Marshall MG250-DFX

[trem]

A friend of mine got one of these for free. Cosmetically it is in very good shape. However, with no guitar plugged in, gain, volume, master, on Zero, the amp oscillates/feeds back real bad. For comparison, it sounds similar to a Fender Twin if you have the OT primaries on the wrong tubes and the amp squeals/howls real bad. And it happens that fast, just as soon as there is current flowing, maybe 2 seconds after the power switch is flipped the amp goes off in a horrible way.
I have never tried to work on a solid state amp before, and I told him I probably could not fix it. But I am willing to give it a shot, but I have no idea where to begin. I pulled the chassis and looked for a burned part or loose wire, but that is about it. Do you guys have any suggestions.?
Thanks

[Ronsonic]

I had one of that amp's cousins a 100HDFX on my bench. The problem turned out to be a film cap that coupled signal from the preamp to the power amp had one lead cut too short and it did not reach through the circuit board to the pretty solder blob under the board. Yes, it was a serious bitch to find.

[trem]

I had one of that amp's cousins a 100HDFX on my bench. The problem turned out to be a film cap that coupled signal from the preamp to the power amp had one lead cut too short and it did not reach through the circuit board to the pretty solder blob under the board. Yes, it was a serious bitch to find.

I am not helped by the fact that I have a scope, but no dummy load, and my signal generator broke into 100 pieces. I am just starting to be able to trouble shoot most of the more common problems with tube guitar amps. Something like this.....I just do not even know where to begin. The oscillation is so loud, and so annoying, that I have not even tried chop-sticking the board/components while the amp is running to see if I can touch something that will improve (or worsen) the situation. Any suggestions on where/how to start would be greatly appreciated. Maybe an amp like this is just beyond my skill level.....?
Thank You

[Jazz P Bass]

Troubleshooting 101
Test the power supply.
Divide the circuit down to isolate the problem.
This amp does not require a load when repairing it.
Disconnect the speaker.
With your meter on Vac you can see the tone on the output jacks.
There are two separate output amplifiers. (stereo?)
See if one or the other (or both) is acting up.
This will help narrow down the problem.
I would check the + & - power rails for proper voltage & lack of Vac ripple.
Same goes for the opamp power supplies.
By checking the power supply you are narrowing down the problem further.
Monitor the Effects Send jack. Is the noise there.
If not, then you have isolated the problem to the power amp.
If it is on the jack then the problem is most likely in the preamp.

[Enzo]

Amen, disconnect the speaker and scope the output instead. You'll see if the thing is howling or not.

You don't need a signal generator to make it stop howling. You only need a generator when you want to see if it amplifies OK. And I rarely fire up mine anyway, I prefer music as a test signal most of the time. CD player, FM stereo receiver. I used to have a cheap little Yamaha or Casio keyboard, some of the patches were pretty clean, and it had an output jack. It worked fine as a test signal too. Push down a key and wedge some cardboard between to hold it there, voila - signal generator.

Are both speakers involved or is it coming only from one of them? I think this has two power amps, one per speaker.

[trem]

Amen, disconnect the speaker and scope the output instead. You'll see if the thing is howling or not.

You don't need a signal generator to make it stop howling. You only need a generator when you want to see if it amplifies OK. And I rarely fire up mine anyway, I prefer music as a test signal most of the time. CD player, FM stereo receiver. I used to have a cheap little Yamaha or Casio keyboard, some of the patches were pretty clean, and it had an output jack. It worked fine as a test signal too. Push down a key and wedge some cardboard between to hold it there, voila - signal generator.

Are both speakers involved or is it coming only from one of them? I think this has two power amps, one per speaker.

Well, I sure am not use to looking at a solid state schem.....But yeah, I see 2 identical power amp circuits, and ribbon type connections. I do not know for sure if both speakers are involved. I will have to just give it a listen. I forgot this is a stereo (or is it 2 x mono) amp.
You guys have given me plenty to get started with.....
Thank You

[Ronsonic]

A solid state amp can scream it's lungs out into no load without harm. So disconnect the speaks and use your scope to witness the output. Simply start at the output jack and keep probing up stream until you get to something that isn't screaming. Break the amp down into blocks. Does the oscillation appear at the FX out jack? Is it affected by any controls? Break the amp into blocks and start to narrow it down.

[trem]

A solid state amp can scream it's lungs out into no load without harm. So disconnect the speaks and use your scope to witness the output. Simply start at the output jack and keep probing up stream until you get to something that isn't screaming. Break the amp down into blocks. Does the oscillation appear at the FX out jack? Is it affected by any controls? Break the amp into blocks and start to narrow it down.

Once again, I should have typed a "qualifier". I bought a scope about a year ago, and the only thing I have looked at with it is the sign wave of the 120AC that comes out of the wall. My audio gen suffered a catastrophic accident eraly on, so I just never got around to using this stuff. So if I can ask a few quick scope questions....?
Can I ground the scope to the sleeve of the speaker jack, and then probe from the tip of the jack, back through the schematic like you said.? Or should I ground the scope to the mains ground of the amp.?
Thanks

[km6xz]

Yes, you can connect the scope as you described. Don't connect speakers again until you believe the amp is stable.
Do any controls effect the noise?
Try plugging in headphones in the Headphone jack. Clean?, if yes than the problem is in the power amps section and if one channel of the PA is acting up, the scope will show the good channel with a clean signal of whatever input you used as a test signal. If both channels are dirty, check the power supply and and ground connections between the power amp module, its inputs and the output

[Ronsonic]

Like he said.

You don't need a sig gen. Use a CD player or FM tuner or whatever the heck produces a signal you can count on. Except for this job you don't need one since your unit is generating signal on its own and you are trying to track down where it is starting.

Break the amp down into blocks and work them one at a time.

[trem]

OK, thanks for all the help and suggestions. I have school today, and will not be able to get back to this 'till tomorrow. Check back when I make some progress......
Thanks Again