Marshall 2205 Reverb Problem

[Gibsonman63]

I have a 1986 Marshall 2205 Head. This is the split channel 50 watt head with reverb. The reverb crackles unless turned all the way off.

Here's what I have tried:
Cleaned the pot. It feels loose the affects the crackling sound when you move it. I cannot find a spot on the pot, other than all the way off, where the amp does not crackle.

The amp crackles with or without the footswitch. I believe the footswitch has issues as well, but that's another project for now.

Cleaned and checked the RCA cables going to the reverb tank.

Checked the reverb tank. No noticable damage.

Checked the amp with the reverb tank unplugged. Still crackles

Chop sticked around in the amp. Nothing seems to be loose.

Substituted the reverb tube with a new tube.

Any other ideas before I order a new pot?

[eddie25]

Have you re-flowed the solder joints on the pot(s), under the board? It's common that they crack since they are connected straight to the board.

[Gibsonman63]

Good thought and thanks. I haven't pulled the board yet. I decided to go ahead and replace the cathode bypass capacitors while I have the board pulled, since it is a PITA to get to.

[selloutrr]

retension the tube socket while you are it.

[Gibsonman63]

The new pot hasn't arrived yet. I took the old one out, took it apart, cleaned it, put it back together and reinstalled it. No change.

I have noticed that even with all the volume and gain knobs off, I still get crackle when I turn the reverb pot.

[Structo]

Could be a leaky cap letting DC get on the pot.

[Gibsonman63]

Thanks, Structo. I can get my probe to the outer legs and meaure 0VDC with the reverb all the way off and about 20VDC when it is all the way up. There is a ceramic disc capacitor between the anode of the reverb tube and the wiper on the reverb pot. I'll check it this weekend when I have the board back out of the chassis.

[selloutrr]

If the sockets, tubes, tank, caps, and pot are good. resistor or transformer would be the next. test all grounds and meter the legs of the transformer. wire placement?

pics?

[selloutrr]

http://www.premierguitar.com/archive/mo ... /08_03.htm

take a look at the second question -


have you tried contact cleaner on the tube pics and sockets?

[Gibsonman63]

OK, I changed out the cap and the crackling is gone, but no reverb. All I had was a silver mica. I believe I have the correct size, but between the cryptic lables and the microscopic print, I always have trouble with them.

That's all I can do for a couple of days. Probably won't get another chance to work on it until Sunday.

Thanks again guys.

[Gibsonman63]

I replaced the silver mica with a 600V mylar and I am back in business.

Thanks, guys.