DC on tone pots

[xtian]

Clearly, I have DC on my pots. They're all very noisy when turned, and not because of dirt or bad traces, it's clearly DC. But I don't really know what we're talking about here, so please teach me.

If I meter the two non-grounded pins on the Bass pot, I read 38 volts. So I replaced all the caps in the tone stack. No difference. Well, I thought I'd move on to the coupling caps. But I don't know how to use my meter to find DC on the pots, nor what to expect about DC voltage across coupling caps.

[xtian]

I compared readings with my working Rocket. No DC voltage across legs of Bass pot. So that's clear, at least. Where does the DC come from when it's broken? The coupling caps?

In my working amp, I measure lots of DCV across coupling caps. I thought these caps were supposed to block DCV. Or are they supposed to block current?

Con-fused.

[Teleguy61]

I'll give it a shot.
DC leakage comes from leaky coupling caps.
When you are measuring DC across caps, you are measuring voltage potential across the cap, not current flow. That would be in (milli)amps, not volts.
Measure the voltage across a cap by measuring each side of a cap in reference to ground.

[diagrammatiks]

leaky coupling caps.

you'll have to check every cap in the tone stack and the cap coming from the pi.

[xtian]

Measure the voltage across a cap by measuring each side of a cap in reference to ground.

Ok! What should I see if good, and what if bad?

[xtian]

leaky coupling caps.

you'll have to check every cap in the tone stack and the cap coming from the pi.

The PI is after the tone stack. Is THAT the cap that might be leaking?

[diagrammatiks]

it's a rocket?

https://tubeamparchive.com/files/taylor ... 08_758.pdf

from that schematic...

you'll have dc on the tone stack if C5, c6, c7, or c8 leak.

you said 40 or so volts DC?

that seems about right for the PI voltage offset.

the other side off the cathode follower should see closer to 100+

[xtian]

I replaced a pair of red Astron coupling caps, and that did the trick! This is the Sano Excelsior amp from my other thread.

[tubeswell]

For future reference, you need to lift/de-solder the non-plate end of the coupling cap from the rest of the circuit, then you clip your +ve DC meter clip onto the loose end of the coupling cap (making sure it isn't going to short on anything else) and the other meter lead to the amp's ground return. Then flick your amp on and see it there is any DC appearing on the meter. If the cap is good there shouldn't be any.

FWIW I've had a couple of silver mica caps go bad recently (and they were new ones too dammit) in BF style tone stacks - but its not just silver micas that can fail.