THE McINTOSH MR 78 SOLID STATE FM/FM STEREO TUNER

THE McINTOSH MR 78 SOLID STATE FM/FM STEREO TUNER
THE McINTOSH MR 78 SOLID STATE FM/FM STEREO TUNER
Reading Time 32 Minutes
Price $1.25
Your MR 78 FM/FM Stereo Tuner
will give you many years of
pleasant and satisfactory
performance. If you have any
questions concerning the operation
or maintenance of this instrument,
please contact:
CONTENTS
GUARANTEE
INSTALLATION
HOW TO CONNECT
BACK PANEL INFORMATION
FRONT PANEL INFORMATION
LISTENING TO THE MR 78 TUNER
PERFORMANCE LIMITS
TYPICAL PERFORMANCE CHARTS
TECHNICAL DESCRIPTION
BLOCK DIAGRAM
CUSTOMER SERVICE
Mclntosh Laboratory Inc.
2 Chambers Street
Binghamton, New York 13903
Phone: 607-723-3512
Take Advantage of 3 years
of FREE Factory Service . . .
Fill in the Application NOW.
GUARANTEE
Mclntosh Laboratory Incorporated guarantees this
Instrument to be capable of performance as advertised. We also guarantee the mechanical and electrical workmanship and components to be free of
defects for a period of 90 days from date of purchase. If such defects occur, Mclntosh Laboratory
or one of its authorized agencies will repair the
defect at no cost to the purchaser. This guarantee
does not extend to components damaged by improper use nor does it extend to transportation to
and from the factory or service agency.
THREE YEAR FACTORY SERVICE CONTRACT
An application for a FREE THREE YEAR FACTORY
SERVICE CONTRACT is included with this manual.
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. The SERVICE CONTRACT is given to purchasers
who live In the 50 United States or Canada only.
6. For your protection Mclntosh selects Its dealers
carefully. Only one dealer in ten qualifies for a
Mclntosh franchise. 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.
7. Your completely filled in application for a SERVICE CONTRACT must be postmarked within 30
days of the date of purchase of the instrument.
8. 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. If the application is not received at
Mclntosh Laboratory, only the service offered
under the 90-day guarantee will apply.
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 or mishandling is not covered by
the SERVICE CONTRACT.
1
Copyright © 1972 By Mclntosh Laboratory Inc.
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15,16
clear any shelves, partitions or existing equipment
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 pencilled rectangle.
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 or 1¼-inch screws for panels ½-inch thick
or over. Place a mounting strip on the back of the
cabinet panel 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.
It is recommended that the MR 78 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
benefit of adequate ventilation is longer, troublefree life.
The MR 78 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 the
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.
5. MOUNT PANLOC SHELF BRACKETS
The installation may be accomplished in six easy steps.
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.
1. POSITION TEMPLATE AND MARK
Position the plastic mounting template over the
area of the cabinet panel where the MR 78 is to be
installed. Be sure that the edges of the template
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
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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 firmily in the cabinet. Depressing the PANLOC buttons a second
time {as with a ballpoint pen) 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
An outdoor antenna is recommended for optimum
performance in all areas. In fringe (outlying) 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. A UHF-VHF-FM 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.
AUDIO OUTPUTS
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 78 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.
CONNECTING AN INDOOR DIPOLE ANTENNA
The flexible folded dipole antenna (300 ohm) is for
use in urban or high strength signal areas.
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 the FRONT
PANEL CONTROLLED jacks should not be less than
47,000 ohms. There is no difference in the signal
quality or maximum output levels available at either
pair of output jacks.
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.
CONNECTING AN FM ANTENNA
CONNECTING A 75 OHM ANTENNA
One of three antenna systems can be used: (1) an
outdoor FM antenna, (2) an all channel (UHF-VHF-FM)
antenna, or (3) the indoor dipole supplied with the
MR 78.
An unbalanced 75 ohm antenna can be connected to
the MR 78. A "type F" connector is used to connect
the 75 ohm coaxial cable to the back panel 75 ANT
input.
