Vibrochamp tone stack problem

[LPStandard]

I have a '68 Fender VC. The volume pot is acting sort of erratic, slight increase in volume from 1-4 then drops off suddenly and stays low for the rest of the sweep with a very minor increase when pegged at 10. With the volume at 4 (loudest I can get it) the treble and bass pots decrease the volume as they are turned up. No noticeable change in tone through the sweep, just decreases volume.

A little background on the amp: The circuit board has been replaced. Original circuit design with a few minor changes to for reliability, bias, and safety. This was all done about 2 years ago with light to medium use since then. I also just replaced the cap can. Tubes are relatively new, but I have no spares on hand to try swapping. It seems like more of a circuit problem than a tube issue to me though.

Any help is appreciated.

I have a DMM and soldering tools. I have some experience with the amp, so I will know how to fix the issue once I know what it is, but I'm not so good at the diagnosis part. I removed/checked all of the pots that seem affected and they tested alright.

[cbarrow7625]

Hmmm....that sort of volume control behavior make me think that you have DC leakage coming through the capacitors. As you turn up the volume and/or bass pot more DC reaches the following grid putting it into cutoff. The signal cannot overcome the cutoff bias and so the volume does not go up any more.

Check for DC on the grid of the stage following the tone & volume controls. If there is any DC, you have a leaky cap or caps in there somewhere. Check them one at a time (unsolder the others from the circuit if necessary) until you find the culprit.

[LPStandard]

Hmmm....that sort of volume control behavior make me think that you have DC leakage coming through the capacitors. As you turn up the volume and/or bass pot more DC reaches the following grid putting it into cutoff. The signal cannot overcome the cutoff bias and so the volume does not go up any more.

Check for DC on the grid of the stage following the tone & volume controls. If there is any DC, you have a leaky cap or caps in there somewhere. Check them one at a time (unsolder the others from the circuit if necessary) until you find the culprit.

There's about .5 mV DC on the grid with the volume at 1 and about 5.4V DC at 10. This seems to be the problem.

Here's a link to a schem, which cap would be the problem/how do I test them. My DMM doesn't have capacitance.

http://ampwares.com/schematics/champ...a764_schem.pdf

I'm assuming it would be either the Orange drops or the silver mica on the right side of the board??

[cbarrow7625]

You don't need a capacitance meter. Just unsolder the tone stack caps one at a time (on the pot end) and check them to see which one(s) have DC coming through. Caps should block DC 100%. If you measure any DC, you have found the leaky cap. Just replace it.

If you want to spend less time on the problem, just replace them all. That should do the trick. Let us know what you find.

[LPStandard]

I figured that out. It turned out to be the 250pf silver mica.

[cbarrow7625]

Weird. I wonder how that happened. Glad you found it.

[LPStandard]

I was wondering that too, what with the board being so new and all. Maybe just a bad cap or some bad luck.

[Bruce / Mission Amps]

Other then tweed amps, I've never seen a stock Fender amp with silver mica treble cap, and, when I replace the cheapy ceramic with a SM cap I'd never install it under the plate load resistor where there is a possibility of the capacitor lead wearing right into the carbon camp resistor creating DC short to the pots!! YIKES>
One more thing, over the years I've seen more shorted/leaky modern silver mica caps (like yours in the picture) then any other new coupling cap. I think they seem to have a higher failure rate.

[LPStandard]

Yeah, the board was completely replaced. When I recieved the amp, the original board was pretty hacked (the amp was not in working condition).

What sort of cap would you suggest then? The closest thing I could find was a 270pf polystyrene. I really dont want to put a ceramic in there.

As for the leads wearing into the resistor, its sort of tough to see in the pic but the turrets are pretty tall. I'm pretty sure that there is enough airspace between the two that it will be ok.

[Dave Curtis, dB AudioTech]

Yeah, the board was completely replaced. When I recieved the amp, the original board was pretty hacked (the amp was not in working condition).

What sort of cap would you suggest then? The closest thing I could find was a 270pf polystyrene. I really dont want to put a ceramic in there.

As for the leads wearing into the resistor, its sort of tough to see in the pic but the turrets are pretty tall. I'm pretty sure that there is enough airspace between the two that it will be ok.

SM caps don't like soldering heat at all, so heat-sink the lead you're soldering. That may account for the failures that Bruce mentioned.

[cbarrow7625]

I've tried polystyrene treble caps in Fenders before. I don't like them at all. Way too "boring sounding". No sparkle, edge or anything.

I think silver mica is the way to go. Just do what Dave suggested & put a heat sink on the lead (like some hemostats or hold the lead with some needle nose pliers) between the cap body & the end you are soldering.

[LPStandard]

I've tried polystyrene treble caps in Fenders before. I don't like them at all. Way too "boring sounding". No sparkle, edge or anything.

I think silver mica is the way to go. Just do what Dave suggested & put a heat sink on the lead (like some hemostats or hold the lead with some needle nose pliers) between the cap body & the end you are soldering.

Will do.