McIntosh MR 71 User manual

FM STEREO TUNER
WARRANTY STATION
MR 71
SERVICE MANUAL
Ifltlntosh

MR 71 FM STEREO TUNER
INTRODUCTION
All McIntosh tuners are carefully aligned and tested at the factory
using the finest available test equipment. All McIntosh tuners will meet
their published specifications when shipped from the factory.
After extensive operation, especially when tubes have been replaced,
it may be desirable to realign the tuner circuits for best performance.
This manual gives complete information on the circuit realignment pro¬
cedure for the MR 71 stereo tuner.
The test equipment listed below (or its equivalent) is necessary to properly
align an MR 71. The accuracy of the alignment will be directly related
to the accuracy and calibration of the test equipment used.
FM Signal Generator (Measurements 210A or equivalent)
VTVM
Multiplex Generator (RCA WR-51A or equivalent)
10.7 MC Generator (Preferably crystal controlled)
Oscilloscope (Hewlett-Packard 120B or equivalent)
Harmonic Distortion Analyzer, desirable but not essential—
(Hewlett-Packard 330B or equivalent)
If the necessary test equipment is not available, alignment should
not be attempted. You may contact the McIntosh Customer Service
Department for additional information.
Customer Service
McIntosh Laboratory, Inc.
2 Chambers Street
Binghamton, New York
Our telephone number is 723-5491
The direct dial area code is 607

MR 71 FM
TUNER
DIAL
SETTING
SIGNAL GENERATOR
INDICATOR
FREQ
COUPLING
MODULATION
TYPE
CONNECTED
TO
Point of no
interference
or signal
10.7MC
Through external
.01MF cap to
pin 7 of
1 2AT7 mixer
cw
VTVM
TP #1
SAME SAME SAME SAME
SAME Pin 6 of T5
SAME
SAME
SAME SAME
SAME Junction of D4
and R42
SAME
SAME
SAME
SAME SAME T6, Pin 6
SAME SAME
SAME
SAME SAME
TP #2
105MC
105MC
300 ohm antenna
terminals with
*matching network
400 cycles
75KC deviation
(100% modulation)
VTVM connected to TP1 and
scope connected to
L or R audio output
90MC
90MC
SAME
SAME
SAME
105MC 105MC
SAME
SAME
SAME
90MC
90MC
SAME
SAME
SAME
Point of no
interference
Scope
L or R output
105MC
105MC
SAME 400 cycles
75KC deviation
(100% modulation)
attenuated to
2.5 microvolts
output
VTVM connected to TP #1
and
Scope connected to
L or R audio output
FM SIGNAL
GENERATOR
ANTENNA MATCHING NETWORK
*
-O
100 OHMS
■ AAA/—
AAA—
150 OHMS
TUNER
300 OHM
ANTENNA
TERMINALS
* IF SIGNAL GENERATOR HAS OTHER THAN 50 OHM INTERNAL IMPEDANCE,
USE A RESISTOR OF 150 OHMS, LESS INTERNAL GENERATOR IMPEDANCE.

ALIGNMENT
ADJUST
TEST
LIMITS
REMARKS
Top (Secondary)
and
bottom (Primary)
cores of T1, T2,
T3, and T4
Maximum
possible
negative
voltage
Shunt to ground the winding not being adjusted with a .01MF capacitor
in series with a IK resistor. Attenuate signal generator until output
voltage at TP #1 is less than 1.5 volts with one IF transformer winding
shunted. IF transformers have terminal #1 marked with a green dot
and are numbered clockwise.
T5 Primary
(Bottom core)
Same as
above
'
T5 Secondary
(Top core)
Adjust for
0 volts
T6 Primary
(Bottom core)
Maximum
negative
voltage
If a distortion analyzer is available, omit this step at this time. Adjust
T6 primary after step 9. At that time, use a strong signal from FM
generator, modulate 100%, and use 75KC deviation. Adjust primary
for minimum distortion. Should be no greater than 0.5%.
T6 Secondary
(Top core)
Adjust for
0 volts
Oscillator
Trimmer
Maximum
negative
voltage
As output increases, attenuate signal generator to keep maximum output
at TP # 1 to a low level. By doing so, precise alignment can be achieved.
Oscillator
Coil
SAME
Repeat steps 6 and 7 until dial calibration is accurate.
Mixer trimmer,
RF trimmer, and
Antenna trimmer
SAME
Mixer, RF, and
Antenna coil
Tuning slugs
SAME
Repeat steps 8 and 9 until output is as high as possible.
Muting adj.
control
Turn muting switch to “in” position. Adjust muting control until
background noise just disappears.
IHFM
sensitivity
2.5 microvolt
for 3% total
noise and
distortion
Step 11 is an overall sensitivity check, and requires a distortion analyzer
and FM signal generator with attenuator.
With 2.5 microvolts input at the 300 ohm antenna terminals, TP #1
voltage should be 3.0 volts or more.

