Sunn 300 Coliseum Troubles

[Euthymia]

I seem to remember rebuilding one of the 15V rails on it. I hope you can get it working again! I don't like to see amps go in the dumpster.

[gbono]

Yes you rebuilt the -15 rail. The +15 is the problem now with only +10V at most of the preamp locations. I assumed it was working for you?

Haven't had time to look at this again. Nothing ever get's thrown out around here

[Euthymia]

Yes you rebuilt the -15 rail. The +15 is the problem now with only +10V at most of the preamp locations. I assumed it was working for you?

I somehow remember the + rail being down even further, and it coming up after I replaced a charcoal opamp, but then that was when I started to suspect that if that one burnt chip wasn't the only thing pulling it down, that there was more work involved in fixing the amp than the client would want to pay for.

[gbono]

Okay I'm chipping away at the preamp and I have a signal up to the input of IC1. Can't seem to get past here. The signal "disappears" at R44 - if I remove the OP amp it appears at that node - replaced the OP amps - checked the capacitors and resistors around the OP amp and +-Vcc are good

BTW this board has had all the OP amps replaced in sockets - its a thing of beauty and excess now

preamp300.pdf

[Jazz P Bass]

I think it is best to check opamps at the outputs.
Looking at the inputs is futile (virtual ground something or other)

[Euthymia]

BTW this board has had all the OP amps replaced in sockets - its a thing of beauty and excess now

Cool! The way they should have built it in the 1st place, I think.

[gbono]

The amp is now functioning though the contour switch seems a bit dodgey. Next up a Sunn 1200 and an Acoustic 370.

[Euthymia]

The amp is now functioning though the contour switch seems a bit dodgey. Next up a Sunn 1200 and an Acoustic 370.

What did the issue turn out to be with regard to the disappearing signal?

[gbono]

Finger trouble I guess -