Marshall 5275 Reverb Problem

[Gerhard Grass]

Hi to all,

I have a Marshall 5275 on the workbench to fix some minor problems.
The amp is in good condition but rather dusty inside.
Have to replace the filtercaps, the footswitch-jack and
a few crackling pots will be replaced.

So far fine with the amp.
But:
If I turn up the reverb, the damn thing starts to hum. Turning it up further, brings up
a wonderfull oscillation...

Traced the signal to the reverb tank and found it good. Disconnecting the tank
from the driver changes nothing. Disconnecting the output of the tank makes the
thing silent.

I try to avoid buying a new tank, if there might be another solution for the problem.

So if anybody has a hint or suggestion for me please let me know.

Thx in advance

Gerhard

[52 Bill]

Is the problem an acoustic feedback or and electronic oscillation?

Try covering the tank with a cloth any difference? Clean and tighten the shield connections on the RCA cables going to the tank.

[Gerhard Grass]

It is definitly an electronic oscillation. Have the tank carefully placed on the workbench.
Did some cleaning on the cables and the RCA-jacks. They looked a bit corroded.

[lowell]

Check continuity of reverb ground to amp chassis circuit ground on BOTH tank jacks. Also, was this hum there before you removed the reverb tank? Reverb tanks are very susceptible to emf from lights and transformers. See if hum changes depending on tank location and orientation. Also Ive found that my soldering iron or even lights in my shop sometimes affect certain amps and noise. Just to eliminate variables see if turning them off reduces the hum.