Marshall AS50R Silence

[Don P]

First, to start off with, I did search through the forums and found some very useful information. Thank You! I posted some of this to an old post, and I was urged to just create a new post, so here goes....

I bought this amp used, and when I plugged in a guitar, no noise what-so-ever came out. First, I checked the speakers, and they are fine. The other post pointed toward power supply, but it's fine, putting out 32 VDC at LT+ and LT- .Another recommendation from that post had me connect the effects send jack to another amplifier to verify the preamp was functional, and that worked. I then connected from the line out jack to the other amplifier. Not only did that work, but both the reverb and chorus circuitry worked.

This is where I got stuck From there, it loks like from the terminal block, there is just a double stacked switch (SW2), and op-amp, and master volume control. Op-amps are usually pretty reliable, and even a dirty volume pot should at least let some noise through. There isn't even a hum, nothing at all.

Please find attached schematic, and Thank you very much for reading. Hopefully, Someone can screw my head on straight.

[g1]

pin 1 of CON7 is the line out, between that and the master is only R79.
The master pot may have other problems than just being dirty. Any signs of cracks on it? If you are running signal through, you can measure AC volts at input and wiper lug of master pot.
Beyond that, you can check if all the DC voltages are ok on the output IC (LM3886), or just replace it if that is easier and you want to gamble. It's a very likely suspect.

[Mick Bailey]

A clip lead connected to the input of the LM3886 or (closest convenient point) should hum loudly when touched. I use a small DIY signal generator that has a test probe connected as a signal injector, the ground side having a croc clip. Makes it easier to troubleshoot this type of fault.

[Jazz P Bass]

Schematic.

[Jon Snell]

The wiper of the master volume control will cause the amp to hum if touched with your finger, if set at 50% and you use a piece of wire.
If it doesn't, the amp section is faulty which will mean the LM3886 IC has been killed.
To check the preamps and effects, connect a jack lead into the send socket then plug that into another amplifier. That will check out the preamps etc.

[Mick Bailey]

Thinking about this, if the signal path is shorted to ground you'd need to eliminate this possibility. Although unlikely, no sound from the power amp could be caused b a shorted C215 or even a defective master volume pot. I once encountered a pot where the wiper was shorted to ground. You could lift the master volume end of C204 and touch that with a finger, screwdriver or piece of wire to see if it hums.