sweep will measure the impedance and the VSWR of
the line across the bandpass of the station. This will be done
first with the line terminated in a dummy load, and then with
the antenna. Someone will need to be at the antenna to open the
line to add the dummy load and then reconnect to the antenna.
If there is anything seriously wrong with the line, it will show
up on this sweep. When the antenna is connected, the
transformer can be trimmed to get the best match across the
bandpass.
Powered Checkout
Once these preliminary checkouts have been done, the
transmitter can be connected up to feed the antenna through
the line. Remember that this is as yet an untried line. It may
have checked out okay with the other tests, but they were
low -power tests.
The line may act differently under power, so feed the
power gradually. Do it in steps. First, apply about one -fourth
the regular power level. Check the VSWR indicator
immediately for standing waves on the line. The indicator
won't be properly calibrated yet. but it will give an indication
if there are any standing waves. Let the system run this way
for a few minutes and watch for any changes. Then, come on
up to about half power. and repeat the procedures. Let the
system run for about 15 minutes on half power. This will give
all the line elements a chance to warm up.
If there isn't any indication of a serious VSWR problem, go
on up to high power. But now, calibrate the VSWR indicator
and check to see what the VSWR actually is. Save this figure
for future reference. Let the system run for about a half -hour
on high power before giving the line a clean bill of health. The
preliminary checkout with the sweep will give a pretty good
indication of line condition, but it is not the same as a check
under full power.
LINE MAINTENANCE
A well selected line with adequate reserve in its specs and
a careful installation will give good service for many years
with only a minimum of maintenance. It does require some
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routine inspections from time to time and a daily check of the
VSWR and gas pressure. Even a properly operating line can
suffer physical damage from falling ice, tools, vehicles
running into the supports, and other mishaps. And electrical
damage can be caused by faults in the antenna itself.
Daily VSWR Checks
The VSWR should be read at signon and signoff. At signon,
the line will be cold, but after a full day of operation under
power. the line will be warmed up by RF heating, and perhaps
the sun and outside temperatures. There may be slight
changes between the two readings, and this is not a cause for
concern. If these readings are gradually getting worse over a
period of time. there is a gradual change in the system. But if
the readings are normal at signon and poor at signoff, there is
something heating up in the line, and problems are developing.
There may be a poor connection that is overheating or
burning, and it will soon break down. There may also be an
antenna problem. and the problem should be uncovered and
corrected as soon as possible.
Daily Pressure Checks
On pressurized lines, there should be a check of the line
pressures each day. If this is a remote location, the line
pressure should be part of the weekly transmitter inspection.
An automatic dry air pump will continue to keep the pressure
up in the line, but if there is a large leak. the unit may run
continuously and burn itself out. If there is a large leak and gas
is used. it can be expensive.
For a large leak.try this :Upon arrival at the site.shut off
the source to the line at the petcock. Note the pressure gauge.
Perhaps a half-hour later, again check the gauge. If there is no
drop. the line is tight or at least the leak is slow. But if pressure
is dropping significantly. there is a large leak. At least the
horizontal run of line can be checked with a soapy water
solution for a leak. If one is found, try tightening the bolts. This
will often correct the leak, as the bolts may work loose. If it
doesn't, then prompt maintenance will be called for after
signoff.
Slow Leaks
It is difficult to detect and find slow leaks. A dry air pump
will usually keep the line under pressure, even though it may
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work harder; but slow leaks usually soon develop into large
leaks. When gas is being used, a slow leak can be detected by
the amount of gas used over a period of time. When a new
cylinder of gas is installed after the inital installation of the
line, keep a record of the dates the cylinder is changed. A slow
leak will use more gas. and it will be necessary to replace the
cylinder more often. You can check the records and see what
the normal usage is. and if the time between changes
suddenly start to shorten up, a leak has occured.
Rigid lines put pressure on all the joints during expansion
and contraction. This can help loosen some bolts. Any leak that
is small in the summer, when the line is more expanded. will
become a large leak in the winter, when the line is
more contracted. Contraction pulls or attempts to pull the
joints apart, while expansion pushes them tighter together.
Physical Damage
The line should be inspected occasionally for physical
damage. If there has been severe icing, and when the melting
comes and the field around the tower looks like an ice floe,
keep an eye on the VSWR. Watch for sudden changes. If the
VSWR suddenly jumps and stays up, at the next safe
opportunity inspect the line for damage. I said safe, because it
is dangerous to be wandering out under the tower if the ice is
falling off in sheets or slabs.
Hot Spots
Another good indication of a high VSWR on the line are hot
spots appearing at various multiples of the carrier
wavelength. If the VSWR indicator is working and properly
calibrated, it should be indicating this.
Aside from heating by the standing waves, other problems
can be occurring that will produce heating. Check especially at
the elbows and flanges on a line. When a flange or the area of
the line within a foot of the line is hot, there may be a poor
connection at the inner connector that is heating or burning.
This can only get progressively worse until it fails. At the first
opportunity, take the elbow or joint apart and check the bullet.
Look for actual burning, but if the conductor hasn't progressed
this far yet, check the copper inner conductor. The one running
the hottest will discolor. Take the elbow apart if you can and
check the inside of it. Replace the bullets or the whole elbow if
necessary.
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Intermittents
Intermittents in a line are just as much a headache as they
are in any other piece of equipment. They are just as difficult
to find and, in some cases. more so. I once worked at a 5 kW,
6-tower. directional AM station where an intermittent frying
noise developed on the signal. After many and sundry checks,
it was determined that the problem was in the antenna system
somewhere. After signoff one night. I left the carrier on at the
full 5 kW in the directional pattern, with no modulation, and
took a transistor radio along on a tour of the antenna system,
listening to the carrier. The frying noise would come and go.
Beginning at the one main feeder line, I banged on the
coax line as I went, listening for something to happen. About
halfway through the system, at or nearing one of the center
towers. when the line was banged. the noise stirred up
considerably and would occur every time the line was hit. (By
the way. the banging was done with the heel of the hand, not
the club I wanted to use.)
Going back and turning off the power. I opened up the line
at that point. The end terminal had a bad solder joint inside
and was burnt pretty badly. After cleaning this and
resoldering. I found the noise was gone for good.
The technique was really no different than checking out a
tube-type amplifier by going down the string of stages tapping
on the tubes and listening for microphonics, except in this
case. the "set" was spread out over a large area.
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Chapter 11
Tower and Antenna
The final control a station has over its signal is at the antenna.
A good antenna system is required so that the RF signal can be
radiated in the most efficient manner. Antennas for AM and
FM are physically different because of the carrier frequencies
and propagation characteristics. They must also have
somewhat different operating and maintenance techniques.
THE AM ANTENNA
Wavelengths in the AM band are rather long and require
physically long antennas. One full wavelength in the middle of
the AM band at 1000 kHz, for example, is 984 feet. This would
require an antenna almost as long if full -wave antennas were
used.
Propagation of the signal is by both the sky wave and
ground wave. The ground wave is more reliable for local
coverage, while the sky wave is best for long- distance
communications. In broadcasting, the sky wave is
undesirable, as it causes interference to stations on the same
channel that are located many miles from the local station.
The wavelength of the antenna that is used affects the
direction of radiation of this sky wave, so a length is chosen
that will reduce the sky wave as much as possible.
The Other Half
The antenna is actually made up of the antenna itself and
its ground system. The ground system may be buried in the
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ground beneath a tower out in a field, or a counterpoise
arrangement if located on top of a building. The ground system
should always be considered as half of the antenna for
maintenance and installation purposes. That is, half of the
antenna is in the air, the other half under ground. This is
somewhat analogous to a half -wave dipole stuck in the ground.
Actually, it is only a loose analogy, for neither the actual shape
or size approaches that of the dipole; but it does help illustrate
the ground system as an important part of the antenna.
The ground system is not used in the sense that other
grounds in the station are used. In those applications, we want
to tie a shield or component to a zero-voltage reference point.
At the antenna, however, we tie the tuning coils and coax outer
conductor to the other half of the antenna. If you will visualize
the antenna and ground system in this manner, then you will
be apt to take as much care in making connections to the
ground system as you do the antenna proper.
Electrical Values
The tower is insulated from ground, and its length is some
part of a wavelength at the carrier frequency. It will exhibit
RF resistance and reactance, depending upon this
relationship. Besides the height, the actual physical shape of
the structure, as well as large metals objects nearby, will
affect these values. This is an important fact to remember
when considering erecting another tower or some large metal
object nearby.
Aside from the ability to tune or resonate the tower in use,
power-consuming factors such as high- resistance joints can
enter the picture. reducing the actual radiating efficiency of
the antenna. So when making connections to the tower or
ground system. remember to keep these as low in resistance
as possible.
Tuning Unit
The RF power from the transmitter will be fed to the tower
over a transmission line. When coax line is used (as in most
cases). the line exhibits its own characteristics and
"temperament" when not terminated properly. To obtain the
most efficient transfer of power through the line, the line must
be terminated in its natural impedance and with zero
reactance.
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It would be a rare case indeed to find an antenna that
would exactly match the line directly. Consequently, a tuning
unit is used to match the two together Fig. 11 -1) . This unit will
usually contain a T- network made up of series and shunt coils
and capacitors. Besides matching the impedance of the line
(
Fig. 11 -1. The AM antenna must be matched to the transmission line at the
base of the tower. Shown is a unit that contains a full T- network that can
handle 1250W. (Courtesy Harris Corporation.)
and the antenna resistance, the circuit will also resonate the
tower and act as a filter to attenuate any harmonics of the
carrier that might be present.
Bandpass
Antennas are usually tuned to obtain the greatest
efficiency in radiation. Tuning for greatest efficiency, while it
will produce a higher radiated RF signal, will also increase the
Q of the system. High -Q systems become narrowband, so
sideband clipping will take place. This will limit the station's
bandwidth.
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The AM station is permitted an occupied bandwidth of 30
kHz. This means that, theoretically, the transmitter can be
modulated out of 15 kHz audio. In practice, however, most of
the older rigs will not pass this high an audio frequency. The
new transmitters, however, can pass well over 10 kHz.
To obtain the greatest fidelity, the antenna system should
be broadbanded. Included in this broadbanding are the
harmonic filters and tuning coils at the output of the
transmitter proper. The match at the tuning unit should also
match the coax line impedance across the bandpass, or there
will be a high VSWR at the unmatched areas of the bandpass.
These can introduce distortion. Lack of broadbanding limits
the station's potential fidelity.
Larger System
The antenna and its ground system are often a part of a
larger antenna system. Many stations use directional
antennas all or part of the time. While each tower in the
system must perform its own role, the very close proximity of
the towers make them all very interdependent. When
something abnormal occurs on any one tower, whether this be
a fault or a deliberate change, it will affect the entire system
and the radiated pattern. This is something to bear in mind
when making repairs to a directional antenna system.
Installation
Station personnel do not erect the towers, but they will
very often install the ground system. The design of the ground
system will be spelled out in the construction permit, so it
must conform to this.
The installation should be done in a careful manner. If
done in a haphazard manner, it is possible the finished job will
end up with irregularly spaced radials. or worse, not enough.
Before starting, plot out the installation and stake it out. The
best way is with a surveyor's transit. If this is not available or
you don't know how to use one, then it can be done another
way.
First, compute the length of the chord between the two
terminating points on the circumference of the circle
described by the radials (Fig. 11 -2A). Measure off a length of
ground wire this length or use a tape measure. Next, measure
off one length of wire for a radial. Take the end of this radial
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straight out from the tower until it is taut. Drive a stake into
the ground. Now either use a tape measure or the other length
of wire you measured and anchor this to the stake. Take the
free end and the end of the radial wire and move to a point
where both wires are taut. This is where the next radial
belongs, so drive a stake in there. Continue this process until
all the radials are plotted out.
Plow It In
The ground wires must not be left on top of the ground, but
must be plowed into the soil about six to eight inches. Either
construct a simple plow arrangement or use one of the
commercial wire plows to do the job. All that is needed is a thin
slit in the ground and something that will insert the wire in the
bottom of the slit as the plowing is done.
Plow a straight line, and on the return trip, run the tractor
wheel over the slit to close it back up. Make sure the end of the
radial is buried and remove the stake. There is no point in
advertising where the end of the wires are. If the stakes are
left in or the wire ends sticking out, this is an invitation to
thieves or vandals to pull up the ground system.
Connections
All the radials and the copper screen below the tower will
have RF currents in them. The currents at the base of the
tower will be high just as they are at the base of the antenna
proper). Wherever two wires cross or touch each other or the
copper screen, make a good mechanical and solder
connection. On the screen, make these at least every two feet.
Intermittent or corrosive connections can cause arcing noise
in the program or introduce an intermodulation components.
The ground system will be down for a good many years, so
use a hard solder, such as silver solder. This is more difficult
to work with, but it makes a stronger connection. If there is a
wind. arrange some type of shielf, or the wind will slow up the
process. Use a torch with a narrow flame for best and quicker
results.
1
Tuning Unit
Take as much care with the connections to the ground
system here as you do to the antenna proper. Use heavy
copper strap and hard solder. And when attaching to the
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tuning -unit box, make sure the paint is cleaned off to get a good
metal contact. Some boxes have a second inner chassis upon
which the components are mounted, and their ground
connections terminate on this chassis. Make sure this chassis
and the outer box are securely bonded together and to the
ground system. If there is a poor or loose connection here, the
antenna system will become erratic or unstable. This is
because half of the system is "floating" because of the
intermittent ground connections.
FM ANTENNAS
The radiating elements of the FM antenna are much
smaller than those in the AM system; the wavelengths are
much smaller. For example, at 100 MHz, in midband, a full
wavelength is 9.84 feet. contrasted with the 984 feet of the AM
band.
(A,
C-.))
HORIZONTAL FIELD
VERTICAL FIELD
('h INPUT POWER)
ai
(B)
HORIZONTAL FIELD
(1/2 INPUT POWER)
Fig. 11 -3. (A) A single FM antenna bay has a power gain of 1 and radiates a
horizontal field. (B) Adding vertical polarization will divide the bay input
power into each field so that the gain in each field is 0.5.
Propagation characteristics are also different, in that the
FM signal tends to travel in a straight line horizontally from
the antenna. The FM antenna is required to be horizontally
polarized, whereas the AM antenna is vertically polarized.
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Other polarizations may be added to the horizontal
polarization, but FCC approval is needed.
Power Problems
Each FM antenna is a dipole that is either formed into a
circle. or into a V- shape. These are the basic shapes, but a
vertical element may be added. or the elements may be bent
into an oval shape. These additional shapings are used when
additional polarization is added.
Each basic dipole has a gain of approximately 1. When
vertical ploarization is added, the gain is reduced to 0.5 in each
of the polarizations. The total gain for the antenna is still 1, but
it will divide the power into two separate fields, the horizontal
and vertical fields.
The receiving antenna will usually only receive one or the
other field. depending upon its polarization. So the voltage in
each field is now less.
You don't get something for nothing. If you want to divide
the power into two fields, then you must double the input power
to the antenna to recover the original radiated power in one
field. Of course. some vertical polarization is most desirable
because it provides a better signal to automobile FM radios.
Stacking
The small physical size of each FM antenna lends itself to
stacking. When individual units (called bays) are stacked,
each will contribute its gain figure to the total gain. Typical
high -gain antennas of 12 bays are often used, and this will
produce a power gain of approximately 12. Thus, if 1 kW was
fed to the antenna, it would radiate 12 kW. This is called ERP
or effective radiated power.
Stacking has its penalties also. The beam is more narrow,
and it can become highly directional just like a multitower AM
system. Directional FM antennas are not used (except in
special instances), rather an omnidirectional antenna is
desired.
Careful phasing of the individual elements produces an
omnidirectional pattern. The narrow beamwidth can produce
holes in the close -in coverage area. Again, special phasing is
done to reduce this effect. Twelve bays seems to be about the
practical limit for stacking. as the entire unit can become very
long. A 12 -bay unit for some FM channels can be well over 100
feet in length.
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1.
Pattern Distortion
Although all the bays are carefully phased to produce an
omnidirectional pattern, this will only hold if the antenna is
mounted on a very slim pole mounted above the tower. But a
great many FM antennas are side -mounted on a tower. The
proximity of the tower will affect this pattern considerably. So
when an antenna is to be side -mounted, then the manufacturer
must have the dimensions of the tower structure so that
appropriate compensations can be made to the final tuning to
produce the desired pattern. Remember that these are still
only theoretical calculations, and when the actual mounting is
made on your tower, there can still be some distortion of the
circular pattern.
Matching
The antenna is fed by coaxial transmission line, so the
antenna must present a purely resistive, zero -reactance load
to the line. The tuning unit for this purpose is a coaxial
transfomer. which will mount at the base of the antenna
feedpoint. There are covered openings that can be taken off,
and shorting or tuning slugs that can be moved to trim up the
impedance match. Once the tuning has been accomplished, the
covers are replaced and the line made gas tight so that the
whole system can be gassed if desired.
The tuning should be done with special sweep equipment
that will measure the VSWR across the bandpass of the
channel. If only the VSWR indicator is used in the transmitter
room, a match can be achieved; but this will be mainly at the
carrier frequency. This is because the carrier is the main
power element, and that is what the VSWR indicator will
measure. Special test equipment will measure the match all
across the bandpass, and this is important. If there is a
mismatch in the outer reaches of the bandpass, there will be
VSWR reflections to the carrier swing and sidebands when
modulation is applied. This can cause amplitude response
problems at higher levels of modulation, and phase shifts to
the subcarrier and sidebands, which affect stereo signals.
Installation
The antenna is assembled at the factory and tuned up on
the station's channel. Then it will be disassembled and
shipped. During this process, it is possible some of the
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elements will be damaged or lost. So look all the components
when they arrive. If tuned elements are bent, it will affect
overall tuning the pattern of the antenna.
Where the bays were originally installed, there should be a
mark of some kind. It is important that they are put back
together. not only in their original line up, but so that one or
more bays are not turned over. This reversal would reverse
the phase of that bay by 180 degrees. Any of these situations
will affect the pattern, impedance match, and operating
parameters of the antenna. So put it back together carefully.
Now, you won't be up the tower installing the antenna, but
make certain the erection crew understands the importance of
all these factors.
Alignment
Not only must all the parts of the antenna be put back
together properly, they must be lined up perfectly in the
vertical position, and the aperture at the face of the array
should be straight (gap between the ends of the horizontal
elements). If the whole antenna tilts forward or backward,
mechanical beam tilt is introduced, and this is not permitted
unless the construction permit specifically allows a definite
amount.
Before everything is finally bolted tightly into place, use a
surveyor's transit and view the antenna from the side. You
should be able to tilt the transit from top to bottom of the
antenna and find every bay on the same position of the
crosshairs. Then go to the front of the antenna. Again, you
should be able to tilt the transit top to bottom and find the gap
at each bay on the same position on the transit's crosshairs.
