Altec 9442a poweramp service manual

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ALTEC ®

9442A and 9442A/ SA

ANNIVERSARY SERIES
POWER AMPLIFIER

SERVICE INSTRUCTIONS

***CAUT|ON***

No user serviceable parts inside. hazardous volt-
age and currents may be encountered within the
chassis. The servicing information contained
within this document is for use only by ALTEC
LANSING Corp. authorized warranty stations and
qualified service personnel. To avoid electric
shock, DO NOT perform any servicing other than
that contained in the Operating Instructions
unless you are qualified to do so. Refer all
servicing to qualified service personnel.

-13-

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I 8.0 SERVICE INFORMATION

CAUTION

No user serviceable parts Inside.
Hazardous voltages and currents
may be encountered within the
chassis. The servicing information
contained within this document is
for use only by ALTEC LANSING
Corp. authorized warranty sta-
tions and qualified service per-
sonnel. To avoid electric shock
DO NOT perform any servicing
other than that contained in the
Operating Instructions unless you
are qualified to do so. Otherwise,
refer all servicing to qualified ser-
vice personnel.

NOTE
Modifications to ALTEC LANSING
products are not recommended.
Such modifications shall be at the
sole expense of the person(s) or
company responsible, and any
damage resulting there from shall
not be covered under warranty
or otherwise.

Figure 6 is the main amplifier
schematic. Two trimpots for each
channel are provided for adjust-
ment. Resistor R15 (and R115) ad-
justs the DC offset at the output of
the amplifier. and resistor R27 (and
R127) adjusts the bias. The mini-
mum amount of test equipment re
quired is a DC voltmeter capable of
reading voltages in the millivolt
range.

8.1 DC Offset Adjustment

The following procedures are re-
commended.

1. Make sure the amplifier is in the
dual mode of operation (not
bridge) and that the power is off.
Rotate both input level controls
fully counterclockwise.

2. Remove the 14 screws securing
the top cover. Refer to Figure 1
for screw locations.

3. Connect a DC voltmeter across
the output of channel 1 and apply
power.

4. Carefully adjust R24 for a read-
ing of less than 10 mV DC.

5. Turn power off and connect DC
voltmeter across the output of
channel 2.

6. Turn power on and adjust R124
for a reading of less than 10 mV
DC. Turn power off.

8.2 Bias Adjustment

The bias may be adjusted by one of
several methods.

8.2.1 Bias Adjustment Without the
Use of Test Equipment

1. Turn power off and rotate both
input level controls fully counter-
clockwise.

2. Remove the 14 screws securing
the top cover. Refer to Figure 1
for screw locations.

3. Adjust R41 to its two oclock posi-
tion when looking at it from the
front of the amplifier.

4. Adjust R127 to its two oclock
position when looking at it from
the front of the amplifier.

NOTE
The bias is intentionally limited to
prevent overbias. The clock posi-
tions In the preceding steps serve
as a good compromise between
low distortion and ease of adjust-
ment.

8.2.2 Bias Adjustment Using a
Distortion Analyzer

1. Turn power off. Rotate the input
level controls fully clockwise.

2. Remove the 14 screws securing
the top cover. Refer to Figure 1
for screw locations.

3. Connect the output of the oscil-
lator to channel 1. Connect the
output of channel 1 to an 8 0 load
with short pieces of 16-18 GA
wire. Connect the input of the
analyzer across the load.

4. Apply power and adjust the out-
put level of the oscillator to read
28.3 vrms at 1 kHz across the load
on the analyzer. Change the fre-
quency of the oscillator to 20
kHz

5. Adjust R41 for the lowest distor-

..14_

tion reading.

6. Using the same setup. repeat the
procedures for channel 2. Adjust
R141 for the lowest distortion
reading.

8.2.3 Bias Adjustment Using DC
Voltmeter

1. Turn power off. Rotate the input
level controls fully counterclock-
wise. If the DC voltmeter is ac
powered, float the ac ground
wire.

2. Remove the 14 screws securing
the top cover. Refer to Figure 1
for screw locations.

3. Connect the voltmeter probes
across R42 (270 Q. 1 watt) and
apply power.

4. Adjust R41 for a reading of 1.1
volts across the resistor. Turn
power off.

5. Connect the voltmeter probes
across R142 (270 Q, 1 watt) and
apply power.

6. Adjust R141 for a reading of 1.1
volts across the resistor. Turn
poweroff.

8.2.4 Bias Adjustment by Averaging
Emitter Resistor Voltage
Drops

1. Turn power off. Rotate the input
level controls fuily counterclock-
wise. If the DC voltmeter is ac
powered. float the ac ground
wire.

2. Remove the 14 screws securing
the top cover. Refer to Figure 1
for screw locations.

3. Apply power and sequentially
measure the voltage drop across
each emitter resistor (R49 and
R52) while adjusting R41 for an
average voltage drop of 5 mV DC.
Turn power off.

4. Apply power and sequentially
measure the voltage drop across
each emitter resistor (R149 and
R152) while adjusting R141 for
an average voltage drop of mV