Sporatic DC on volume controls

[Paul P]

The volumes pots of both channels of my amp are very scratchy, the Vibrato channel quite a bit more than the Normal channel.
The pots are PEC's, close to new. I've read about them being noisy on their own, but for the moment I'm presuming they're ok.

The preamp circuits are from brownface Fenders of the Bandmaster variety, like the 6G7-A.
Normal and vibrato preamps are identical except for their output configurations.



Inputs are isolated from the chassis and all grounds are connected to a local star point (ref. Aiken).
Signal carrying leads are shielded with the shield grounded at only one end.
The 68.1k resistor below the Treble control is to simulate the tap on the pot in the original amps.

I've checked for DC in and around the volume and tone controls and found some, but it's far from constant.

Here is the situation as I've been able to determine so far :

• in general, there is no DC on any of the pots and no noise out of the amp. A sine wave going in produces a nice sine wave at the speaker
• turning the Volume knob generates a lot of crackling, louder with the Vibrato channel volume pot
• for the Normal channel, only moving the Bass pot can I get DC to appear along the connection between Bass, Treble and Volume pots. This DC is constantly varying and around 0-15 mvDC. This usually stops as soon as I stop moving the Bass pot, but I can sometimes set it so that it keeps going.
• for the Vibrato channel, while the volume knob is turned, DC appears on the grid of V1b (maybe also on Normal channel, but a lot less)

There is also a lot of constantly varying DC along the connection between the output of the Vibrato preamp and the input to the Vibrato circuit, varying around -40 to +40 mvDC.
This wire is shielded at one end, about 5in (12cm) long and runs along the wall of the chassis.



Preamp circuits are at the right, vibrato in the middle, reverb on the left (the photo is upside down)

[Helmholtz]

What's the DCV at the grid of V1b with vol. fully up?
Please specify if positive or negative wrt ground.

Do you use new tubes?

[Paul P]

DCv at grid of Normal channel V1b is zero with volume steady anywhere including fully on.
I can get it to vary between 0 and +1 mvDC while the knob is being turned slowly, up to +2-3 mvDC if I move the knob a lot back and forth.
But it always goes back to zero when I stop anywhere. Turning the Bass knob can generate a bit of DC as well, not the Treble.

DCv at the grid of the Vibrato channel is the same, but moving the volume a lot I can get it up to about +8 mvDC which will go back to zero when I stop.
Sometimes I can get it to stop at +1 or +2 mvDC when I stop moving. Both Treble and Bass knobs do nothing.

Tubes have been used very little (less than an hour or two).

[Enzo]

A noisy pot will create a noise signal, and your meter can react to that as a small voltage, which it is.

Substitute a different pot, preferably one you KNOW is not noisy. DO not assume a pot is not noisy just because it is new or a "good brand".

[Helmholtz]

DCv at grid of Normal channel V1b is zero with volume steady anywhere including fully on.
I can get it to vary between 0 and +1 mvDC while the knob is being turned slowly, up to +2-3 mvDC if I move the knob a lot back and forth.
But it always goes back to zero when I stop anywhere. Turning the Bass knob can generate a bit of DC as well, not the Treble.

DCv at the grid of the Vibrato channel is the same, but moving the volume a lot I can get it up to about +8 mvDC which will go back to zero when I stop.
Sometimes I can get it to stop at +1 or +2 mvDC when I stop moving. Both Treble and Bass knobs do nothing.

Tubes have been used very little (less than an hour or two).

Good, it seems there is no steady DCV at the pots.

To make sure there's no leaky cap, one after the other remove right side ends (referring to schematic) of 250p, 0.1µ and 0.02µ tone stack caps from circuit (leaving the other end connected) and measure DCV from the caps' open ends to ground.

BTW, it should be possible to open and clean the PEC pots.

[vintagekiki]

Try with capacitor, it can't hurt.
If the pots are still noisy, replace the suspicious pot. Although it looks new, we do not know when it was produced and how long it stood in the warehouse, there is a possibility that a fine oxide has formed on the sliding (metal) part.

Check that the metal housing pots are grounded.

[Paul P]

Thanks for the suggestions. I'll be taking them one at a time.

When I had the tubes out and with the amp off, I connected a signal generator to the input jack, jumpered across v1a to the volume pot and read the ouput of the pot on my scope.
Turning the pot generated a lot of noise so I guess they may well be dirty or corroded. I'll open up the vibrato channel pot and see what it looks like.

[Paul P]

This thread will be one more about new PEC pots that are dirty inside.

I took apart the vibrato volume and wiped the track and contact with a Q-tip and alcohol.
There were small bits of carbon on the Q-tip afterwards.

Put things back together and there's absolutely no crackling. I'm amazed again.

I now will have to take all my pots apart. Luckily they can be retracted from the chassis without de-wiring them.
I did have to remove two disc caps from the Vibrato LFO to retract all three channel pots together.

I suggest anyone considering PEC pots to take them apart for cleaning before doing anything with them.
That may avoid headaches down the road.

There are some very small pieces inside the pots that become loose when you remove the retaining ring on the shaft.
It may not be completely necessary to take the track and wiper apart for cleaning, which would be a lot easier and safer.
Maybe put a piece of paper between contact points to clean between them.

Onwards...

[loudthud]

Never mind, seems like you found the problem.

[Helmholtz]

Just curious, how much did you pay for one of these military grade, "high end" pots?

[Paul P]

I paid around 9 $US each, back in 2007.

Precision electronics

[g1]

I have had similar experience with new (NOS?) PEC pots. Mostly I just avoid them now.

[vintagekiki]

This thread will be one more about new PEC pots that are dirty inside.

About repairing potentiometers

http://www.geofex.com/article_folders/potsecrets/potscret.htm
The Secret Life of Pots

https://www.element14.com/community/community/project14/simpleelectronicrepairs/blog/2017/10/24/potentiometer-repair
Potentiometer Repair

https://www.radiomuseum.org/forum/repairing_old_potentiometers.html
Repairing old potentiometers

https://www.apogeeweb.net/article/145.html
Potentiometer Circuit: Potentiometer Problems and Repair

https://support.newgatesimms.com/potentiometer-maintenance-and-lubrication-case-study/
Potentiometer Maintenance and Lubrication-Case Study


Resistive track and metallized slider with cotton wool and isopropyl alcohol clean from dirt.
Metal slider, degrease with solvent from accumulated dirt and clean with Glass Fibre Pen.
Application field Glass Rubber
Before assembling pot, resistive track treat with some contact spray.

[vintagekiki]

I forgot, clean potentiometer in the ultrasonic bath.

[vintagekiki]

I have had similar experience with new (NOS?) PEC pots. Mostly I just avoid them now.

https://en.wikipedia.org/wiki/New_old_stock
New old stock (NOS), or old stock for short, refers to aged stock of merchandise that was never sold to a customer, but still new in original packaging.

#6
Although it looks new, we do not know when it was produced and how long it stood in the warehouse ...

[g1]

I don't think we should have to worry about keeping our stock of pots 'fresh', like electrolytic capacitors. Especially when they are sealed, 'military grade' pots.