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technics se-9200 bias adjustment?

T

TrevorBarten

Well-Known Member
I am currently working on a se-9200 power amp that really does not need that much work. I adjusted the dc offset on the speaker leads to 0.00 volts however I also wanted to adjust the bias but I couldn't really find anything about this in the service manual. I was expecting maybe a target voltage or measuring point but unless I am not looking correctly I can't seem to find any of this. Could someone tell me what I should do ? Is there a standard or can I use context clues to decide? Also another thing I noticed is that the dc voltage on each channel is not the same for the right and left speaker output. There is a difference of about 0.04 V. I couldn't really find how to adjust that either but I assume that just has to do with shifted resistor values and wasn't made to be adjustable or not?
 
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I downloaded the service manual.

Observations:-
1) On each power amp board the DC voltage offset at the output is adjusted at VR401 (mounted close to TR401 & TR402). A value close to zero is ideal, but anything below 20mV is probably O.K. Don't fret too much about it unless the minimum achievable value is above 30mV.
2) On each power amp board the bias current setting is made using VR402 (mounted close to TR406 base connection). The setting for this is not shown in the manual (unusual).

I would be inclined to set the bias as follows. The amplifier topology is a fairly standard AB amplifier with a differential pair front end. Nothing unusual.
The power transistor emitter resistors (RR422 & R423) are 0.47ohms. Connect a DC voltmeter set for millivolts across R423 (not the speaker terminals). Adjust R402 until a voltage of about 14mV is achieved. This will give a quiescent current of about 30milliamps on the output transistors which is a safe value and probably close to what the manufacturer originally intended.

Both set up tests should be done with nothing connected to the amplifier and with it left switched on for about 30 minutes so that it is at normal working temperature.
 
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