Peavey Mark IV output cuts out

[JamesonStyles]

Hi everyone, new poster here. I recently purchased from my local music-go-round a Peavey Mark IV bass head, and while it puts out the most beautiful, fat growl I have ever heard, its got a small problem. Occasionally, within the first few minutes of using it, the output will cut out or down to almost nothing, with occasional pops or whispers of a tiny bit of the signal getting through. I can sometimes correct this by turning up the pre and post gain, which seems to have the effect of forcing enough juice through to restore normal operation, although this doesn't always work. So, it doesn't seem mortally broken, but this is still a big problem, (imagine if it happened onstage!).

So, I've got a theory. It could be a problem with the limiter circuitry, which I've read about with other Mark IV's having some limiter related problems. I suspect that something would be triggering it to limit the output completely, but why? I know that the limiter circuits can be removed fairly easily from these amps, maybe I should give that a shot?

I'm not too experienced with amp troubleshooting, and although I'm comfortable with opening it up and taking a look, I'm really not sure where to go from here. What do you guys think?

[Chuck H]

I don't think it's the limiter circuitry because those circuits don't attenuate. They limit. Your problem could be much simpler. Try cleaning all the jacks, switches and pots. A guy brought me a Peavey bass head with similar problems and a good cleaning fixed it right up. No promises, but it's cheap to try it.

Chuck

[JamesonStyles]

That's a good idea. This thing may not have been cleaned inside since 1982. This is going to sound really stupid but I don't really know how to clean pots. Do you use compressed air?

Edit: Boy, a good googling set me straight. Most people seem to say Caig Labs Moving contact Lubricant, or DeOxIt if the pots have rust. Sound right to you guys?

[Chuck H]

I just use whatever "contact cleaner" I can get at the nearest place that sells such things. Both brands you list have a good rep. I might avoid using anything that has "lubricant" in the description for cleaning jacks since it seems to me that there would be some kind of residue left behind that could pick up debris and eventually cause gunk buildup.

If you ever use such cleaners on guitars be sure to keep it off the finish and don't allow it to soak into any bare wood in control cavities.

Chuck

[Gtr_tech]

Sounds like it could be the coil on the spkr output. Those can break off that the standoff. It still might look OK, but try moving it and watch the wire at the standoff, it may be cracked. Diming the vol sends enough current to it where it will arc and weld itself back together...temporarily. Just resolder it if this is the case, but clean all the jacks/pots/switches first and see if that takes care of the issues.

[Enzo]

Gtr tech might be right, but my money is on CHuck. You power amp jack is probably frotzed. Plug a spare cord from preamp out to power amp in. See if that cures it.