When this is the case, lock everything in place.
Transmission Line
The coax line will run on up the tower to the antenna
matching unit. If this is a flexible line, band it to the tower leg
every two feet. A rigid line must use hangers every ten feet. If
the coax line has an outer insulating covering, then peel this off
at several places and bond the copper outer conductor to the
tower. If the tower is an AM antenna, the bare line must be
connected tightly to the tower to prevent arcing, and if the line
is insulated. then the outer conductor must be connected to the
tower to break up its length relationship with the AM carrier.
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Isolation
When the tower is an insulated AM tower, another
problem presents itself. The coax line must be isolated or it
will short out the AM antenna. This same holds true for any
metallic conductors that cross the base, whether it is AC
power or coax cable to a remote pickup antenna, or coax to
sampling loops. These must be RF insulated, not simply DC
insulated. The fact that there may be an insulated covering on
the cable will not prevent it from shorting out the AM tower.
There are at least two methods used to insulate the FM coax
line.
Coupler Isolation
An isolation unit ( Fig. 114A) is preferred. It is a metal box
that is gas tight and completely insulated at its input and
output. That is. both the inner and outer conductors of the
transmission line are insulated. There is a loop coupling to the
two sides of the circuit. By special design, the line impedance
is not altered. so that the VSWR with the unit in the line is very
small. Since there will also be RF voltage stress from the AM
carrier across the unit, it is rated for different levels of AM
carrier power as well as the FM power.
There are mechanical stresses as well as RF stresses on
the unit. Pressure from the line expansion and contraction can
break the insulation and allow a gas leak. And the RF carrier
from the AM can melt the ceramic insulators. If the line is
ungassed and a leak occurs, moisture can get in and rust the
box.
Bazooka
This is a tuned line section and makes use of the high
impedance of a quarter -wave line section that is shorted at one
end. In this method. the coax line does cross the base insulator
intact. But a quarter- wavelength up the AM antenna, the coax
line must be insulated from
the tower. This
quarter-wavelength is figured at the AM frequency.
These tuned sections work out pretty well, but the spot
where the coax shorts to the tower is reasonably critical if the
full advantage of the section is to be realized. Past the
quarter -wave insulated section. the line is bonded directly to
the tower the rest of the way up to the FM antenna. The inner
conductor of the coax is not affected, since it is shielded from
the AM carrier by the outer conductor of the coax. As far as
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the AM carrier is concerned, that coax line is open at the tower
base.
Problems can occur if insulators break and allow the line
to touch the tower, or if something shorts out an insulator. This
destroys the tuned-line effect and, of course, the insulating
qualities. When the tower is painted, these insulators must not
be painted, and anything that can cause leakage, such as ice or
corrosion across the insulators, can reduce the tuned line's
effectiveness and affect the AM system.
Whenever an FM antenna is added to an existing AM
tower, a new measurement of the base impedance of the tower
is required. and this must be filed with the FCC. Additional
equipment on the tower, plus crossing the base insulator, will
probably affect the parameters of the antenna.
If the station is to be an AM -FM station, even though the
FM may be some time later getting on the air, mount all the
FM gear on the AM tower before the base measurement of the
AM tower is made. If you do not, then when you do it some
time later, a new set of measurements must be made.
TOWERS
The tower is an important structure that requires a large
capital investment by the station. A properly designed and
installed tower, given the usual care, will stand and serve for
many years. There are many AM towers standing today that are
over 40 years old.
Wind Loading
A tower is designed to withstand a certain amount of wind
pressure, and this is figured with a certain amount of ice
coating. The figures are usually conservative. The tower is
usually specifically designed for heavy duty or light duty. As
long as the original conditions exist, the tower will give many
years of service.
But many factors can change these original conditions.
One factor is additional loading on the tower. When antennas,
microwave reflectors, and similar equipment are added to a
tower, this changes both the weight- loading factor and
wind -loading factor. Some of the smaller FM antennas don't
weigh much or add much windloading, but microwave
reflectors and heavy antennas do.
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Dead weight-that is, the vertical weight of the
equipment -is not as important as the wind -loading, unless of
course, the tower is a very light- weight design and the load is a
very heavy antenna. Most towers are carefully designed for
the dead weight they can carry and are designed for certain
specific purposes. such as mounting microwave dishes or
heavy TV antennas. If a tower is loaded with all sorts of things
it was not designed to carry, it may be overloaded, and a gust
of wind may cause it to collapse. Before mounting anything
large on the tower, consult the factory and get their opinion.
They will be able to determine if the tower can carry the load
or not.
Unless a tower is inspected at regular intervals and any
faults corrected, it can deteriorate quickly. Remember that
the tower is exposed to all types of weather, heating and
cooling, wind stresses, chemicals born in the air, and salt near
oceans. Rust can set in. braces work loose, bolts fall out. On
guyed towers, the guys may lose tension or break. All these
factors change the original conditions, and when a tower is
neglected. these factors can weaken the tower and cause it to
come down in a high wind.
Joint Use
It is common practice today for different broadcast
companies to share the same tower, for example. a TV, FM
station, or both, on an AM tower. In the past, each user of the
tower was responsible for the lighting, painting, and other FCC
requirements. Agreements must be worked out among all
parties and approved by the FCC. The approved agreement
must be kept on file at each of the stations for the radio
inspector to see. All the other stations then don't have to
concern themselves with the requirements of lighting,
painting, and logging of the tower. The one that assumes the
obligation Ithis is usually the tower owner) must comply with
all the FCC requirements.
Painting
Towers must be painted in a prescribed manner to provide
greater visibility to aircraft during daylight hours. The
prescribed colors are international orange and white. All
towers are not required to meet this section of the rules. ( All
these requirements will be found in part 17 of the rules. )
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All towers more than 200 feet above ground level
must be
painted. Towers less than 200 feet may or may not be required
to be painted. This depends upon their location in relationship
to airports and air corridors. In all cases, the prescribed
painting will appear in the station's construction permit and
license. When the paint begins to fade, the tower must be
repainted. If a station has approval to use the new
high- intensity lighting, the tower need not be painted.
Even a short tower whose top is over 200 feet above ground
must be painted. This would be the case if a large part of the
support structure were a tall building.
Banding
The color of the paint bands and the number and width
limits are prescribed in the rules. The bottom and top bands
must be international orange. For towers up to 700 feet, there
must be 7 equal bands of alternate orange and white. Towers of
greater height than this must have additional bands since the
maximum width can only be 100 feet, and at 700 feet, the
maximum width-100 feet -has been reached. The minimum
width is 1' 2 feet.
To compute the number of bands and their width Ifor
towers up to 700 feet), divide the tower height by 7 to obtain the
width of each band. For example, a 350 -foot tower must have 7
bands of 50 -foot width. If the tower is 700 feet. each band is 100
feet. the maximum. There will be 4 orange bands and 3 white
bands.
Tall Towers
Since the maximum width of each band can be 100 feet,
towers greater in height than 700 feet must have more than 7
bands. For these towers, the width of each band and the
number of bands must be computed differently. To determine
the total number of bands required, add two bands for each 200
feet of tower above 700 feet. If there is a fractional part less
than 200 feet. add two bands for that fraction. In all cases, the
top and the bottom bands on the tower must be orange, so
there will always be an odd number of bands in the total figure.
For example, a 900 -foot tower would have 7 bands for the
first 700 feet plus 2 more bands for the next 200 feet, for a total
of 9 bands. In another case, let's assume the tower is 990 feet.
You have already worked out 9 bands for the first 900 feet. For
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that additional 90 feet. there must be 2 more bands. This would
make a total of 11 bands for the 990 -foot tower. The width of
each band in the first case will be 100 feet (900/9 = 100). In the
second case, the bands will be 90 feet wide (990/11 = 90)
.
STANDARD LIGHTING
A tower which is 150 feet or more above ground level must
be lighted according to the FCC rules. The particular pattern
prescribed will be determined by the tower height or special
lighting requirements. Towers less than 150 feet may need
lighting. In all cases, the prescribed lighting for the tower will
be shown in the construction permit license.
Essentially, there is a flashing code beacon on the top of
the tower, and at different intervals on the tower, side marker
lights or additional code beacons are required according to the
tower height. These side lights must be seen from all
directions, so they will be mounted on the outside of the tower
leg at each interval, one on each leg. For towers of 300 feet or
less, there will be one flashing beacon at the top and a set of
marker lights at midlevel. Changes occur in the requirements
after every 150 feet of tower height, with a different
arrangement of marker lights and additional code beacons.
Lamps
The flashing code beacons must contain two lamps wired
in parallel. Each lamp must be rated at 620W or 700W. Having
two lamps in parallel is a safety feature. for if one burns out,
there will still be one lit even though the output will be cut in
half. The lamps themselves have clear glass envelopes. The
beacon contains a red screen to produce the red color, and the
outside glass of the beacon is a lens which focuses the light into
a horizontal plane.
The marker lights are single -lamp fixtures and must
contain a 116W or 125W lamp. These also have clear glass
envelopes. The fixture contains a red screen and the outside is
a lens.
Lamp Control
The lamps may burn continuously, or they may be
controlled automatically by a photocell control Fig. 11 -5). The
photocell must face the north sky, and when daylight drops to
35 foot -candles, it should turn the lights on. When the north sky
-
on
(
388
M
brightens in the morning to 58 foot -candles, then the photocell
may turn the lights off.
The photocell must have an unobstructed view of the north
sky. If it is shaded by a tall tree or building, it will turn the
lights on too early and off too late. If the photocell should fail, it
must turn the lights on, and they must burn continuously until
the photocell is repaired. This is called a fail -safe
arrangement. Towers which are less than 150 feet and which
must be lighted may use a photocell, a clock, or manual control.
Flashing Beacons
The flashing of the beacons must be within the limits
prescribed by the rules. There can be no less than 12 flashes
per minute or more than 40. The on or lighted time must be
twice the off or unlighted time. For example, if the lamp is
flashing 20 times per minute, the total on -off cycle will be 3
seconds. The on time will be 2 seconds, and the off time will be
1 second. This 2-to-1 ratio must be maintained, as must the
number of flashes within the prescribed limits.
The flashing mechanism will require some maintenance.
Expect to have problems with the rotating mechanism and
switching part of the unit. The flasher can get the station a
citation if the code flashing is out of limits. The inspector does
check this.
Installation
The tower lighting will draw heavy current. The wiring
runs are long, so heavy cable should be used to prevent or
reduce voltage drop at the top of the tower. Although 120V may
be entering the cable at the bottom, several hundred feet later
there may be only 100V left. When the system is first installed
and in operation, have the voltage measured at the socket of
each fixture in the system. The bulbs should be in place and
drawing their share of current. Make a note of these
measurements for future reference.
If the socket voltage is too low for the rating of the lamp,
its light output will be lower than normal. The rules require
that the rated voltage of the lamp be within 3% of the socket
voltage. If the condition existed as just mentioned, the lamp
must be rated within 3% of 110V and not 120V.
HIGH-INTENSITY LIGHTING
When a station desires to use high- intensity lighting, it
must receive approval from the FCC. When the FCC does
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authorize the station to use this system, then it will replace the
requirement for both the standard lighting and the tower
painting requirements of the rules. It must, however, conform
to the rules as they apply to high- intensity lighting.
The high- intensity lamp (Fig. 11 -5) will produce a light
that approximates daylight, with color response from infrared
Fig. 11 -5. Flash Technology's
Model FTB -205 Electroflash units. Top unit is one of the lamp
fixtures, and the lower unit is
the control and photocell arFlash
rangement. (Courtesy
Technology Corp. of America.)
to ultraviolet. This daylight spectrum is what makes it so
white and contributes to its brightness. The lamps are quartz
lights filled with xenon gas. The presence of the gas and the
pulsing of the light creates a high peak light output and high
efficiency. The special fixtures with their reflectors also boost
the light output so that it carries a terrific punch.
Light Pattern
The arrangement of lights on the tower depends as much
upon the height above ground as it does in the conventional
390
lighting (Fig. 11-6). Breakover points are at 300, 600, and 1000
ft. The height in this case is of the tower itself and does not
include any antennas that are mounted on top of the tower. At
each of the height levels on the tower where they are required,
(including the top of the tower), at least three fixtues will be
required, and maybe more.
SINGLE ALL DIRECTION
FIXTURE
OVER 20'
3 OR 4 LAMP FIXTURES
300'
OR
TOWER
LESS
EARTH
Fig. 11-6. High -intensity light system requires lights at the top of tower and
at different intermediate heights on towers above 300 ft. If antenna projects above tower more than 20 feet, then an omnidirectional lamp must
be used on top of the projection.
The same light output must be radiated horizontally from
each level in a 360- degree circle around the tower without
obstructions (Fig. 11 -7). This typically requires three fixtures
on a three -leg tower and four on a four -leg tower, but it
depends upon the width of the tower and the ability of the
particular fixtures to provide even lighting.
Figure 11-8 shows the relationship of the components of a
strobe lighting system. If an antenna or anything projects
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3- LEGGED TOWER
I
\
1
0 a
\
Fig.
11
\
200,000 I
CANDELAS
(DAYLIGHT)
-7. Light must be dist ibuted around
LIGHT
FIXTURES
/
tower equally in 360 -degree
horizontal plane.
above the tower more than 20 feet, a special, single,
omnidirectional lamp must be mounted on the top of this
projection. This is different from the other fixtures, since it
projects light in a manner similar to the conventional code
beacon.
Light Output
These lamps will burn continuously around the clock, but
at different intensity levels. The intensities will change at
twilight, at dark, and again in the morning. A photocell is
required to monitor the north sky and operate an automatic
changing device.
During daylight, the lamps must produce an output of
200.000 candelas in the horizontal plane, equally around the
tower, at each level. (The candela is a measure of light output,
as against the foot-candle which is a measure of illumination.)
At twilight, when the north sky dims to a light level
between 30 and 60 foot -candles, the light output from the
fixtures will be reduced to 20,000 candelas; and when the north
sky dims to a light level between 2 and 5 foot -candles, the
fixture output is reduced to 4000 candelas.
When a top omnidirectional lamp is required, its output
will be 20.000 candelas during daylight and twilight hours and
must drop to 4000 candelas at night. This light is controlled by
the photocell as are the other lights.
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Should the photocell or its control device fail, the lights
must go to either the daytime brilliance of 200,000 candelas or
the next light step higher than is required for that amount of
daylight.
All the lights in the system must flash simultaneously. The
prescribed flashing rate is 40 flashes per minute. The actual
duration of the flash (on time) is 10 milliseconds during
daylight and 250 milliseconds during night.
Power Supply and Control
High-intensity type of lighting requires a power supply and
control circuitry. Both of these are solid -state units that mount
at the ground level. They may be mounted in a building at the
tower base or on the tower itself (at the base). The units are
housed in a weatherproof cabinet so they can be mounted
outdoors. Monitoring units are also available so that the lights
can be monitored and operated by remote control.
TOWER AND ANTENNA ICING
In many areas of the country. sleet and ice can cause
many problems with antenna systems during the winter
months. Antennas for AM are not affected as much as FM
antennas. Ice also presents a problem to the towers, guys,
people and structures on the ground below the tower.
Physical Effects of Ice
A heavy coating of ice increases the size of the tower
members and changes the wind -loading rating. The tower
members now offer a greater resistance to wind. Tower
designs will take a certain amount of ice coating into account
when developing the wind -loading rating of the tower. The real
problem comes when a tower has been overloaded with
additional elements it was not designed to carry. such as extra
antennas and microwave dishes.
With a guyed tower, the guys can also become heavily
covered with ice. This adds wind -loading to the guys and also
extra weight on them. A tower that is overloaded and then
becomes heavily coated with ice can easily collapse.
Electrical Effects of Ice
Many AM antennas can take ice in stride without any
noticeable effect, yet others may go haywire. It depends upon
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the operating parameters of the particular antenna. Those
with a very high base resistance will be significantly affected,
as can those that use top-loading arrangements or an isolated
quarter -wave section for the FM coax line at the base of the
tower. On guyed towers, ice will bridge the insulators in the
guys and create leakage paths, as well as coat the base
insulator.
Icing of FM Antennas
Although FM antennas are broadband, heavy icing will
cause some detuning. When the antenna detunes, it no longer
presents the correct load to the transmission line, so the VSWR
will increase on the line. The high voltages and currents set up
in the line by standing waves can cause damage to the line and
also to the output stage of the transmitter. These standing
waves will also cause phase shifts in the carrier and its
sidebands, and the phase shifts are detrimental to the stereo
signal. The transmitter can become very unstable.
Ice Formation
Ice can form on the antenna even though there is none on
the ground. Air temperatures at ground level are warmer than
those at the FM antenna location on top of the tower. Moisture
can be freezing on the antenna and tower even though that
reaching the ground is in the form of mist or rain.
The mass of metal in the tower and the antenna are slow to
change temperature. While the air temperature may be above
freezing. that of the metal can yet be below freezing. Any
moisture present will freeze on the metal surfaces. These
same conditions can prevail for some time after the icing
condition has passed on the ground.
The greatest icing occurs when a sleet or freezing rain is in
progress. The antenna and towers will continue to build up a
coating of ice all during the storm. The next day when the sun
comes out. the whole tower may look as if it is made of glass.
Recognizing the Icing Problem
An AM system that is affected by ice will show inaccurate
antenna current readings, and the transmitter output stage
efficiency will deteriorate. The plates of the PA tubes can be
running much redder than normal because the stage is
detuned and dissipating more power within itself.
396
The output stage in the FM transmitter will become less
efficient also, and its tuning may become unstable or erratic.
The transmission line monitor will show an increase in VSWR
on the line. If this rise in VSWR is high enough, the line
monitor will shut the transmitter down.
Icing and Operating Techniques
The AM station that is affected should take some steps to
protect the tubes in the transmitter and coax line. Pull the
power back at least 10%. If the coax line has high standing
waves on it, and if the modulation is at 125% positive and the
line is normally close rated, pull the power back more than
10%.
You might try taking a wooden stick and chipping off the
ice at the base insulator, but be very careful not to break
anything or get burned by RF. If you try to chip ice off any of
the low guy insulators, be careful not to set up vibrations on
the guys or start them swinging. If the horn or ball gap is solid
with ice. chip it out carefully, also the insulator to the tuning
house. Retune the PA for its most efficient operation under the
conditions present. It will need tuning as conditions change
outside.
At the FM transmitter, pull back the output power. Derate
the transmitter according to the VSWR figure. Even though
the line may be heavy enough to carry the VSWR, the
transmitter tubes must be protected. Simply divide the
transmitter power output by the VSWR reading to get the
operating power to use. Make sure the plate circuit is kept in
resonance.
Ice Control Methods
There is no control equipment for AM antennas, but there
is for FM systems. Control is in the form of heaters that melt
the ice or keep it from forming. The entire antenna may also
be enclosed in a radome. The heaters are the less expensive
method and do not add to the wind loading. A station located in
an area where ice will be experienced would do well to invest
in heaters.
