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THE McINTOSH MR 74 SOLID STATE AM FM/FM STEREO TUNER
READING TIME: 28 Minutes
Price $1.25
Your MR 74 AM FM/FM STEREO
TUNER will give you many years
of pleasant and satisfactory
performance. If you have any
questions please contact:
CONTENTS
CUSTOMER SERVICE
Mclntosh Laboratory Inc.
2 Chambers Street
Binghamton, New York 13903
Phone: 607-723-3512
SERVICE CONTRACT
INSTALLATION
HOW TO CONNECT
BACK PANEL INFORMATION
FRONT PANEL INFORMATION
LISTENING TO THE MR 74
PERFORMANCE LIMITS AND RATINGS
PERFORMANCE CHARTS
TECHNICAL DESCRIPTION
BLOCK DIAGRAM
FM STATION LOG
WARNING: TO PREVENT FIRE OR
SHOCK HAZARD, DO NOT EXPOSE
THIS UNIT TO RAIN OR MOISTURE.
Take Advantage of 3 years
of FREE Factory Service . . .
Fill in the Application NOW.
THREE YEAR FACTORY SERVICE CONTRACT
An application for a FREE THREE YEAR FACTORY SERVICE CONTRACT is included with this manual
or mishandling is not covered by the SERVICE CONTRACT.
The terms of the contract are:
4. The SERVICE CONTRACT is issued to you as
the original purchaser. To protect you from
misrepresentation this contract cannot be
transferred to a second owner.
1. Mclntosh will provide all parts, materials and
labor needed to return the measured performance of the instrument to the original performance limits free of any charge. The
SERVICE CONTRACT does not cover any shipping costs to and from the authorized service
agency or the factory.
5. For your protection Mclntosh selects only
dealers who have technical competence to
guide purchasors fairly, and provide service
when necessary. To receive the SERVICE
CONTRACT your purchase must be made
from a Mclntosh franchised dealer.
2. Any Mclntosh authorized service agency will
repair all Mclntosh instruments at normal
service rates. To receive the free service under
the terms of the SERVICE CONTRACT, the
SERVICE CONTRACT CERTIFICATE must accompany the instrument when taken to the
service agency.
6. Your completely filled in application for a
SERVICE CONTRACT must be postmarked
within 30 days of the date of purchase of
the instrument.
7. To receive the SERVICE CONTRACT all information on the application must be filled
in. The SERVICE CONTRACT will be issued
when the completely filled in application
is received at Mclntosh Laboratory Incorporated in Binghamton, New York.
3. Always have service done by a Mclntosh
authorized service agency. If the instrument
is modified or damaged, as a result of unauthorized repair the SERVICE CONTRACT
will be cancelled. Damage by improper use
1
Copyright © 1972 By McIntosh Laboratory Inc.
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16
ment located behind the panel. With the template in place mark the six "A" and " B " holes
and four small holes locating the corners of the
cutout. Next, join the four corner marks with
pencil lines; the edge of the template may be
used as a straight edge.
2.
DRILL HOLES
Holding a drill perpendicularly to the panel, drill
the six "A" and " B " holes using a 3/16 inch drill.
THE SIX HOLES MUST BE DRILLED BEFORE
MAKING THE CUTOUT.
3.
SAW CUTOUT
Using a saw carefully cut the rectangular opening on the inside of the pencil lines.
4.
SECURE MOUNTING STRIPS
Secure mounting strips (supplied in the hardware package) on the inside of the cabinet
panel.
Insert two screws (supplied in the hardware
package) into the center holes ("B" holes on the
template). Use the ¾-inch long screws for
panels under ½-inch thick or 1¼-inch screws
for panels ½-inch thick or over. Place a mounting strip on the back of the cabinet pane! as
shown. Align it with the three holes in the panel
and tighten the screw. The screw head should
pull slightly into the wood panel. Attach the other
mounting strip by repeating the procedure.
5.
MOUNT PANLOC SHELF BRACKETS
Attach the Panloc shelf brackets to the cabinet
panel using four screws of the proper length.
Place the template over the mounting screws.
