The problem is obviously revolving around the op-amp which is made up of Q15 and U2.
The two gates of Q15 should be balanced at all times, otherwise the output of U2 would be stuck at the +12 or the -12V supplies (because U2 is supposed to have infinite gain).
So you could do this: in a situation where the two gates of Q15 are NOT balanced, look at the output voltage of U2, does it saturate ? What are its two input voltages ?
Does it control the output current ? Are you seeing any voltage drop across R43 (in series with U2's output) ?
I think we're getting close to the "Aha !" moment !!!
Joel
