Oscillating Marshall 5275 SolidState

[Agent_Smith]

Hi all!
I'm new here. Hope to have a good time here ;-) and hope you guys can help fixing my amp :-)

So, here I have this 80s Marshall 5275. 75w Solidstate rocker. Schematic is found here (although I discovered some small differences in my circuit, but I think that happend sometimes with marshall at that time): 5275

Here are the symtomes:
*There's pretty much noise on both channels. The noise is about as loud as the guitar signal
*the amp oscillates like crazy. level and frequency of oscillating reacts to gain and volume (boost channel) and to volume (clean channel). Also little reaction on EQ (boost).

Through all the noise and oscillating the guitar signal comes pretty normally. To my ears the way it should sound. So, signal path work all the way through.
Reverb works fine; Channel switching works.

When I plug the guitar into the fx_return the guitar sound loud and clear. Oscillating sounds through a bit though. But power amp section seems fine I guess.

What have I done so far:
*c48 and c49 were fried so I replaced them along with c38 - c41.
*The I worked my way back to the beginning of the circuit by pulling (and replacing the same) coupling caps. Here's what happend:
c36 pulled --> no noise; geen hum; oscillation sound weak on the background
c35 pulled --> same
c16 pulled --> same
c11 pulled --> same

Then (with complete circuit ofcourse) I pulled c5. Clean channel was fine. Much less noise and no oscillation. Boost channel didn't work anymore ofcourse.

So, with my limited knowledge I tend to think maybe both problems (noise and oscillation) are located somewhere between c4 and c5, since IC1a is shared for both channels.

Any suggestions people? IC1b is shot? Diodes? A cap or something? You think I'm in the right direction here?

Hope you guys can give me some help!

Thanks in advance!

[soundguruman]

If the capacitors really were fried, then there are probably other things wrong, and in oscillation-ville, you might have broken circuit connections, dirty controls...
It sounds like you should find somebody with a scope and signal generator, clean the controls and jacks, but it's hard to say until you can actually trace through it. Dirt can be a big factor in an old amplifier.
"no noise; geen hum; oscillation sound weak"
yup, that geen hum...it's hard to say

[Jazz P Bass]

I agree on cleaning the controls & jacks.
Just got off of a Marshall 5100 that was oscillating like crazy.
I used Deoxit D5 on all off the pots, cleaned all of the jacks with Deoxit GN5 & lastly replaced the intermittant master pot.

[Agent_Smith]

yup, that geen hum...it's hard to say

Sorry about the typo ;-) I'm from europe and english is not my native language, but I'm trying to write in english as good as I can. :-)

Well, about the electrolytic caps: I said they were fried. Maybe not the correct word. Only the two big one were a bit leaky. So I decides to replace all caps in the powersupply section to start with. There was a little dirt on the board's component side. A cleaned the board on both sides and it looks fine.

I do have a scope and signal generator, but I'm not quite confident on how to trace the signal. I'll give it a try later this day. I guess I'll have to trace the inputs and outputs of the opamps and see where the noise and oscillation comes in? Sounds pretty obvious as I think about it ;-)

I already cleaned the pots and jacks. Sorry, I forgot to mention. But maybe one or more needs to get replaced. I'll have a look.

Thanks so far and I report later about the scope tracing.

[Agent_Smith]

Okay, I fired up the scope and amp today and tried to follow the signal and the oscillation.
But it is pretty confusing to me...

With the signal generator plugged in but switched off the amp oscillates like crazy. On the output of the first opamp (IC1a) I don't 'see' the oscillating on the scope. On the output of IC1b (and ofcourse on the diodes) the oscillation is very strong visible. So I guess it must be, like I thought before, somewhere around IC1b.

But, when I turn on the signal generator (1000 Hz) it completely 'overrules' the oscillation signal. It's stil there but hardly noticable.

The even more confusing part is: the oscillating also reacts on the footswich. And on the pull switch in VR1 (pulling the switch switches in the master EQ on the boost channel)
**sidenote: these amps, like JCM800's, can be switched by just plugging in a dummie connector. First step is the reverb switching; second the channels.**

I'll try to describe it clearly. Sorry if it comes out confusing...:-(
On the boost channel: pull switch not activated--> crazy oscillation
On the boost channel: pull switch activated--> pretty much no oscillation

On the clean channel: always oscillation regardless of the pull switch

Sorry, I think it hard for you guys to give any sensible clue from this story. But I hope someone'll have a idea or suggestion how to continue on the amp.

Thanks in advance everyone!

[Agent_Smith]

Thanks everyone for the input! The amp is alive and kicking!
I replaced IC1 (LM1458) and that solved the problem.
Happy camper :-)