Heaters are installed inside the radiating elements of the
antenna and are out of the RF field (Fig. 11 -9). Melting of ice
takes place by conduction of the heat from the heater to the
outside metal of the antenna element. There are no heater
397
HEATER
(A)
(B)
HEATERS
120V,
120V
HEATER
120V 120V
NEUTRAL
230V
Fig.11 -9. Each element of the FM antenna hasa heater mounted internally.
All heaters are connected in parallel. Divide load on both sides of a 230V
circuit.
currents flowing in the metal of the radiating elements. Each
of the heaters will operate on 120V AC and will be about 150W
to 200W each, or 300400W per bay.
There is a heater in each of the radiating elements of a
bay. on both sides. The AC power is fed to the heaters at the
rear of each bay, and the AC harness must be anchored so that
it does not come loose and get into the RF field. This will also
be shielded cable.
Heater Control
The heaters may be operated manually from the ground,
or they may be tied into the tower lighting photocell so that the
heaters come on at night with the lights. If this is done, then a
manual shutoff should be provided so that the heaters do not
run all during the summer months.
The best arrangement uses a thermostat control mounted
at the base of the FM antenna. This will sense air
temperatures at the antenna location and then turn the heaters
on or off automatically.
Heater Power
When a multibay antenna is used, the heaters should have
their own power circuit. If they are tied to the lighting power,
398
they can reduce the voltage available to the lamps, and they
may not work efficiently because of low voltages. If each bay
draws 400W and there are 12 bays, that is 4800W. This is too
much additional current to draw through the lighting cables.
Run a new circuit all the way from the base of the tower to the
heaters. This may be 240V, single phase, neutral ground.
Balance the 120V heaters on both legs of the 240V circuit.
Heater Installation
When the antenna is first installed, turn on the heaters. A
tower worker should feel each of the radiating elements in
each bay to see if it is warm. Remember that each side of the
bay will have separate heaters, so check them both for
warmth.
After these have all been checked and they are working
properly, measure the current drawn in each leg of the power
feed at the base of the tower or in the transmitter room at the
contactor. Then turn off the heaters and measure the DC
resistance of the circuit on each leg. Make the same
measurement after the heaters have been warmed up ( and
shut off). This will provide a hot- and cold- resistance
measurement of the system. Save these resistance and current
measurements for later reference. If any of the heaters burn
out, the resistance figures will change and so will the current
drawn.
LIGHTNING PROTECTION
Antenna towers are likely targets for lightning discharges.
Such discharges can produce havoc with the equipment on the
tower, at the tower base, and inside the transmitter building.
Static Drain
Protection is based on the static drain principle. That is,
charges building up in the vicinity of the tower are drained off
to ground before they can develop full damaging potentials.
Lightning strikes nearby can also induce strong currents in
the tower, so these currents must be drained off before they
can cause damage.
Grounded Towers
The first requirement is a sharp, pointed lightning rod
above the highest part of the tower and any equipment
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projecting above it, for example, the beacon. Some towers use
three rods in a cluster. These rods must be securely bolted to
the tower and should make a very good electrical contact.
During tower inspections, these rods should also be examined
for looseness. Constant flexing by the wind can sometimes
work the bolts loose or cause the rod to break at the bolt
connection.
Antennas and transmission lines for FM should be bonded
to the tower at various places. At the tower base, a heavy
copper strap. or one on each leg, should connect the tower to
ground rods driven several feet into the soil. The concrete pier
is not a good ground connection, so use copper strap. This
connection to earth is very important, and it should be as low a
resistance as possible.
Insulated Towers
The insulated AM tower presents more problems since the
whole tower is insulated from ground (Fig. 11 -10). Protection
at the top and down the tower is the same as for the grounded
tower, with lightning rods and good bonding. But now the
problem is to keep the discharge out of the equipment and get
it to earth without providing a metallic path across the
insulator.
Lightning will follow the line of least resistance, so try to
provide this. In the feed line between the tuning house and
tower, add at least one or two loops about 12 inches in
diameter. This will make that path slightly inductive. Across
the insulator, provide horn or ball gaps. These gaps should be
as close together as is practical; the closer the gaps. the lower
the gap resistance. But you don't want modulation peaks to arc
over, so adjust the gaps with full 125% positive modulation and
set the gaps slightly beyond the point where they will arc over
on peaks. The ground side of these gaps must be connected to
the ground system with a heavy ground strap.
Lightning is unpredictable. It isn't possible to eliminate all
current surges. so it is important that the antenna meter is
removed from the circuit by its shorting switch and the line
meter is not left active in the tuning unit. A heavy surge will
invariably burn out these meters if they are left in the circuit.
And this will take the station off the air, since the
thermocouple is in series with the line circuit when active. If it
burns out, the circuit will open.
400
ROD
ABOVE
BEACON
-p
BOLT TO TOWER
BOND
ANTENNA
TO TOWER
---
BOND
LINE
TO
FM ISOLATION
UNIT
TO FM
TRANSMITTER
LIGHTNING GAP
HEAVY STRAP
AM
TUNING
UNIT
12" LOOP
IN FEEDER
I/ //
GROUND SYSTEM
Fig. 11-10. Standard lightning protection.
Arc Suppressors
When a discharge does occur across the ball gap at the
base of the tower, this will effectively short out the tower and
put a load on the transmitter. The transmitter needs protection
in the form of an arc suppressor or other means.
The arc suppressor is simply a relay circuit in the
transmitter whose coil return is provided through the arc on
the line when it occurs. That is. the arc across the ball gap
provides the switch action. This relay doesn't shut the
transmitter off, but it does remove the RF carrier by removing
the RF drive or plate voltage.
Once the arc starts. the RF would sustain it unless the RF
is shut off. During that brief period before the overloads can
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shut off, the output tubes take a heavy load. If this relay is DC
operated. be careful when making changes in the tuning unit
so as not to add series capacitors to the line, or the arc
quencher will not work.
Lightning Protection for FM Antennas
Side -mounted antennas should be bonded to the tower with
good electrical connections, and so should the transmission
line. If the coax has an insulated covering, this should be
opened at a number of places and the copper outer conductor
bonded to the tower. (If the tower is an AM tower, this will
have already been done for other reasons.) Where the line
crosses the insulator at the base of an insulated tower, ground
this outer conductor shortly after it crosses over the base
(toward the transmitter) to the nearest ground point. If there
are any currents flowing on this outer conductor, get them to
earth immediately after leaving the tower so that they are not
carried back into the transmitter building.
Guyed AM Towers
The guys should be broken up with insulators at various
intervals for reasons already explained. Thus, there are
insulated wires hanging up in the air, and they can have heavy
currents induced in them from discharges. Since there is no
place for it to go, it will jump the insulators. Carbon paths can
build up from these discharges, or the insulators can crack and
eventually fall off. Any damaged insulators should be
replaced.
The techniques that have been discussed are not 100%
guarantees there will be no lightning damage. Elements
should be inspected from time to time in the AM tuning unit
after there has been a heavy electrical storm. The tuning
house or the ends of the guys are not a safe place to be during a
storm or the period immediately preceding a storm. It is also
unwise to have someone in the tuning house calibrating
antenna meters or working on the tower itself during the
period immediately preceding the storm -let alone during the
storm itself. Very often the atmosphere is very highly charged
just prior to the storm.
GENERAL MAINTENANCE
With the antennas and towers exposed to the elements, the
system needs periodic inspections to detect damage or general
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erosion. When signs of failing are discovered, they should be
corrected as soon as possible, or depending upon the problem,
keep a closer check on its progress. Small problems in the
antenna system can suddenly become very large and
expensive problems. The following are a few areas to consider
in the station's outside maintenance program.
Transmission Lines
The horizontal run can be inspected more often for
physical damage. Look for dents, holes, loose ground straps,
and faulty conditions of support posts and hangers. All this
requires is a walk along the line, watching closely for anything
that may be out of normal. Some problems, such as large dents
in the line, can show up as high VSWR. And if gas leaks occur
on a gassed line, then the gas consumption will increase or the
dry air pump will run continuously. Check and correct as
discussed in the chapter on lines.
Tuning Unit
Inside the tuning unit, check for loose and burned
connections on the coils and ground connections. Observe for
tar or other dielectric and sealing material leaking from
capacitors. If these checks are made immediately after
signoff, then feel for heating of capacitors and connections. A
poor connection will show signs of heating without showing
signs of burning. Capacitors going bad can also be heating
without showing other signs yet.
Also check for the presence of mice. If there are holes in
the tuning box, mice will get in. Close up the openings, or if
they are needed for air circulation, then use screen wire.
Grounds
Keep a check on all ground straps around the tuning unit of
the AM tower. These are necessary to tie the ground system
into the antenna and to provide a low-resistance path to earth
for lightning discharges. If any of them breaks, replace it as
soon as possible. and if a solder connection comes loose, get it
resoldered. Keep all the grounds tight. If these become loose,
the antenna system will become unstable.
Gaps
Check out the ball or horn gaps at the base of the tower.
Insects will build nests here and then get scorched. This can
403
also provide a leakage path across the base of the antenna.
Other debris may also accumulate and do the same thing. If
leakage paths aren't created, then the gap width can become
more narrow and arcovers can occur on modulation peaks.
Clean out these gaps with a wooden stick. Be careful when the
power is on so as not to get an RF burn. When the debris has
been cleaned out, check for carbon or metal deposits from
lightning that may have gone across the gap. When power is
off, clean out or polish up the gap with emery cloth.
Arc Quencher
If the transmitter has an arc quencher, check it out
occasionally. This will take two people. and it is best to test
with the RF power off but the transmitter otherwise turned on.
Take a large screwdriver and short the gap momentarily and
have someone observe the relay in the transmitter for
operation. If it doesn't work, check out the relay circuit. Also
check the tuning unit to see that no series capacitor has been
added.
Grounds Keeping
The fence around the base of the AM tower must be kept in
good repair so that unauthorized personnel cannot touch the
tower and perhaps get a RF burn. The FCC requires the fence,
and that it be in good condition. When the inspector is around
and goes down to read the antenna meter, he will also observe
the condition of the fence. If it is falling down or the gate
missing. the station will get a citation. Also, make sure it is
kept locked, just as the tuning house must be kept locked.
Another thing the inspector will look for is weeds growing
around the base of the tower. The weeds must be kept down. If
they grow up over the insulator and get wet, leakage paths are
created for the RF. and this can affect the tower operation.
Getting rid of weeds is easier said than done. Most chemical
treatments are temporary. If nothing else works, at least take
a whip and cut them down.
Construction
If any construction takes place at the base of an existing
AM antenna, such as a sewer line run across the field, the
wires in the ground system can be cut off. It is important that
these be soldered back together, or part of the system is left
404
floating. Explain the purpose of the wires to the workmen
doing the job. They may not understand, but make them at
least understand you want to resolder any wires that are cut.
Turnbuckles
Guyed towers have some type of tension -adjusting
arrangement at the anchors. If this is a turnbuckle, the flexing
of the guys have a tendency to turn them loose. These must be
kept at the proper tension for tower support. One technique for
this is to run a loop of the guy wire through the turnbuckle
after it is properly tensioned (Fig. 11 -11). This can be a scrap
of the guy or the loose end of it. After looping through the
turnbuckle, clamp it together. This will keep the turnbuckle
from being turned, and the guy will stay at the proper tension.
CLAMP
GUY WIRE
CLAMPS
ANCHOR
TURNBUCKLE
Fig. 11 -11. Use a scrap of guy wire or the end of the guy to Loop through
the turnbuckle to keep it from turning.
Corona Discharge
This is a condition that can take place with high -power FM
transmitters and when vertical polarization is used. Unless the
tips of the vertical units are rounded or capped off with a
spherical device, corona can take place at the end. It can also
happen at the gap in the horizontal part. This will usually
happen during misty weather or when it is raining. Corona
can be seen from the ground as a white light at one place on
the FM antena.
At the transmitter, corona discharge will affect the
loading, and the VSWR will change erratically. If this happens,
pull the transmitter power back below the point where the
405
corona ceases. At the earliest opportunity, have someone
inspect the antenna for a missing end cap.
High VSWR
If the VSWR is intermittent or just plain high, and there is
no apparent reason, such as a damaged line or antenna or
moisture in the line, have someone check out the system with
sweep equipment for VSWR across the bandpass. It will be
necessary to open and terminate the line at the antenna so that
a determination can be made as to the source of the problem,
the antenna or the line. A station will seldom have this test
equipment available, so the work will need to be hired out.
Don't let the system operate too long if the VSWR is
running consistently high. Much permanent damage can be
done to the lines, antenna, or the transmitter.
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Chapter 12
Required Inspections
broadcast stations are required to make certain in spections periodically. These inspections are essentially for
preventive maintenance, and the FCC does not spell out the
exact procedures to use. The station can make the time spent
productive if these inspections are carried out in some
systematic manner geared to the station's needs, rather than
simply fulfulling a legal requirement. In the pages that follow,
a variety of suggested procedures are offered that you may
find useful at your station.
All
TRANSMITTING- EQUIPMENT INSPECTION
The station is required by the FCC to inspect its
transmitting equipment and associated monitoring equipment
once each calendar week. There must be at least five days
between each inspection. The inspecting operator must hold a
valid first class radiotelephone license. At the completion of
the inspection, he must make an entry in the station's
maintenance log which certifies that the station is operating
within the technical requirements of the FCC as determined by
the inspection. When any adjustments or repairs are required,
that also must be included in the log entry.
Routine Inspection
While the inspection can be made only of those
components of the system that would fall in the category of
407
transmitting equipment and associated monitoring equipment
and still fulfill the requirement, I suggest broadening this
inspection so that it may be more productive as a routine
maintenance practice of the station. That is, include the entire
transmitter plant in the inspection. If done in a systematic
manner. this would require very little extra time but yield far
more information. Inspecting in a systematic manner requires
planning. Develop a regular routine in a checklist form. This
will serve as a reminder of what to inspect and later as a
reference. Make up copies of the form. Leave a blank space so
the operator can check off each item and a space for
comments or notations beside each item. For the transmitter
proper. make up a meter -reading sheet of all the meter
positions on the transmitter, plus a few others for its operation
(Fig. 12 -1). Again have copies run off for worksheets. All that
should be necessary are blanks along side each item to enter
the meter reading, the rest should be printed out on the form.
Other simple forms may be added and left at the transmitter
site.
Keep a File
Aside from the maintenance log entries that must be
made. all these other forms should be kept in a file for future
reference. They can be of great value when troubleshooting,
and many months later they can show definite trends or
deteriorations in the system. Even on a short -term basis these
week -to -week comparisons can show problems developing (or
now present). These comparisons are easier if the sheets, such
as the meter -reading sheets, are so arranged that each week's
set of readings are in a vertical column and that several of
these weekly columns are on one page. This places a meter
position's readings directly across the page, and any changes
will stand right out. If the set of readings that were taken for
the transmitter when it was installed are available, present
readings can be compared against those to indicate what
changes have taken place. If major modifications are made to
the transmitter, that may change some of the meter readings;
the new set taken immediately after the modifications will
then serve as the latest reference.
Transmitter Inspection
The first thing to do with the transmitter is to observe its
physical appearance and listen to its sounds, that is, its
408
p.
AM TRANSMITTER METER READINGS
DATE
2 -2 -77
2 -9 -75
2 -16 -77
MULTIMETER
AUDIO
IK
mA
OSO
IK
mA
1ST BUFF
lc
mA
1ST BUFF
IK
mA
807
Ic
mA
807
IK
mA
PA
1K
mA
MOD
le
mA
PA
le
mA
PA
EP
RF
FIL
TIME
I
E
V
A
V
hr
COMMENTS:
Fig. 12 -1. Typical transmitter meter -reading sheet. Make up one that suits
your particular transmitter.
acoustical sounds. After an engineer becomes familiar with his
transmitter and has made a number of inspections, he can
quickly sense when something is not normal. A slight change
in the sounds might indicate the air system problems, such as
plugged air filters or back pressure from a building exhaust
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fan. Or he may detect relays beginning to hum, buzz, or
chatter.
Many transmitters provide small windows or observation
ports so that tubes and other components can be observed
while the transmitter is in operation. Look at the power tubes
and modulators (if AM). Observe the redness of the plates, the
brightness of the filaments, and note especially if there is any
blue glow among the elements. This would indicate the
presence of gas (in a "hard" vacuum tube) and that the tube is
going bad. If the PA plates are redder than usual (or white),
the stage is either out of tune, or something has happened to
the load. If the tubes are modulators and the plates are
running hotter than normal, the tube or bias characteristics
have changed. and those tubes are also dying. If the tubes are
ceramic types, look for discoloration of the plate or cooling
fins, which can indicate excessive dissipation. Observe if there
are any burn or arc marks where flashovers may have
occurred around the tube or socket. Then observe any other
components that may be seen through the windows. Again,
look for signs of overheating, arcing, insulation peeling off
wiring, and other abnormal conditions.
The transmitter cabinet should be checked by placing a
hand on its panels and feeling for vibrations, heating, and air
leaks. A relay or contactor, for example, can be developing
loose laminations or the mounting screws may be loose, and
this can set up vibrations that may as yet not be very loud but
can be felt through the panels. If heating is abnormal,
components inside may be overheating and radiating the heat
to the panel; or the air exhaust or internal air system may be
faulty, making the inside cabinet temperature hotter than
normal, and this is transferred to the metal panels. Air leaks
at the seams or joints in air ducts will soon develop dust
streaks on the outside surface of the duct near the leak. Look
for such dust streaks when observing through the window as
discussed earlier. If the duct is accessible, you can feel for the
air leak. Leaks in the system should be tightened up as soon as
possible and not allowed to get too large. If the air pressure
drops too low, the air interlocks can open and shut the
transmitter down.
Meter Readings
Naturally you will observe the PA voltage, current, output
power, and antenna current to see if these are within
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specifications. There should be places on the meter sheet for
these entries since the sheet should record all the meter
readings that are occurring at that time.
Take a full set of readings of the transmitter and record
them on the meter -reading sheet. There will usually be a
switchable meter that can monitor most if not all of the lower
stages in the transmitter and. perhaps, separate filament
voltage and line voltage meters and a running -time meter. The
sheet should be laid out so that the entries required are in the
same direction as the rotation of the selector switch. Lay out
the sheet in the same order the meters appear on the
transmitter, also. In this way a straightforward routine can
be used to take all of the readings without jumping back and
forth between positions or meters.
As discussed in the chapter on transmitters, the original
set of readings should be those that are realized in the specific,
on -site installation. rather than the factory checkout sheet.
This set should be the standard for comparison, and
comparison can prove invaluable in maintenance and
troubleshooting. For example, some lower stage may be far
off its reading from the previous week (although the
transmitter output appears normal) and may require
maintenance before the stage fails altogether, taking the
station off the air.
