McIntosh MAC 4275 Service Manual

Page 1

MAC 4275

AM/ FM RECEIVER

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CONTENTS
Performance Specifications ................... 2-3
Mechanical Views ............................. 4
Block Diagrams ............................. 5-6
Section Locations ............................. 7
Section 1 - Audio .......................... 8-13
Section 2 - Tuner ......................... 14-18
Alignment Procedure ....................... 19-20
_ Repacking instructions ....................... 22

IIItInlnsli Service Manual

Page 2

Performance
Specifications

AMPLIFIER SECTION

STEREO POWER OUTPUT

75 watts into 8 ohm loads or 100 watts
into 4 ohm loads is the minimum sine wave
continuous average power output per channel,
from 20 to 20,000Hz, with both channels
operating.

POWER BANDWIDTH
20 to 20,000HZ

TOTAL HARMONIC DISTORTION

0.03% maximum at any power level from
250 milliwatts to rated power per channel,
20 to 20,000Hz, both channels operating.

INTERMODULATION DISTORTION

0.03% maximum at any power level from
250 milliwatts to rated power per channel,
for any combination of frequencies from 20 to
20,000Hz, with both channels operating.

FREQUENCY RESPONSE
+0, -O.5dB from 20 to 20,000Hz at rated
power.

INPUT SENSITIVITY
Phono(MC) 0.12mV
Phono(MM) 2.5mV produces rated output
Tape, Aux ZSOmV produces rated output
Power Amp 1.5V produces rated output

SIGNAL TO NOISE RATIO, A-WEIGHTED
Power Amp ioOdB below rated output
Tape, Aux 95dB below rated output
Phono 90dB with 10mV input

MAXIMUM INPUT SIGNAL
Phono(MC) 4.1mV
Phono(MM) SOmV
High level 8V

INPUT IMPEDENCE
Phono(MC) 100 ohms
Phono(MM) 47k ohms
Tape, Aux 20k ohms
Power Amp 18k ohm

DAMPING FACTOR
Greater than 50 at 8 ohms

TAPE OUTPUT
Phono ZSOmV with rated input
Tape 250mV with rated input
Tuner 1.0V at 100% modulation (FM)

PROGRAM EOUALIZER
tide at 30, 750, and 10kHz

FM SECTION

USABLE SENSITIVITY
11.25dBF which is 1.0uV across 75 ohms

50dB QUIETING SENSITIVITY
Mono 15.2dBF which is 1.6UV across
75 ohms
Stereo 27dBF which is 6uV across 75
ohms

SIGNAL TO NOISE RATIO
Mono BOdB
Stereo 75dB

FREQUENCY RESPONSE
Mono +0.5, -0.5dB from 20 to 15kHz
Stereo +0.5, -0.5dB from 20 to 15kHz

HARMONIC DISTORTION
Mono 0.15% at 100Hz
0.1% at 1k
0.1% at 10k
Stereo 0.1% at 100Hz
0.1% at 1k
0.25% at 10k

INTERMODULATION DISTORTION
Mono 0.1%
Stereo 0.2%

CAPTURE RATIO
1. SdB

ALTERNATE CHANNEL SELECTIVITY
60dB

SPURIOUS RESPONSE
95dB

IMAGE RESPONSE
90dB

AM SUPPRESSION
60dB

STEREO SEPARATION
35dB at 100Hz
45dB at 1k
40dB at 10k

SCA REJECTION
65dB

Page 14

19

Alignment
Procedure

FM ALIGNMENT

A. SYNTHESIZER FREQUENCY ADJUSTMENT
1. Connect a Frequency Counter between TP106 and ground.
2. Tune the Receiver to 98MHZ.
3. Adjust C209 so the local oscillator frequency shown by the
Frequency Counter reads 108.700MH2.

B. FRONT-END ALIGNMENT
1. TUNING VOLTAGE
a. Connect a DMM to TP1O/4.
b. Tune the Receiver to SBMHZ and adjust L6 if the voltage is not
3.6V +/-O.5V.
c. Tune to 108MHz and check that the voltage is within 21 to 25
volts.

