SS Marshall 5212 Problems

[Just Mike]

The complaint was that it shut down after a few minutes of playing. I plugged it in and tested it (not loud) and it seemed to work OK. It didn't shut down for me. After I spoke to the owner, he said it happens at Gig volume. So I opened it up and reflowed all the solder connections. It also turns out that the owner only used the clean channel. When I tested the Boost channel, it passes signal but the gain knob does nothing. It stays clean. I know it's all op amp distortion so my first guess is a bad op amp (IC2-TL072). I don't have a scope, so I can't trace the signal.
So I've got 2 problems;
1.- amp reportedly shuts down when played at high volume. Can I run a SS amp into an attenuator safely? If so, I can replicate the problem. Can the shutting down be due to a bad thermistor?
2.- No boost in the boost channel. Bad Op amp? How can I troubleshoot without a scope?

Thanks.

[loudthud]

I don't see any kind of thermal cutout on the schematic, so... first suspect is the effects loop switching jack problem. Amp should be OK into an attenuator, but it could be that it's the sound pressure level that makes it fail, not the power. For the channel switching I'd first look at the logic that turns on those transistors. Just check for .6V on the base. Looks like TR4 (hard to read schematic, next to the one that drives LED) is the transistor that decides which channel is active.

[Just Mike]

Thanks Loudthud. As far as the channel switching, it switches channels ok, all of the boost channel knobs work, it's just that the boost channel has no distortion, so I don't think it's the switching transistors?

[Jon Snell]

TL072.4558 etc Op amps either work or they don't, they don't go low gain. I would look for a low value electrolytic as they are used in the gain stages and coupling.
The output stage uses Darlington Pairs and they are quite reliable.
There is no thermal measurement in these so reflowing the solder has probably fixed that. The pots can go open circuit with corrosion, a good clean with switch cleaner will sort those out.
Use a resistive load and a 2.2W car dashboard lamp in series with the cable to your monitor speaker. That will allow you to wind it up and keep the volume to a couple of watts in the monitor.
Like this;

[Jazz P Bass]

Here is a later version schematic.
Much easier to read!
Although the component designations are different.
The only thing different that leaps out is the positioning of the .22 capacitor (unlabeled no less) that is coming off of the collector of TR4.
The newer version has it switched by a transistor.

[drewl]

Those pots tend to go bad after many years.
try cleaning it and measure it's resistance and that it sweeps correctly.

[Just Mike]

Hey Jazz P Bass, I don't see the schematic.

[vintagekiki]

Hey Jazz P Bass, I don't see the schematic.

https://music-electronics-forum.com/filedata/fetch?id=924701