Trouble shooting scratchy pot on JCM800 style build

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mmmoser
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Trouble shooting scratchy pot on JCM800 style build

Post by mmmoser »

So I have been asked to track down a scratchy volume pot on a JCM800 clone circuit amp. Normally I suspect either just a pot that needs some cleaning or replacement or a leaky cap flowing some DC when it shouldn't.
After cleaning then replacing the pot (this is the first volume pot, not the master volume pot) I then started looking for the DC. I did measure some DC (-.5V up to -1V) after cap feeding the volume pot when using the low gain only channel. I figured this was the culprit and replaced the cap. No joy. That DC was still there. Further tracing of voltage reveals the voltage coming from pin 2 of the first preamp tube. It varies from virtually 0 at no volume up to -1V at full volume.
This is causing a rather noisy pop when the volume knob comes off full counter rotation and the pot is scratchy through its entire rotation. This is with no input signal by the way. I am scratching my head over this scratching noise. I cant get rid of it with any number of different preamp tube swaps either, so its not a tube specific issue. Any ideas from the collective community?
So far I have replaced the pot twice just to make sure I was not dreaming and I used an Alpha, a CTS and now have a PEC pot in there. I replaced the caps, the plate resistors and even the preamp tube socket. Cant get rid of the noise.
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Re: Trouble shooting scratchy pot on JCM800 style build

Post by dorrisant »

Will the scratchiness stop if you insert a cap between the coax and wiper of the volume pot?
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pdf64
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Re: Trouble shooting scratchy pot on JCM800 style build

Post by pdf64 »

Some tubes have excessive grid current, which can cause such scratchiness; try a different tube.
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mmmoser
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Re: Trouble shooting scratchy pot on JCM800 style build

Post by mmmoser »

I will try the cap trick. I have tried about 6 different tubes already. Some NOS, some new issue. It has not helped so far.

Thanks for the pointers however.
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Baddog01
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Re: Trouble shooting scratchy pot on JCM800 style build

Post by Baddog01 »

What schematic did you use for the preamp?
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Re: Trouble shooting scratchy pot on JCM800 style build

Post by Baddog01 »

Looks like a 2203. Check all resistor values that go to V1. Try running the volume pot ground to the back of the pot instead of the buss wire. Try seporating the signal wires that go to V1. Used individual shielded wire .

Keith




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mmmoser
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Re: Trouble shooting scratchy pot on JCM800 style build

Post by mmmoser »

Baddog01 wrote:Looks like a 2203. Check all resistor values that go to V1. Try running the volume pot ground to the back of the pot instead of the buss wire. Try seporating the signal wires that go to V1. Used individual shielded wire .

Keith




T
Hi- Yes, its a 2204 circuit. I have changed out all the resistors going to V1. Each lead to V1 carrying signal now has its own shielded cable grounded at one end as per usual, same problem. I changed the caps, I changed the socket, I changed the pot (3 times), I changed the input jacks. I tried 12 different preamp tubes. All with no end to the scratch.
I will try the changing of the pot ground to the pot back. I did also try a small cap at the pot to ground, no joy either. I even tried a diode from the center lug of the pot to ground- don't ask me why, I just did. No change there either.
After this ground swap, I am out of things to try. There has got to be a reason for this as I have never run into it before on any past build. Somewhere the electronic devils are laughing.
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JazzGuitarGimp
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Re: Trouble shooting scratchy pot on JCM800 style build

Post by JazzGuitarGimp »

Do you have measurable DC on the pot with V1 pulled?
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lovetone
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Re: Trouble shooting scratchy pot on JCM800 style build

Post by lovetone »

It might also be the component board conducting I have had a similar fault on a Marshall 1959 that has a PI fault

On that amp I removed the components from the turret tag and it still had volts leaking from an near by terminal.
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Re: Trouble shooting scratchy pot on JCM800 style build

Post by Baddog01 »

I had that same issue with my bassman 50. Had to seporate the two boards :)
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Re: Trouble shooting scratchy pot on JCM800 style build

Post by Cameron »

Could be a layout issue ....I would separate the input signal from the signal going to the second gain stage...each should have it's own shielded cable ...the way it is ...could be what's causing the scratchy pot....
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mmmoser
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Re: Trouble shooting scratchy pot on JCM800 style build

Post by mmmoser »

Cameron wrote:Could be a layout issue ....I would separate the input signal from the signal going to the second gain stage...each should have it's own shielded cable ...the way it is ...could be what's causing the scratchy pot....
Yes- I have rewired the leads to their own shielded cable.

I do have measurable DC from pin 2 to volume pot:
I did measure some DC (-.5V up to -1V)
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Post by Stevem »

I have had a reissue Fender SR with a bad board that was arcing in the PI section intermitantly.
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mmmoser
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Re: Trouble shooting scratchy pot on JCM800 style build

Post by mmmoser »

So the board itself is not leaking DC. I am reading up to -1V on pin 2 of the preamp tube. This is the source of the DC making the pot scratchy. Its feeding that DC back to the pot.
Today I did several experiments. I tried grounding the scratchy pot to back of pot rather than the buss ground. No difference. I actually pulled the buss ground and did the back of the pots grounding scheme. Why? why not. But no change. I have rewired the jacks as well. Why? why not.
So, I still have this dc coming off pin 2 and I have swapped many many different tubes in there. Old tubes, new tubes, lower gain tubes, all with no change. I cannot figure out what it might be.
Currently stumped and frustrated. Something somewhere is blinding me with science.
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JazzGuitarGimp
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Re: Trouble shooting scratchy pot on JCM800 style build

Post by JazzGuitarGimp »

Why not AC couple the pot to the tube and be done with it?
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