5G9 DC on Volume pots

[hylaphone]

Hi folks, just buttoned up a 5G9 and I am having static issues on both volume pots, ONLY with the trem engaged.
I'm measuring .2v through -.02v on the tops of the pots, the wipers reach -1v turned all the way up.

Surely it suggests a leaky cap, but I am not understanding how -DC from the bias supply or trem circuit could find its way back to V1..
Can anyone shed some light or perhaps suggest possible culprits in the circuit?

Here are a few pictures, and the attached schematic for reference.

https://imageshack.com/i/5e1ek2j
https://imageshack.com/i/mwhijbj


Only changes I made were lower V1 coupling caps, and higher (.01, .02, .04) caps in the LFO to slow down the trem a touch.
It has a bias pot added as well.
Voltages are higher than the original, I'm running a 330-0-330 PT, through a 5R4GYB
V1 1/2 - 184v
3/8 - 2.91v

V2 1 - 226v
7 - 222v
3/8 - 31v

V3 1/7 - 283v
3 - 2v
7 - 285v
6 - 425v (B+)

V4/V5 1 - 425v
3 - 420v
4 - 424v
5 - -38v
8 - 24ma

[hylaphone]

Sorry for the superfluous attachments. I am no computer whiz.

[loudthud]

The voltages on the tubes look good and because the volume pots feed an LTPI with an input coupling cap, the fault has to be a conductive eyelet board. To confirm, remove the output tubes and disconnect the bias tap so there is no negative voltage generated. Now power up the amp and look for negative voltage on the pots. Unfortunately, replacing the eyelet board is the only likely cure.

[hylaphone]

The board is G10 but I did the test anyway and the -volts are still there.

[Bruce / Mission Amps]

Sometimes the actual grid of the preamp tube "leaks" DC voltage to the previous stage.... tried a few different preamp tubes?

[hylaphone]

Yes I've swapped out V1, V2, and V3.
Just had a look on a scope and you can see it clear as day - wiggling DC on the tops and wipers of each volume pot, changes with the trem speed, ceases with the trem off.

[Chuck H]

Yes I've swapped out V1, V2, and V3.
Just had a look on a scope and you can see it clear as day - wiggling DC on the tops and wipers of each volume pot, changes with the trem speed, ceases with the trem off.

Ceases wiggling or ceases to be there? Is there voltage present that isn't wiggling while the trem is off?

You might have a poor ground or a long ground lead that's shared by the trem and volume pot or some other such circuit relationship.

[hylaphone]

You're right Chuck, the voltage is there regardless, about 40mv. Engage the trem and it starts dancing around..
The static only happens turning the volume pots with the trem engaged.

I have the grounds split - preamp, PI and tone controls to the input jack ground, then Trem, Bias, Output, Center Taps, etc. to a dedicated ground lug over on the other end. All via sturdy bus bars.
Amp plays great otherwise, and is virtually hum-free.

[hylaphone]

Correction, the voltage is only there with the trem engaged. Sorry, had an errant reading.
Wiggles from 20mv to -20mv, and changes speed with the speed knob.

It's present on the tops and wipers of each volume pot.
I can live with the scratchy pots, but would really like to understand whats going on.
I have triple checked the grounding, lead dress, etc., swapped tubes.
Any other wild ideas folks?
Thanks

[Chuck H]

Your photos are small, but it looks you have two ground buss wires with the preamp, inputs and the last filter on one side and the trem, PI, power amp, remaining filters and power supply on the other. Is this correct?

Is there still voltage on the pots when they're turned all the way down?

Just to be clear, there is no voltage on the pots when the trem is off?

Are you using a grounded AC power cord?

Engaging the trem UN-grounds the oscillator signal. I think most other Fender bias wiggle trems engage by grounding one of the phase legs. I don't know that it's relevant except that lifting that ground places voltage on the volume pots. It would help to have more, larger photos of the actual innards to better see how everything is grounded.

[hylaphone]

Messed up the picture attachments, these links should take you to the real thing:
https://imageshack.com/i/5e1ek2j
https://imageshack.com/i/mwhijbj

Anyway, you are correct on the grounding. Yes indeed - grounded power cable!

Zero voltage on the pots with the trem off.

With the power tubes back in, voltage on the pots wiggles +.75v to -.75v

There is ~10mv on the pots turned down.

[Jazz P Bass]

I took notice that the Tremelo plate voltage is coming off of the screen tap.

Could it be, that what you are seeing is the Tremolo oscillating the whole power supply?
If the supply was wiggling, it would pass through the .1uf V1 coupling caps.

I would piggy back another cap on the screen cap to see if that helps.
If that does not help, try a cap across the 310V side, after the 10K R.

[hylaphone]

Jazz, glad you mentioned that. Had crossed my mind as well.
However, I clipped in an extra 10uf, then 22uf, before the 10k and then after it. No change to the dancing voltage!

[Jazz P Bass]

Can you see the ripple on the plate side of V1?

If so, that Will get through the coupling cap.

You stated that you changed the V1 coupling cap value.
To what?
And why?

[hylaphone]

Yes the wiggle is there on both V1 plates.
At the moment I have .05uf coupling cap on one channel, and the stock .1uf on the other. The owner of the amp plays humbuckers.