Sunn Coliseum 300 - loud hum

[acorkos]

This amp got banged around, with a preamp coupling cap breaking free. I got that put back together, and it is working, except now there's a loud low hum through the speaker. i measured dc voltage at the big 4700uF e-caps...one measures +56V, the other -38V. shouldn't they be about the same (+vs. -)? maybe one of these caps is bad?

http://music-electronics-forum.com/a...-sunn300pa.pdf

[52 Bill]

Although there are a few Sunn amps that have non-symmetric power supplies this one should have the same + and - dc. What ac voltage reading do you get across the filter caps?

[Enzo]

I agree with Bill, look for AC on those power rails. To make 56vDC one rectifies and filters roughly 39vAC. Suspicious that your low DC voltage looks about the same as the AC it comes from. I'd wager the cap for the -56vDC is either bad/open or not connected.