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FM ANTENNA
OPTIONAL
75
ANTENNA
LEAD-IN
PREAMPLIFIER
POWER AMPLIFIER
RIGHT SPEAKER
LEFT SPEAKER
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Back Panel Information
It sets the output level of the tuner FRONT PANEL
CONTROLLED AUDIO OUTPUT jacks. The FIXED
OUTPUT jacks are not affected by the VOLUME control.
TP1 and TP2
Test points TP1 and TP2 are used with the Mclntosh
Maximum Performance Indicator.
The AC power switch is part of the VOLUME control.
Turning the volume control fully counterclockwise
turns the AC power off.
120 V AC POWER OUTLET
TUNING DIAL AND METER
Provides 120 volt AC power up to 400 watts for additional equipment such as amplifiers, etc. This outlet
is not fused. It turns on and off with the front panel AC
power switch on the VOLUME control.
The MR 78 has two dial scales:
1. FM scale - marked 88 to 108 MHz
2. 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 the dial pointer has been illuminated
to increase the ease of tuning. An FM station is tuned
correctly when the TUNING meter pointer is in the
black area of the meter scale. The tuning indication is
independent of the station signal strength. Maximum
left and right pointer swing, which occurs as you tune
through a station, is dependent on the setting of the
SELECTIVITY switch.
AC POWER CORD
Connect the AC power cord to a 120 volt, 50 to 60
Hz power line receptacle. The power used by the
MR 78 is 35 watts.
FUSE
A 0.5 AMP fuse protects the tuner circuits. This fuse
does not protect additional equipment connected to
the back panel AC power outlet.
ANTENNA CONNECTION STRIP
STEREO FUNCTION INDICATOR
Provides easy push type connectors for 300 ohm FM
antenna and a ground connection.
The STEREO FUNCTION indicator lights red when
the dial pointer is tuned to or crosses a station broadcasting the 19,000 Hz pilot signal for stereo. The special
circuit used will light only when the 19,000 Hz multiplex pilot is present. The indicator will not light on
noise pulses or interference. The STEREO FUNCTION
indicator will light regardless of the MODE switch
setting.
FM ANTENNA -75 OHM
Provides a "type F" connector for a 75 ohm unbalanced FM antenna.
AUDIO OUTPUTS
The left-hand pair of AUDIO OUTPUT jacks provides FIXED OUTPUT audio signals. The program at
these jacks is not affected by the MR 78 front panel
VOLUME control. Use these output jacks to connect
the tuner to a stereo control preamplifier or other
equipment which has its own volume control.
MODE SWITCH
The MODE switch has three settings.
STEREO ONLY - Only stations broadcasting the
19,000 Hz pilot signal for stereo can be heard in this
position. All monophonic stations and background
noises are suppressed. The TUNING meter and SIGNAL
STRENGTH meter function will indicate monophonic
broadcasts when tuning but no program will be heard
from the loudspeakers.
The right hand pair of AUDIO OUTPUT jacks provides a FRONT PANEL CONTROLLED audio signal.
The program at these jacks is controlled by the MR 78
front panel VOLUME control. Use these output jacks
to connect to external power amplifier, tape recorders,
or any equipment where continuous front panel control of tuner output volume is desirable.
MONO-In this position of the MODE switch, all broadcasts will be heard monophonically. The automatic
stereo switching is bypassed. The STEREO FUNCTION
indicator will light when a station is transmitting
stereo, but the program heard from the loudspeakers
will be mono.
Front Panel Information
STEREO AUTO - In this position of the MODE switch
both mono and stereo broadcasts are heard. The tuner
automatically switches from mono to stereo mode of
operation, depending on the type of broadcast being
received.
The MR 78 Tuner has the most flexible control system ever designed in a stereo FM tuner. Use of these
controls will give you a higher level of performance.
VOLUME CONTROL AND AC POWER SWITCH
MUTING SWITCH AND FUNCTION INDICATOR
The VOLUME control has been precision tracked
throughout the listening range (0 to -65 dB) for accurate stereo balance.
Muting suppresses the background noise and hiss
normally heard between stations. Turn the control
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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. This allows
tuning of weak stations, but the noise and interference
will be present.