The screws should be centered on the "A" and
" B " holes in the template. If necessary, loosen
the screws and push the mounting brackets into
alignment then retighten.
It is recommended that the MR 74 tuner be mounted in a normal or horizontal position. However, with
adequate ventilation the tuner can be mounted in
any position.
Adequate ventilation extends the trouble-free life
of electronic instruments. It is generally found that
each 18° F rise in operating temperature reduces the
life of electrical insulation by one half. Adequate
ventilation is an inexpensive and effective means of
preventing insulation breakdown that results from
unnecessarily high operating temperatures. The
direct benefits of adequate ventilation is longer,
trouble-free life.
The MR 74 tuner requires a mounting space that is
at least 15 inches deep, 17½ inches wide and 6
inches high. Provide additional space for the air flow
across the bottom of the tuner and a means for warm
air to escape at the top.
Remove the tuner, shelf brackets, parts bag and
mounting template from the carton. Remove tuner
from the plastic bag and place the tuner upside
down on the shipping pallet, then remove the four
plastic feet fastened to the bottom of the chassis.
The installation may be accomplished in six steps:
1.
POSITION TEMPLATE AND MARK
Position the plastic mounting template over the
area of the cabinet panel where the MR 74 is to
installed. Be sure that the edges of the template
clear any shelves, partitions or existing equip2
6.
INSTALL THE UNIT
Thread the power cord through the opening on
the cabinet panel. Carefully slide the instrument
into the opening so the rails on the bottom engage the track on the mounting brackets. Slide
the instrument in until it stops at the adjust position latches. Press the latches in and continue
to slide the instrument until its front panel is
flush with the cabinet panel. At the bottom front
corners of the instrument are the PANLOC buttons. Depressing the PANLOC buttons will lock
the instrument firmly in the cabinet. Depressing the PANLOC buttons a second time will
release the instrument. You can then slide the
instrument forward to the adjust position. Depressing the adjust position latches will allow the
instrument to be removed from the cabinet.
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How to Connect
AUDIO OUTPUTS
CONNECTING AN INDOOR DIPOLE ANTENNA
Use the FIXED OUTPUT jacks to connect to a stereo
control preamplifier or other equipment which has
its own volume control. The VOLUME CONTROL
does not affect the output of the tuner at the FIXED
OUTPUT jacks.
The output impedance at the FIXED OUTPUTS is
600 ohms. Longer cables than are normally supplied
can be used to interconnect the MR 74 with other
equipment. The length of the cable is limited by the
capacity of the cable. The total capacity must not
exceed 1600 pF. For instance: cables with a capacity
of 32 pF per foot may be 50 feet long; 16 pF per foot
cable may be 100 feet long.
Use the FRONT PANEL CONTROLLED jacks to
connect to equipment such as power amplifiers or
tape recorders where control of volume at the tuner
is desired. The load impedance connected to FRONT
PANEL CONTROLLED jacks should not be less than
10,000 ohms. There is no difference in the signal
quality or maximum output levels available at either
pair of output jacks.
The flexible folded dipole antenna (300 ohm) is for
use in urban or high strength signal areas.
Connect the two leads from the dipole to the 300
ANT (red) terminals. The flexibility of the thin flat
wire assembly permits it to be placed under a rug,
tacked behind the stereo . . . or, placed in any other
convenient location. In some cases, it may be necessary to "position" the antenna for best signal reception. This should be done before it is permanently
located. Avoid locating this antenna next to other
wires or metal objects. This antenna may not prove
effective in houses having metal siding or metal-clad
insulation.
AM ANTENNA
A high-quality loop-stick antenna is provided. It
can be rotated through nearly 180° in all directions
for maximum performance, optimum signal reception
or minimum interference. With this mobility you will
not suffer loss of sensitivity regardless of the angle
at which the instrument is mounted. A back panel
antenna jack is provided for connecting an external
antenna if desired.
CONNECTING AN FM ANTENNA
One of three antenna systems can be used: (1) an
outdoor FM antenna, (2) an all channel (UHF-VHFTV) antenna, or (3) the indoor dipole supplied with
the MR 74.
