Amplifier bias question help

[reddish75]

Hi, 

I've just replaced the output transistors and emitter resistor which was cracked on my Jungson amplifier and now the bias on the repaired channel is around 10mv compared to the 'good channel' which is ~90mv, it does have the same value (560r) resistors in both channels adjustable position.

My question is how would I change fixed resistor R269 to an adjustment trimmer, would I just put the trimmer where the resistor was and connect the wiper to one of the legs?

Many Thanks

Chris

[w2aew]

Hi,

I've just replaced the output transistors and emitter resistor which was cracked on my Jungson amplifier and now the bias on the repaired channel is around 10mv compared to the 'good channel' which is ~90mv, it does have the same value (560r) resistors in both channels adjustable position.

My question is how would I change fixed resistor R269 to an adjustment trimmer, would I just put the trimmer where the resistor was and connect the wiper to one of the legs?

Many Thanks

Chris

I wouldn't replace it with a trimmer because you'd never adjust it near 0 - much of the range of the trimmer won't be use which will make adjustment tricky.  If you want to temporarily replace it with a trimmer to determine an appropriate value to get the bias you want, then that's OK.  If you want the trimmer to be permanently installed, I'd put a trimmer in series with a fixed resistor.  The best solution is to determine an appropriate value for R269 and install a fixed resistor.

[Damianos]

I think that the bias is adjusted via the RW3. The R269 is to adjust the temperature coefficient of the Q89/D29 to match that of the other transistors.
To be sure, it is better to wait also for the advise of someone else.