Marshall JCM2000 TSL100 issue

[bratfink]

Didn't know whether to put this in here or the tech forum (admin please move it if needs be).

So I have a reoccuring problem with my TSL100. When I switch it from standby to on it immediately goes into a feedback hum.

I have tried all the usual stuff of reseating tubes etc, often just cycling it on and off a few times gets it to work without the feedback hum.

But yesterday it seems to have taken a permanent set to this problem and it won't go away. I had it working and was playing through it, then it clicked and the feedback hum came on strong. I cycled it a few times and got it to go away, then it came back. Now I can't turn it on without having theis feedback hum.

I have taken it to be looked at professionally probably 3 times, and although I have had all the other faults fixed the guy can't seem to replicate this issue on his bench.

I probably would have thrown the amp in the trash by now if the tone wasn't so good!

Any ideas? Anyone else had Marshall reliability issues (I know that will probably open the flood gates)?

[DonP]

Reliability?
My DSL100 has been rock solid since I bought it in 1998. I've replaced one power tube. Asking about reliability issues for a manufacturer that makes a zillion different products doesn't seem to make sence, since you amp is much more complicated than my amp. And the same could be said of guitar makers or anyone else that mass produces.

The issue:
Feedback issues (if you are correct in the diagnosis) are caused by high powered signals "jumping" over into low level inputs and the loop keeps feeding back. It's hard to explain but your large amplified signal is getting fed back to where the signal started (low level).

This is usually caused by wiring or circuit traces. If you had a wire in you amp with a highly amplified signal and it was laid alongside a wire with a low level signal, the high powered signal would jump over.

The Solution:
This might be as simple as moving some wires around inside the amp to keep the signal from jumping. Amp builders call this "lead dress" and some do it great (custom builder that don't mass produce) and some don't because they are making mass produced amps. More severe cases require using shielding on low level wiring. And worse is if circuit board traces were laid out badly.

Taking the case of you amp, Marshall designed it so that it wouldn't have feedback from the start, because if it did, they wouldn't have sold any of these. So I can only guess that something was done to your amp to cause this and you just need to undo whatever was done to put it back to the way it was.

[Rupe]

I was a Marshall dealer when the DSL and TSL amps were released. We saw a crazy number of them (> 25%) come back for service.
That said, what you are describing sounds like a preamp tube...have you tried replacing them? I would try V1 first and move on from there, swapping in a new tube at each position until you solve the issue (or not). Beyond that, get it onto the bench of somebody reputable...Dave Sarge in State College, PA would be my recommendation.