PA Efficiency
Always compute the PA stage efficiency after taking the
meter readings. This output efficiency figure is a good
barometer of changes that have taken place. There may be a
load problem. a drive problem, an output meter out of
calibration, or simply deterioration in the tube itself. So
compute the efficiency-but don't become elated to discover
the old rig seems to be improving with age, delivering a better
efficiency than it ever has in the past. Don't believe it.
Remember that efficiency is a ratio of power output to plate
power input. An efficiency that looks "too good" can mean the
output meter or the remote antenna current meter is out of
calibration, thus not telling the true facts. If the meters are in
calibration, then something in the load may have changed. In
this case. that super efficiency actually means that the station
is radiating less than normal power, and not the indicated
power you used in the formula (Fig. 12 -2). If the efficiency
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ANTENNA
CURRENT
TRANSMITTER
ANTENNA
I2
SANTENNA
R
x
100 = EFFICIENCY
1p
ORIGINAL:
ANTENNA R = 50 f1
ANTENNA = 4.47 A
P
= I2R = 1000W
I
ANTENNA R HAS CHANGED:
ANTENNA
ANTENNA HELD CONSTANT: ANTENNA
I
R
I
= 45 f1 p =
= 4.47 A
12
R
= 900W
Fig. 12 -2. The PA efficiency can be a good barometer of problems. In this
example, if the antenna resistance changed from 50 to 45 ohms but the
original antenna current were maintained, the efficiency would look good,
but the output power would actually be 10% less.
looks especially good or bad, this calls for some after -signoff
maintenance, or it may call for outside help to measure the
load with special instruments. Always investigate a change in
efficiency. whether a gradual trend or a sudden event.
Coax (Indoors)
Depending upon the type station and its operation, there
may be only a single line, or there may be a maze of coax
sections. elbows, and other fittings in the transmitter room
(Fig. 12 -3). Check the lines for hot spots caused by standing
ANTENNAt
TRANSMITTER
I
st
HOT SPOTS
Fig. 12 -3. Check the line, flanges, and elbows for heating and hot spots.
412
waves. Because of the long wavelength for AM, there would be
one at most, but at the shorter FM wavelength you can have
one or several inside the building. You need to feel the line for
this. and also feel all the flanges and similar connections. If
there are any problems, including high VSWR, there can be
heating at the flanges. If you detect places that are abnormally
warm, you may have some serious problems that require
early maintenance before something burns up. Room
temperatures as well as conducted heat in the copper can
warm the lines, but after a few inspections, the operator will
soon learn what is normal.
Harmonic Filter
The FM transmitter will have a harmonic filter
immediately following its output. Since this unit is dissipating
any harmonic content in the signal, it will run warm. If the unit
is running hotter than normal, look for problems. This can
mean that there is a problem inside the filter itself, but it can
also mean that there has been a change in the transmitter
tuning or other filtering units that are now either generating
harmonics or allowing others to get through. If the units
running hotter than normal, look critically at the meter
readings that have been taken and note the position of the
tuning controls. This can call for some after-signoff
maintenance. You may need to check out the multipliers and
other internal filter circuits.
VSWR Indicator
The VSWR indicator may be part of the output- measuring
arrangement. or it may be a separate line monitor. At least
one AM transmitter now has a built -in VSWR indicator, but
generally these are found in FM stations. These indications
should also be observed and noted on your record sheet. Show
both the forward and reflected power if it is not a calibrated
VSWR indicator.
Whether or not the unit is a calibrated one, the important
thing is the reflected -power reading and its trend. Take into
consideration the weather at the time of the reading. During
cold or very hot weather, the metal in the FM antenna can
expand or contract a small amount so that there is some slight
detuning, thus a little higher VSWR. Of course, if it is raining
and the VSWR is high, that indicates a problem also. Be on the
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lookout for a drift to higher readings from week to week; there
may be problems developing on the antenna or in the line.
You can make DC resistance remasurements on the line and
antenna (after signoff) to compare with the original
measurements. If this doesn't show much change, then plan
on getting some outside help to check out the system with
specilized test equipment. Don't allow the situation to go on
too long, or it can get to the point that line or antenna damage
is caused, resulting in the line monitor intermittently shutting
down the transmitter.
Line Pressures
When pressurized coax line is in use, then the line
pressures should be on the list for inspection. Check the line
gauges for pressure. If this is zero and the pressure pump is
running continuously, there is a wide -open leak in the line.
Shut off the line petcock; the unit should shut off. If a gas
cylinder is used, you will hear the gas hissing into the line. The
line has opened up in some manner. This will call for checkout
and repair. The repairs may have to wait until signoff, but
check it out before leaving the site.
When the coax line is leaking pressure slowly, this is more
difficult to detect. Notice if the pressure pump seems to run
quite a bit when you arrive, and listen for it during the time
you are at the site. The pump may be on and off several times
during the inspection. Suspect a medium -to-small leak in the
line. If it hasn't run at all and the pressure is up, the line is
tight. When a gas cylinder is used for pressurizing the linès,
keep a chart next to the cylinder. Show when it was installed
and the pressure in the tank. You can make an entry on the
chart each week of the pressure. This chart, then, can be a
good indicator of gas usage. By having the chart right at the
cylinder, it will be easy to compare from week to week, and
also. when a cylinder is exchanged, there will be little excuse
for not putting the pressure reading on the chart.
Power Panel
Inspect the power panel for tripped circuit breakers.
There may be some that have tripped, even though they do not
directly affect on- the -air equipment. For example, they may
be on room heaters. Reset any that have tripped and check
why. Aside from breakers that may have tripped, feel the front
414
breakers in the panel. Some may be overheating.
Sometimes breakers will run hot before they fail.
of the
The Building
number of things should be inspected or observed about
the building itself, both inside and outside. This is particularly
important for the remote, unattended transmitter site.
Inspection of the building should be a part of the regular
inspection routine. When approaching the building, be alert
and have your mind on the inspection and not occupied with
other matters. Look over the building, the grounds, the tower,
etc.. as you make the approach. Before going into the building,
take a walk around the outside. Observe the windows, doors, or
any opening or ventilator for signs of attempted forced entry
or plain vandalism.
Once the door is unlocked and unlatched, let go of the knob
and pause momentarily. If the building has an air exhaust
system that is intermeshed with the transmitter cooling
system. the system may be fouled up and two things can
happen: I1) If the system is in full exhaust, but the outside air
intakes are closed or clogged, the room pressure will be low.
When you let go of the door knob, the rush of outside pressure
to equalize that lower pressure in the room will cause the door
to swing inward. (2) If the thermometers are calling for full
exhaust and the outlets are closed. then the room pressures
will build up. When you let go of the door knob, nothing will
happen: but when you push the door open, you will feel
resistance against the door swinging into the room. By being
A
AIR PRESSURE
OPEN
INTAKE
CLOSED
TRANSMITTER ROOM
Fig. 12 -4. An air system in full exhaust but with its intakes closed
create a low pressure in the room.
will
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alert to such things you can quickly gain clues that something
is amiss.
Temperatures and Odors
At your first entrance into the transmitter, be alert for
abnormal room temperatures and odors. These can be clues of
problems either present or developing. If the room temperatures are too warm or too cold, something is wrong with
the heating and cooling systems. A circuit breaker may have
tripped, or there may be mechanical problems with the
louvers -a motor burned out, etc. When components are
overheating, they will give off odors. If the odors are very
strong. then something is in the process of overheating; but
also be alert for fainter odors. Something may have burned out
a couple of days earlier.
Water Leaks
The roof may have sprung a leak, or there may be leaks
around outside air vents, windows, or doors. Observe the
ceiling for water stains or wetness, and the same around any
of the openings. Leaks in the roof can allow rainwater to drip
(or run) into the transmitter and equipment. The leak itself
doesn't have to occur immediately above the equipment; it
can run down rafters from some other section. And the opening
it finds may be right over the transmitter. Roof leaks can
occur at any time, but be especially alert in the spring. Falling
ice from the tower may have damaged the roof, and with the
spring rains, come the large leaks.
Security Lights
When all has been inspected inside the building, it is time
to go out to the tower. Turn on the outside security lights first.
These may normally come on with the photocell, but you may
wish to check them by turning them on manually. This may
require a pass around the outside of the building, and if any are
out. they should be replaced. If you wish, go back in and turn
them off, and when you get to the tower )assuming the
photocell is here). cover up the photocell to note if the security
lights do come on.
Walk the Line
On the way out to the tower, walk the transmission line
run. Observe the supporting posts or structure, the ice shield
416
over the line, the line hangers, and the line itself. If everything
inside the building looked okay, then the line check can be
casual. But if there were indicated problems, then take a close
look. If VSWR was high, feel the line for hot spots and the
flanges for heating. If there were pressure leaks, check the
flanges and solder joints for leaks with a soap or chemical
solution.
Doghouse
The AM station may or may not have a "doghouse" ( house
for the tuning apparatus). but it will have a tuning unit at the
base of the tower. For a doghouse, inspect the outside and the
door for signs of attempted entry or vandalism. If the
tower- lighting photocell is on the tuning house, inspect it also.
An unprotected photocell can become the target for
stone -throwing vandals. If the photocell is okay. cover it up to
see if the tower lights come on; if the security lights are also
connected to this circuit, note if they come on also. Note if the
door is actually locked -the last one at the tuning house may
have forgotten to lock it! The FCC requires that the tuning
house and gate around the tower be kept locked and that the
key be accessible to the operator. There should be a key at the
control point and also one inside the transmitter building -in
case whoever came out to check the place forgot to bring the
key along.
Be alert for things amiss, just as you were in the
transmitter building. Odors can warn that something is
overheating, such as a capacitor or lighting choke (if used).
There can also be other odors: mice that got into the unit and
were roasted. Feel any coax lines near their end terminals or
flanges for heating. Inspect all the ground straps for tightness
and see that they are in place. Remember that these tie the
system to the antenna ground system and are very important.
Tuning Unit
Open up its door and take a look inside. Note if the antenna
meter switch has been left in a position that would allow the
meter to read. Check the antenna current while you are at it,
and note any calibration charts that are present. Calibration
charts should be posted at the tuning unit or in the doghouse,
although this can be a copy of the actual chart.
Leaving the line meter active in the circuit is just as
hazardous as leaving the antenna meter active. Lightning
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discharge can blow both of them out and take the station off
the air. If this meter has been left in the circuit and it doesn't
have a switch arrangement but a plug instead, then before you
leave, arrange with the control room to turn off the
transmitter long enough to make the change. If you work out
your cues properly, it should only take a few seconds. Then
observe the capacitors for overheating or leaking. With power
on, don't feel the capacitors. As a matter of fact, it is best to
keep your hands out of the box altogether. Also note the coils,
especially the taps, for discoloration or signs of burning and
arcing. There may be discoloration, or there may be soot from
an arc. Observe the static drain choke; this may be cooking
down. Before leaving, make sure the antenna meter and line
meter are out of the circuit.
QUARTERLY LIGHTING-EQUIPMENT INSPECTION
There must be a daily observation of the operation of the
lights and if any are out, this must be reported to the FAA
tower or Flight Service Station. The FCC also requires that all
the tower -lighting equipment receive an inspection at intervals
not exceeding three months. This is a preventive maintenance
requirement. The fact that the light operation is observed
every day and that it is operating will not substitute for this
quarterly inspection. Either the station personnel may make
this inspection, or an outside service company may be hired.
Outside Service Companies
There are many companies that specialize in towerlighting service and maintenance. As in anything else, there
are some good ones and some not -so-good ones. Always keep in
mind that it is the licensee that is held responsible for
compliance with the FCC -not the outside service company.
The licensee's responsibilities cannot be delegated to anyone.
Check up on the work done by the service company, and
don't pay for work unless you are reasonably certain it has
been performed. I had one experience where the serviceman
who was supposed to inspect and relamp the tower simply
drove by after dusk when the lights were lit. He noted that all
were lit and filed his report as having inspected and changed
all the lamps. An easy way to make a buck, but he didn't get
away with it. They had to send a man back and actually do the
job.
418
Don't let anyone on the tower unless he reports to the
control operator why he is there. The serviceman should come
to the control room and talk to the operator. If the transmitter
is an unattended site in the country, then someone from the
station should go to the site and observe that the work is being
done. In all cases. check on the work being done. Many of these
service companies will leave the old lamps that were taken out
during the relamping.
What to Inspect
The FCC requires that all mechanical or automatic
controls and other devices associated with the tower lighting
be inspected to insure they are operating properly. So what to
inspect actually depends upon the individual system in use at
the station. There are relays, contactors, transformers, circuit
breakers and panels, flashers, and the associated wiring (Fig.
12 -5). If there are other relays that tie into the lighting control,
they should be inspected so that a malfunction will not
adversely affect the tower lighting. If the station has a
contract with an outside service company, they will also
relamp the tower each time they inspect the fixtures.
TOWER
LIGHTS
OPERATING
PROPERLY
PHOTOCELL
f
10
AC
/
CIRCUIT
BREAKER
I/
i
i
l
', i
F
%
I/
1
f_
/
Ì
/
I
i
I
/
.
'/
CONTACTOR FLASHER
COIL &
CONTACTS
LI GHTING
CHOKE OR
TRANSFORMER
Fig. 12-5. Things you are required to check on Inspection.
Photocell-All towers except those under 150 feet must
have a photocell to control the lighting. This is for standard
lighting as well as high -intensity lighting. Cover up the
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photocell to observe that it does turn on the lights. Listen for
any noisy internal relays. Look over the glass enclosure for
cracks, and note if there is moisture condensation on the inside
of the glass. These are designed to mount outdoors, being
sealed with gaskets. If the gaskets have deteriorated, moisture
may be getting inside.
Relays-The photocell does not usually switch the lighting
directly. It uses relay interfacing that operate contacts. The
photocell operates the coil of the interfacing relay. Check out
the appearance of the 120V coil for heating signs. The coil
changes color or the wrappings crack when overheated. You
can feel the coil also, but be careful of the 120V. If the
mountings or laminations become loose, the relay can hum
and buzz. This is no real problem, but it can be a nuisance if
the relay is mounted where someone has to listen to it all day.
Tighten down any loose mountings and inspect the contacts.
The contacts switch the load, producing a high -current drain.
So check for burned and pitted contacts. Clean and dress them
as needed, or replace the contacts if they are beyond repair.
Check all of the relays that are directly associated with the
lighting. If the system is a complicated one, then a checklist
should be made up so as to not miss any.
There will most likely be other relays connected to the
photocell which interface to other functions, such as outside
FROM PHOTOCELL
ï
LIGHTING
TA;TOR
C
TO TOWER
LIGHTS
120VAC
ALARMS
OR
SECURITY
LIGHTS
EXTRA
RELAY
i
120V AC
Fig. 12 -6. Check other relays that may operate from the photocell.
420
security lights and alarm systems IFig. 12-6). Having security
lights tied to the photocell insures they will turn on
automatically at dusk. Alarms will warn the operator that the
lighting has turned on. It is important that any additional
relays that are attached to the photocell control be inspected,
especially the coil. If this coil should short out, it will short out
the photocell and lighting control. If it opens up, the security
lighting or alarms won't work.
Flasher-Code beacons require a flasher unit. This may be
mounted at the tower base or off the tower. Listen to its
operation. The rotating machinery can become dry and noisy.
Add a drop of oil or grease where required and observe the
mechanical operation to see if it is smooth or jerky. The motor
or bearings may be becoming defective and may soon need
replacement. Check out the fail -safe features on the particular
unit. If the unit has dry contacts, inspect these for pitting and
burning. Replace any defective contacts.
A simple test device can be made up to test the flasher unit
(Fig. 12 -7). At the base of the tower or in the tuning house, it is
difficult or impossible to also see the top beacon while
observing the flasher unit. Get one of those rubber -covered
outdoor sockets that has pigtails on it. Attach alligator clips or
test leads to the pigtails and insert a regular light bulb. Attach
the leads to the load output of the flasher to observe the
operation of the test lamp. If the light only flickers on and off
r
MERCURY SWITCH
!LOAD
NEUT
TO
BEACON
L
FLASHER
TEST LEADS
WITH CLIPS
RUBBER -COVERED
SOCKET
REGULAR
LIGHT
BULB
Fig. 12-7. A simple flasher test unit.
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and the switch is a mercury tube, the mercury is probably
parted or the tilt mechanism is improperly adjusted. Take a
stopwatch along to time the flashes. The on time should be
twice as long as the off time, and there should be no more than
40 nor less than 12 flashes per minute.
Lamps & Fixtures-When the towers are relamped, the
serviceman will also inspect the fixture, socket, wiring, lenses,
and color screens. If any are cracked or broken, they should be
replaced. Moisture in the unit can cause shorts. If a drop of
cold rain water splashes onto the lamp while it is lit, it will
shatter. Any condensation that occurs should be allowed to
leak out, or at least the unit should be allowed to "breathe."
There will be small "weep" holes for this purpose. These
should be kept open.
Logging
When the lighting- equipment inspection has been made, an
entry is required in the station's maintenance log showing
when it was done. If there were any repairs necessary, then
these should be part of the entry. If the problems were found
that couldn't be corrected immediately, this should be
entered on the log also. When the necessary parts are
installed, make an entry on that day's maintenance log.
Besides the log entry, you may desire to make a notation
on your calendar or similar record of the date the inspection
was made. This method will serve as a reminder of when the
next one is due and saves leafing through a stack of logs to find
some specific date or entry.
ANNUAL INSPECTION
The station has a large investment in its tower, antenna,
and transmission line. That these expensive system items are
constantly exposed to the elements 24 hours a day would seem
to indicate that they should receive close attention and
periodic inspections to insure they will serve faithfully for
many years. The tower and antennas should be inspected at
least once a year.
It is doubtful the station would have either the equipment
or competent personnel to do an adequate inspection.
Consequently, a tower erection company or service company
should be retained to do the work. Spell out what you want
done. Ask for bids from at least three companies. Major
422
companies will bid reasonably close so long as you state what
is to be done. When the bids come in, make certain they spell
out what they are proposing to do. Be careful of a very low bid.
This outfit may not be planning to do half of what you want.
Don't accept a simple term such as "tower inspection" on the
proposal; have it spelled out. Then there is no question, either
when the work is in progress or afterwards.
Include in your letter asking for bids the height, number of
towers, and whether they are guyed or self- supporting. Specify
what you want done: Inspect the condition of the paint; inspect
tower members for rust, galvanizing condition, loose or
missing bolts, and bends; check the lightning rod, lighting
fixtures, conduit, guy insulators, and guy condition; measure
tension and adjust guys; check the plumb of guyed tower;
inspect the condition of the FM antenna, grounds,
transmission line, and hangers. Specify the time of year to do
the work (allow some leeway) and that evidence of adequate
insurance coverage be provided. Also state that a full report
must be made after the inspection has been completed.
Repairs
Ordinarily, this will only be an inspection and will not
include any repairs (except minor ones). The service company
will not have the equipment along to make any major repairs.
