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 Sense and Emitter junction. Adjust R128 until a
reading of 10mV ±±1mV is obtained.
5. Measure DC mV across TS105 Sense And Emitter junction. Adjust R127 until a
reading of 10mV ±±1mV is obtained.
6. Recheck AcrossTS109 as above. Reset if necessary to 10mV.
7. Check S, Ejunction 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 acrossTS9 Sense And Emitter junction. Adjust R28 until a reading
of 10mV ±±1mV is obtained.
9. Measure DC mV acrossTS5Sense And Emitter junction. Adjust R27 until a reading
of 10mV ±±1mV is obtained.
10. Recheck AcrossTS9 as above. Reset if necessary to 10mV.
11. Double check S, Ejunction 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.
