DC on volume pot fix?

colinesquire

Member
Messages
1,121
Hello gents. I recently picked up a 74 deluxe reverb and have been doing some upkeep. I've changed the problem caps & resistors but still getting DC cracking on both volumes. There is some stray voltage on the board itself. Does anyone know a quick fix to tame that down? Is there a cap or resistor I can add inline with the volumes?

I'm not ready to fully rebuild on new board but would like to keep quiet on the cheap for now. I'm a layman but can follow instruction well!

Thanks.
 

phsyconoodler

Member
Messages
4,315
A) clean the controls with control cleaner
B) replace the treble capacitors.Usually 250pf/400v-630v

I've changed the problem caps & resistors but still getting DC cracking on both volumes
AND WHICH ONES WOULD THOSE BE EXACTLY?
 

colinesquire

Member
Messages
1,121
Hi-thanks. Did both of those and no dice. I've read that the black fiber boards can attract moisture and eventually become conductive. I believe that is my issue as I'm getting voltage by measuring on the board.
 

Tone Meister

Member
Messages
3,268
If your getting voltage on the board. I'd say you need a new board.
If it's actually the fiber board, you can remove excess moisture with a dehumidifier in a small room. There may be more elaborate suggestions, but I know from personal experience that it will work if it's indeed moisture absorbed by the fiber board.
 

Kyle B

Supporting Member
Messages
5,035
Are you sure there's actually DC on the pots and they're just not getting old and scratchy??? Did you measure this stray DC with a voltmeter???
 
Last edited:

colinesquire

Member
Messages
1,121
Just to follow up that I solved it tonight. Replaced all the 25uf electrolytic caps and one or more must have been leaking. All quiet now!
 

zenas

Member
Messages
8,228
Just to follow up that I solved it tonight. Replaced all the 25uf electrolytic caps and one or more must have been leaking. All quiet now!
In your OP you said you replaced all the problem caps. Those 25uf caps are problem caps so I though they were new.

I replace EVERY electrolytic cap on an amp like yours. It makes alot of problems go away.
Saves alot of tail chasing and gives you more time to well chase tail.
 

colinesquire

Member
Messages
1,121
Thanks-I should have been more specific. I replaced them all but 2 while i was waiting for a few other caps to change in the vicinity. Wouldn't you know it was one of those that was bad. Anyway satisfying to work out myself. I was dreading that the board needed replacement. Still getting some stray dc on board with voltmeter but no more scratching. Is it a terrible idea to duct tape in a few silica packs to keep moisture down? It's pretty humid in my house.
 

ylo

Member
Messages
698
FYI, silica gel IS NOT a dessicant, it acts as a buffer. The stuff absorbs water vapor until it is in equilibrium with the ambient humidity. If the humidity goes down in your guitar case or amp where you have the silica gel, the silica gel will slowly give up water, which helps to slow the change in humidity locally. The same happens in reverse as the humidity increases.

To use silica gel as a (temporary) dessicant, put it in the oven at around 200 degrees F and dry it out. Then remove it and put it in a closed container containing your guitar or guitar amp (for example in a garbage bin sealed with tape or in a sealed garbage bag). This will dry out the guitar or amp -- not necessarily a good thing as you could crack the wood if you go too dry.

Again, silica gel eventually equilibriates with the local humidity. It just slows down changes in humidity. It works like a capacitor for moisture.
 

pdf64

Member
Messages
6,917
I can't think how a bad cathode bypass cap could cause Vdc type crackling on the volume pots, unless it was a near short, which could result in the grid leakage current increasing. But the issue then would have been horrible tone.
The point is that if the root cause was something else then the problem could return.
Pete
 

mmrxcsr01

Member
Messages
140
In my area, Home Depot sells a product called "DampRid", for wet basements, etc. Used one in my car once after leaving a window down overnight in a rainstorm, it's VERY good at pulling moisture out of a room. Try a package lying in your open chassis- is MAY pull enough moisture from your board. Worth a shot for the few bucks...
 

Carol-AnnAmps

Member
Messages
4,854
Hello gents. I recently picked up a 74 deluxe reverb and have been doing some upkeep. I've changed the problem caps & resistors but still getting DC cracking on both volumes. There is some stray voltage on the board itself. Does anyone know a quick fix to tame that down? Is there a cap or resistor I can add inline with the volumes?

I'm not ready to fully rebuild on new board but would like to keep quiet on the cheap for now. I'm a layman but can follow instruction well!

Thanks.
The following can all cause this:

1. Ground loop issues inside the amp. The brass grounding plate is a known problematic area.

2. The fiber boards can soak up moisture over a long period of time. Easy to know, they become all bent and wavy. THe only solution is rather dramatic. You have to strip everything off them and put them in the oven on warm for a few hours. Putting them in a room with a dehumidifier has never worked for me.

3. Bad coupling or tone control cap. The ceramics in those Fenders tend to become leaky before anything else.

4. Bad lead dress can cause this issue especially around the controls.

5. First stage preamp tube issues can also cause it, the Chinese 12AX7A's tubes are especially good at creating this fault.
 




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