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tony Ma 9th January 2013 11:39 PM

Bias, not same output etc .... Pls help
Dear All,

I have B&K St-140 that I replace the electrolytic capacitor with elma silmic.

One side (R channel total dead) so I replace with new transistor J50 and K135.

Just FYI: I measure secondary wiring is 46V x 2 and the DC input to PCB is 65V

There are 3 cable going to PCB. RED 65V, Green 0V, Black (-)65V. (I put the multimeter to DC with black tip to amp chassis)

Last night I found this thread

Amplifier Distortion, DC-Offset, and You! - Home Audio Stereo Discussion Forums

I did measure mine with no load meaning just amp itself and I set my Multimeter to DC with + to RED speaker out and - to black speaker out

When it start R = (-) 89mv and L = (-)87mv

After 20 minute R = (-)87mv and L = (-)94mv

My questions are

1. Why I got negative reading?
2. How to set bias? which Pot? there are 2 pots here (I only have Multimeter)

Can some one guide me step by step please??


tony Ma 9th January 2013 11:42 PM

4 Attachment(s)
pls see picture

madtecchy 10th January 2013 12:46 PM

You mention DC offset and also Bias ?????

bobodioulasso 10th January 2013 01:49 PM

P1 sets offset voltage.
P2 sets the bias current of the output stage.Setting P2 is dangerous for the amp if you don't know exactly what you are doing

Edit: +/- 65V looks high for only one pair of output devices

madtecchy 10th January 2013 01:51 PM

I see no presets in the piccy just blank spaces.. Unless they are hiding underneath the pcb

bobodioulasso 10th January 2013 01:57 PM

Can be seen on 4th pic.

madtecchy 10th January 2013 02:08 PM


Originally Posted by bobodioulasso (
Can be seen on 4th pic.

And now i see .. thankyou .. Hi ho hi ho its of to spec savers i go

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