An outdoor antenna is recommended for optimum
performance in all areas. In fringe areas, best results will be obtained with a highly directional FM
antenna used in conjunction with a rotator. Rotate
the antenna until the best reception is obtained. Connect the 300 ohm antenna to the 300 ANT (red)
terminals.
An UHF-VHF-TV antenna is often effective but the
antenna must be designed for both FM and TV reception. Connect the leads from the UHF-VHF-TV
antenna to the 300 ANT (red) terminals.
CONNECTING AN AM OUTDOOR ANTENNA
For best long distance AM reception, use a copper
antenna wire 50 to 150 feet in length. Suspend the
wire in a straight line as high as possible. Attach the
wire at each end with suitable glass or ceramic insulators. Connect a lead-in wire at any convenient
point on the antenna. It is recommended that a lightning arrester be used with an outdoor AM antenna.
The arrester should be well grounded to a suitable
water pipe or a ground rod sunk into the ground.
Connect the lead-in wire to the AM ANT push connector on the antenna terminal strip on the back
panel. Set the AM ANT slide switch to EXT position.
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FM ANTENNA
ALTERNATE
AM ANTENNA
A high-quality loopstick antenna
is provided. It can be rotated for
maximum performance, optimum
signal reception or minimum
interference.
The antenna is rotatable through
nearly 180 degrees.
POWER AMPLIFIER
PREAMPLIFIER
RIGHT SPEAKER
LEFT SPEAKER
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Back Panel information Front Panel information
TP1 and TP2
TUNING DIAL
Test points TP1 and TP2 are provided for tuner
circuit alignment. They are also used in conjunction
with the Mclntosh Maximum Performance Indicator.
Provides 120 volt AC power up to 400 watts for
additional equipment such as amplifiers, or other
equipment. This outlet is not fused. It turns on and
off with the front panel AC power switch on the
VOLUME control.
The MR 74 has three dial scales:
1. AM — Marked 55 to 160
2. FM — Marked 88 to 108
3. Logging scale — Marked 0 to 100
The logging scale can be used to accurately retune any station. You may find it easier to keep a
record of your favorite stations by use of the logging
scale.
A small portion of dial pointer has been illuminated
to increase the ease of tuning.
AC POWER CORD
INDICATORS
Connect the AC power cord to a 120 volt, 50 to 60
Hz power line receptable. The power used by the
MR 74 is 30 watts.
The MR 74 has four indicators on the dial panel:
STEREO indicator, MULTIPATH indicator, SIGNAL
STRENGTH meter, and the FM TUNING meter.
FUSE
STEREO INDICATOR
A 0.5 AMP SLO-BLO fuse protects the tuner circuits. This fuse does not protect additional equipment connected to the back panel AC power outlet.
The STEREO indicator lights red when the dial
pointer is tuned to or corsses a station broadcasting
the 19,000 Hz carrier for stereo. The special circuit
used will light only when the 19,000 Hz multiplex
carrier is present in the signal. The indicator will not
light on noise pulses or interference.
120 AC POWER OUTLET
AM ANT SWITCH
Adjust the MR 74 AM ANT switch to match the particular type of AM antenna used. EXT position is
used for an outdoor antenna. LOOP position is for
the built-in ferrite loopstick.
MULTIPATH INDICATOR
The MULTIPATH indicator is an exclusive Mclntosh development.
The proper use of the MULTIPATH indicator
makes it possible to improve FM reception with
FM antenna positioning.
An electron ray indicator is used to show multipath reception. When rotating the antenna, observation of the multipath indicator will show best orientation for the FM station being received. Multipath
distortion causes the two beams on the indicator to
fluctuate rapidly. When the antenna is rotated to the
best position, the indicator beams tend to remain
steady. The directional antenna then picks up the
desired signal and rejects the reflected multipath
signals. In some locations it is possible for the best
reception to occur when the antenna picks up a
strong reflected signal rather than the direct signal.
Multipath distortion is practically independent of signal strength.
ANTENNA CONNECTION STRIP
Provides easy push type connectors for an AM
antenna, a ground connection, and a 300 ohm FM
antenna.
