DC voltage on pots?

[punkykatt]

 Hello all, got a Victoria clone of a Fender Bandmaster 5E7  on the bench  with noisy scratchy pots, cleaned all the pots  a half dozen times  no improvement. Got the meter out and there is up to several vdc on every pot ( the presents pot does and should have vdc). I disconnected all the coupling caps to the volume pots and tone stack pots one at a time to find the culprit, with all of them disconnected I still get several vdc so I disconnected the standby switch .05 cap to ground thinking that might be leaking vdc with no change. Set my meter for lowest ohm setting and checked the eyelet board  for continuity, did not get a reading. Did any one here ever have this problem? Any  ideas or suggestion are much appreciated.   Thanks you in advance.  Punky

https://el34world.com/charts/Schematics/files/Fender/Fender_bandmaster_5e7_schem.pdf

[66Strat]

You might try lifting the end of the .1 uf cap on the board and testing it for leakage.

[punkykatt]

Thanks for the reply. I already lifted all the coupling caps to the volume pots and the bass and treble pots. Even the cap to ground on the standby switch.
still has dc voltage on the pots.

[66Strat]

Did you lift the cap that I circled?

[punkykatt]

Yes, the only coupling caps that are not lifted are after the PI.

[sluckey]

Pull the rectifier tube. Still got dc on pots?

[punkykatt]

thanks Steve, That will be my 1st task tomorrow morning.

[punkykatt]

With the rectifier tube pulled and amp turned on with the pots turned full CW I get the following:  Bass -728mV, Treb -931mV, Vol brite -337mV, Vol norm -280mV.   if I turn the pots CCW  the voltage goes to 0 vdc, when I turn the amp off reading go to 0 vdc.

[sluckey]

Really? I wonder where that voltage is coming from. Unplug the amp from the wall and check again. What kind of meter do you have?

I know this must sound like I'm funnin' with you and I guess on some level I am, but I'm also serious.

[punkykatt]

My main meter is a BK precision 2703C, I have 2 others for back up, a Southwire 1000305 and a GB GDT-190A.  I tested with all 3 meters, I got different reading but not far off.  With the amp unplugged I got 0 vdc with all 3 meters. 

[sluckey]

With the amp unplugged I got 0 vdc with all 3 meters.

Finally, something that makes sense.

Check grounds on the amp, especially if there is a brass plate under the pots.

[punkykatt]

All the grounds in the amp test good and to double check the brass plate I clipped my clip on test leads from the pot ground lugs to a clean sanded spot on the chassis. No change in voltages on pots.

[sluckey]

Is the board fiberglass or that flimsy waxy stuff that Fender used?

[punkykatt]

the board looks to be the typical fender eyelet board with a backer board but no waxy stuff.

[sluckey]

Just wondering if the board was conductive.

[66Strat]

Maybe an unwanted connection to - bias supply?

[punkykatt]

Im not sure I used the proper method to test the  board for conductivity earlier. I set the meter to lowest ohm setting and put the probes on the board. I did not get a reading. Should I turn on the amp and test for mV`s from board to ground?

[PRR]

> test the  board for conductivity earlier. I set the meter to lowest ohm setting and put the probes on the board

*Highest* Ohms setting. The scale which can tell 1Meg from Infinity.

What is this meter? Battery DMM or a plug-in VTVM?

[jjasilli]

Maybe an unwanted connection to - bias supply?

Good point.  With the rectifier tube pulled the bias supply is the only remaining source of DC in the amp.  AND, it's (-)DC voltage being read. 

It's tempting to suspect the meters (which are identified by punkycatt above); but: the pots are scratchy; and 3 different meters confirm the -DC voltage issue.

[punkykatt]

Thanks PRR,  the meter is DMM. I will do tests on the highest ohm setting.

[punkykatt]

Set my BK 2703C  DMM on 20meg setting and did not get any readings when putting both probes all over the board.  This is good, no moisture conductivity right?

[punkykatt]

Removed eyelet board  part way to inspect the back, looks good. Remove and inspect the insulator board all good.  Reinstall and check for dcv on pots, still there. Unsoldered the wire from right lug of treble pot, measured 22vdc on the wire end, traced wire to the board where the eyelet connects to the 250pf coupling cap that is lifted off the board. With nothing connected to that eyelet but the wire to the treble pot which is now disconnected I read 22vdc. The board has to be conductive. I did some investigating on Victoria amps and read they used the exact same board material in their amps as Fender did in the 50`s that are known for becoming conductive.  Is the board material that Doug sell any better at not becoming conductive?

[sluckey]

Doug's boards are not conductive. Is this your amp? If not I hope your customer has deep  pockets.

[punkykatt]

No, not my amp. A good friend. He has many amps and is in no rush, I will be kind to him, work on it during slow times. Treat others the way you would like to be treated.  Thanks for all the help Steve and all others that chimed in.