MR 71 MULTIPLEX
TUNER
SIGNAL GENERATOR
INDICATOR
STEPS
DIAL
SETTING
FREQ.
COUPLING MODULATION
TYPE
CONNECTED
TO
100MC
100MC
300 ohm
1 9KC pilot only
DC
Pin 7 of
modulated by
antenna
VTVM
6U8
MPX generator
terminals with
approx.
1 000 microvolt
signal
(V14B)
8
SAME
SAME
SAME
SAME
SAME SAME
SAME
SAME SAME
SAME
SAME SAME
SAME SAME
SAME
SAME
Tune to a
strong MONO
FM station
SAME
1KC 100%
modulation
left or right only,
pilot on
Audio
VTVM
Pin 1 or 2 of
38KC transformer
(T8)
L or R
output jack
SAME
Turn off 1 KC
audio modulation
SAME
SAME
MPX
stereo
indicator
light on
tuner
SAME
SAME
SAME

DECODER ALIGNMENT
ADJUST
TEST
LIMITS
On the top of the chassis is an opening labeled "MPX Light Adjust."
Insert a screw driver into this opening and turn the control completely
counterclockwise.
1 9KC phase coil
Adjust for
and 19KC
maximum DC
transformer (T7)
voltage
Adjust "MPX Light Adjuster” control completely clockwise.
38KC
transformer
bottom core
38KC
transformer
top core
Adjust for
maximum
voltage
Adjust for
stable scope
display
1. Turn off 19KC pilot on MPX generator.
2. Adjust top core of 38KC transformer to obtain a stable and uniform
1 KC signal scope display. This adjustment may be critical, so turn
core very slowly.
3. Turn 19KC pilot back on.
1 9KC phase coil 30db separation Modulate left channel and measure right channel output. Adjust 19KC
or more phase coil for minimum right channel output (maximum separation).
Remove all test leads from TP #2 for separation checks.
SAME
Modulate right channel and measure left channel output. Separation in
steps 5 and 6 should be at least 30db.
MPX light adj.
control—R3
This step checks the rejection of 19KC and 38KC frequencies. Residual output should be
at least 40db below modulated output.
Turn control until light comes on. Then back off just enough to cause the
light to go off. Then back off about 14 of a turn more. Light should
operate ONLY on an MPX signal.
5