FM ANTENNA — 75 OHM
Provides a type F male connector for a 75 ohm
unbalanced FM antenna.
AUDIO OUTPUTS
The FIXED OUTPUT pair of AUDIO OUTPUT jacks
provides audio signals unaffected by the MR 74
front panel VOLUME control. Use these output jacks
to connect the tuner to a stereo preamplifier or other
equipment which has its own master volume control.
The FRONT PANEL CONTROLLED pair of AUDIO
OUTPUT jacks provides audio signal controlled by
the MR 74 front panel VOLUME control. Use these
output jacks to connect to external power amplifier,
tape recorders, or any equipment which requires
front panel control of tuner output volume.
SIGNAL STRENGTH METER
The SIGNAL STRENGTH meter indicates the
strength of the signal as received from the antenna.
The higher the indication, the stronger is the signal,
FM TUNING METER
An FM station is correctly tuned when the meter
needle is in the black area of the FM TUNING meter.
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The action of the TUNING meter is independent of
station signal strength.
MODE SELECTOR
Selects any one of three sources:
AM— Connects the AM tuner section of the MR
74 to the output jacks.
FM— The FM position provides monophonic FM
or FM stereo at the left and right channel
audio output jacks automatically. With the
MODE SELECTOR in the FM position a
station broadcasting monophonic program
will be heard in mono. When the station
switches to stereo broadcast, the stereo indicator will light and the MR 74 will automatically switch to stereo operation. If mono
broadcasting is resumed the MR 74 will automatically switch to mono.
FM MUTING
Muting suppresses the background noise and hiss
normally heard between stations. Turn the control to
either LOCAL or DISTANT position for muting. Weak
stations that may not override noise and interference
are also suppressed by the muting. In the OUT position, the muting is turned off to allow FM tuning with
the noise and interference present.
VOLUME
The VOLUME control has been precision tracked
throughout the listening range (0 to —65 dB) for accurate stereo balance. It adjusts the output level at
the tuner FRONT PANEL CONTROLLED AUDIO OUTPUT jacks. The FIXED OUTPUT jacks are not affected by the volume control.
The POWER ON/OFF switch is part of the VOLUME control. Turning the VOLUME control fully
counterclockwise turns the AC power to the MR 74
OFF.
FM-MONO — Connects the FM tuner sections of
the MR 74 to the output jacks. This
position bypasses the automatic
stereo switching. The stereo indicator will light when a station is transmitting stereo, but all programs are
heard monophonically.
PANLOC BUTTONS
At the bottom of the front panel corners are the
PANLOC buttons. After a tuner is installed on the
PANLOC brackets, pressing the PANLOC buttons
will lock the tuner firmly in position. Depressing the
PANLOC buttons will release the tuner. The tuner
can then be slid forward to the inspection and adjustment position. The PANLOC system gives absolute
ease of installation, operation and maintenance.
STEREO FILTER
The STEREO FILTER reduces noise on weak
stereo stations.
Position 1 reduces noise by approximately 10 dB.
Position 2 reduces noise by approximately 20 dB.
SELECTIVITY
In NORMAL, a very low distortion 10-pole filter
is connected to the IF system. Use this position of
the SELECTIVITY switch for normal reception conditions.
In NARROW, a sharp 6-po!e filter is added to the
low-distortion 10-po!e filter. This yields a low-distortion, highly selective 16-pole composite filter. Use
this switch position to reduce interference on distant
stations.
SECONDARY CONTROLS
On the top of the chassis behind the front panel
is the DIAL SCALE INTENSITY switch. Adjust the
brightness of the dial lights by means of this switch.
Set the switch to BRIGHT for maximum dial light; set
the switch to DIM for subdued dial light.
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Listening to Your MR 74
the listening quality will be rather poor due to the
background noise.
If an antenna rotator is used, rotate the directional
antenna for best reception as shown by the SIGNAL
STRENGTH and MULTIPATH indicators.
Adjust the VOLUME control to desired listening
level. If the FIXED AUDIO OUTPUT jacks are used,
the tuner VOLUME control will not affect volume.
