Marshall 8100 with no sound

[Jimmyblacksmith]

Hi,
I have a Marshall 8100 with no sound. I do get sound if I plug into the effects return. The valve is lit (although I think that’s only utilised by the dirty channel). I’ve done an oscilloscope signal trace with a signal generator phone app and found nothing of interest. I’ve tested pretty well every component between the input jack and the effects return with the exception of the ICs. Some of the ICs are marked A5201A, which I’m assuming is a 5201 opamp, which seem quite difficult to find information on.
I’m wondering if anyone can tell me how to test the ICs? Or where I can find info on them? Or what else I could do to troubleshoot?

[Enzo]

You have a working power amp and a preamp that does not pass signal.

I’ve done an oscilloscope signal trace with a signal generator phone app and found nothing of interest.

What does that mean? If you applied a signal to the input, and traced it through the preamp, you either found signal or you did not. It made it to some point in the signal path and then no farther. Or it made it all the way through. Any of those bits of information are of interest.

How do I test an op amp? Mostly by looking at its output pin and seeing a signal there or not.

The M5201 is a dual op amp with switching. it is used for signal routing. Data sheet:
https://www.google.com/url?sa=t&rct=...GAak5ouLCjO8Uv

[Chuck H]

First thing for me would be to identify the fault in the AC signal chain and then check for DC voltage from the supply to the amplifier (opamp, probably) immediately prior. No DC=find out why and make it so there IS DC. If there IS DC then replace the opamp. Of course there is the possibility that some signal switching circuit is failing. This should also be identifiable by AC signal tracing. And again you would check for DC operating voltage for any switching logic first.

[Jazz P Bass]

Here is the schematic if needed.

Marshall valvestate 8080_8100.zip

[g1]

If you have a bad M5201 (obsolete) you can replace with NJM2120D or NJM2121D.

[Enzo]

AES seems to have them still.

https://www.tubesandmore.com/product...-circuit-m5201

[J M Fahey]

I’ve done an oscilloscope signal trace with a signal generator phone app and found nothing of interest.

What does that mkean?
You inject signal at the input jack and it goes through a chain of stages until it reaches the power amp (which luckily works).

Signal must reach each stage input and exit through its output, like Enzo said.

Since the preamp as a whole does not work, then at some point either signal does not reach its input (wiring/track/solder/connector/switch/pot problem) or it does not reappear at the other end (IC or tube problem).

You must find where signal gets lost, randomly pulling parts for measurement is a wate of time and eventually you´ll damage the irreplaceable PCB.
You can buy any part at one shop or another ... except the PCB so pamper it.

Or what else I could do to troubleshoot?

See above.