sorenb

maybe start by checkin PSU rails at the AMP boards entry points, and check that all diodes are properly lit

PM me your address, I have plenty

Sounds like it's not only the +500V having a problem ... Have you measured the drop across the "current setting" resistors for the various 10m90's? Summing up those per rail, might shed some light on the matter ...

The PSU's I have at hand consumes ~25W unloaded Exploding 10m90's indicate massive current draw ... have you checked for any carbonization in the vicinity of those?

@jokerman777 Based on your post's I'd recommend disassembling both angle brackets, making sure all holes are properly deburred, and all the tappings are clean, also clean all semi's. Make sure the screws can go into the tapped holes using only finger force - if not re-tap the hole. Re-assemble, using regular compound (the white stuff). As for the buzz, you stated to have observed a flash in the region of the negative rails, and showed a burn'ed screw from the positive rail, showing that something was cookin here as well, and might have cause excessive current draw from the transformer causing it to buzz

@jokerman777 The thermal compound looks kind of grey'ish ... do you use some exotic compound? The screw needs to be able to slide into the 7721 without any force, otherwise it is too big. Maybe you found the cause of the HV transformer buzzing with "no load" ...

STTH512FP is ready available and fully insulated. If you look through the KGSShv thread, some had troubles with un-insulated rectifier tabs shorting.

@jokerman777looks like you did not use stth512's ?

As I understand, the channel has been working all along, just thrown off by the bal servo. Wonder what the benefit of troubleshooting with the LF353 installed are?

Look at the Uberamp output stage.

.... are D19/D20 the correct value and both oriented correctly (not flipped)? There are some good advice to be found in the posts from @simmconn, make sure you have read those.

Did you test Q24 (J79 in R+ ) ?

To me it seems R+ is not working correctly as it seems to be stucked at ~0V ...does it swing toward B+ at power off? have you tried pull the LF353? does the EL34 heater glow? maybe swap the EL34's in that channel?

Provided the readings are right, I'd look at the right channel LF353 and re-check all the diodes in the right channel servo are properly oriented if everything seems right, pull the right channel LF353 and see what readings you get.

Kevin, wish you a UM66T08