Some work requires winch trucks and tower -rigging
equipment. Such equipment is only sent to the job sites when
needed. If you know ahead of time that some major work must
be done. then advise them of this in your request for a quote.
This work will usually be quoted separately. If there are
needed repairs, detected by the inspection, that can be done
without additional equipment, you will be charged extra. So
make sure all this is clear in the contract.
The best time of the year to have the tower inspected is
during the summer months. Not only does the work go faster,
but if there are any major repairs needed, time is allowed foi
obtaining any necessary parts before bad weather sets in. Try
to spell the time out in the inspection contract
Guys
These cables are what keep a guyed tower standing. They
should be inspected for rust, missing clamps, broken strands,
missing or cracked insulators, and proper tension. Insulators
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are used only on towers where it is necessary to break up the
electrical length of the guy. If an insulator breaks and falls
off. the guy will not come apart, but there will be slack in the
guy. If there are broken strands, the strength of the guy is
weakened; if clamps are missing, the remaining clamps are
carrying the full load.
Tension-It is important that guys at every level have the
correct amount of tension so that the pull is the same on all
sides and levels of the tower. If they are improperly tensioned,
they will affect the plumb of the tower or cause the tower to be
S-shaped. That is, if all guys on one side of a tower are too tight
and those on the opposite side too loose, the tower will tilt. If
they are different tensions at different levels, the tower will be
as crooked as a snake. All these are bad for the tower,
reducing its ability to stay standing during high winds.
Measure the Tension-The tension is measured with a
special device called a dynamometer. If the wind speed is
more than 20 mph. the contractor will not measure the guy
tensions. Measurements will not be accurate if there is too
GUY
1
2
3
4
BOTTOM
2ND LEVEL
3RD LEVEL
4TH LEVEL
Fig. 12 -8. The tension on each guy should be measured and recorded on
the report sheet.
424
I.
much wind. Measuring guy tension should be part of the
contract, plus any adjustments that may be required. These
measurements should be a part of the final report (Fig. 12 -8) .
Shoot the Plumb-Plumb means that the tower is standing
straight from bottom to top and does not tilt in any direction.
The plumb is measured with a surveyor's transit. It should be
measured from two different locations two sides of the tower)
that are 90 degrees apart Fig. 12 -9). The transit should be set
up no closer to the tower than the farthest guy, nor should
there be a building or tree in the way to obscure the view. The
operator must be able to observe each leg of the tower from
(
(
lA
2B
1B
POSITION
2A
POSITION 2
1
Fig. 12-9. Transit should be set
up at two different positions 90
degrees apart. On a triangular
tower, this may be less than 90
degrees. Check opposite legs of
tower in each position.
POSITION
1
bottom to top through the transit. At each of the locations, two
opposite legs are observed (one at a time. The crosshairs of
the transit are centered at the outside edge of one leg, then
tilted up through the full length of the tower to note any
deviation from the starting position. Then the same is done on
the opposite leg. Special note is made of the levels where the
guys attach: improper tensioning can pull the tower out of
plumb. The same procedure should be followed at the other
location 90 degrees from the first transit position. This time
the other pair of opposite legs are measured.
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FM Antenna
It is important to have the FM antenna system inspected.
Changes will show up as VSWR, pressure leaks, and gas
consumption; but there are many mechanical factors which
can be operating that don't show up until failure occurs. Some
of these can be detected from the ground with a pair of
binoculars, but they are not always readily visible even then.
A careful inspection should be made of the antenna for
loose connections, bent elements, discolorations (from arcing
or corona), and parts missing. Check that the heaters are
working properly. Observe carefully the actual feed to each
bay. that is, the strap or short stub which feeds from the center
conductor to one side of the horizontal element. The bolt may
be loose or missing that would seriously affect that bay. For
those stations using vertical polarization also, observe the
horns of the vertical elements for discoloration, which can be a
sign of corona. Corona can also take place across the ends of
the horizontal rings. Look for any missing anticorona devices.
Heaters should be turned on while the man is on his way up
the tower. This will give the elements time to warm up. He
should actually feel each element to make sure it is heating.
The RF power should be turned off for this check. Remember,
there are two heating elements in each bay, so make sure he
checks both sides of each bay. At the ground level, the current
drain of the heaters can be measured and compared against
previous readings.
Transmission Line
The line can be checked on the way up the tower. In an AM
station, check the isolation unit. If an isolation section is used,
it is important that insulators are in place and that none are
painted, cracked, or missing. Any problem here will detune
the AM antenna. Then check the flanges for heating, loose
bolts, hangers. and ground clamps. Also look for dents in the
line due to ice falling. If an isolation section is used, carefully
inspect that important first strap where the coax bonds to the
tower.
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Chapter 13
Tests and Measurements
A station has a sizable investment in its equipment. In
order
that this high-quality technical equipment can be properly
maintained, it is only logical that test equipment of
comparable quality and variety be owned by the station. For it
to be of any real value, however, station engineers must
understand how to use the test equipment to its fullest
capabilities.
TEST EQUIPMENT
Any test unit is basically a device that is external to a
circuit and compares samples of the operating circuit to some
outside value or standard. The test unit should not affect the
operating circuit in any way.
Testing is essentially comparing some value against
another standard or an arbitrary value, then detecting the
deviation. The values may be normally accepted
values -voltage, current, resistance, or waveforms -for the
type of equipment; or they may be more rigidly controlled
standards, such as the frequency of WWV. Throughout the
broadcast station, there are a variety of degrees of accuracy
required. Some may be loose go /no -go comparisons, and
others for strict tolerances, such as the carrier frequency
measurement. The majority will fall somewhere in between
these extremes.
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Portable and Fixed
Measuring devices may be either fixed or portable. Both
types will be in use at the station. Those in the fixed category
will be factory -installed meters on units such as the
transmitter multimeter and the console VU meter. There are
others which station engineers install for remote -metering
purposes or for a running measurement on some unit.
Selecting Instruments
When selecting portable test instruments, there are
several factors to consider. One of these is the quality of the
instrument. Quality depends upon the accuracy of the end
results and the ability of the instrument to withstand the
handling necessary in portable use. Test equipment is
available in a wide range of accuracies, beginning with those
intended for the home hobbyist to laboratory standards. Most
broadcast instruments fall somewhere out in the middle of this
range.
What test equipment to purchase is often dictated more by
preference than need. But we need to be practical, balancing
preference and need against the budget. The end results are
what should be the controlling factor.
Many tests are of the simple go /no -go variety or simple
comparisons that do not require absolute values. For example,
when measuring a 24V power supply bus to operate relays, it is
immaterial if the meter indicates 22V or 26V instead of the true
24V. When erratic switching occurs, however, and
measurement shows the bus indicating 14V instead of the true
16V, this is really material. A standard multimeter will help
the engineer troubleshoot this problem; it is not necessary that
a lavoratory instrument be available to indicate the precise
voltage.
Most of the test instruments for broadcast work are the
better instruments used in service shops. Seldom are
laboratory- accuracy instruments required. When some
particular measurement does require such accurate
measurements, accurate instruments can be rented, the work
performed by the station's consulting engineer, or one of the
service companies can be contracted. Making the
measurement of the AM antenna resistance, for example.
requires accurate, specialized equipment. A number of
stations with a directional antenna do own an RF bridge; this
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First, a few factors involved. In the AM process, the peak
of the audio signal will be exactly equal to the peak of the RF
signal at 100% modulation. To derive a noise figure, a 100%
reference point is necessary. The regular AC voltmeter and
the noise meter will read in RMS voltage; they have been
modified so they indicate in this fashion. The detector will be a
peak- reading device, and the DC output voltage will be equal
to the peak RF carrier voltage.
Making the Noise Measurement
First. use the detector and measure the RF carrier
voltage in the recovered DC value by a regular DC voltmeter
IFig. 14 -13). Adjust the RF input coupling to produce
approximately 3V to 4V DC. This represents the peak RF
carrier. Now multiply 0.707 times this DC value to derive the
RMS value. Use the signal generator with a low- frequency
audio tone and feed this to the noise meter until that computed
RMS value is obtained. Then read this value on the decibel
scale. This calibrates the noise meter with an audio signal
whose peak is equal to the RF carrier peak (100% AM
modulation).
Do not change any setting on either the detector coupling
or the noise meter, since both are now calibrated. Remove the
audio tone from the noise meter and connect the calibrated
noise meter to read the output of the detector rectifier. Reduce
the meter sensitivity pads until a reading is obtained on the
meter. The meter reading and the pads will then give the noise
value. If this is high or borderline, use the scope on the output
of the meter to identify the noise element. In the majority of
cases. it will be coming from the power supplies, although it
can be from cathode -to-filament shorts in one of the power
tubes.
Graphs
The audio response curves of the system will follow the
standard 75 µsec preemphasis curve. That curve is not easy to
draw. As a suggestion, you may have the standard curve as
shown in the FCC Rules taken to a print shop and have them
print you up a batch of sheets which have both the graph
divisions as well as the curve tolerances drawn in. Or if you are
good at this type of drawing. then make up a master form by
drawing the curve limits on a graph sheet and run this through
a copy machine for worksheets ( save the master) .
www.americanradiohistory.com
1
2 ADJUST GEN. OUTPUT
TO RMS CALCULATED
ADJUST RF INPUT
AND MEASURE D.C.
VALUE
RECTIFIED C.C.
RF-
3V
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PEAK
DETECTOR
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D.C.
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AC
AUDIO
SIGNAL
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RMS
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NOISE ON CARRIER
3 CALIBRATE
NOISE METER
RF ---10
/RMS
AUDIO
SIGNAL
GENERATOR
/2.1 VAC
PEAK
DETECTOR
Fig. 14-13. To measure AM noise on FM carrie : (1) Measure DC output of
the peak detector. (2) Compute 0.707 of this DC value and set output of
signal generator to this value. (3) Use this AC signal to calibrate the noise
meter. (4) Connect calibrated noise meter to detector to measure any
modulation components on carrier. For example shown, RF was adjusted
to obtain 3V.
When drawing in the actual curve you measured,
remember that the input levels actually describe a 75 µsec
deemphasis curve, the opposite of the system response. But
you must plot the system response. So change every sign to the
opposite sign. For example, if your measured signal level were
-15 dB (from 1 kHz reference) at 15 kHz, change this to +15
dB for plotting purposes.
Other Measurements
Besides the required measurements, you may make any
other measurements you desire. One good example is a power
486
measurement. If the station uses direct power measurement,
then the output power meter must be calibrated at least every
six months. There is no requirement that it be done at proof
time. but this seems like a logical time to make one of the
required calibrations.
Remember to restore all the equipment to its operational
mode. Switch the processors to their active mode, and make
sure all patch cords and terminating plugs are out of the
system. If the transmitter was switched to the dummy load to
make a power measurement, restore it back to the antenna. If
a temporary patchup of the recording booth or similar source
was used to replace an automation system, then it is advisable
to run a little music programming through the system after it
has been restored to make sure that it really is restored. You
can use an audio identification out of the system for the final
station identification also.
STEREO PROOF
The technical requirements for stereo are found in
73.322 of the rules and highlighted in Table 14 -3.
Part
Proof Requirement
The rules do not require that an annual proof be made for
stereo. When a station is operating in stereo, the technical
requirements of Part 73.322 must be met all the time, just as
must all the other technical requirements of the rules. But the
only proof requirement is that of the monaural proof as
described in Part 73.254. The actual wording is somewhat
vague in this regard. After consultations with several people at
the FCC and in the industry, I learned that the only
requirement for a proof is that of the monaural proof.
Good Engineering Practice
It would seem that a variety of measurements must be
made to demonstrate lunder controlled conditions) that the
stereo system does meet the technical requirements of the
rules. especially if the station is full -time stereo. During the
annual monaural proof of performance, there is no prohibition
against making additional measurements to the system for the
station's own purposes. It would seem logical that a set of
stereo measurements be made at this time to obtain a broader
view of the technical operation than that derived from the
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Table 14-3.
STEREO TECHNICAL STANDARDS.
AUDIO RESPONSE, DISTORTION, NOISE
Identical with those of monaural except modulation percentages
include 10% pilot.
SEPARATION
Measurement to be made of left and right audio channels, 50 Hz
to 15 kHz.
29.7 d9 (each channel).
Limit:
-
CROSSTALK
Measurement of the crosstalk from the main channel into the
subchannel.
(Main channel modulated with 400 Hz, subchannel unmodulated.)
Measurement of crosstalk from subchannel into main channel.
(Subchannel modulated with 400 Hz, main channel unmodulated).
40 dB below 90 %modulation.
Limit:
-
SUBCARRIER SUPPRESSION
(38
Measurement of the degree of suppression of the subcarrier
the
subchannel.
of
kHz) 'without modulation and with modulation
Limit: 1 %(or less) of main carrier modulation.
PILOT
Within 2 Hz of 19.000 Hz.
Modulation: 8%10°/ of main- carrier modulation.
SUBCARRIER
38 kHz, 2nd harmonic of 19 kHz Pilot.
Amplitude modulation, double sidebands with suppressed carrier.
mono proof. For our purposes, we will call this a
proof." but remember that it is not required.
"stereo
Method of Operation
for
As with the monaural proof, it may be difficult
automated stations to find their main microphone input, but if
the station is on full -time automation, this is really a moot
question anyway. Of course, if the operation is live, then there
is no problem, and the test should be run through the console
microphone input jacks.
For full -time automation, I would recommend that the
the
signal be applied to the input of the system where
test
automation system feeds it. That is, substitute the
generator for the automation system (Fig. 14 -14). I do not
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recommend running test tapes through the automation system
for the proof figures. Test tapes are subject to stretching. edge
damage, etc., so that the results are not always repeatable or
accurate, and they rely on the head and characteristics of the
particular machine that is playing the tape. Check out the
automation system by itself and get it where it should be in
technical specs; but for the stereo part of the system, feed the
signal in place of the automation system.
Equipment Required
The test equipment required is the same as for the AM and
FM proofs, that is: the signal generator, audio voltmeter and
pads. and noise -distortion analyzer. The station in stereo
must have a FCC type- approved modulation monitor in use.
To obtain type approval, the monitor must also include
special test provisions so that the stereo measurements can
be performed. An oscilloscope is also necessary for phasing
adjustments of the stereo generator and can be used for the
other purposes described earlier.
Special Arrangements
The test equipment is monaural, so to make some of the
special stereo measurements, some means must be provided
that will allow the monaural test signal to feed both channels
at the same time. At other times, phase reversals will be
necessary to make some measurements. The engineer may
find it desirable to build up a switching arrangement in a box
to handle these functions, or they can be done with patch
cords.
When patch cords are used, there should be at least three
jacks wired together, which will, in effect, place the two stereo
inputs in parallel across the single output of the signal
generator (Fig. 14 -15). This is the method I use, and I find it
practical and easy to set up. Make sure the test cords are the
same length and mark the plugs to show which will be right
and which left. Use the notch on the side of the plug for
polarization.
One more precaution: Use an ohmmeter and check
through the patch cords to make sure the plugs haven't been
put on in reverse. This can happen if they were apart for
repairs and the engineer was careless about polarity when
putting them back together.
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SHOWS POLARITY
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SIGNAL
GENERATOR
300
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IMPEDANCE AS
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MULTIPLED
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PATCH
CORDS
Fig. 14 -15. Use patch cords and multiple jacks to strap generator
output
to feed both left and right channels together. Change generator output
impedance to match the resulting impedance as closely as possible.
There is also an impedance problem when paralleling the
two 600 -ohm stereo inputs. This results in a load impedance on
the generator of 300 ohms. So use the 150 -ohm or nearest
impedance available on the signal generator output.
Phasing
The first preliminary checks of the system should be of its
phasing. The phasing is of two types: 180 -degree phasing and
lesser degrees of phasing. The 180- degree phasing will switch
channels if the channels are both reversed; but if the polarity
of one channel is 180 degrees out from the other, this can shift
the audio of one into the other channel (according to the
phasing), leaving little or nothing in the opposite channel.
Lesser degrees of phasing will affect the separation
measurements and the actual channel separation.
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The channel phasing should be checked first. Set up the
signal generator to feed the left input of the system with a
very low-amplitude 400 Hz tone. Connect the vertical input of
the scope to the stereo generator composite output, and
synchronize the scope horizontal sweep with 19 kHz. This may
be obtained from the 19 kHz test jack on the generator. Adjust
the horizontal sweeps so that only one cycle of the 19 kHz is
displayed. If you do not use too much audio input to the left
channel and the phasing is correct. then small sections of the
400 Hz will be seen in the first and third quadrants of the 19 kHz
cycle IFig. 14 -16). Observe the modulation monitor. It should
show that the audio is in the left audio channel.
(A)
ONE CYCLE OF 19 kHz
DIVIDED INTO ITS 4 QUADRANTS
(B)
LEFT CHANNEL ONLY
(C)
RIGHT CHANNEL ONLY
Fig. 14-16. (A) Divide one cycle of 19 kHz into its four quadrants. (B)
Modulation will appear in quadrants 1 and 3 when left -channel -only signal
is fed. (C) Modulation will appear in quadrants 2 and 4 when right channel -only signal is fed.
Now you may check out the right channel if desired. This
time, feed the audio to the right audio input of the system. The
audio will appear in the second and fourth quadrants of the 19
kHz cycle displayed on the scope, and the audio will indicate on
the right -channel meter of the modulation monitor.
492
Polarity
The previous test showed that the audio channels were in
the correct position but didn't show if one of the channels had
its polarity reversed. This can be checked at the output of the
stereo generator or at the composite output of the modulation
monitor. Since you are already set up with the generator, go
ahead and use that. First, feed 400 Hz to both the left and right
channels simultaneously, in phase; that is, L = +R. When
both channels are in phase, the matrix in the stereo generator
will sum the two and send them into the main channel of the
transmitter. That is, they will be an audio signal only to the
transmitter. So the output of the stereo generator will supply
an audio sine wave of 400 Hz, equal in peak -to -peak value to the
normal composite output. But if one of the channels is out of
phase. then the matrix will see out-of-phase or L = -R
signals at its input and will send these all into the subchannel.
At the output of the stereo generator, you will have
displayed two halves of the 400 Hz cycle, but each half -cycle
will be filled in with 38 kHz waves (really sidebands of the
subcarrier). If there is a polarity problem, go down the jack
field and feed the in-phase audio to both channels at different
places, and this will quickly run the problem down to the
offending unit. Look for the wiring to have been replaced
incorrectly either at the input or output terminals.
Pilot Phasing
The pilot and 38 kHz switching must be properly phased, or
there will be separation problems. Leave the scope monitoring
the composite output of the stereo generator and synchronized
to the 19 kHz. Feed audio to the left and right channels out of
phase. that is; L = -R. (Simply turning over one of the
channel patch plugs will do this.) This produces the typical
butterfly pattern on the oscilloscope. Expand the scope display
so as to better view the center, where the wings connect
together. The horizontal points of each wing should face each
other and be lined up on the horizontal axis through the
display. If they do not, there is a phasing problem. Adjust the
pilot phasing control on the stereo generator to align these two
points.
