10K trimmer potentiometer failure, or something else?

[Paul Birkeland]

Interesting, so your 431 chips seem OK, and disconnecting the regulator still doesn't bring OA up, which is somewhat unexpected.  My next suggestion would be to check for shorts between pins of each transistor on the B side C4S.  By process of elimination, this seems to be the problematic area.

All of the parts are still in stock and a replacement Paramount Soft Start half kit can be made for you if that's helpful.  The flat rate repair service is also an option.

[Brent]

I’m not getting shorts between any transistor pins on either A or B side.

If the problem can be isolated to the board, maybe it’s simpler just to replace it at this point. FWIW, I’ve resoldered all the terminals around and connected to the 9 pin socket too.

[Paul Birkeland]

Yes, the B side C4S appears to have some issues.  If I had your amp in for repair, I would just build and install a new PCB.

[Brent]

I’ve sent an email to order the half kit. I’ll follow up when I’ve installed it just to confirm it fixed the issue. Thanks again for your help.

Brent

[Brent]

I received the half kit last week and installed it a couple days later. Voltages are normal and the amp is back in business. Woo hoo!

I have a couple of observations about the newly installed board. First, both amps used to 'squeak' a few seconds after turning them on, about the same time the normal background hiss came up (my speakers are about 100dB, so the hiss is easily within normal operation for the amps). The amp with the new board doesn't squeak anymore. Second, regarding that background whitish noise, it's noticeably quieter on the repaired amp.

Both of these observations have me curious. The only visually obvious parts difference was that the new board had a plastic PN2097A instead of the metal can 2N2097A transistor that came with the amp. Would this account for either or both observations? I recall reading on this forum about different batches of LM431 correlating with better and worse noise levels, so it could also be that (despite no obvious visual differences there, though I didn't compare part numbers on the LM431s).

I ask because one of the reasons I was previously using 6SN7s in place of the 5670s was the reduction in self noise from the Paramounts. It wasn't terrible, but with 100dB speakers it was noticeable when things were quiet. However, if I could get both amps as quiet as the one I just repaired, I don't think I'd bother with 6SN7s. There is still a bit more gain than I need with the stock 5670s, but that's not such a problem with the BeeQuiet on my BeePre 

Thanks as always!
Brent

[Paul Birkeland]

The 431s that we got that were bad have "WS" written on them.  I don't think the PN2907 should be any quieter than the 2N2907, but it could be!