LISTENING TO AM
Turn the MODE SELECTOR switch to AM and the
SELECTIVITY switch to NORMAL. Turn the tuning
knob to the desired station. The SIGNAL STRENGTH
meter will indicate the relative strength of the particular AM station being received.
Adjust the VOLUME control to desired listening
level. If the FIXED AUDIO OUTPUT jacks are used,
the tuner VOLUME control will not affect volume.
LISTENING TO MONOPHONIC FM
LISTENING TO STEREO FM
Turn the MODE SELECTOR to FM MONO to listen
to monophonic FM. In this position all programs will
be mono. The stereo light will be seen on stereo stations but the program heard will be mono.
Set the STEREO FILTER switch to OUT
Set the MUTING switch to OUT
Set the SELECTIVITY switch to NORMAL
Turn the tuning knob to the desired station. The
Station is properly tuned when the FM TUNING
meter pointer comes to rest anywhere in the black
area of the meter scale. While tuning across the dial
you may notice movement of the tuning meter without hearing a station. This is cuased by a weak station that does not over-ride the background noise.
With muting in operation signals that are marginal
are automatically suppressed. To hear these weaker
stations, set the FM MUTING switch to OUT. Usually
Turn the MODE SELECTOR to FM
Set the STEREO FILTER switch to OUT
Set the MUTING switch to OUT
Set the SELECTIVITY switch to NORMAL
Turn the tuning knob to the desired station. The
station is properly tuned when the FM TUNING meter
pointer comes to rest anywhere in the black area of
the meter scale.
When the STEREO indicator is lighted, the station
is broadcasting a 19,000 Hz carrier for stereo and the
MR 74 will automatically switch to stereo. If a station
is broadcasting a monophonic FM program, the
STEREO indicator will remain off and the tuner will
automatically switch to mono.
Adjust the VOLUME control to the desired listening
level. If the FIXED AUDIO OUTPUT jacks are used,
the tuner VOLUME control will not affect volume.
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Performance Limits
PERFORMANCE GUARANTEE — Performance limits
are the maximum deviation from perfection permitted
for a Mclntosh instrument. We promise you that the
MR 74 you buy must be capable of performance at or
exceeding these limits or you get your money back.
Mclntosh is the only manufacturer that makes this
guarantee.
STEREO FILTER: 10 dB noise reduction in Position 1
20 dB noise reduction in Position 2
AM SECTION
SENSITIVITY: 75 µV (external ant.)
SIGNAL TO NOISE RATIO: 55 dB at 100% modulation; 45 dB minimum
HARMONIC DISTORTION: 1% at 30% modulation
SELECTIVITY, ADJACENT CHANNEL: 35 dB in
NORMAL position; 45 dB in NARROW position
IMAGE REJECTION: 65 dB 540 kHz to 1600 kHz
FREQUENCY RESPONSE: SELECTIVITY SWITCH IN
NORMAL position
3500 Hz, 6 dB down
NARROW position
2100 Hz, 6 dB down
All tuner performance limits were measured with selectivity switch set at normal; unless otherwise stated
FM SECTION
TUNING RANGE: 88 to 108 MHz
ANTENNA INPUTS: 300 ohms balanced; 75 ohms
unbalanced
INTERMEDIATE FREQUENCY (IF): 10.7 MHz
SENSITIVITY: 2.5 µV at 100% modulation (± kHz
deviation) for 3% total noise and harmonic distortion
SIGNAL TO NOISE RATIO: 70 dB below 100% modulation
HARMONIC DISTORTION: MONO — 0.3% at 100%
modulation + 75 kHz deviation; STEREO — 0.5%
at 100% modulation
DRIFT: 25,000 Hz for the first two minutes; thereafter 5,000 Hz at 25°C in 24 hours
FREQUENCY RESPONSE: ± 1 dB 20 Hz to 15,000 Hz
with standard de-emphasis (75 µ sec.)
