Seriously this time...Dead pots!

[Cliff Schecht]

I've been going over a guys Peavey MKIV Bass Amp (uses the 400BH series power amp) which started oscillating in one of the channels. When you adjust the parametric mids knob nothing happened and the shift knob changed the frequency of oscillation. After chasing down the problem (and a proper schematic) for a few days I finally realized that the parametric mid pot had died completely. More specifically, the pot reads open from one outer lug to the next and from the inside lug to either outer lug. I've never really seen a pot fail like this either, especially a quality one (MIJ Alps). I put in a temporary 50k linear pot and the oscillations died immediately.

The best part is it's a really funky pot design with a sort of unusual mounting system so it's going to be a nightmare trying to retrofit something else into this amp (because of how it's built). If the customer wants to pay the money to have a $100 amp worked on then I will do so but I'm expecting this amp ends up not getting fixed. There's a least a few more pots (if not all of the non dual-ganged pots) that need replacing so it's a lot of work and will cost more than the amp is worth.

So rant over, but has anyone else seen a pot fail like this? I pulled it apart and saw nothing wrong visually..

[Zippy]

If the customer wants to pay the money to have a $100 amp worked on then I will do so but I'm expecting this amp ends up not getting fixed.

How 'bout setting a nominal fixed value (instead of a pot) with a pair of resistors?

[NickC]

Sometimes the intrinsic worth of a thing is irrelevant. I'd argue the amp, in its' present state, isn't even worth $100. Properly repaired, the owner might never recoup the investment in the repair. However, an amp "made right" by someone of your knowledge and skill, in my mind, has much greater intrinsic value than $100. What the owner will end up with is different than that type of Peavey in stock form.

I started down a similar path with a Sovtek MIG50. It was due for a cap job, and I wanted to replace the cheesy plastic pots with good panel mounted ones while I was in it. The flimsy PC board, and board mounted pots, turned into a mess. The solder pads were lifting off the board whilst removing the 3-point pot connections. Ultimately I scraped the PCB and rebuilt the amp on a G10 Garolite, turret construction board. I learned a lot.

If I could do it over again differently, I'd have installed some mini-turrets on the PCB and effected the repairs that way.

Good luck with whatever direction this project takes.

[Leo_Gnardo]

The best part is it's a really funky pot design with a sort of unusual mounting system so it's going to be a nightmare trying to retrofit something else into this amp (because of how it's built).

Peavey has a pretty good reputation for supporting their gear, even fairly old stuff. Sorry I don't have their # or email but should be easy enough to find. Ask, the worst that can happen is "sorry no gots no more" then you're on to other solutions.

[Cliff Schecht]

Wow! I am thoroughly impressed with Peavey's support! They don't have the original part but they have suitable replacement pots. I'm going to go through and find all of the bad pots tonight and order what I need tomorrow.

I measured the pot threadings to be 7mm, definitely an unusual size. Peavey is asking $5 a pop for the correct parts which is the same as the best online price I could find.

I agree with Nick. I take on a lot of jobs that the repair will cost more than the unit is worth. With that said, if that piece of equipment is essential to your sound and you can't find another working one easily, I'm your best friend in the world .

[Leo_Gnardo]

Wow! I am thoroughly impressed with Peavey's support! They don't have the original part but they have suitable replacement pots. I'm going to go through and find all of the bad pots tonight and order what I need tomorrow.

Good going Cliff, you ARE the circuit P.I.