Hello diyAudio! This is my first post but I've lurked for quite a long time even before registering. Boatloads of info here
!
So my friend gave me a Marshall MG100RCD to fix for him (I know I know... flame away). I'm able to get sound through it from a guitar but if the gain or channel volume is turned up past a certain point it starts to oscillate (definitely not feedback) even with the master volume turned almost all the way down. I can crank the master volume to 10 without causing oscillation so it appears to be something in one of the preamp sections. The same thing happens with nothing plugged in to the amp. The pitch of the oscillation varies when tone controls are adjusted and also changes depending on whether or not a guitar is plugged in. This happens on both channels. I went through the amp and resoldered all of the pot connections/power resistors as well as any other questionable connections I could find. Reverb and FX Loop knobs have no effect. All ICs are getting power (-16.8 and +17.4).
This is my first time troubleshooting a real issue that didn't end up being something simple like a bad ground connection or broken jack. Does anyone have any suggestion as far as where to start? FWIW I have a TEK 2465 scope, a decent DMM and another working MG100RCD as a reference. I also have a handful of spare 4558D OpAmps. I could probably suss it out by blind process of elimination but I'd prefer to learn how to troubleshoot it because having the exact same model amp laying around won't always be a luxury.
Much thanks for any pointers!
!So my friend gave me a Marshall MG100RCD to fix for him (I know I know... flame away). I'm able to get sound through it from a guitar but if the gain or channel volume is turned up past a certain point it starts to oscillate (definitely not feedback) even with the master volume turned almost all the way down. I can crank the master volume to 10 without causing oscillation so it appears to be something in one of the preamp sections. The same thing happens with nothing plugged in to the amp. The pitch of the oscillation varies when tone controls are adjusted and also changes depending on whether or not a guitar is plugged in. This happens on both channels. I went through the amp and resoldered all of the pot connections/power resistors as well as any other questionable connections I could find. Reverb and FX Loop knobs have no effect. All ICs are getting power (-16.8 and +17.4).
This is my first time troubleshooting a real issue that didn't end up being something simple like a bad ground connection or broken jack. Does anyone have any suggestion as far as where to start? FWIW I have a TEK 2465 scope, a decent DMM and another working MG100RCD as a reference. I also have a handful of spare 4558D OpAmps. I could probably suss it out by blind process of elimination but I'd prefer to learn how to troubleshoot it because having the exact same model amp laying around won't always be a luxury.
Much thanks for any pointers!
First step, try to find a schematic for your amp, post a link here so everybody is on the same page 
Next, use the scope to look through the signal path (generally pins 1 and 7 of the 4558 opamps) to find where the oscillation is and is not. Determine what controls affect the signal at various points. This will give clues to where the actual fault is. Does the Master Volume have to be up part way or will it oscillate when the Master is all the way off?
It's good that all the ICs are getting power, but check those voltages with the scope. Sometimes a fault in the power supply will allow unwanted feedback to travel along the power supply lines. You can also check that there is no fault in the ground. Use the Ohm function of the DMM to make sure everything that should be grounded is grounded.
Report back what you find.
Next, use the scope to look through the signal path (generally pins 1 and 7 of the 4558 opamps) to find where the oscillation is and is not. Determine what controls affect the signal at various points. This will give clues to where the actual fault is. Does the Master Volume have to be up part way or will it oscillate when the Master is all the way off?
It's good that all the ICs are getting power, but check those voltages with the scope. Sometimes a fault in the power supply will allow unwanted feedback to travel along the power supply lines. You can also check that there is no fault in the ground. Use the Ohm function of the DMM to make sure everything that should be grounded is grounded.
Report back what you find.
DOH! Forgot to attach a schematic in the first post. I'll check out what you've suggested tonight and post what I find. It does oscillate even with the master all the way down though and when the channel gain and volume low enough I can turn the master all the way up without oscillation. Thanks so much! 
Ok so this is what I have so far...
Power to all ICs is +17.1V / -16.4V and clean with the exception of an 8.6mV spikes at 120hz (noise from the bridge rectifier?)
Those same spikes also appear on IC1 pin 1 when the gain is low enough not to oscillate.
IC9B (pin 7) is the first to start to oscillate cleanly going from looking like low level noise to nearly rail to rail instantly, nearly a perfect sine wave. Putting my hand or a probe anywhere in that airspace (but not physically touching anything) will reduce or stop the oscillation. Turning up the gain slightly more causes oscillation to appear across the entire signal path at various levels.
Mid and Treble controls vary the pitch quite a bit. Anywhere from 2khz to 14khz.
Shorting the signal wire from the reverb tank to ground kills all oscillation but only if the Normal Channel is active.
I'll still need to go over all the ground points but it's getting a tad late for that lol. It's almost 3am.
Power to all ICs is +17.1V / -16.4V and clean with the exception of an 8.6mV spikes at 120hz (noise from the bridge rectifier?)
Those same spikes also appear on IC1 pin 1 when the gain is low enough not to oscillate.
IC9B (pin 7) is the first to start to oscillate cleanly going from looking like low level noise to nearly rail to rail instantly, nearly a perfect sine wave. Putting my hand or a probe anywhere in that airspace (but not physically touching anything) will reduce or stop the oscillation. Turning up the gain slightly more causes oscillation to appear across the entire signal path at various levels.
Mid and Treble controls vary the pitch quite a bit. Anywhere from 2khz to 14khz.
Shorting the signal wire from the reverb tank to ground kills all oscillation but only if the Normal Channel is active.
I'll still need to go over all the ground points but it's getting a tad late for that lol. It's almost 3am.
Try This-
Try following the signal path and removing one leg of each coupling component to isolate each gain stage.
Inject from the PA backwards and as each stage passes the test add it back into the test path.
I think I would remove the reverb path until I had checked everything else.
Just a suggestion: Check the feedback path for a problem also, particularly if the problem stays with the PA section.
Ccat.
Try following the signal path and removing one leg of each coupling component to isolate each gain stage.
Inject from the PA backwards and as each stage passes the test add it back into the test path.
I think I would remove the reverb path until I had checked everything else.
Just a suggestion: Check the feedback path for a problem also, particularly if the problem stays with the PA section.
Ccat.
I had a similar problem years ago.
If my memory is wel (and some dought it...) it is the grounding ring that sits in front of the input jack.
Make sure it's there and it makes a good contact with the frontpanel.
and that's it...
or isn't it?
Oh, my memory...
No, serious, have a look at that ring...
If my memory is wel (and some dought it...) it is the grounding ring that sits in front of the input jack.
Make sure it's there and it makes a good contact with the frontpanel.
and that's it...
or isn't it?
Oh, my memory...
No, serious, have a look at that ring...
The input jack on these amps is plastic and insulated from the chassis. I finally had to dig my amp (of the exact same model) out of the garage to do some comparisons. What I found was that even my known good amp oscillates with no input and it's WAY worse if the reverb tank is not plugged in. All of the symptoms were completely identical with the exception of the point that the "known good" amp starts to oscillate (I have to turn the gain up quite a bit more). I'm going to replace the jack on my friends amp and call it a day. Tooling around with an amp design this prone to issues doesn't seem like a smart place to start learning about troubleshooting an amp.
I'll be damned... Replaced the jack, used minimal length of hookup wire (previous jack had way too much) and the thing works like a champ! I can't even get it to oscillate with everything turned up to 11! Not sure if it was a bad jack, capacitance in the wire used to hook it up or both but I'll take it! Thanks so much everyone!
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