ITEM
NO.
USE
MR 71 PARTS LIST
TUBES
IDENTIFICATION
NUMBER
VI
RF Amplifier 1
6DS4
V2
RF Amplifier 2; Mixer
12AT7
V3
Oscillator
6BN4A
V4
IF Amplifier 1
6AU6
V5
IF Amplifier 2
6AU6
V6
IF Amplifier 3
6AU6
V7
IF Amplifier 4; Limiter 1
6AU6
V8
IF Amplifier 5 , Limiter 2; Muting
6CS6
V9
Multipath Indicator
6HU6/EM87
V10
Muting Amplifier; Muting Detector; A&C Clamp
6BN8
VI 1
Left Audio Amplifier
6BL8
VI 2
Right Audio Amplifier
6BL8
VI 3
Voltage Reference Diode
ST2-27S
VI4
MPX Amplifier, MPX Indicator Control
6U8
VI 5
MPX Oscillator
12AU7
Q1
TRANSISTOR
MPX Indicator Lamp Switch
2N3053
Dl, D2
DIODES
Selenium Rectifiers, High Voltage Power Supply
GE#6RS20PH6RGD1
D3
Variable Capacitance
Amprex S-254
D4, D5
Narrow Band Discriminator (Matched Pair)
Type 1N542 Germanium
Diode
D6, D7 Wide Band Discriminator (Matched Pair)
Type 1N542 Germanium
Diode
D8, D9
Balanced Detector for MPX Indicator (Matched Pair)
Type 1N542 Germanium
Diode
DIO, Dl 2 Balanced MPX Detectors—Left Channel (Matched Pair)
Type 1N542 Germanium
Diode
Dll, Dl3 Balanced MPX Detectors—Right Channel (Matched Pair)
Type 1N542 Germanium
Diode
Dl 4
Low Voltage Rectifier Type INI 21 7 Silicon Diode
ITEM
NO.
CONTROLS
FUNCTION RESISTANCE
IDENTIFICATION
NUMBER
R1
Automatic Freq.
Control
500K
R147 A103
R2
Muting Adj. Pot.
10K
R50, 000-6F
R3 MPX Light Adj. IK Wirt #807
R4
Volume Control
20K (Dual)
R105-164
R5
Rear Panel Output Adj. Control
40K (Dual) R147-A102
SWITCHES
ITEM
IDENTIFICATION
NO.
FUNCTION DESCRIPTION
NUMBER
SI
Muting
3 position
S147-B130
S2
Mode Selector
3 position with
S147-B105
on/off switch
S3
Panel Light Dim- Bright
SPST Stackpole SS-26
TRANSFORMERS
ITEM
IDENTIFICATION
NO. FUNCTION
NUMBER
T1 FM first IF
T107-134A
T2
FM second IF
T107-133A
T3
FM third IF T107-133A
T4
FM fourth IF T107-133A
T5
FM fifth IF, Discriminator
T107-135B
T6
FM Discriminator
T107-I35B
T7
19KC Amplifier
T129-101A
T8
38KC Oscillator
T129-102A
T9 Baiun
T10
Power
T123-133A
CAPACITORS
ITEM
IDENTIFICATION
NO.
DESCRIPTION
CAPACITANCE
VOLTAGE TOLERANCE NUMBER
Cl
Variable FM
C147-C101
C2
Antenna Trimmer
1 - 8 pf
NPO
C3
RF Trimmer 1 -8pf
NPO
C4
Mixer Trimmer 1 -8pf
NPO
C5
Oscillator Trimmer 1 -8pf
C6
Ceramic Disc lOpf
20% NPO
C7
Ceramic Disc
6.8pf 20% NPO
CIO
Ceramic Feed Thru
lOOOpf
Cl 2
Ceramic Tubular
5pf
±.25pf N330
C14
Ceramic Tubular
3pf ±.25pf NPO
Cl 5
Ceramic Tubular
1,5pf
±.25pf NPO
Cl 6
Ceramic Feed Thru
lOOOpf
Cl 8
Phenolic .18pf
10%
C19
Phenolic 3.9pf 10%
C20
Ceramic Feed Thru
lOOOpf
C22
Ceramic Feed Thru
lOOOpf
9

CAPACITORS
ITEM
NO.
DESCRIPTION
CAPACITANCE
VOLTAGE
TOLERANCE
IDENTIFICATION
NUMBER
C23
Phenolic
,33pf
10%
C25
Ceramic Feed Thru
lOOOpf
C27
Phenolic
.33pf
10%
C30
Ceramic Feed Thru
lOOOpf
C31 Phenolic
2.2pf
10%
C32
Ceramic Disc
6.8pf
20% NPO
C33
Ceramic Feed Thru
lOOOpf
C34 Ceramic Feed Thru
lOOOpf
C35
Non-Polarized lOmfd
25V
+ 100-10%
Electrolytic
C37
Ceramic Disc ,47pf
20% NPO
C38
Ceramic Tubular 5pf
±.25pf N330
C39
Ceramic Tubular
5pf
±.25pf N150
C40
Ceramic Disc 22pf
20% N470
C41
Ceramic Feed Thru lOOOpf
C42
Ceramic Disc
470pf
20%
C44
Ceramic Disc lOOpf
10% N1500
C46
Paper
,047mfd
200V
20%
C47
Ceramic Feed Thru
lOOOpf
C49
Ceramic Disc 47pf
20% N470
C50
Ceramic Disc 22pf
10% NPO
C54 Ceramic Disc
6.8pf
20% NPO
C55
Ceramic Disc
lOpf
20% NPO
C56
Ceramic Disc
27pf
20% N470
C57 Ceramic Feed Thru
lOOpf
C58
Ceramic Disc
2 x .01
900V AC
C59 Electrolytic 40mfd
200V
C60
Electrolytic
4 x 50mfd
200V
C61
Ceramic Feed Thru
lOOOpf
C62
Ceramic Feed Thru
lOOOpf
C63
Ceramic Feed Thru
lOOOpf
C68
Ceramic Disc
lOOpf 10% N1500
C70
Paper
.0022mfd
400V
10%
C71
Silver Mica
2700pf
100V
5%
C72 Ceramic Disc lOOpf
10 % N1500
C76
Ceramic Disc 220pf
20%
C78
Silver Mica
4700pf
100V 5%
C79
Silver Mica
4700pf
100V 5%
C85
Ceramic Disc 330pf
10%
C86
Ceramic Disc
330pf
10%
C87
Ceramic Tubular
430pf
5%
C88
Ceramic Tubular
430pf
5%
C93
Electrolytic
1 mfd
150V
Cl 24-129
C94
Electrolytic
1 mfd
150V
Cl 24-129
C95
Electrolytic
lOmfd
3V
C96
Electrolytic 1 Omfd
3V
COILS
ITEM
IDENTIFICATION
NO.
DESCRIPTION
VALUE
NUMBER
LI
Antenna Coil
L107A141
L2
RF Coil LI 07-207A
L3
Mixer Coil LI 07-208B
L4
Oscillator Coil LI 07-206A
L5
1 9KC Trap
LI 29-1 03
L6
Coupling Link
L7
RF Choke
.47 Micro H
L8
RF Choke 2.2 Micro H
L9
RF Choke
1.2 Micro H
LI 0
RF Choke
LI 0, 004
LI 1
RF Choke
1.2 Micro H
LI 2
RF Choke
75 Micro H
LI 3
RF Choke
1.2 Micro H
LI 4 RF Choke
2.5 Micro H
M-7060
• LI 5 RF Choke
75 Micro H
LI 6
Peaking Coil
LI 29-1 23
LI 7
RF Choke
2.2 Micro H
LI 8
RF Choke
2.2 Micro H
LI 9
RF Choke
1.2 Micro H
L20
RF Choke
1.2 Micro H
L21 RF Choke
1.2 Micro H
MISCELLANEOUS
Lamp, Festoon: 7 volts, 2 watts, 7 x 43mm Meter, Tuning: #M146B148
Lamp, Stereo: No. 1850 LDR Network: #R146-A143
Lamp, Meter: No. 1 847 Cable, Coaxial: 50 ohm, Amphenol #21 -598
Fuse: 1 Amp. Slo-Blo, 3AG SCA Filter: #Z146B135
Meter, Signal Strength: #M146B146
10

