Ampeg VH-140C Volume cutting out / can't find any solutions

[kylekienitz]

So I'm doing some work on an Ampeg VH-140C where the volume drops out after 15 or 20 minutes of playing. Sometimes it will cut back in and sometimes I just let it rest, after which it always works normal for a little bit. I have replaced the effects loop jacks, so it's not that. I have done a lot of research online and this seems to be a very common problem, but I haven't found any solutions. What could this be? What should I look for?

I visually inspected the PC boards and resoldered some questionable joints, but the problem persists. What's my next move?

Thanks

[52 Bill]

Because they can be caused by physical problems as well as electrical problems, finding intermittent problems is one of the hardest things to do.

Does physically moving any components or wiring cause or cure the problem? If not, then start reading voltages and compare them when the amp is working and when the amp is not working. Hopefully the odd or missing voltage will lead to the problem.

[kylekienitz]

Thanks Bill! I'll do some more searching and get back if I find anything useful

[nevetslab]

I had to look this amp up first, as I haven't serviced any of these. But, I did notice on the rear panel it has a Preamp Out/Power Amp input jack set. Have you tried putting a patch cord in to externally connect the Preamp Out to the Power Amp in. Those Cliff jacks (presumed) are a common fault over the years, sometimes being a tension issue, other times oxidation.

The Preamp output is ALWAY active....you could patch that to another amp, so it would allow you to see if the problem is in the power amp stage or the preamp stage.

[g1]

The Preamp output is ALWAY active....you could patch that to another amp, so it would allow you to see if the problem is in the power amp stage or the preamp stage.

Good point. Even though you already replaced the jacks, you can still use them to isolate whether the problem is in the pre or power amp. When the fault occurs, see if you still have signal at the pre out.

[Jazz P Bass]

Ampeg VH-140C Schematic

Well it appears that there are 4 jacks.
The Line Out (FX) & the Power amp .
The Power Amp in is a normally closed jack, which could be corrupt.

[kylekienitz]

Hey thanks, I'll give that a try first.

[Jazz P Bass]

Running a signal cable jumper from Power Amp In to Power Amp Out will suffice to see if that is the problem.

[kylekienitz]

Just want to leave an update... That's exactly what it was, the shorting jack for the power amp in was faulty. I replaced the jack and it's rockin again with no problems. Thanks again for the help!