Unable to bias right channel properly TA-N55ES....

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Hello all,

I'm having a problem with setting bias to 7mv on the right channel on a Sony TA-N55ES.. I can only set the bias as low as 9.6mv i cannot get it any lower.

I have changed the bias pots on both channels and new (original) output transistors on both channels.

I do not get the problem with the left channel.

Can anybody tell me where to start looking ?

Thanks for any help in advance.
 
The measurement is made from the idling adjustment terminals on the mainboard just behind the output transistors and not from the speaker outputs.

I cannot set the 'idling adjustment' below 9.6mv for the right channel, sony state that the idling current should be set to 7mv .
 
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There are two things that might be happening:
1. The driver stage drawing too little current because of a bad resistor or solder joint. Inspect the driver / output stages and make sure no resistor has drifted high and that all joints are OK. (Note: Checking emitter resistors of a fraction of an ohm can be tricky.)
2. High-frequency oscillation fooling the multimeter. In this case, inspect the Zobel network (the RC to ground) at the output.

Have you verified that the bias pot itself is doing roughly what it should? That would be option number 3, it might be in need of cleaning.
 
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