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Marshall 100v 8100U Guitar Amp Hums
Marshall 100v 8100U Guitar Amp Hums
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Old 29th January 2007, 02:20 PM   #1
DreadPirate is offline DreadPirate  United States
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Location: Athens, GA
Default Marshall 100v 8100U Guitar Amp Hums

I'm guessing somewhere out there is a forum for guitarheads that like to work on gear, if anyone knows of one, please let me know.

Amp is 1995 vintage. This guy came with broken volume and contour pots, pretty beat up. I replaced with identical Marshall parts and I think I did this right, at least the hi and lo values match up with the others. The only other thing I did was add solder to C62, it looked like there wasn't any on one of its legs.

I don't play guitar and do not have the proper connectors to test. I used walkman headphones in both the line out and rear output jacks all I get is a hum/buzz. I plugged a microphone into the input jack, no result on the outputs, same noise. I adjust all controls and push buttons, no effect, just hum/buzz. Am I just testing this wrong?

What is the function of the send/return jacks?

I did take a dc measurement at the output jacks, got steady 18mv.

The schematics are here, the pdf files are easily expanded and they look pretty good actually:


Three General Questions:

1. The tube is there, it is original and lights up. Is there any way by looking at it to determine if it is operating properly? I do not have a spare tube.

2. I hear talk of filter caps, are these C61 and C62 and usually close to the transformer? These checked out with the esr meter, they are good. Note comment about added solder.

3. I also hear there are "coupling caps", how do you find these on the schematic? Do they come in pairs like filter caps?
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