Metropoulos Forum

I own a Marshall 2003 from 1977 which I use to play with a Les Paul. For more power stage overdrive I also use the Marshall Power Brake. I don't use any distortion nor overdrive pedals. I just like that real Marshall overdrive sound! Since a few weeks there is a strange oscillation to hear, especially if I play the strings hard. Whit a frequenzy generator and a cathode oscilloscope i found out (1200 sinus wave cycle / 150 millivolts) that i get this problem, when I set preamp more then 6, presence and middle more then 7, the master more then 3. Power Brake is around middle and Min. See also the picture from oscilloscope in the attachment. The red marked area is the oscillation.

What could the source of this behavior be and how can I fix this?

Great troubleshooting! You have identified the occilation. Now comes the hard part- getting rid of it. Try these things, in roughly this order:

tubes

bias

grounds

leaky signal caps

wiring layout

Best of luck. Let us know what you find.

George

Sorry for my long absence. But now it seems I've got the reason! Thanks to George for his suggestions!

What I did was:
- I first changed the tubes (OK).
- Then I checked the bias (OK)
- Opened the preamp-section from the power stage. Whit frequenzy generator and cathode oscilloscope I checked the preamp separately (OK) and then the power stage seperately (OK) for oscillation
- I read the schema diagram and compared with the amps wiring. So I saw the presence 0.68u cap modification, original is 0.1u (EXCITED)
- Then I put in the originally 0.1u cap => The oscillation was almost eliminated (INTERESTING)
- At least I read in this forum (jtm45 squeal) that I have to tuck away the purple wire, which is wired to the presence pot, from the other tone control wires (ELECTROMAGNETIC COMPATIBILITY).
- So I put in the 0.68u cap again and tuck away the purple wire (THIS IS THE REASON). Now I can keep the oscillation under control.

I hope it helps for someone else with same troubles.
Freebird