JCM2000 DSL401 lost bias voltage on V4, V5

[Prairie Dawg]

I've got a JCM2000 DSL401 (4x EL84) that's lost bias on one pair of tubes. V4 and V5. I've got no voltage on R85 and 84, and I have voltage on R86 and 87.

I changed out C79 and C80, and I'm about to go in and redo C79 on the off chance that it is a bad cap. Meanwhile to Office Despot for some more tags because I'm out.

Anyone ever have this happen before? Should I be looking at the bias adjust pot PR1?

UPDATE: I've tracked the problem down to the master volume control. In certain positions it goes open on one side which effectively lifts the grid signal and also the bias voltage, which explains why one side was redplating. Checking the operation of both sections of the pot confirms that this is what's happening. Now, all I gotta do is find one. It is a Piher A200k stacked pot notated as VR12A and VR12B on the schematic. Anyone have any idea what the part number is and where it can be obtained from
?

Thanks

[Enzo]

Well, you beat me to it, I was going to suggest master vol, since the bias goes through it.

Call parts is parts and tell them what you need, they should be able to look up the part number.

It is a dual pot. I could be wrong, but it seems to me last time I bought those I got them from MOuser.

[Chuck H]

+1
A little searching may be in order but Mouser has pots in most sizes and shaft types for dual ganged stereo control type applications.

[Prairie Dawg]

This thing's a real odd duck and not the usual double ganged pot. These are dinky. It's a little wider than the rest of the pots but the same depth, six or seven leads coming out the bottom through the board...dang it, I wish I had some R12-that would probably clean it. Here's a couple pics.

If you have any idea where I can find one of these p-lease chime in.

[Prairie Dawg]

Parts is parts says they have em, so I ordered two, one for this amp and a spare I may use as an earring. Thanks for the tip, Enzo. FYI their part number is POTP00076 and they're $7.50 a copy.

Just as a point of interest I found a site in Hungary that sells them. The language is Hungarian but the part numbers and pictures marry up.

[Chuck H]

Mouser has them in the paper catalog. I can't seem to find them at their site though. If I have time later I'll look up the number.

[Enzo]

Thank you, yet another example of the value of having the paper catalog instead of relying on the web site. Online, you have to know what you are looking for, but you can browse the paper and spot things. sometimes the value is finding out what they CALL something. Aha! 14mm pot!

You are thinking old school when you talk about dual deck pots for stereo or something. This is a common appraoch these days, inside the pot, one pot resistive strip is the inner lane, while the other is the outer lane. Think of a divided highway going around a curve. Then the wiper assembly is two sets of feelers next to each other.

Mouser stock 754-0109, I have a bag of them in front of me. I think they are what you want.

http://www.bourns.com/pdfs/PTV142.pdf

[Prairie Dawg]

Looks about right. Mouser's out of them but I did snag a couple from Parts is parts. The Bourns info looks good because I need to scare up some Piher equivalents for my much neglected Kittyhawk.