If you have already run the monaural proof, then the
system is within normal tolerances; but you may still want to
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19 kHz PILOT
38 kHz
(A)
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HORIZONITAL
AXIS
"WING" POINTS
ON HOR ZONTAL
AXS
(PHASING IS CORRECT)
Fig. 14-17. Measure composite output of stereo generator. (A) Obtain the
typical butterfly pattern. (B) Expand center area. Points of wings should be
on the same horizontal axis. If not, adjust pilot phasing in generator.
take a couple of sample measurements to make sure the new
arrangements have not affected impedances or introduced
noise or hum into the sytem.
Level Setting
Set signal levels throughout the audio system so that the
left and right audio channels are balanced throughout. First,
feed a 1 kHz tone into the left -channel input. Have the correct
impedance matching. This level should be at -50 to -55 dB, as
in the monaural proof. Use the audio voltmeter and measure
the output of each unit in the left chain so that it is within its
normal range. The output signal will probably be 0 or +4 dB,
depending upon the stereo generator. Now allow this to
modulate the transmitter, and adjust for the correct
494
percentage of total modulation This will be 100q. which also
includes 10rß pilot.
With the left channel properly calibrated, move the
generator to the right- audio -channel input and again measure
and adjust levels throughout the channel. But this time, adjust
each unit so that its output matches that of the level you
measured on the left channel at that point. In this way, the two
audio channels will apply equal -amplitude signals to the stereo
generator. Once the two channels are set up and calibrated, do
not make any changes in them.
Separation
Now that the audio levels throughout the left and right
channels have been calibrated, you may wish to check out the
separation. To do this, feed 400 Hz to either the left or right
channel and measure the composite output of the stereo
generator with an oscilloscope (Fig. 14 -18). If desired, you
may shut ¿ff the pilot for this check, as that will make a
clearer pattern on the scope.
BASELINE
B
POOR
SEPARATION
SEPARATION =
20 log
A
B
FLAT BASELINE
GOOD SEPARATION
Fig. 14 -18. Check the separation of the stereo generator with a scope.
Feed left or right channel. Base line should be flat for good separation.
Adjust separation controls on stereo generator to flatten baseline.
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Check the baseline of the pattern. It should be perfectly
flat. If it is not, touch up the separation controls in the
generator to flatten out the line. Now feed the other channel
and again flatten out the line if necessary. It may take a few
tries, as the two channels and the adjustments are
interrelated. Restore the pilot to its normal value.
Stereo Monitor
If there is any question about the stereo monitor, then go
through the appropriate checkouts and adjustments as
recommended in the instruction manual. If it does need
adjustment, then the composite output from the stereo
generator will be needed for a direct feed to the monitor. If the
preliminary test runs check out okay and the stereo generator
itself seems to be in correct adjustment, then the monitor is
probably okay.
It is important that the stereo generator be adjusted for
separation and phasing with a scope, and not the monitor. If
the monitor is out of adjustment and the stereo generator is
adjusted to the monitor, it is a case of the blind leading the
blind. The measurements may appear okay, but they are not.
Once the generator checks out all right with the scope, go
ahead and adjust the monitor against the generator.
Response and Distortion
These measurements are made in the same way as in the
monaural proof, with the exception that the left or right
channel is measured singly (Fig. 14 -19), and the total
modulation will be 10% less; that is, there will be 90%
modulation and 10% pilot. When you do make the
measurements, terminate the input of the opposite audio
channel with a resistor so that the channel does not allow
open -circuit hum to foul up the measurements. Actually, it is
best if this termination is applied at the input of the stereo
generator.
For the distortion measurements, connect the distortion
analyzer to the distortion output of the left or the right channel,
as the case may be, on the modulation monitor. Make sure to
switch in the 75 µsec deemphasis on the monitor, if it is a
switchable item.
Separation
The separation measurements should be made
immediately following the response and distortion
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measurements. You measure the response at 100%
modulation. After recording the input level, check the
distortion for that frequency. Without changing anything,
make the separation measurements on the monitor. How they
are actually performed may vary from one monitor make to
another, but with the modulation at 100 %, the separation
switch is placed in the opposite channel, and the meter pads
are reduces so as to measure whatever audio is in the idle
channel. So if you were feeding the left channel, then the right
channel would be read for any residue Fig. 14 -20) The meter
sensitivity pads and the meter reading will give the value of
separation in decibels. Separation measurements are only
made at 100% modulation.
.
1
Crosstalk
Crosstalk is measured in two directions -that is, the
crosstalk from the main channel into the subchannel, and the
L
TRANSMITTER
STEREO
GEN
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LEFT TOTAL MOD RIGHT
RF
100
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IN THIS CASE
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LEFT RIGHT
STEREO
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MONITOR
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SEPARATION
METER SWITCH
AND SENSITIVITY
'PADS
Fig. 1 -20. Measure separation according to the monitor in use. Shown is a
typical monitor measuring the separation of the right channel from the left
channel.
498
crosstalk from the subchannel into the main channel Fig.
14 -21). During these measurements, only the main channel or
subchannel is modulated; the other is idle. The main channel
here is the modulation spectrum of the main carrier in the
audio region of 50 Hz to 15 kHz. The subchannel is that
spectrum area of 23 kHz to 53 kHz. What is being measured is
spillover. harmonics. or spurious signals that may originate
in one channel and cross over into the other channel.
(
MAIN
PILOT
¡CHANNEL
J
rI
50 Hz
CARRIER
l
I
15 kHz
t
STEREO
SUBCHANNEL
l
23 kHz
¡-SCAR
I
I
l
53 kHz
19 kHz
Fig.14 -21. The sideband spectrum of composite modulation of the FM carrier. Crosstalk is the spillover of spurious and harmonic elements from the
main channel into the subchannel, or the subchannel into the main channel.
Feed audio at 400 Hz into both audio channels in phase
(L = +R). The matrix in the stereo generator will cause all
this to appear in the main channel, and nothing into the
subchannel. The total modulation should be at 100%. Set the
function switch on the monitor to read crosstalk of main
channel into subchannel, and then on the appropriate meter
(usually right) of the monitor, set the meter pads to increase
meter sensitivity until a reading is obtained. The meter
reading and the pads will give the value of crosstalk from the
main channel into the subchannel.
Then reverse one of the input patch cords so as to feed the
400 Hz into both channels out of phase (L = -R). This
out-of -phase signal will be sent into the subchannel, and
nothing into the main channel. At the modulation monitor, set
the function switch to measure crosstalk of subchannel into
main channel, and then adjust the meter sensitivity pads
(usually the left meter) so as to obtain a reading. The meter
pads and meter reading will give the value of crosstalk from
the subchannel. All these readings are in decibels.
499
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Subcarrier Suppression
The 38 kHz subcarrier itself is not transmitted, only its
sidebands (Fig. 1422). This test will show how well the stereo
generator is performing this function. The tests are made first
without modulation of the subchannel, and then with
modulation of the subchannel with audio tones from 5 kHz up to
15
kHz.
100%
-
z
¢W
w
U
n.
z
O Q 50%-
5
0
MAIN
45% MOD
î 10%-
O
PILOT
10%
SUBCHANNEL
SIDEBANDS
45% MOD
1%s
50 Hz
15 kHz 19 kHz 23 kHz 38 kHz 53 kHz
SUBCARRIER
(LESS THAN 1 %)
Fig. 14 -22. The 38 kHz subcarrler must be suppressed to less than 1%
modulation of main carrier's total modulation.
For the first measurement, terminate both inputs of the
stereo generator with a resistor. At the monitor, set the
function switch to measure 38 kHz suppression. Then adjust
the meter (usually right) pads to increase the meter
sensitivity and obtain a reading. The meter reading and pads
will give the value of subcarrier remaining, or rather how well
it is suppressed, in decibles.
Using the same setup on the monitor, feed 5 kHz out of
phase to the left- and right -channel inputs and modulate to
100%. Read the subcarrier suppression as was done without
modulation. Make this measurement for each of the
modulation tones.
Noise
You may now make the noise measurements. Actually,
they could have been made during the response and distortion
500
measurements, when the routine got around to 400 Hz. The
noise measurements are made in exactly the same way as in
monaural, but in this case all measurements are made on the
left channel, and then on the right channel. Expect to lose 1 dB
of noise in these measurements as compared to the monaural
measurements. This is because you are now only modulating
90% (plus 10% pilot), whereas in the mono proof you used the
full 100%. Look at the modulation meter-this 10% represents 1
dB.
You may make AM noise measurements if you wish, but I
have never seen any change between the measurements made
in composite modulation and monaural modulation.
Putting It All Together
When you plot the graphs, plot the left and right channels
on the same curve (Fig. 14 -23). To identify the curves, either
use different colors, or draw one curve in a solid line and the
18
16
14
12
100% MODULATION
(90% + 10%PILOT)
10
8
6
LEFT CHANNEL
RIGHT CHANNEL
1
r
4
2
STAN DAR D P R EEMPHA SIS CURVE
o
REF
2
ER E NC E
4
FREO RESPONSE LIMIT
6
2
3
4 5 6
789100
2
3
4
5 6 7 8 9 1000
2
3
4
5 6 7
89 0,000
2
Hz
Fig. 14 -23. Plot the left and right channels on the same graph. Use two colors, or make one a dashed and the other a solid curve. This will make the
difference stand out.
501
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other in a dashed line. When two colors are used, any
difference between the curves will be very striking.
Don't forget to restore everything for normal program
operation. Check the AGCs, limiters, and other processors to
see that their action is switched back on. Check the patch
panels for stray cords or plugs, the console for switches open,
and. especially, make sure the transmitter is not on dummy
load. If there is an automation system, make sure it works
normally. After being up all night, you certainly don't want the
signon man getting you out of bed because the system won't
work!
502
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Appendix A
Simple Resistor
Branching Pad
This is designed for 600 -ohm balanced circuits. Signal at
input A will divide equally into outputs B and C. The three
resistors should be identical. There will be a loss of 6 dB from
the input level to either output. The pad will work in the
reverse direction equally well, and there will be no crossover
between B and C inputs when used in this manner. Circuit is
useful when a single output must be fed to two different buses,
and when used in reverse, will mix two sources into a single
bus.
O
dB
503
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PADS IN TANDEM
When it is necessary or desirable to split a signal into three
channels, then the simple resistor pad can be used, that is,
two pads in tandem. The signal loss is 6 dB across each pad,
so the output level at D or E will be -12 dB.
504
THREE-INTO -ONE ARRANGEMENT
The simple resistor pad can also be used to mix three
sources into one input. Two pads are worked in reverse. There
is a 6 dB loss across each pad, so the output at D is down 12 dB.
The output of one source must be adjusted 6 dB lower than the
other two if the levels at D are to be the same. This is useful
when it is desired to feed a three -tray cartridge machine into
a single fader on the console, for example.
-.
TRAY 1
A
-
6
dB
600
il
600
fl
D
600f1 -
600
TRAY 2
0 dB
TRAY 3
0dB
-
B
C
600
fl
600
fl
600
fl
12dB
fl
505
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BRIDGING PADS
When a transformer is not available, or if not needed for
the application, simple bridging pads can be used. The pad in
A will produce a loss of approximately 29 dB at its output. It
bridges a normally terminated program bus. The pad in B will
produce a loss of approximately 20 dB, under the same
conditions as in A. When the bus does not feed anything else
but the level is too high, then the bridging pad can be used
with a 600 -ohm resistor across the input of the pad, as in C, to
properly terminate the source.
600 n
BUS
6600
n
6800
n
2200
n
(A)
600 n
600 n
BUS
600
n LOAD
APPROX 29 dB LOSS
(B)
600
n
2200 n
600
n LOAD
APPROX 20 dB LOSS
(C)
600 n
SOURCE
600 n
600
n
600
n LOAD
APPROX 20 dB LOSS
506
1.
PHOTORESISTOR AS GAIN CONTROL
The photoresistor can be used as a remote gain control. If
stereo circuits are involved, then use a dual or stereo unit. The
audio circuits themselves do not have to be routed away from
the amplifiers or the rack. All that is needed is a DC control
circuit which controls the voltage to the lamp in the photo resistor. This allows the control to be placed at any
location -for example, at the console -while the photo unit is
at the amplifiers in the rack or even another room
-
-
PHOTORESISTOR
(STEREO UNIT)
LEFT AUDIO
I
LEFT OUTPUT
I
I
I
RIGHT AUDIO
I
I
RIGHT OUTPUT
1
I
tt
1
I
1
_1
CONTROL
507
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RECORDING FROM REGULAR TELEPHONE LINE
For news reports, remote broadcasts, and others that are
done over the regular telephone, a simple connection can be
arranged at the studios. In A is a simple connection. The series
capacitors block out any DC voltage on the line, the
600 -600 -ohm transformer provides isolation. If there is not
enough level then an amplifier can be inserted as at B. It is
often desirable to play back a commercial announcement to a
client over the phone. The circuit in A can be used in reverse,
as is shown at C.
A)
REGULAR TELEPHONE
CIRCUIT
1µF
600/600
SI
o
I
TO CONSOLE
o
I
1µF
(B)
REGULAR TELEPHONE
LINE
1
µF
600/600 f).
*--{
o
AMPLIFIER
I
1
I
1
TO
CONSOLE
I
1µF
(C)
REGULAR TELEPHONE
LINE
1
p
600/600 f2
I I
FROM TAPE MACHINE
OL AMPLIFIER OUTPUT.
1
pI
508
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CASSETTE OR MOBILE RECEIVER MATCHING
The simple circuit shown will match the speaker output of
a cassette tape recorder or the receiver output of a mobile unit
to the station's regular equipment. Use a speaker -to -line
transformer, which will provide isolation. If the level is too
high. insert a balanced loss pad between the transformer and
the audio system.
TO MOBILE RECEIVER
OR TAPE CASSETTE
SPEAKER OUTPUT JACK
4 8 f2
60012
TO CONSOLE
600 f2
4
SPEAKER -TO -LINE
TRANSFORMER
INSERT LOSS PAD
IF NEEDED
TERMINATIONS
For test purposes, a handy termination at the jack field
can be a big convenience during maintenance or even for
operation. Simply wire a resistor across either the jack
terminals themselves or down on the audio block. Have one of
each of the common impedances that are most likely needed at
the jack field, that is, of those used in the station. A patch cord
can then easily patch in the termination when needed.
JACKS
>
600
S2
JACKS
150í2
509
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MICROPHONE CORD TESTER
This simple test unit can be built up to test for continuity of
each wire in a mike cable and also to check for shorts across
the cable wires.
Use a two-layer rotary switch of at least seven positions.
The whole unit should be mounted in a metal box and attached
to the wall at the workbench. Use a regular ohmmeter for the
test.
In CAL position, a short is provided to calibrate the
ohmmeter.
Positions 1 -2-3 checks for continuity of cable wires
1
-2 -3.
Positions 4-5-6 checks for shorts across the wires in
the cable.
Flex the cable throughout its length during each test to
check for intermittents
USE OHMMETER
TEST
JACKS
I
/
6,`
CAL.
1
4
5
6`
CAL.
1
5
3
3
l
MALE
3 MIKE
tRECEPTACLE
i
510
MIKE CORD UNDER TEST
3
FEMALE
MIKE
RECEPTACLE
TRANSFORMERS ON JACKS
For isolation and bridging purposes, transformers
mounted in a rack and with all their windings appearing on
jacks can be very helpful both in operation and for test
purposes. Bring both the primary and secondary windings to
jacks. but do not try to use more than one set of taps at a time.
-
)
-¿ =
20K
>
Is''
600 n
_
r---
150
)
(
150n
-i
60o
CI
(
--4.--,
41I
_(15,0n
600
n
CAPACITORS AS TIMERS
The charge stored in a capacitor can be used for timing
purposes in control circuits. Estimate approximately two
seconds of delay (or hold) per 400 uF. Use good electrolytics.
Capacitor and Resistor to Delay Pull-In of Relay
The circuit shown is designed for a continuous control
provide
voltage applied. The RC combination will
approximately two seconds of delay before relay will pull in
and operate.
loon
+
24V DC
CONTROL
A.AA.
-I.
T3o0N F
i
RELAY
511
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Capacitor and Resistor to Delay Dropout of Relay
The circuit shown is for a continuous control voltage.
Values should provide about a 11/2- second delay in dropout.
Increase size of capacitor to obtain a longer delay.
24V DC
CONTROL
VOLTAGE
RELAY
Capacitor to Delay Dropout of Relay
The circuit shown is for a pulse-type (DC) control. The
capacitor should provide for a little over one second of delay in
the relay dropout. Increase capacitor value for longer delay.
(C)
+ 24V
DC
500µF
m
JL
24V DC
CONTROL
PULSE
512
47
S2
CAPACITOR AS MOMENTARY START CIRCUIT
This circuit is useful when the control voltage is on
continuously, but the circuit requires only a momentary start
pulse. In effect, this circuit transforms a continuous control to
a momentary control.
While the relay is idle, the capacitor charges from the 24V
DC supply voltage. When the control voltage comes on, the
capacitor is transferred to the outgoing control line, and the
charge in the capacitor is dumped into that line. Value shown
will provide approximately a 2'/2- second DC start pulse for
whatever is desired to be started. Since relay stays on while
the continuous control is on, capacitor cannot recharge and
thus cannot provide additional pulsing to the momentary
circuit. When relay relaxes, capacitor will charge again,
awaiting for the next call.
+ 24V DC
(MOMENTARY)
START PULSE
TO DESIRED
CIRCUIT
24V
DC
1000µF
4712
/T7
+ 24V
DC CONTROL
VOLTAGE
(CONTINUOUS)
513
DIODE AS GATE
The diode will act as a one -way gate in DC circuits. This
property can be very useful in control circuits where it is not
desirable to have the DC circuit voltage feed back into the
control circuit.
In the circuit shown, the start pulse will pass through the
diode to the relay coil. When the coil pulls in, the holding
voltage will be present, but this holding voltage cannot feed
back into the control circuit. If the control voltage is negative,
reverse the diode.
+
24V DC
1
n
HOLDING CONTACTS
M
DC CONTROL
PULSE
TP
DIODE AS TRANSIENT SUPPRESSOR
Inductive devices will kick up a strong transient at turnoff.
The diode can be used as a suppressor. The diode must be
placed in the circuit properly or it will conduct on the control
voltage. As shown, the diode will not conduct when the control
voltage is turned on. But when the control turns off, the
inductance will try to maintain the status quo and will create a
voltage from its collapsing field that is opposite to the control
voltage. The polarity of this transient voltage will be such that
the diode will conduct and short it out.
Always remember to place the diode in the circuit so that
the cathode is facing the positive side of the control voltage.
