Fender Ultimate Chorus power amp repair

[rybeka]

Update. The new 14 piece control set I put in had a bad pot that was in the rate position. Put in a new 100K pot and the chorus works correctly now. I can observe the frequency change with the control but alas, the squeal remains.

[rybeka]

Changed C41 and the squeal is greatly reduced. Played about 15mins with no problem. Shut down and powered back up and the right side went out again taking CR46, CR47 and Q10 with it. Q10 now has a physical burn trace across the face of the transistor from the base to the emitter. The fuse did not blow. I am really scratching my head over this one.

[g1]

At this point (if you haven't done yet) I would try replacing U6, then U13.

[rybeka]

I have replaced both of them 2X. The signal that is taking out the right side is coming from the chorus section. Would it be OK to set the bias at 6.5V instead of 8V since it quite at that setting?

[g1]

Does it sound ok? I think it might create a bit of distortion of the chorus, and also U6 both halves will not be running at their supply mid-point, so a bit less headroom of signal there.
Is it your amp? If not, chances are that someone else tweaks it to 'make it right' and it blows up on them. At very least, leave a note in there.

[rybeka]

It is mine. I will tried the lower bias setting. I am still determined to discover why only Q10 is taken out (with the other components previously mentioned) and not Q11. I should not have to compensate for behavior that was not in the original design. The amp sounds ok but as you mentioned it really lacks the headroom and does not sound as full and rich as it should.

[g1]

I would guess that the glitch getting to the outputs has more of a positive DC component, so Q10 is conducting hard while Q11 does not.
Sorry I have no other ideas as to what could be causing the glitch. If you have not replaced the filter caps for the +/-16V supplies, I guess that is worth a shot.

[rybeka]

I was thinking the same that some type of positive signal is driving the right amp to severe clipping. Changed the caps and the zener diodes in both supplies. There is a point when one has to decide if any more time and money should be invested in a project and I am there. Probably going to just cut my losses and part it out. Perhaps there are some different ideas that others could share. Thanks to all for you insight and help.