2. TRACKING ADJUSTMENT

a. Connect the FM Generator to the 75 ohm antenna input, and a
Detector Probe to Pin 1 of 1C105 and ground. (See how to
make the Detector Probe in figures 5 and 6 on page 20).

b. Tune the receiver to 106MHz and adjust the generator so a
waveform is displayed on the Oscilloscope.

(2. Adjust C2, C9, C11 and C14 for maximum waveform height.

d. Tune to BOMHz and adjust the Generator so a waveform is
displayed on the Oscilloscope.

6. Adjust L1, L2, L3 and LI» for maximum waveform height.

f. Repeat above steps a few times as necessary to obtain the
maximum waveform height.

C. IF ADJUSTMENT
1. Tune the Receiver to 98MHz and adjust the Generator to display
a curve on the Oscilloscope.
2. Adjust L5 in the front-end for maximum output, and L101 for a
symmetrical output curve, using as little input as possible such
as 2uV at the antenna input.

D. DETECTOR ALIGNMENT

1. Remove the Detector Probe and connect the tuner output to the
Oscilloscope and Distortion Analyzer.

2. Connect a DC Meter across TP102 (-) and TF'103 (4-).

3. Tune the Receiver and Generator to 98MHz feeding 54uV to the
antenna input.

A. Adjust L102 secondary for minimum distortion.

5. Adjust L102 primary for UV center.

6. Repeat above steps for best result.

7 Adjust L5 in the front-end for minimum stereo L or R
distortion.

8. Adjust L101 in the same way for the same result.

9. Repeat above steps for best result.

10. Check to confirm that the DC Meter reads 0V +/-50mV.

E. DISTORTION
1. Tune the Receiver and Generator to 98MHZ and feed 50uV to the
antenna input.
2. The Distortion should be within the following limits:
Less than 0.12%: MONO
Less than 0.1%: L+R, L, R, L-R

F. SEPARATION

1. Tune the Receiver and Generator to QBMHZ and feed SOuV to the
antenna input.

2. Modulate L Channel only and adjust R119 for minimum output on
R Channel VTVM and Oscilloscope.

3. Modulate R Channel only and adjust R119 for minimum output on
L Channel VTVM and Oscilloscope.

4. Readjust R119 if necessary to obtain the some reading on both
channels.

5. Check and confirm that the reading is more than AGdB on each
channel.

nn

G. AUTO BLEND

1.

2.
3.

1:.

Tune the Receiver and Generator to 97.50MHZ and feed 56uV to
the antenna input.

Check and confirm that the S/N ratio is more than 60dB.

Adjust the attenuator of the FM Generator so the S/N ratio
reads GOdB.

Adjust R157 so the S/N reading is ZdB down.

H . SIGNAL METER

1.

'7

Tune the Receiver and Generator to 97.50MHz and feed 1AdBu to
the antenna input.
Adjust R199 so the second LED lights at this input level.

1. AUTO STOP AND STEREO LEVEL

Tune the Receiver and Generator to 97.50MHZ and feed 3.2uV to
the antenna input.

Connect a DMM to TP101.

Adjust R109 to obtain a high level.

Next, adjust R151 so the Stereo Indicator LED lights.

AM ALIGNMENT

A. TUNING VOLTAGE ADJUSTMENT

1.
2.

3.
1:.

Connect a DMM to TP105.

Tune the Receiver to 520kHz and adjust L109 and C150 so the
voltage reads within 1.6 to 1.7V.

Tune to 1710kHz and check that the voltage is within 27 to 28V.
Repeat above steps until within speciiied voltage.

B. TRACKING ADJUSTMENT

1.

2.
3.
l».
5.

Tune the Receiver and Generator to 1h00kHz and feed 10uV to
the antenna input.

Adjust C148 and C149 for maximum output.

Tune to 630kHz.

Adjust L107 and L108 for maximum outpLJt.

Repeat above steps as necessary.

C. SIGNAL METER

1.
2.

Tune to 1000kHz and feed 100W to the antenna input.
Adjust R179 so the fifth LED lights.

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FROM
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SCHEMATIC DIAGRAM OF DETECTOR PROBE

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PICTORIAL DIAGRAM OF DETECTOR PROBE

Keep leads as short as possible to minimize stray signal pickup.