U3 Voltage Problem

[leo]

I built two C84s a while back and have had problems with one. They are revision 1.6. Comparing the two I find that U3 on the bad C84 has approximately -15v going to pin 1 that is not present on the other C84. Also, I am seeing approximately -8v on pin 8 of U3. I checked polarity on the caps and I checked that the diodes were oriented correctly. I also checked all of the other compenents that are polarity sensitive and have not found any issues.

Is there somewhere I should start looking? I have been reviewing the schematic and I have working backwards from the pins that I have had problems with without success yet.

Any guidance would be appreciated.

Thank you.
Joe

Update: I checked the voltages again and this time they are positive instead of negative. Also, I am getting voltages at Q3, 4, 7, 8.

[leo]

One more thing I noticed, when I compare the good C84 and the bad on my scope, the top part of the sine wave is blurry on the bad C84. I don't know if this helps at all.

Thank you for the help.

[leo]

Bump. I am still working on this problem. I believe I have narrowed it down to the transistors, but I could be wrong. I believe I oriented them correctly, but I may have damaged them at some point. I have checked all of the important resistor values around the op amp that has the problematic readings.

Tim, do you think I am on the right track with the transistors, or am I missing something.

Thank you
Joe