CAPTURE RATIO: 1.5 dB minimum
GENERAL
AUDIO OUTPUT: front panel controlled: 2.5 volts into
47,000 ohms. Fixed Output: 2.5 volts into 47,000
ohms; 1.0 volt into 600 ohms
POWER REQUIREMENTS: 120 volts, 50/60 Hz, 30
watts
SEMICONDUCTOR COMPLEMENT: 5 FETs, 17 transistors, 2 ICs, 30 diodes, 1 indicator tube
MECHANICAL INFORMATION
SIZE: Front panel: 16 inches wide (40.64 cm) by
5-7/16 inches high (13.81 cm); Chassis: 15 inches
wide (38.1 cm) by 13 inches deep (33.02 cm), including PANLOC shelf and back panel connectors;
Knob clearance: 1½ inches (3.81 cm) in front of
mounting panel
WEIGHT: 25 pounds (11.34 kg) net; 37 pounds (16.78
kg) in shipping carton
FINISH: Front Panel: Anodized gold and black with
special gold/teal panel nomenclature illumination;
Chassis: Chrome and black
MOUNTING: Mclntosh developed professional PANLOC
SELECTIVITY:
Switch Setting
NORMAL NARROW
Adjacent Channel:
6 dB
15 dB
Alternate Channel:
68 dB
88 dB
SPURIOUS REJECTION: 90 dB
IMAGE REJECTION: 95 dB; 88 to 108 MHz (IHF)
MUTING: 50 dB noise reduction in LOCAL position
MUTING THRESHOLD: DISTANT position 3 µ V ; LOCAL position 10 µV
SCA FILTER: 50 dB down from 67 kHz; 275 dB per
octave slope
STEREO SEPARATION: 35 dB at 1,000 Hz
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FM SIGNAL PERFORMANCE
0
10
RELATIVE OUTPUT IN dB
Performance
Charts
20
30
40
50
60
70
0.1
1.0
10
100
10,000
1000
RF INPUT SIGNAL IN MICROVOLTS ACROSS 300 OHMS
FM - IF CHARACTERISTIC
0
-10
-30
-40
-50
-60
-70
AMPLITUDE RESPONSE IN dB
-20
PER CENT HARMONIC DISTORTION
FM HARMONIC DISTORTION
3
2
1
0
10
-90
FREQUENCY IN kHz
0
100
200
300
1000
FREQUENCY IN Hz
-80
-100
100
400
10
10000
20000
RELATIVE
OUTPUT IN dB
AM FREQUENCY RESPONSE
0
-10
-20
-30
-40
20
100
1000
10000 20000
FREQUENCY IN Hz
AM SELECTIVITY AT 1000 kHz
RELATIVE
OUTPUT IN dB
0
-20
-40
-60
-80
-85
-100
1030
1020
1010
1000
990
FREQUENCY IN kHz
11
980
970
are in the AM section. By interleaving the sections
(FM-AM-FM-etc.) spurious responses and oscillator
pulling are significantly reduced. The four FM sections of the variable tuning capacitor provide a high
degree of RF selectivity and excellent spurious rejection. Use of the latest "state of the art" field
effect transistors with a well-designed variable tuning capacitor provides an excellent RF front end.
All of the RF circuits, including the selectivity circuit and the AM sections of the variable capacitor
are encased in a metal module. Each FM-RF section
is isolated in a separate compartment by metal
shielding. Careful design and manufacturing increase
the protection against radiation and interference.
The MR 74 exceeds the FCC requirements for suppression of local oscillator radiation.
A dual insulated gate metal oxide silicon field effect transistor (MOSFET) is used as first and second
RF amplifier. Each gate of the transistor is internally
protected by back-to-back diodes against incoming
transients. Use of MOSFET's greatly reduces the
cross-modulation products over a wide dynamic
range. A wide dynamic range permits the input circuits to accept extremely strong signals without
overload. Since both RF amplifiers have insulated
gate configurations, external neutralization is not
required. This design results in a very stable RF
amplifier circuit.
Low temperature coefficient components for the
FM local oscillator prevent frequency drift. The frequency stability inherent in the local oscillator makes
automatic frequency control (AFC) unnecessary. The
rate of drift of the local oscillator is less than ten
parts per million per degree centigrade.
The mixer design uses a junction field effect transistor (JFET) for high sensitivity and freedom from
overload. The mixer delivers the composite FM signal at the 10.7 MHz intermediate frequency. The path
of the IF signal is controlled by the front panel
SELECTIVITY selector switch.