SELECTIVITY switch to NORMAL connects a very low
distortion 8-pole Rimo filter in the IF circuit. Use the
NORMAL position of the SELECTIVITY switch for
listening to local broadcasts.
Setting the SELECTIVITY switch to NARROW adds
a sharp 8-pole Rimo filter to the NORMAL IF filter to
yield a low distortion (less than 0.2%), highly selective
16-pole composite IF filter. Use the NARROW position of the SELECTIVITY switch to reduce any interference during the reception of distant stations.
The MUTING FUNCTION indicator lights amber
whenever the MUTING switch is set to DISTANT or
LOCAL It will extinguish when the MUTING switch
is set to OUT.
Setting the SELECTIVITY switch to SUPER NARROW adds a 4-pole 4-zero crystal filter to the two
previously mentioned filters. Use the SUPER NARROW
position to receive stations from distant cities which
are on channels adjacent to local stations. There may
be usable signals never heard before with ordinary
FM tuners.
FILTER SWITCH and FUNCTION INDICATOR
The FILTER SWITCH reduces noise on weak stereo
stations.
Position 1 reduces noise by approximately 10 dB.
Position 2 reduces noise by approximately 20 dB.
The FILTER FUNCTION indicator lights green whenever the FILTER switch is set to 1 or 2. It does not light
when the FILTER switch is set to OUT.
The SELECTIVITY indicator lights blue. The IHF
Adjacent Channel Selectivity characteristics are:
NORMAL 7 dB; NARROW 22 dB; and SUPER NAROW 55 dB.
SIGNAL STRENGTH/MULTIPATH SWITCH AND
METER
The SIGNAL STRENGTH/MULTIPATH meter can
be switched to indicate either relative signal strength
or relative amount of multipath interference. With
METER switch set to SIGNAL STRENGTH, greater
pointer deflection indicates a stronger signal.
SECONDARY CONTROLS
With the METER switch set to MULTIPATH, there
is little or no multipath interference when the meter
indication remains steady and at the left end of the
scale. If there is severe multipath interference, the
meter indication will follow the audible signal in much
the same manner as a VU meter. When the receiving
antenna is rotated to minimize multipath interference,
the meter indication will decrease and become steady.
If the multipath interference is eliminated, the meter
indication will be near zero and steady.
DIAL SCALE INTENSITY SWITCH
On the top of the chassis behind the front panel is
the DIAL SCALE INTENSITY Switch and ANTENNA
MATCH SELECTOR switch.
Adjust the brightness of the dial lights by means of
this switch. Set the switch to BRIGHT for maximum
panel light; set the switch to DIM for subdued dial
light.
ANTENNA MATCH SELECTOR SWITCH
Under most conditions the ANTENNA MATCH
SELECTOR switch should be set to HIGH GAIN ANT.
position. Only in extreme fringe areas or when using a
low-gain (indoor) antenna should this switch be set
to LOW GAIN ANT. position. When set to LOW GAIN
ANT. position, background noise on weak stations
may be reduced.
SELECTIVITY SWITCH AND INDICATOR
The SELECTIVITY switch allows stereo reception
even under severe receiving conditions. Setting the
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MR 78 will automatically switch to stereo. If a station
is not transmitting the 19,000 Hz pilot for stereo,
the STEREO indicator will remain off and the tuner
will automatically switch to mono.
Listening to the MR 78 Tuner
MONOPHONIC FM PROGRAMS
To receive monophonic FM broadcasts set the:
When the MODE switch is set to STEREO ONLY,
monophonic broadcasts and background noise between stations will be muted. Broadcasts with the
19,000 Hz pilot signal will activate an automatic stereo
sensing circuit. The stereo indicator will light and the
broadcast will be heard.
MODE switch to MONO
MUTING switch to DISTANT
FILTER switch to OUT
SELECTIVITY switch to NORMAL
USING VARIABLE SELECTIVITY
Adjust the tuning knob to the desired station. The
station is properly tuned when the TUNING meter
pointer comes to rest anywhere over the black area
on the meter scale. While tuning across the dial you
may notice movement of the tuning meter pointer
without hearing a station. This is caused by a station
so weak or distant that it does not over-ride the background noise. Extremely weak stations may be heard
by setting the MUTING switch to OUT.
