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