23/11/99 Issue 1
Preset Adjustment PCB 9323
To reset the amplifier bias following service or device replacement, follow these steps.
1. Before powering up the unit for the first time after replacing devices, Turn
R27,R28,R127 and R128
(All 1K presets) fully clockwise. This will bias the output
devices into minimum conduction. Failure to do this may result in the amp
appearing to draw excessive current, Especially if powered through a “Current limit”
device.
2. Reset amplifier DC offset for Channel 1 by measuring between
P2
(0V) and
P107
(Speaker). Set to
0V DC.
±±
1mV
, by Adjusting
PS102
.
3. Reset amplifier DC offset for Channel 2 by measuring between
P2
(0V) and
P7
(Speaker). Set to
0V DC.
±±
1mV
, by Adjusting
PS2
.
4. Measure DC mV across
TS109
S
ense and
E
mitter junction. Adjust
R128
until a
reading of
10mV
±±
1mV
is obtained.
5. Measure DC mV across
TS105
S
ense And
E
mitter junction. Adjust
R127
until a
reading of
10mV
±±
1mV
is obtained.
6. Recheck Across
TS109
as above. Reset if necessary to
10mV
.
7. Check
S
,
E
junction of all four devices on channel 1 (
TS105,TS109,TS111,TS114
)
This Should be
10mV
±±
3mV
. This completes Channel 1 re-biasing.
8. Measure DC mV across
TS9
S
ense And
E
mitter junction. Adjust
R28
until a reading
of
10mV
±±
1mV
is obtained.
9. Measure DC mV across
TS5
S
ense And
E
mitter junction. Adjust
R27
until a reading
of
10mV
±±
1mV
is obtained.
10. Recheck Across
TS9
as above. Reset if necessary to
10mV
.
11. Double check
S
,
E
junction of all four devices on channel 2 (
TS5,TS9,TS11,TS14
)
This Should be
10mV
±±
3mV
. This completes the amplifier re-biasing.
In practice, It may be easier to disconnect the Pre-Amp loom and facia from the rear chassis.