With the increasing number of stations crowding
the FM broadcast band, an FM tuner with variable
selectivity has become a necessity. Local stations are
received best with the SELECTIVITY switch in the
NORMAL position. This position gives the lowest
distortion. Use this position for critical taping of live
local broadcasts.
If you experience "crosstalk" or "splatter" on a signal from a strong adjacent or alternate channel, use
the NARROW position of the SELECTIVITY switch
to reduce the interference.
Rotate the directional antenna for best reception as
indicated by a maximum pointer deflection on the
SIGNAL STRENGTH meter and a minimum pointer
deflection on the MULTIPATH meter. Then adjust the
VOLUME control to the desired listening level.
The "SUPER NARROW" SELECTIVITY position is
used to find clear signals from distant stations that
cannot be heard from ordinary tuners.
STEREO FM PROGRAMS
When you use SUPER NARROW SELECTIVITY,
you may find that reception of distant stations adjacent to a channel transmitting SCA (background music
programs) is difficult. Stations transmitting SCA
often have wide sidebands. These sidebands spill over
onto adjacent channel stations. If you tune the MR 78
to one of the sidebands of the SCA transmission, you
may be able to hear the background music program
simultaneously with the regular program.
If you wish the tuner to automatically switch between stereo and mono broadcasts, set the:
MODE switch to STEREO AUTO
MUTING switch to DISTANT
SELECTIVITY switch to NORMAL
In this mode of operation, setting the FILTER switch
to position 1 or 2 will reduce noise on weak stereo
stations.
When the STEREO indicator is lighted, the station is
broadcasting a 19,000 Hz pilot signal for stereo and the
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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 78 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.
MAXIMUM SIGNAL INPUT: 12 volts across 300
ohm antenna terminals will not increase harmonic
or intermodulation distortion
AUDIO HUM: 70 dB down from 100% modulation
MUTING: 70 dB noise reduction between stations
TUNING RANGE: 88 to 108 MHz
MUTING THRESHOLD (Typical): Distant position
5µV; Local position 20µV
ANTENNA INPUTS: 300 ohms balanced; 75 ohms
unbalanced
SCA FILTER: 50 dB down from 67 KHz to 74 kHz;
275 dB per octave slope
INTERMEDIATE FREQUENCY (IF): 10.7 MHz
STEREO SEPARATION: 40 dB at 1,000 Hz
SENSITIVITY: 2µV for 35 dB of quieting; 2.5µV at
100% modulation (±75 KHz deviation) for 3% total
noise and harmonic distortion.
STEREO FILTER (Typical): 10 dB noise reduction in
Position 1.
20 dB noise reduction in Position 2.
SIGNAL TO NOISE RATIO: 75 dB below 100% modulation.
HARMONIC DISTORTION: 0.2% mono or stereo at
100% modulation to 15,000 Hz. Typically, 0.05% at
1,000 Hz
AUDIO OUTPUT: Front Panel Controlled: 2.5 volts
into47,000 ohms.
Fixed Output: 2.5 volts into 47,000 ohms; 1.0 volt
into 600 ohms.
DRIFT: 25,000 Hz for the first two minutes; thereafter
5,000 Hz at 25°C in 24 hours.
All tuner performance limits were measured with
SELECTIVITY switch set at NORMAL, unless otherwise
stated.
FREQUENCY RESPONSE: MONO - ±1 dB 20 Hz to
20,000 Hz with standard de-emphasis, (75µS);
STEREO - ±1 dB 20 Hz to 15,000 Hz with standard
de-emphasis <75µS)
GENERAL INFORMATION
POWER REQUIREMENT: 120 volts, 50/60 Hz 35 watts
SEMICONDUCTOR COMPLEMENT: 3 JFET's, 2
MOSFET's, 17 Bipolar Transistors, 43 Diodes, 4 Integrated Circuits
CAPTURE RATIO: 0.25 dB detector only; 2.5 dB complete tuner
MECHANICAL INFORMATION
SELECTIVITY: IHF
SELECTIVITY SWITCH SETTING
SIZE: Front Panel: 16 inches wide (40.65 cm) by5-7/16
inches high (13.8 cm); Chassis: 15 inches wide (38.1
cm) by 13 inches deep (33.1 cm), including PANLOC
shelf and back panel connectors; Knob clearance"
1-1/2 inches (3.85 cm) in front of mounting panel.
