Peavey Decade amp, noisy pots, replaced with new, still noisy?

[Kenrod]

schematic attached.

I took in this Peavey Decade for a few noisy pots. After much spraying with contact cleaner, the "low" and "high" pots still made some noise when turned. I went so far as to diassemble the pots to clean them as thoroughly as possible, still noisy when sweeping the knob. I ordered replacement pots from Peavey and installed, same noise still exists. Is it something else in the circuit? the "low" pot makes a lower freqency noise and the "high" make a higher frequency noise, not quite the typical generic "scratchy dirty pot" sound.

[allante666]

Sounds like they have DC on them?

[Kenrod]

Sounds like they have DC on them?

I saw that mentioned on a similar thread, but not sure what to look for. I've never encountered that that I am aware. It's usually just a dirty pot.

[52 Bill]

I saw that mentioned on a similar thread, but not sure what to look for. I've never encountered that that I am aware. It's usually just a dirty pot.

This amp has a single polarity power supply, so all of the op amps will be biased at a positive voltage of about 1/2 the full value of the power supply, in this case 11 volts dc.

To keep this bias voltage out of the tone controls there are blocking caps. If one of these caps becomes leaky and allows a little dc to pass through, there will be noise when the pots are turned. Try and measure the dc voltage to ground from the different terminals of the noisy pots. If there is any dc voltage there, try and trace back through the circuit to either C13, C14 or C15 and see if any are leaking dc voltage.

[Kenrod]

Thanks 52 Bill! Exactly the kind of info I was after.
I'll post my findings tonight after I get home and back on it.

[Kenrod]

Yep, about 2.5V on the "low" pot and 4.5 V on the "high" pot. I'll look at the caps next

[Enzo]

Those 2.2uf electrolytics are the primnary suspects

[Kenrod]

Those 2.2uf electrolytics are the primnary suspects

thanks for the shortcut!

[Kenrod]

Fixed it, but it was C15 as I suspected from the info 52 Bill provided.
But I did swap out the pair of 2.2uF first as Enzo suggested, because I also explicitly trust him as well, but that didnt clear it up.
I may swap the original pots back onto the board. Only because I had to mount the new pots off board (mounted on the chassis/faceplate) and use jumper wires.
I could leave well enough alone. They are new pots, probably needed eventually anyway.

[Enzo]

I hope you meant implicitly, but I appreciate the compliment. I mentioned those, because those particular caps were used in MANY PV amps of that era, and almost all of them I encounter have dried up since and need replacement. As I said, they were suspects, but apparently not the perps this time.

Looking at the schematic more closely this time, I'd have to agree that C15 is a better suspect.

[Kenrod]

Uhh. Yeah. That's what I meant to say. I blame auto correct?

[Enzo]

Oh yeah, I have one of those too. Every time I say something, my wife automatically corrects what I said wrong.

[Kenrod]

I have one more minor nagging problem. When I put it all back together in the cabinet and hooked up the speaker and turned it on, I was met with a prominent hum. The pre and post gains were wide open, but when I rolled the post gain from 10 to 9 it's like the noise "switched off". That abrupt. Go past 9 toward 10 and its instantly back.
What might that be?

[Kenrod]

I was ready give this back, but not when I heard this problem which I think is new. Maybe I should look closer at everything involved when I put the chassis back in the cab.

[52 Bill]

Something that you said that you did earlier was to remove and replace the pots. Have you checked to see if you lost a ground connection somewhere?