Fender M80 hum

[kkatarn327]

I'm working on an old Fender M80 (non-chorus) head. Initially the only problem was that the volume pot on the clean channel was broken off so I went ahead and replaced it. After putting it back together and playing it to test it, it worked fine but about a minute or two into playing the sound suddenly cuts off and loud, incessant humming resumes. Nothing else comes through besides the hum. I looked for shorts, reflowed solder in some places, and even tapped around to see if I could interrupt it but the hum was totally solid throughout (the hum is also not dependent on the volume).

Any ideas of what this could be? I have no personal experience of these amps so I don't know what the usual supsect(s) may be. Thanks!

EDIT: Upon further inspection, the signal seems to be coming out of the PREAMP OUT jack fine. I suppose then it is a power amp issue?

[Enzo]

When the loud hum comes out, look closely at the speaker. Does the speaker cone move from rest position to either all the way in or out and STAY THERE? Or better yet, disconnect any speakers and measure to see if there is a large DC voltage on the output of the amp.

Loud hum is almost always from DC on the output or a loss of filter cap. Filter caps can fail, but WAY more often it is cracked solder to them. If no DC on output, measure for excess ripple on the two power amp power rails.

[kkatarn327]

The filter caps were one of the locations I resoldered. There seems to be -20mVDC on the output.

EDIT: Also come to think of it, before I opened up the amp I was playing on it and noticed a rather significant fluctuation in volume on the overdrive channel. It was momentary at times but other times it would stay at one volume level for a minute or two before gradually going up or down to another volume level and staying there, etc. I attributed this to dirty pots/bad solder joints at the time but could this be indicative of a similar condition? Unfortunately the amp didn't last long enough after I worked on it to check on this condition.

[kkatarn327]

Is the voltage I measured (-20VDC) significant enough to point to the filter caps as the issue? Seeing as the appropriate ones are almost $13 each I would like to be sure. Thanks!

[Enzo]

-20v or -20mv?

[kkatarn327]

Sorry, -20mv, my bad.

[kkatarn327]

Well I finally got around to replacing the filter caps today but the hum persists. No detectable change in volume, pitch or anything. Same conditions as before: hum is independent of volume or any controls, input(s), effects loop, or channel. Any ideas??? This amp is driving me nuts.

Just to note... the diodes marked CR25 and CR26 show heat discoloration as well as the ceramic resistors around them but research shows this is to be expected. I also ensured that the ceramic resistors have not broken off at the board.

[Enzo]

Are both +15 and -15 volt supplies up to snuff? Those zeners run hot, and will discolor the board, but they can also fail. Check them.

PLug the guitar into the power amp in jack, but dial it down to zero. Hum remain? We need to isolate hte problem to the power amp or the preamp.