SUPER
NORMAL NARROW NARROW
Adjacent Channel
Alternate Channel
7 dB
55dB
22dB
80dB
55dB
80dB
WEIGHT: 27 pounds (12.25 kg) net, 39 pounds (17.69
kg) in shipping carton.
SUPURIOUS REJECTION: 100 dB IHF
FINISH: Front panel: Anodized gold and black with
special gold/teal panel nomenclature illumination;
Chassis: Chrome and black
IMAGE REJECTION: 100 dB, 88 to 108 MHz. IHF
INTERMODULATION DISTORTION: 0.2% mono or
stereo for any combination of frequencies from 20 Hz
to 15,000 Hz with peak modulation equal to 100% or
less. Typically 0.1%
MOUNTING: Mclntosh developed professional PANLOC
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Technical Description
provides maximum sensitivity for a low-gain antenna.
In metropolitan areas crowded with many strong signals, a sharp RF input circuit greatly reduces spurious
responses and achieves maximum sensitivity when
connected to a high-gain antenna.
TUNING MECHANISM AND DIAL DRIVE
The RF bandpass of the antenna matching circuit
can be modified by switching the resonator L1 C1 in
or out of the circuit. Broad band pass is obtained by setting the ANTENNA MATCHING switch to the LOW
GAIN ANT. position. This connects+24 volts to the circuit turning diode D1 off and diodes D2, D3 on. In this
configuration the antenna is connected directly to the
RF amplifier input resonator L2 C2. Diode D3 is tapped
on L2 to yield optimum noise, while D2 shorts resonator L1 C1 so its loss is not coupled into the circuit.
In the MR 78, the 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.
Sharp bandpass is obtained by setting the ANTENNA
MATCHING switch to the HIGH GAIN ANT. position.
This connects -12 volts to the circuit, turning diode
D1 on and diodes D2, D3 off. The RF input circuit
becomes a double tuned filter with high Q in both
resonators, which provides sharp bandpass. Diode D1
is tapped on L1 to yield optimum selectivity. The oscillograph below shows the two bandpass curves of the
RF input circuit.
FRONT END
FM signals enter the tuner through the antenna terminals. A coaxial cable conducts the signal to the RF
amplifier. In the amplifier the stations are separated,
amplified, and converted to a common 10.7 MHz IF
frequency. Instead of using a conventional easily
overloaded transistor or FET as a straight RF amplifier, the MR 78 uses a rugged Junction Field Effect
Transistor as an impedance converter to drive a 5-watt
power transistor. This combination (a cascode circuit) makes the RF amplifier virtually impossible to
overload or cross-modulate. As an example, if you
are tuned to a 3 microvolt signal at 96.3 MHz, the MR
78 will reject signals elsewhere on the dial which are
at least 4,000,000 times stronger. Thus, a 12 volt signal
received at 104.3 MHz would not bother the signal at
96.3 MHz. When tuned to 104.3 MHz, the tuner will
not overload. All MR 78 tuners must pass the 12 volt
overload test.
The MR 78 has an antenna matching circuit in the
RF input. In fringe areas a broad low-loss RF circuit
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Another new design in the MR 78 front end is the
integrated circuit balanced mixer. This mixer is practically impossible to overload. Oscillator pulling,
crossmodulation, and other types of distortion so common in ordinary transistor or FET mixers is minimized.
A balanced bifilar transformer couples the 10.7 MHz
output to the IF amplifier.
Completing the MR 78 front end is the oscillator.
It uses a high-Q ceramic form tank coil. The oscillator
is free form spurious radiations, and operates at high
efficiency. AFC is not required.
