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 felt 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. I removed the pot, cleaned it and put it back together and reinstalled. It really didn't look bad inside.

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 connectors going to the reverb tank.

Checked the reverb tank. No noticable damage. Measured 34 ohms across the input transformer and 202 ohms across the output transformer.

Checked the amp with the reverb tank unplugged. Still crackles

Chop sticked around in the amp. Nothing seems to be loose or creates the crackle sound.

Substituted the reverb tube with a new tube.

I ordered a new reverb pot, but I am not convinced that it is the problem. Any other suggestions?

[Gibsonman63]

I changed out the coupling cap from the reverb driver. The cracking is gone, but no reverb. All I had was a silver mica in that size. Between the cryptic labeling and the microscopic print, I always have trouble with capacitor values, so I am not 100% sure I have the correct value in there.

[txstrat]

I think the value of the cap doesn't matter for troubleshooting. After you changed the cap the crackling was gone. That implies the old cap was bad unless the crackling source lies before the cap and the new one is bad as well (I hope this makes sense).
With the new cap you have no sound. I'd try to clip another cap parallel to the new one and see if it changes something.

[g1]

Which cap do you mean? The reverb driver has a cap from plate to cathode, this is the one I would suspect, sometimes they short right out. The reverb recovery has a coupling cap which I think is the one you mean.
Did the reverb work after you put the pot back in but before changing the cap?
Does it have the transistor switching for the reverb pot?
Is this the right schematic?
http://www.schematicheaven.com/marsh...h_50w_2205.pdf

[Gibsonman63]

That is the correct schematic. I replaced C29. I picked up some ceramic disk capacitors at lunch today. I should have some time tomorrow afternoon for more troubleshooting.

I believe that I had reverb after I cleaned the pot. It was very weak in comparison to the crackling noise. These amps don't have stellar sounding reverbs, but I would like to get everything functioning correctly.

The switch was working before I changed the cap because I could turn the crackle on and off.

[Gibsonman63]

The reverb has transistor switching. TR2 shorts the reverb signal to ground.

[txstrat]

The schematic says the reverb consists of two triodes from different tubes (V3B and V4B). Did you replace both tubes? The crackling might be the other tube.

[Gibsonman63]

The cracking went away when I changed C29... but so did the reverb. The tubes are fairly new, but I have plenty of others I can stick in there to test with. I'll give that a try tomorrow before I start disassembly again.

[rockin roy]

Just shooting from the hip, I think the silver mica cap value may be to small. They are generaly just pico farids. I can't help but wonder if this cap may be the same as on off swith.

[g1]

Ooops! That was the wrong schem. You said you have transistor switching rather than the IC so it should be this one:
http://www.schematicheaven.com/marsh...v_50w_4210.pdf
The values of some caps are different, should try again to read the value on the one you replaced.
The cap I was talking about is called C47 on the IC version schem. They can get crackly, sometimes short out and fry the reverb drive xfrmr.

[Gibsonman63]

I misunderstood. I have the IC version, but there is a transistor (TR2) that looks like it bypasses the reverb pot to me. Anyway, I stuck a 600V mylar cap in there and all is good.

Thanks for the suggestions.

Note to self... buy an assortment of capacitors.