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'KfCISIQN KiN.HC. • *«*A--TO*
MOOtl «B?I f* STEREOPHONIC TUNE*
PRECISION 5TEREO BY flftlllto&h
PHOT W»P INTFNSITY
MUTE ADJUST
CONTROL
MR 71 TOP and BOTTOM VIEWS
- WIDE BAND
DISCRIMINATOR
TRANSFORMER
TRANSFORMER
rd IF
■ RANSFORMER
TRIMMER]
MUTE ADJUST
CONTROL V
DISCRIMINATOR
TRANSFORMER s
5th IF--
TRANSFORMER
4th IF-
TRANSFORMER
MIXER COIL
KC
PHASE COIL
3rd IF
TRANSFORMER
—- 19 KC
TRANSFORMER
1st IF--
TRANSFORMER
MPX, LIGHT
ADJUST
TRANSFORMER
OSCILLATOR
ANTENNA

SPECIFICATIONS
Useable Sensitivity
2.5 microvolts at 100% modulation (±75KC
deviation) for less than 3% total noise and
harmonic distortion in accordance with
IHF standards.
Audio Frequency Response
Within Vidb from 20 to 20,000 cycles.
Distortion
Less than 0.5% at 100% modulation
±75KC deviation.
Capture Ratio
1.5db at 100% modulation.
Muting
IF injected ultrasonic muting: at least 60db
noise reduction between stations.
Oscillator Drift
Less than 25KC with AFC disabled; negligible
with AFC in operation.
Image Rejection
Better than 80db at 90MC; better than 70db
at 105MC.
Hum
Better than 70db below 100% modulation.
Output
Approximately 2.5 volts; low impedance.
Antenna Inputs
300 ohms balanced; 75 ohms unbalanced.
RF Amplifier
Cascode with 6DS4 Nuvistor in first stage.
IF Stages
Five, with 200KC bandwidth
Limiters
Two.
Radiation
Substantially below FCC requirements.
Multiplex Channel Separation
Better than 30db at 1000 cycles.
Multiplex Filter
Greater than 48db suppression of 19KC pilot
and 38KC carrier.
Multiplex Indicator
Front panel multiplex stereo light activated
by 19KC carrier-only.
Multiplex Type
Peak-detecting, self-matrixing detector.
SCA Filter
50db down at 67KC to 74KC 275db per
octave slope.
12

U.5.A. 9642MSM
HMntosb
LABORATORY INC.
2 Chambers St., Binghamton, N. Y.
Made in U.S.A.
Phone—Area Code 607-723-6491
Design subject to change without notice.