At the NORMAL position, the SELECTIVITY switch
directs the signal through an IF preamplifier stage
that uses a JFET and a double-tuned IF transformer.
The signal then goes to the FM-IF and discriminator
module for further amplification. Setting the SELECTIVITY selector switch to NARROW routes the signal
through two double-tuned transformers, a ceramic
filter network, and a single-gate MOSFET. The sides
of the IF curve are compressed by this circuit narrowing the IF bandpass. In this mode of operation
weak stations adjacent to strong stations can be
tuned.
TUNING MECHANISM AND DIAL DRIVE
In the MR 74, unique design and careful manufacture of the mechanical dial drive assembly gives
smooth flywheel tuning.
By controlling the relationship of mass and mechanical resistance, and by dividing the workloads
in the dial drive system, it becomes nearly impossible to detect any backlash. Yet, the entire dial drive
is a model of mechanical stability.
For added ease and increased tuning accuracy, a
section of the dial pointer is illuminated.
FM
Antenna connections for either 300 ohm twin lead
transmission line or 75 ohm coaxial cable are provided on the back panel. The normal input impedance of the RF amplifier is 75 ohms. Impedance
match to 300 ohms is provided by a negligible loss
balun transformer designed by Mclntosh. Connec-
The Radio Frequency (RF} section houses the
complete FM-RF front-end and part of the AM-RF
circuit.
A seven-section variable capacitor is the heart of
the RF section. Four sections of the variable capacitor are in the FM front end and the remaining three
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tions for both 300 ohm twin lead and 75 ohm coaxial cable are made with push type terminals.
FM muting in the MR 74 operates by detecting
ultra-sonic noise. When noise is present the muting
circuit can be activated or defeated by the use of
the muting switch on the front panel.
FM-IF AND DISCRIMINATOR
The MR 74 uses linear-phase IF filters. The IF filters have equal time delay in their pass band region.
Any error in time delay causes FM distortion. All
other IF filters have delay distortion, some as much
as 100% of the 10.7 MHz transit delay. The MR 74
has less than 1.0% delay distortion from antenna
input to discriminator output. This makes possible
the overall low distortion performance limit for the
FM tuner and multiplex section.
Amplification of the IF signal is provided by two
high gain integrated circuits, each containing 16
transistors, 3 zener diodes, 5 diodes, and 23 resistors, all on a single monolithic silicon chip. The exceptionally high gain of the integrated circuit assures
"hard limiting" at very low levels of input signals.
A "phase" or "Foster Seeley" discriminator has
been designed to complement the integrated circuit
IF section. The detected output signal of the discriminator is extremely low in distortion content.
Paralleling the main signal path through the FM-IF
and detector module, a secondary amplifying and
de-modulating process is used to activate the signal
strength meter and to provide test point TP1 with a
signal to be used with the Mclntosh Maximum Performance Indicator.
The STEREO FILTER is provided to reduce noise
when listening to weak stereo stations. Careful design permits an ideal compromise between channel
separation and noise rejection.
AM
The AM-RF amplifier circuit includes a three section variable tuning capacitor in the metal enclosed
shielded RF module which also houses the FM-RF
front end. A three section variable capacitor is used
for greatest spurious rejection. The RF amplifier is
unique. The circuit has constant sensitivity, constant
selectivity, and high image rejection across the complete AM band. Ordinary AM-RF circuits cannot do
all of these simultaneously. The circuit design
achieves high sensitivity even at the low end of the
band. Spurious, image, and intermediate frequency
rejection are all superior. The same circuit delivers
equal selectivity across the entire band.
In addition, there is no loss of audio frequency
response at the low end of the band, common in AM
receivers. Another advantage of the Mclntosh circuit
is freedom from cross modulation and overloading
by strong local stations.