+
24V DC
CONTROL
r
514
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TURNOFF
TRANSIENT
ALARM MUTE CIRCUIT
When announcing takes place in a control room it is
undesirable for acoustical alarms to go off in the middle of an
announcement when the mike is open. All alarm devices can
be run through relay contacts on a relay that is operated from
the regular speaker mute relay voltage for the control room.
When the mike is on, the alarm bells cannot sound. The alarms
should also have a visual indicator so that the announcer
knows the alarm is sounding.
V
SPEAKER
MOBILE RADIO
o
Y
BELL
TELEPHONE
o
`T
BELL
ALARMS
o
ELECTRONIC
ALARM
TELEPRINTER
lE
I.
CONSOLE
SPEAKER
MUTE
RELAY
CONTROL
VOLTAGE
515
ALARM SYSTEM
A variety of functions may have alarms, and when more
than one fires at a time, it can be difficult determining which
one is on.
In the circuit shown, only one bell (or electronic alarm) is
needed. and each alarm circuit has an indicator lamp to
visually indicate which circuit has alarmed. The coil of the
relay for any alarm source can have the desired voltage value
for the circuit, since the source supplies the relay voltage.
Li
120V AC
120V AC
BELL
TRANSFORMER
¡gip
6V
l'J
BELL
-t.
INDICATOR
LAMPS
SOURCE
1
SOURCE2
"... SOURCE3
OTHER SOURCES
OTHER LAMPS
516
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REMOTE MICROPHONE SWITCH
There are several places where it is desirable that the
announcer be able to switch his mike on and off, such as in an
announce booth. The circuit shown will permit this. The
console key must be on and the fader open. When the key is in
its off position, a short is placed across the audio input to the
console. thus not allowing hum or noise. When the key is
thrown, the short is removed and the mike is fed into the
console through the switch.
The additional set of switch contacts operate the channel
mike mute and other relay functions associated with the
console switch. A capacitor is placed across the contacts to
soften any "pops" that might occur.
TO CONSOLE
MICROPHONE
INPUT
t
MICROPHONE
t
,
0.47 µ
F
4,4 LJ4
TO CONSOLE
START KEY
MULTICONTACT
LEAF SWITCH
517
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TURNTABLE REMOTE START SWITCH
This circuit will allow the turntable be started from the
regular key on the console. The circuit is simply an outboard
relay whose contacts supply the same action as the normal
turntable start switch. Mount the relay in the turntable
cabinet. The normal start switch can still be used for cueing.
etc.
120V AC
,
REGULAR
TURNTABLE
f
TURNTABLE
MOTOR
_.
START SWITCH
I
1
1
FROM
CONSOLE START KEY
24V DC
-y
518
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ALARM WITH IC INTERFACE
This is a suitable alarm circuit when the source is an IC
which can supply only low voltage and low current. The
transistor is used as the interface and as the switch to turn on
the relay. Contacts on the relay supply 24V to both external
alarms and to a local alarm. The variable resistor will control
the loudness of the local electronic alarm.
The circuit as shown assumes that the reset is done to the
IC source itself. If the initial alarm is only a single pulse, then
add holding contacts to the relay, and bring the return for the
relay through a normally -closed switch which can be used as
a reset button.
+ 24V DC
+ 24V DC
24V DC
INDICATOR
LAMP
TO EXTERNAL
ALARMS
MALLORY
SC628
ALARM
(LOCAL ALARM)
519
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AUDIOTAPE MACHINE AUTOMATIC REWIND
In those cases where a tape machine is unattended, the
circuit as shown can provide for automatic rewind at the end
of the tape and, at the same time, provide an alarm to warn the
operator that the tape is rewinding.
A foil switch must be added to the tape machine, and a
length of conducting foil applied to the tape at the position the
tape normally stops. This provides the start of the switching
action. The capacitor and resistor on Kl provide a delay of 24
seconds, approximately, which allows the machine to come to
a full stop. After the delay, Kl will operate. and its contacts
will switch the machine into reverse and apply a start impulse
to the fast control position. These contacts do the same as the
machine pushbutton switches.)
Contacts on Kl also apply supply voltage to K2, which
sends out an alarm voltage to a remote alarm at the operator's
position. The loudness of this alarm is adjustable. Alarm reset
is through the normally closed ground return circuit switch) .
(
(
TAPE MACHINE CIRCUITS
I
I
REVERSE
FAST
COMMON
FOIL
SWITCH
I
I
330
nTAPE
- 24V DC
1000µF
K7
2!V DC
r
1
REMOTE
ALARM
MALLORY
SC628
ONO
520
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RESET
SWITCH
RELAY POWER SUPPLY
The station will build many small alarms, indicators, or
relay circuits that need power, and it is not always possible or
desirable to obtain power from the regular power supply in
some unit.
A small power supply can be built for this purpose. Use a
transformer which has a 1A, 25V secondary. The bridge
reduces the amount of ripple. The bleeder is not necessary, but
does hold the unloaded output voltage down. If better
regulation is desired, then a zener can be added. But for most
common purposes, the circuit as shown is adequate. If the
circuit is often unloaded, use a 50V capacitor, as the DC
voltage can rise to about 40V ( depending upon line voltage) .
24V DC
521
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Appendix B
Useful Formulas for
Broadcast Engineers
Ohm's Law
DC Circuits
Circuits
E =12R
E =12Z
E
E
R
I
Z
=E
1=E
Z
1
Power
P=I2R=
E2
R
=1E
Impedance
Series Circuits
Z=
`/R2
+ (XL -Xc)2
Parallel Circuits
RX
Z
+
X2
Equal Resistors in Parallel
RI value of one resistor)
R.
_
n
522
(
number of resistors)
Two Unequal Resistors in Parallel
R1 R2
RI
R1
+R2
Power Factor
pf =
Q of Circuit
P (true power)
El apparent power)
Z
I
=X
Q
R
Reactance
Xi. =
277-
1
Xc =
JL
2n fC
Transformers
NP
Ep
Is
ZP
Ng
ES
IF,
ZS
Decibels
dB = 101ogio
P
P2
dB = 20 log io
dB = 201og io
E
I
Ei
}
(Equal resistance
or impedance)
12
Sine-Wave Relationships
RMS =
peak x 0.707
peak x 0.637
Average =
Peak = RMS x 1.414
Peak = average x 1.57
523
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Constants
it =
2zr =
Fahrenheit =
Celsius =
3.14159
6.28319
(Celsius x 9/5) + 32
(Fahrenheit
- 32) x 5/9
Wavelength (Free Space)
X
(meters) _
(feet)
-
Quarter -wave (ft)
-
A
300
f (MHz)
984
f (MHz)
Antenna Length
Half-wave (ft) =
234
f (MHz)
468
f (MHz)
Antenna Electrical Length in Degrees
Length (degrees) = length(feet) x f(kHz) x 1.016 x
Chord of a Circle
c2= a2
+b2- 2ac cos C
where a and b are the two sides, C is included angle
Bandwidth
= 2M
= 2M + 2D
FM:
where M = highest audio -modulating frequency
D = deviation to one side of carrier
AM:
BW
BW
524
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106
x
360°
Loss Pads
Formulas are for a T -pad. For balanced circuits, use an
H -pad; divide the input resistor value in half and place half in
both legs (do the same for the output side).
R3
- 2 N-1
RI
= Z,
R2
=
IN.
IN+1)
IN-1)
IN
Z2
+1)
(N-1)
R3
R3
where N = desired loss (dB)
Zi = input impedance
Z2 = output impedance
525
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Index
A
AC
power input
power. transmitter
ripple measuring
Accuracy maintenance
Adjustment audiotape bias
AFC loop. transmitter
Air
cue. console
flow. transmitter
Interlocks. transmitter
Alarms teleprinter
Alignment
audiotape cartridge
audiotape head
FM antenna
28
297
445
442
189
336
153
322
323
214
186
184
382
AM transmitter
13.31.373
antenna
374
antenna electrical values
374.377
antenna tuning
254
audio
375
bandpass
313
modulation. asymmetrical
292
modulation
monitor
power amplifier lineup
power calibration charts
power expanded-scale meter
power measurements
power meters
proof
proof requirements
signal processing
super
systems
Amplificaticin. console
335
300
304
374. 377
83
404
401
Area
studio
19
transmitter
Arrester
installation
lightning
maintenance
selection
27
97
94
98
97
ASCII code
Assignment. terminal block
209
77
Asymmetrical
modulation. AM
modulation. AM. forced
313
313
modulation. FM. forced
signal identification
Attenuation. line
Audio
AM
313
319
345
255
filtering. transmitter
336
466
313
350
292
152
301
349
distortion
118
plot) Ing
Annual inspection
Antenna
AM
457
installation. FM
380
connect ions
cont
line
304
465
219
246
302
FM power distortion
gain
height
icing. FM
380
circuit
mismatch
portable
efficiency
FM
240
302
300
check. base
coupling units
coverage
381
304
AM power. tuneup
FM power. tuneup
alignment. FM
tower
tower general maint
tower icing
243
254
25
Amplifier
power. grounded-grid
Amplitude
derabng. line
maintenance
matching. FM
polarization
power. FM
stacking. FM
tuning. AM
Anticipating space needs
Arc
quencher numi
suppressors. lightning
421
13.31.373
382
373
402
393
244
32
240
239
13.32.379
381
238
238
396
381.426
75
input
input proof measuring
line FM
monitor basics
monitor classes
monitor console systems
monitor high current circuits
monitor impedance matching
monitor level control
monitor speakers
otorntor systems
problems. automated system
response. inverse Junction
response proof
terminal blocks
Audio ape
bias adjustment
capstan and pinch roller
capstan motor
cartridge alignment
cartridges
clogging
equalization
erasure
false indications
heads
head alignment
head removal
head replacement
machines
playback
playback heads
29
455
253
217
216
219
219
218
217
217
216
276
454
471
76
180
189
188
189
186
207
188
182
182. 198
187
180 205
184
186
186
179
180
181
526
www.americanradiohistory.com
record nears
residual magnetism
181
tests
204
205
track identification
worn heads
Automated system
audio problems
booth
cable abrasion
cables
cartridge machines
cleaning
lake switching
grounds
head replacement
head substitution
installation
level setting
maintenance
master amplifier
MOS program control
PC board connectors
phasing
pilot lamps
playbacks
readouts
response
silence sensing
switch failure
systems
183
187
15
276
268
280
269
275
281
278
269
279
279
268
270
275
270
283
280
269
282
272
282
272
280
277
15
lore switching
turnkey
Automation
expansion
planning
program
273
space
266
267
267
267
266
265
storage
Average power. line
245.351
B
Balance push-pull stage
Ball gap selling
Bandpass
AM
radio link
Base antenna check
Basic
meter movement
processors
transmitters
transmitter divisions
Batteries
Battery
charger maintenance
current drain
maintenance
118
468
375
240
244
434
311
291
321
251
197
251
97.441
Baudot code
Bazooka. FM
Beacon
210
Installation
tower
Bending line
389
389
358
383
Bias adjustment. audiotape
Bidirectional microphone
Binary-coded decimal BCD,
Bleeding the line
Block diagrams
Bonding line
Booth
automated system
location
recording
recording equipment
Bridge values, calculations
Bridging
monitor
transformer
transformers. building
Broadcast
equipment. industrial
loops
use. consumer items
Building
bridging transformers
inspection
land line equalizer
pads
Bullets. line
Bus
console mixer
high or low
Bypassing remedy. RFI
189
165
284
365
71
362
268
79
198
199
136
audio
board test
breakers. transmitter
protection. power line
QKT
(lasses. audio monitor
Cleaning
automated systems
console
25
441
326
95
24.236
216
281
164
turntable records
Cleanliness. transmitter
(logging. audiotape
179
324
188
Coax
225
131
136
inspection
transmission line
Coaxial line check
412
343
244
Code.
241
24
105
136
ASCII
Baudot
teleprinter
transmission. teleprinter
Common building ground
147
Comparison test
Components
defective
proof conditions
Computer maintenance
Connoting Jumpers
451
Connect ions
415
230
133
367
141
209
210
209
90
437
118
463
286
101
audio
line
377
turntable output
175
75
abrasion
automated system
frequency cheek
identilicatiom. electrical
identification. mechanical
installation
insulated
lacing
multicondutor
plug problems
separation
shielded
ties
Cabling modifications. console
Calculations
bridge values
space
Calibrating meters
Calibration
charts. AM power
83
Console
139
280
269
448
99
100
98
85
102
75
170
85
84
102
159
136
80
338
304
air cue
amplification
cabling modifications
calibration
153
channel selection
cleaning
contact problems
cueing
custom
designation
151
152
159
162
164
163
150.154
154
151
drilling
161
dual.channel
fader problems
faders
functions
grounds
input selector
installation
155
Jacks
157
maintenance
master gain
metering
162
147
170
150
145
157
147
156
152
153
console
FM power meter
frequency
162
prat
462
miser bus
mnditicaiton. relays
modifications
monitoring
307
power supply modification
158
337
preamphtication
stereo
147
required. FM power
transmitter FM monitor
Capacitor
distortion influences
maintenance. transmitter
Capstan
motor. audiotape
pinch roller. audiotape
ardwd microphone
Carrier
deviation
shift proof
Cartridge
alignment. audiotape
audiotape
machines
turntable
Channel selection. console
Charts. AM power calibration
Check. coaxial line
Checkout
line
remote
Circuit
307
309
126
330
189
188
165
337
473
186
207
275
175
ISI
304
244
386
250
159
158
153
155
stock
switch modifications
switch problems
system. audio monitor
voltage modifications
wiring problems
nstant.voltage
C
design variables
system
system design
Consumer items. broadcast use
Contact
bounce
problems. console
.
transmitter
Contracted
measurements
service inspections
Control
icing heater
ladder. transmitter
2a5
397
21.145
388
transmitter
26
Controlled shield ground
Cooling
and heating. transmitter
systems
Counter. frequency
Coupler isolation. FM
Coupling units. antenna
Coverage. antenna
Crosstalk measurement
88
298
31
309
383
32
240
498
Cueing. console
150.154
Current
drain. battery
251
measurements. weather line 213
Custom console
154
Cutting line
358
211
Considerations. space heat
C
Cabinet RFI protection
Cable
line. audió
methods. ice
room
tower lamp
154
D
Damage to line
DC voltage measurement
Decibel measuring
Defective
components
IC isolation
Derating. line
Designation. console
Detection. signal
Deviation. carrier
Diagrams
block
371
443
444
118
450
349
151
55
337
71
pictorial
74
schematic
Diffusion. signal
Direct FM power measurement
Distortion
amplitude
equalizer
frequency
general causes
influences. capacitor
influences. resistor
intermxdulating
71
259
306
118
127
116
172
126
126
120
Jacks
122
anse analyzer
461
phase
112
power supply
proof
response
signal
transistor stage
turntable stylus
122
472.463
496
112
125
178
turntable tracking
Division. system
178
Doghouse inspection
Dolby B system
Dressing wire
Drilling console
Dual-channel console
417
metering
15
315
IDI
161
155
340
161
162
219
160
163
221
220
221
105
450
103
330
308
418
398
296
E
Earphones
KISS Equipment
Effects. temperature
248
25
118
Efficiency
antenna
spare
239
81
Electrical
maintenance. transmitter
values. AM antenna
Environmental RFI protection
Equalization
audiotape
land line. on-air
Equalized circuit. land lines
Equalizer
distortion
land line. building
325
274
139
182
230
229
127
230
527
www.americanradiohistory.com
Equipment
FRS
25
maintenance. remote
250
manual marking
52
placement
79
recording booth
199
updating
67
Erasure. audiotape
92.199
Errors. newsroom operator
196
Excessive voltage test damage
439
Expansion. automation
267
Experimental period. tuneup
300
Externals. transmitter
297
r
Faders
console
problems. console
False
indications. audiotape
Fan-ins
150
163
187
102
FCC
notification. tuneup
rules. remotes
rules. transmitter
Ferrite bead remedy. RFI
Fields
jack
strength measurement
Flies for Inspect ions
Filters. transmitter
Flasher inspection
Flexible
line horizontal run
line installation
299
242
296
143
76
243
408
298
421
362
-
modulation
monaural proof
monaural proof requirements
monitor
monitor calibration
power
power amplifier tuneup
power calibrations required
power indirect measurement
power measurement. direct
power meter calibration
power tolerance
propagation
signal processing
stereo
system
Frequents
calibration
check. cable
counter
distortion
distortion. reactance
logging
measurement. FM
stereo pilot check
Functional errors in tests
Functions. console
Fusing. lightning
transmitter
238
462
363
27
122
402
402
194
138
320
29
66
331
302
399
87
289
90
157
88
139
91
89
89
298
Guy
inspection
tension inspection
Guyed AM tower
lightning protection
423
424
402
361
FM transmitter
13.32.379
antenna
382
antenna alignment
296
antenna icing
426
antenna inspection
381
antenna installation
antenna lightning protection 402
381
antenna matching
380
antenna power
381
antenna power distortion
380
antenna stacking
362
antenna transmission line
253
audio line
33
antenna weather effects
383
bazooka
corona discharge correction 405
308
frequency measurement
383
isolation
limiters
G
Gain
antenna
change proof conditions
Caskets and barriers. line
Gear. transmitting
General
causes. distortion
maintenance. antenna
maintenance. tower
newsroom monitoring
remedies. RFI
transmitter maintenance
Generator. stereo
Gradual wear records
Grounded
grid output stage
grid power amplifier
towers
Grounding
automated system
Ground
common building
console
controlled shield
lead Rt'l protection
lightning protection
microphone
points
315
H
Harmonic Filter inspection
413
Head
alignment. audiotape
audiotape
audiotape playback
audiotape record
print. teleprinter
Headroom
Heater
control. icing
installation. icing
power. icing
Healing and cooling.
