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! - AudioKarma.org 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??

tks