DC on Pot, Scratch, Scratch

[ChrisM]

Just finished a DC-30 style amp.

On the EF86 channel the volume pot is very scratchy, any movement and you hear it. I figured it was either a bad pot or DC on the pot. Sure enough I measured it tonight and found 2.5V DC on the wiper (forgot to measure the "hot" lug).

The DC from the plate should be blocked by the contour knob caps. No matter what the setting on the contour knob I get DC though. I am just unsure where this DC is coming from.

Could one bad cap in the contour switch leak DC and cause the issue?


Thanks...

[Deric]

I would guess that one bad cap on the switch would be enough to cause a problem. Disconnect the switch from the PI input cap and measure again. If the DC is there...I would guess it's coming from the EF86. If not...check for DC at the PI input.

Good luck.

[azatplayer]

Thats my call too, input cap on the phase inverter.
Been having some crappy luck with mica caps lately too. Really gotta start measuring the beggers.

[ChrisM]

Thanks for the advice. Will do some more investigation work tonight when I get home.

Question though,
The PI input cap is after the volume pot. You think DC from the PI output is leaking back through that cap and to the volume pot?

[azatplayer]

hmmmmm. All that cap love. I wanna say its one of the mica caps giving grief.
Had some trrouble with micas lately. I chased dc for ages on a rocket build, that turned out to be the PI input .047 cap. It was dc on the tone pots, and it was coming back thru the coupling cap into the treble wiper, there back to the other pots to a degree. I would drop out that .01? on the input PI first, try another cap. Then its on to the micas if that caps ok.
Good luck!!

[nee]

Had the exact same thing on a DC-30 clone. It was the 180pf cap in the tone switch that had gone. I just removed it, as it's in parallel with a .01uf.

[ChrisM]

On mine it was the 10nF. Got rid of it and the amp is sounding great.

[AllenBradley]

Don't know much about the DC-30. Where was the 10nf located in the circuit?

[ChrisM]

It was on the rotary switch.

Since the EF86 can't really handle our typical tonestack the designer decided to have a rotary switch that selects 6 different coupling caps coming off the plate.

One of those caps was this bad 10nF. Mallory btw. FUnny thing is I only used Mallorys for the rotary cause they are small. Everywhere else I used 6PS.