A high-quality loopstick antenna is provided. It can
be rotated for maximum performance, optimum signal reception or minimum interference. In each
MR 74, the loopsticks are individually tuned for optimum performance. After tuning, the loopstick is
sealed. This custom matching of the loopstick to the
AM-RF front end maximizes the performance of the
loopstick antenna. The antenna is rotatable through
nearly 180 degrees. With this mobility you will not
suffer loss of sensitivity regardless of the angle at
which the instrument is mounted. A back panel connector is provided for an external antenna. The antenna is coupled to the tuner through a carefully
designed matching transformer which provides optimum performance regardless of antenna length.
To maintain the excellent image rejection and lack
of spurious cross modulation of the AM-RF amplifier
an autodyne converter circuit was used in producing
the AM-IF signal.
FM STEREO MULTIPLEX
The 19,000 Hz pilot signal, broadcast by an FM
station, is filtered from the composite stereo input
signal, amplified by a special limiting amplifier,
doubled to the 38,000 Hz carrier frequency, and then
amplified again by a limiting amplifier. The composite signal minus the 19,000 Hz pilot is combined with
the 38,000 Hz carrier signal. The new combination
of signals is fed to the special detector circuit mentioned above. Balanced full wave detectors are used
to cancel the 38,000 Hz components in the output.
The SCA (Subsidiary Communication Authorization)
signal is removed from the composite output. This is
accomplished by the use of a new "Image Parameter" band elimination filter that has been computer
designed. The SCA filter rejects SCA signals without impairing stereo performance.
When the 19,000 Hz carrier of a stereo signal is
received, the automatic FM stereo switching circuit
activates the multiplex decoding circuit. This lights
the stereo indicator. The circuit switching is all done
electronically. The automatic stereo switching can
be defeated by setting the mode selector switch to
FM MONO. (In this position the stereo indicator will
still light to indicate the presence of a stereo signal.)
On monophonic transmissions the stereo switching
is inactive, assuring optimum signal to noise ratio.
The stereo switching circuit has been designed so
that noise will not activate it.
AM-IF uses two double tuned IF transformers designed to obtain a high degree of selectivity yet allowing good audio fidelity. With the SELECTIVITY
switch in NARROW, a narrow band ceramic filter is
added to the AM-IF amplifier.
A 10,000 Hz active filter eliminates the 10,000 Hz
whistle and irritating noise caused by an adjacent
station. Because an active filter is used, the output
level at 10,000 Hz, or the whistle frequency, is down
over 20 dB or one hundredth of what it would be
13
without filtering. With the SELECTIVITY switch in
NARROW position, the active filter cutoff frequency
is lowered. The filter then effectively suppresses the
5,000 Hz whistle from nearby television receivers.
The AVC (automatic volume control) system was
designed to prevent blasting when the AM is tuned
through a strong signal. Distortion at low audio frequencies is minimized by using two AVC filter sections instead of the conventional one.
AUDIO AMPLIFIER
The audio amplifier increases the level of the program adequate to drive a preamplifier or other accessory equipment. It consists of two, two-transistor
amplifiers, one for each channel. The design uses
considerable negative feedback to help achieve low
distortion, wide frequency response, and excellent
stability. Each audio amplifier delivers 2.5 volts to
the FIXED OUTPUT jacks. A second pair of outputs
are available where level can be varied by the VOLUME control.
POWER SUPPLY
Special design attention has been given to the
power supply section. Three separate power supply
circuits are used. The first is a 24 volt regulated supply. This 24 volt regulator is very elaborate in design,
using a specially selected transistor and associated
circuit. The regulator uses electronic filtering to insure the lowest possible background hum level,
maximum stability and extremely good regulation. All
signal stages are powered from this regulator.
The second circuit is a half wave rectifier and
filter for the DC high voltage needed for the anode
of the multipath indicator. The third power circuit is
a half wave rectifier which supplies DC to the AM
section.
14
Block
Diagram
FM STATION LOG
CALL
LETTERS
FREQUENCY
LOCATION
CALL
LETTERS
ANTENNA
DIRECTION
16
FREQUENCY
LOCATION
ANTENNA
DIRECTION
McINTOSH LABORATORY INC.
2 CHAMBERS ST., BINGHAMTON, N. Y. 13903
607-723-3512
Design subject to change without notice.
Printed in U.S.A.
038-839
BE112003
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