transmitter
lash
current precautions
intensity lighting. tower
184
180.205
181
181
215
111
398
399
396
296
321
389
451
306
307
or low bus
voltage capacitor maint
voltage meter maint
voltage supply maim
Hoisting line
Homemade pad test
Horizontal
line runs. coax
run. flexible line
Hotspots. line
Hum measuring
305
33
314
253
IC defect isolation
Ice control methods
450
294
478
467
335
337
305
301
307
30
292
309
448
309
116
117
310
306
448
440
145
94
Icing
electrical effects
FM antenna
heater control
heater installation
heater power
operating techniques
328
442
326
361
134
257
362
371
446
397
393
396
398
Impedance
matching
matching. audio monitor
matching. transformers
microphone
mismatching
natural. line
sweep. line
turntable
Indicator. signal
Indirect measurement. FM power
Industrial broadcast equipment
Input
audio
balance test
selector. console
Inspect ion
annual
building
coax
contracted
doghouse
files
flashers
FM antenna
guys
guy tension
harmonic filter
lamp and fixture
lighting equipment
line
line pressure
meter records
color
photocells
power amplifier records
power panel
relays
routine
temperature
tower plumb
transmission line
transmitter
transmitting equipment
tuning unit
VSN'R indicator
water leaks
Installation
arrester
automated system
168
127
344
368
177
451
30
241
209
147
432
415
412
418
417
406
421
426
423
424
413
422
418
416
414
410
416
419
411
414
420
407
416
425
416
408
407
417
43
416
cable
console
flexible line
361
FM antenna
icing heater
large system
line
planning
recording booth
tower beacon
381
transmitter
turntable
399
378
354
70
202
389
296
175
weather line
212
weather line selector
213
Insulated cable
85
Interconnecting links
352
Interlocks. transmitter air
323
Intermittents. line
372
Intermodulating distortion
120
Internal problems. microphone
170
Insulated towers
400
Interference
IIS. 137
radio frequency
fir
393
396
transformer
Identification. RFI
Identifying
cables. electrical
cables. mechanical
141
397
131
97
268
96
156
physical effects
symptoms
398
218
29
438
circuity teleprinter
Isolation
coupler. FM
FM
tower
399
117. 127
I37
383
383
34
131
J
99
100
528
www.americanradiohistory.com
Jacks
console
187
distortion
fields
field wiring
maintenance
mountings
natural impedance
peak
er
phasing w
plowing
power rating
powered checkout
pressure checks
pressure inspection
pressurization
pressunzing equipment
propagation velocity
122
76
101
land line
maintenance. monitor
Joint use. tower
Jumper connections
K
Knowledge updating
232
224
386
101
66
fine
Jacks
use losses
fine low -level
output
inequality
Large system installation
Leaks in line
Level
control. audio monitor
program
102
388
422
388
227
229
232
228
231
227
376
366
217
103
setting. automated system
270
setting. proof
494
turntable signal
177
Light
output. tower
392
pattern. tower
390
Lighting
high.intensity. tower
386
tower
N.398
Lightning
arc suppressors
401
arrester
equipment inspection
fusing
protection
protection. FM antennas
protection grounds
protection. guyed AM lowers
protection. lower
static drain
transients
Limiters
344
344
347
377
353
369
370
414
367
364
346
256
KM
rigidity
L
Lacing cables
Lamp
control tower
fixture inspection
ower
Land
ines
fine equalized circuit
256. 369
356
418
355
runs. horizontal. coax
size. transmission
skin affect
slow leaks
solderin
standing waves
standing wave ratio iVSNRi
support and shelter
357
352
360
J70
J60
termination
transmission
transmitter transmission
344
30
297
tuned sections
types
348
354
VSRchecks
370
Vas
g
interconuwrti
microwave
radio
Live operation
load impedance. turntable
lagging
347
348
357
252
257
237
12
177
booth
79
tnggm
uency
maintenance
310
422
f
tan
gne network hookups
term troubleshooting
loops. broadcast
lasses. land line
Low-level output. land line
237
40
24
228
231
94
M
399
402
91
402
34
Main mike input proof
Machines. audwtape
Magnetism audiutape residual
Mainenanre
399
93
FM
315
peak
29.311
Line
attenuation
345
audio
255
average power
345.351
bending
356
bleeding
365
bonding
362
breathing
367
bullets
367
checkout
366
connections
377
cutting
358
FM antenna transmission
382
253
FM audio
damage
371
derating
349
gaskets and barriers
363
hoisting
361
hotspots
371
identification
447
impedance sweep
inspection
368
410
installation
intermittents
354
land
leaks
losses. transmission
227
372
360
352
478
179
183
442
antenna e
243
arrester
98
automated system
batteries
battery
battery charger
capacitors
computer
console
general transmitter
grounded-grid output
high -voltage capacitors
high-voltage meter
high -voltage supply
line
logging
microphone
monitor lacks
monitor muting relay
monitor speakers
monitor system
newsroom
operational
pickup system
preventive
probes
records
remote equipment
routine
scheduling
techniques
275
442
197
197
441
test equipment
403
tower gaps
403.404
tower grounds
transmitter AM carrier shil t 333
332
transmitter AM loading
333
transmitter AM modulator
transmitter arc quencher
transmitter bearings
transmitter. electrical
transmitter. electronic
transmitter. mechanical
transmitter multiplier
transmitter primary AC
transmitter shoring bars
404
322
325
329
321
329
326
330
transmission line
403
403
tuning unit
177
turntable
Marking equipment manual
52
Master
amplifier. automated system 270
152
gain. console
Matching
enra
361
impedance
117. IV
131
transformer
Measurements
AM power
302
308
contracted
243
field strength
308
FM frequency
306
FM power. direct
30
indirect. FM power
peak -to-peak
peak value and RMS
431
p
voltmeters for audio
302
96
432
weather
213
438
er
power line
line current
Memory
storage
troubleshooting
Meter
accuracy
AM power
284
288
437
304
338
307
435
calibrations
calibration. FM power
checking
expanded scale. AM power
dial faces
movement. basic
pads
reading
records for inspection
RFI
VU
Metering
console
dual power
Microphones
304
436
434
104
338
410
436
104
26
153
340
164
330
bidirectional
66
286
cable and plug problems
70
162
cardioid
ground
65
320
331
328
442
326
256.369
422
197
224
225
224
224
196
441
242
61
443
63
250
61
61
443
19
impedance
internal problem
maintenance
168
omnidirectional
patterns
11S
phasing
physical structure
problems
test setup
room noise problems
sports
system
wind Screen*
Microwave
link
signal path
Mismatch
amplifier
tolerance
Mismatching. impedance
reactive components
170
197
166
168
166
170
169
Ill
167
21
167
X57
x58
219
128
127
126
529
Miser bus. console
Modifications
automated syslen=
147
159
181
180
261
transmitter
transmitter modulation
175
Panel
336
219
218
217
217
217
225
216
336
224
224
225
224
216
224
447
283
180
27
153
193
335
30
356
75
258
248
Mutes monitor
224
237
339
I90
196
193
194
operatorerron
198
speakers
switching
telephone recording
196
191
193
Noise
distortion analyser
distortion proof
measurement
measuring
461
467
465
446
473, 484
prod
234
remote
building
479
245
Mountings. line
Multi-conductor cables
Mullshop relays
Multimike remetes
N
Network hookups. long-line
Neutralisation. transmitter
Newsroom
maintenance
monitoring
monitoring. general
PA efficiency. transmitter
PC board troubleslwsiing
Pad
meter
resistor
Paint
bands. tower
requirements. tall tower
313
313
MO6
lineup. program controller
Motor. audiotape capstan
Monitoring
console
newsroom
P
158
156
AM transmitter
audio. console systems
audio. impedance matching
audio level control
audio speakers
basin. audio
FM transmitter
jacks maintenance
mutes
muting relay maintenance
speaker maintenance
systems. audio
system maintenance
test pal*
315
158
Modifying automated systems
Modulation
AM. asymmetrical
AM forced asymmetrical
monitor check
percentage
Monaural turntables
Monitor
classes. audio
176
Qermodulauon. trarwnutter
161
console
console cabling
console power supply
Insole relays
console switch
console voltage
bridging
Output connections. turntable
meters for teat
meter test
transmitter power
Path
length. stereo
microwave signal
Patterns. microphones
Parameters. transmitter
Peak
averages. waveforms
limiters
limiters
Odor inspection
Ohmmeters for test
416
433
Omnidirectional microphone
On-air land lineequalizatwn
Operation maintenance
Operating techniques icing
Operation
live
peripheral
stereo
Operator errors. newsroom
165
230
441
367
12
la
13
196
°collator
transmitter. FM
problems. remote
(g-o(-band proof
291
133
474
133
104
132
387
387
434
440
297
254
258
166
30
107
29
311
344
power. line
431
-to-peak measurement
value and RMS measurement 430
Performance proof
Peripherals
operations
Phase distortion
Phasing
automated system
microphone
stereo heads
stereo. proof
Phasors. transmitter
Phototcell inspection
Physical
checkout. transmitter
453
is. I90
is
112
269
168
205
491
297
419
299
structure. microphones
Pickup system maintenance
Pictorial Diagrams
Pilot
168
lamps. automated systems
phasing proof
Placement. equipment
282
493
79
242
14
Naming
automation
installation
Plotting amplitude
Playback
audiotape
automated system
heads. audiotape
Polarity
checking
teleprinter
Polarization. antenna
Portable
amplifiers
amplifier test
Power
amplifier, grounded grid
calibrations required. FM
O
330
287
rating. lire
teleprinter
tolerance. FM
PreamphlicatsontonMde
Precautions
high-current
transmitter high- voltage
checkout. line
circuit entry. 'WI
268
70
457
180
272
181
441
215
240
246
247
302
307
369
144
distortion. FM interim
381
FM
FM antenna
king heater
input. AC
line circuit protection
line measurements
measurements
measurement. direct. FM
meter calibration. FM
panel inspection
305
380
396
28
96
96
302
306
307
414
297
panel. transmitter
122
supply distortion
supply modification. console 156
530
www.americanradiohistory.com
Preemphasis proof
Pressure checks. hne
Pressurizing
equipment. line
line
Preventive maintenance
763
217
3011
147
331
340
462
3770
364
363
61
Primary
326
AC balance. transmitter
AC maintenance. transmitter 326
315
Pnnt heads. teleprinter
Probe
maintenance
test
Problems
console contacts
console fader
console switches
443
432
console wiring
microphone
microphone cable and plug
microohone internal
microphone room noise
remote site
remoteoscillalor
sampler
Procedure. test
Processing
AM signals
FM signals
signal
signals
Processors
basic
setup. signal
Program
automation
controller MO6 lineup
level. remote
levels
signal waveforms
Proof
AM
313
314
24
311
311
317
265
283
248
103
105
465
454
arrangements
490
arrangements. stereo
471
audio response
462
calibrations
473
carrier shift
463
conditions. components
462
conditions. gain change
457.472.483
distortion
472
distortion
463
equipment
486
equipment. stereo
457
forms
486
graphs
493
pilot phasing
level setting
main mike input
measurement
method. stereo
noise
of performance
494
478
470
488
437.473. 484
outofband
453
474
planning
preemphasts
469
482
preliminaries
power
records
reference
response
450.479
4E0
458
482
481
466
467
requirements. AM
requirements. FM monaural
196
separation
470
setup and levels
487
stereo
493
stereo polarity
462
system conditions
453
test equipment
Propagation
FM
radio link
velocity. line
Protection
lightning. tower
33
237
346
34
surge
93
33
tower
Pulling cable
99
Push-pull stage balance
Psychology. troubleshooting
118
40
Q
QRTcircuil
protection. ground leads
protection. shielding
T-pad remedy
Ribbonless paper. teleprinter
Ripple measuring
RMS and peak value
138
138
143
216
446
438
Room
control
noise problems. microphone
Routine
inspections
maintenance
maintenance dens
Rumble. turntable
21
171
437
61
66
Rack
space
%
transmitter
Radio
frequency interference l RFl
298
l
11
15
pa
link propagation
Reactive components
impedance mismatch
Reading meters
Readouts. automated system
Record
cleaning. turntable
gradual wear
heads. audiot ape
maintenance
proof
Recording
237
128
338
282
179
65
181
63
458
booth
booth equipment
booth installation
tooth standards
newsroom telephone
I%
turntable
172
Recorders. tape
Relay
inspection
modification. console
mutt hop
Remotes
checkout
control and STL
equipment maintenance
FCC rules
noise
multimike
oscillator problems
program level
resistor terminations
site problems
talk-back hookup
Reproducing. turntable
Required
FM power calibrations
inspections
Residual magnet ism. audiotape
Resistor
distortion infuencm
199
202
203
193
72
420
159
258
24
250
259
250
242
234
248
233
248
233
232
235
172
307
407
783
Sampler problems
Scheduling mainerance
Schematic diagrams
Selection
processing
processing. AM
processing. FM
processor setup
teleprinter
tracing
tracing. system
waveforms. program
139
169
84
agnaldistonion
122
special proof arrangements
technical standards
490
488
175
13
254
491
448
493
487
488
488
55
259
112
319
451
177
258
24.311
313
314
317
209
53
58
105
automation
calculation
efficiency
heat considerations
rack
Spare parts updating
265
Speakers
audio monitor
maintenance. monitor
newsroom
81
61
80
67
217
224
1%
179
259
167
66
26
380
106
wave ratio1SNRt
31
waves line
347
wave ration VSNRi. line
348
Static drain. lightning
399
Weather
Station selector.
Service 212
FM
generator
head phasing
136
Super AM
Support and shelter.
Surge protection
Switch
350
253
29
2%
11
9
178
174
560
357
93
failure. automated systems
modifications. console
newsroom
problems. console
standing waveratiol
System
audio monitor
automated
constant- voltage
cooling
design. constant- voltage
division
microphone
pickup maintenance
signal tracing
standard levels
NM
277
161
191
162
31
216
15
228
31
721
15
21
242
58
106
T
T -pad remedy. RFI
Talk-back hookup. remote
Tall lower paint requirements
143
215
387
Ta
180
recorders
22
Techniques. troubleshooting
36.42
Telemetry.
255
Telephone recording. newsroom 193
Teleprinters
208
alarms
214
codes
code transmission
Input circuitry
polanty
power
print heads
nbbonless paper
signal
Temperature
effects
inspection
209
211
209
215
215
215
216
209
118
415
Terminal
block assignment
blocks. audio
77
76
Termination
line
test
344
439
Testa
audiotape
156
19
26
lire
audio
so
turntable
Sports microphone
Spot checking
Stereo
console
154
Strapping tranfonnera
Studio
area
transmitter like i STLU
transmitter. separate
transmitter combined
Stylus
distortion. turntable
turntable
Sebcarner suppresson
39
339
267
284
Stock console
139
360
370
360
Sanding
139
470
Skin effect. line
Slow leaks. line
Soldering line
Space
137
256
436
85
496
495
diffusion
distortion
identification. asymmetrical
Indicator
levels. turntable
path. microwave
272
496
483
141
147
Signal
detection
remote control
studio transmitter link
Slacking FM antennas
Standard levels. system
138
151
microphone test
Shielded cable
Shielding protection. RFI
Short-term troubleshooting
Shorting bars. transmitter
Sit
143
97
Setup
levels. proof
233
144
71
prod
terminations. remote
141
M
console channel
Selector. console input
Separation
cable
measurement
132
automated system
distortion
proof
RF interference
RFI
bypassing remedy
entry. power circuit
fernte bead remedy
general remedies
identification
line
meters
protection. cablineta
protection.environnlental
336
arrester
pads
Response
261
automation
memory
$
Speed.
126
over STL
path length
phasing. proof
pilot frequency
pofanty. proof
proof
proof equipment
proof method
Storage
227
p
298
turntables
173
24.236
Quality. land lire
monitor
operation
circuit bawd
comparison
204
441
437
Test
equipment
427
531
www.americanradiohistory.com
equipment maintenance
equipment. proof
equipment selection
equipment types
equipment use
excessive voltage. damage
functional errors
homemade pad
input balance
panel meters
portable amplifier
procedures
signal tracing
setup. microphone
MI
termination
transformer
439
439
453
428
430
429
439
440
134
438
440
247
437
54
169
'Des. cable
102
Tolerance
FM power
mismatch
305
128
Tone
arms. turntable
Towers
antenna
general maintenance
antenna icing
beacons
beacon installation
176
33.385
373
402
393
389
389
404
403
construction around
gap maintenance
403.404
ground maintenance
399
grounded
389
high -intensity lighting
400
insulated
34
isolation
386
joint use
388
lamps
388
lamp control
392
light output
390
light pattern
34. 388
lighting
34
lightning protection
387
paint bands
386
painting
425
plumb inspection
33
protection
385
wind loading
54
Tracing test signals
178
Tracking distortion. turntable
205
Track identification. audiotape
452
Transients
93
lightning
Transformer
131
bridging
131
impedance matching
131
isolation
131
matching
135
strapping
test
Transistor
stage distortion
switching check
Transmission
line. coax
line. FM antenna
line inspection
line losses
line maintenance
line. transmitter
line selection
line size
lines
439
airflow
532
AM monitor
AM modulator maintenance
AM systems
335
arc quencher maintenance
area
audio filtering
basic
bearing maintenance
blower maintenance
capacitor maintenance
circuit breakers
cleanliness
contact maintenance
control
control ladder
cooling and heating
divisions. basic
electrical maintenance
electronic maintenance
externals
FCC rules
404
filters
FM modulation
FM monitor
FM monitor calibration
FM system
grounds
high -voltage precautions
inspection
installation
maintenance. general
mechanical maintenance
modulation AM
modulation monitoring
monitoring
multiplier maintenance
neutralization
oscillator. FM
output tuning
overmodulation
PA efficiency
parameters
phasors
physical checkout
power panel
primary AC balance
primary AC maintenance
racks
grounded-grid output
separate power
shorting bars
studio combined
studio separate
transmission line
tube life
tuneup
tuneup. FCC notification
FM power amplifier
301
transmitter
299
Tuning
AM antenna
374
unit inspection
unit maintenance
transmitter output
Turnbuckle maintenance
Turnkey automated system
333
292
27
336
291
322
322
330
326
324
333
26
296
298
321
325
329
297
296
298
294
335
337
292
298
320
408
296
320
321
292
30
335
329
329
294
245
335
330
30
297
299
297
326
326
298
331
Turntables
cartridges
output connections
distortion. stylus
distortion. tracking
installation
load impedance
maintenance
monaural
record cleaning
recording
reproducing
rumble
signal levels
178
175
177
177
175
179
172
172
173
177
177
U
Updating
equipment
knowledge
spare parts
U.S. Weather Service
67
66
67
211
V
Vce isotation
449
Voltage
443
measurement. DC
160
modification. console
olVSWR1 31
standing wave
432
Voltmeters
VSWR
checks. line
correction
indicator inspection
voltage standing wave ratio
VU meter
370
370
413
31
104
297
330
9
11
297
332
299
299
Water leak inspection
Waveform
peaks and averages
program signal
Weather
alert
effects. FM antenna
line current measurement
line installation
line selector installation
service station selector
Wiring
jack fields
problems. console
Wind
348
loading. tower
screens. microphone
Wire dressing
Work patterns
Won heads. audiotape
40
288
287
40
39
42
tuneup
297
336
322
178
176
332
30
175
wiring
Tube life. transmitter
Tuned section. line
352
21
174
174
407
352
267
175
equipment inspection
gear
Troubieshooting
long term
memory
PC boards
psychology
short term
techniques
297
405
179
Transmitting
382
426
352
403
245
stereo
stylus
tone arms
125
343
417
403
speed
447
Transmitter
AC power
AFC loop
323
air interlocks
AM carrier shift maintenance 333
332
AM loading maintenance
AM power amplifier
300
efficiency
experimental period
301
300
turntable
416
107
105
211
33
213
212
213
212
83
101
163
177
385
167
101
74
187
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