RIMO FILTER IF AMPLIFIER
The MR 78 uses linear-phase IF filters. The Rimo
filter, designed from a FORTRAN computer program,
is the most general class of minimum-phase constantdelay filter. The filter design has great mathematical
complexity. Using numerical integration in the S-plane,
an "IBM" 1130 high speed computer spent eighteen
minutes on the mathematics for the design of the IF
filter. It would have taken an engineer, working twenty-four hours a day, seven days a week, and working
error-free three-hundred years to perform the same
mathematical calculations.
The Rimo filter has flat time delay in its pass band
region. Any error in time delay causes FM distortion.
All other IF filters have delay distortion, as much as
100%. The MR 78 filter has less than 1.0% delay distortion from antenna input to discriminator output.
For almost all reception conditions the tuner provides stereo signals that have the lowest possible
distortion.
Constant delay in the IF filter bandpass (which is
equivalent to linear phase) is essential for low distortion FM reception. The MR 78 tuner uses linear phase
(Rimo) filters at NORMAL and NARROW positions of
the SELECTIVITY switch settings.
Constant delay design techniques are used in the
4-pole 4-zero crystal filter used in the SUPER NARROW
position. However due to the extreme selectivity offered, delay distortion is very slightly increased over
the other selectivity positions.
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The MR 78 has the narrowest IF bandwidth ever used
in a stereo tuner. It is the correct width to let just one
FM station through.
The excellent selectivity of the MR 78 {210 kHz
wide at 60dB down) permits tuning stations that are
impossible to receive on ordinary tuners.
Ordinary stereo filters use a single capacitor. The
MR 78 uses an elaborate twin-T bandpass filter. This
circuit is much more effective in suppressing noise
and still preserving the stereo image.
LINEAR-PHASE
The audio amplifier increases the level of the program adequate to drive a preamplifier or other accessory equipment. It consists of two, two-transistor ampifiers, 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.
BRIDGE
AUDIO PREAMPLIFIER SECTION
DISCRIMINATOR
The advantages of the Rimo filter would be lost if
it had to work into an ordinary FM detector. A new
detector had to be developed. It is the linear phase
bridge discriminator. The linear-phase bridge discriminator uses a balanced transmission line bridge
in conjunction with a differential voltage-doubling
rectifier to acheive nearly distortionless demodulation
of FM signals. Mclntosh has a US patent on this new
FM detector.
POWER SUPPLY
The resistors R1 and R3 form a Wheatstone bridge
in combination with the two transmission line sections.
The RF voltages E1 and E2 at the junctions of the resistors and the transmission lines will vary with frequency in such a manner that their RMS arithmetic
difference (E1 - E2) is a straight-line function. Thus
these RF voltages are peak rectified by an amplitude
detector, and the difference taken, producing a distortionless recovery of the FM information.
Special design attention has been given to the power
supply section. Two separate power circuits are used.
The first is a 24 volt regulated supply.
The 24volt 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 is a half wave rectifier with electronic
filter which supplies DC to the multiplex decoder.
BASIC LINEAR PHASE BRIDGE DISCRIMINATOR
Performance of the MR 78 bridge discriminator has
reduced distortion to nearly the theoretical zero. In
addition to its excellent distortion-free performance,
the bridge discriminator also exhibits a capture ratio
close to 0 dB.
STEREO DECODER
An integrated circuit audio amplifier with over 120
dB of negative feedback in two loops is used to drive
the stereo demodulator. This new circuit keeps the
distortion low in the stereo mode of operation. Highly
stable pilot recovery circuits are used to minimize
19,000 Hz intermodulation beats. MUTING, STEREO
ONLY, and AUTOMATIC STEREO/MONO switching
are functions performed within the stereo decoder.
The STEREO FILTER is connected in the audio amplifier to reduce noise when listening to weak stereo
stations. Careful design of the STEREO FILTER circuit
permits an ideal compromise between channel separation and noise rejection.
14
McINTOSH LABORATORY INC.
2 CHAMBERS ST., BINGHAMTON, N. Y. 13903
607-723-3512
Design subject to change without notice.
Printed in U.S.A.
038-580
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