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What happens to the volume output if the “Volume Knob” loses earth or is not connected at all? Will “Any” volume still be present or will it not work at all? For that matter, what would happen to volume if the unit loses or struggles for good earth anywhere within the unit?

I have an amp head that fades with its volume during use. It does it at random with variable intervals between failings anywhere from 2 minutes to 12 months. If I put it in the car and take it for a drive, it will now work again properly when I get it home. (Hence music shops can never find a problem with it because it’s had a car ride to get to the music shop and works again when tested).

If I take the head out of the box and plug into it lying raw on the floor and it fades the volume it will return if I touch it somewhere but the area to touch is also random. Eg: One day it faded and I touched the power plug in the back and it sprang to life as soon as I touched it but volume faded again a few minutes later. Trying the same trick on the same spot didn’t work this time, instead I touched the input jack from the foot switch and it sprang back to life again the instant it was touched, but again only for a short time. 3rd time touching the power plug didn’t fix it nor did the foot switch jack. Instead this time it sprang back to life the instant I touched one of the circuit boards. (Not the terminals, just the very tip corner of the board itself). It’s like it’s using me as a substitute earth! Could this be correct?

And bumping along in the car is also tickling something back to life, hence the endless music shops keep giving me a bill and telling me there’s nothing wrong with it. I get it home and it works fine for a while and then the game is back on once more.

It really is frustrating and getting expensive for no result. It has two volume knobs and I think one or both are losing earth but I just can’t confirm that.

When the volume dies, it’s always proportional to about 80-90% loss of whatever its current position setting is, but never loses volume completely. That’s what’s leading me to believe that volume will work with power alone and no or limited earth but it would be very, very weak?

Any clues would be helpful here. The next trip to the music shop I’ll be getting both the volume switches changed out even if they tell me there’s nothing wrong with again. I have to start changing something to begin some chain of elimination. Cheers.

What amp head is it? Have you made the connections correctly? Can you post a circuit schematic?

The reason for the above questions is to assist with diagnosis. The cause you suggest, bad earthing, usually causes excessive mains hum, 50Hz or 60Hz depending on your continent of residence. Another possibility, bad pot sliders, could cause excessive loss of signal. But most amp heads are quite complex and full of active components. Loss of power is another possibility but would cause distortion on the way down. Bad connection of input cables would also cause mains hum. Partial shorting of input cables might cause the input attenuation that plagues you. As you can see, taking the amp head to the store might leave the fault at home, to recur on your return.

The head is a Marshall and yes it’s a complicated looking sucker inside. It’s a Bi-chorus 200 (8200E). They are not full valvestate as with other Marshalls. They are only a single valve as well as solid combo.

The other clue I’ve discovered since my original post is it does appear to return volume correctly if I turn the “Dirty/Crunch” channel off and return it to “Clean”. What does the crunch operate from, the pre-amp board or is that coming from the reverb board? All the knobs go through the pre-amp board leading me to still believe it’s an error with one or both volume knobs, but the foot switch plugs into the back of the reverb board and that turns the clean and crunch on and off.

I don’t have a schematic for it sadly. In fact I have little clue on how any electronics work.

I have worked out to “Not touch the brown wire”. (Ding, ding, ding…..Jackpot)!

If I could add pictures I would but I cant see where this allows that. I have pictures of inside it but I doubt that helps you much and I can't post them anyway. https://www.youtube.com/watch?v=iY8g6XAg4J0

This link shows what one looks like from the outside.

Marshall schematics are readily available online. Looking at the Dirty/Crunch foot switch, this is a very simple passive (resistor-capacitor) item and should be simple to fix. Suggest temporarily disconnecting your foot switch and trying a manual switch instead.

Is the Echo Send/Return wiring okay? A poor connection could lose signal when Echo is not in use.