DC on tonestack, possible causes?

[francospence]

Hi there,
This has been a problem on my Express for some time now, scratchy sounding pots are the result, and it would be great if i could get this issue fixed. I have replaced the silver mica 500pf with various caps (polystyrene and pio) this seems to help some times for a very short time, turn it next time and the Dc is back on the tonestack. I have swapped the bass and mid .022uf caps out, to no change. Changed the pots (to pec) and the grounding ( a buss bar near the pots that is earthed near the input jack) the input jack is isolated and earthed to the same point. Ive also tried numerous tubes in V1 to no change. The dc on the treble pot varies between 80 mv to 200 mv, max readings are with the pot on 0 , min readings are when its on 10, when it has read 2 mv on the treble pot (after i swapped a pio treble cap in) the scratchiness was gone, next time i turned the amp on it was back. Any other ideas would be most welcome, thanks in advance,
Best regards Lee Spence

[markr14850]

What are the circuit boards made of?

I ask because I once ran across a batch of leaky FR4. My guess is that it some of the "anti static" variety got mixed up with the normal, non-conductive variety. DC would magically appear in strange/impossible places.

https://tubeamparchive.com/viewtopic.php?t=7756

Good luck,
--Mark

[francospence]

Thanks Mark, seems you were spot on, the boards are the holed brown fiberglass variety i purchased, and if i measure dc in the hole next to the turret to ground, nearly a volt! Strange thing is the amp is incredibly quiet, stable and sounds amazing, the dc on the pots have been the only issue. No easy fix to this one i suppose! Well picked up on your behalf, thanks,
cheers Lee

[azatplayer]

Wow, would never have guessed that in a million years, i had similar trouble with a rocket build a while back, ended up redoing a lot of the amp as a result, replacing the board was one of the things i did, fixed the issue, maybe it was the board material?
Where did you source the board from? Good to know to steer clear of in the future, however i have for some time now been using tagstrips, another great reason to continue!

[DonMoose]

Odd. I'd have thought at least one of the grounded connections to the board - standoff, Rk/Ck, something - would drain it.

We know the black waxy boards (old fenders) absorb moisture -Ah! All those perf holes let moisture seep in? Does that seem likely?

[azatplayer]

Well i take that back regards the tagstrips, had a leaky cap on my express thats tagstrip, turned out the damn tagstrip is conducting!!
Bugger, i have to rework the B+ rail now.