Marshall JCM 2000 TSL 100 -- hum problem

[xtian]

What happens if you disconnect the jumper cable at CON 11 without shorting C28 to ground?

To which CON does the other end of the cable connect?

If I disconnect CON11 without grounding anything, hum and hash increase dramatically (acts as antenna). The other end of the cable is CON12 on the Crunch/Lead PCB.

[Helmholtz]

As V1 and V2 are DC heated, heater hum would have to come from V3. I would try a different ECC83 here.

[xtian]

Appreciate that. I mentioned I have tried two complete sets of tubes, and also shuffled all preamp tubes around. No changes.

[Helmholtz]

Ok, then scope you hum back starting from the PI input.

[xtian]

The part of the circuit I suspect--with its multiple pathways, switching, mute circuit, parallel/serial FX loop, etc.--is so complex between the FX Send jack and the PI input, and some is on the OD PCB which I can't reach easily with a probe. I've gone mental trying to trace the signal thru the schematic.

Another big clue has taken a while to gain traction in my brain. Now I can confirm that, when cold, amp sounds ok for the first few minutes of operation. But after 5-10 minutes, the bias begins to drift, and the hum starts to get louder. This feels like confirmation that the main PCB, a 2002 JCM2-60-00 iss 5, suffers from the bias drift bug, and should be replaced.

[xtian]

But after 5-10 minutes, the bias begins to drift, and the hum starts to get louder.

With this in mind, I ran a test from a cold start. After 1 minute, bias test point on one side (two tubes per side) reads 75mV, hum nominal. After 12 minutes, bias rose to 84mV, and hum loud. I hit the bias components with freeze spray, and got the bias back down to 77mV, but that didn't quiet the hum.

[Mars Amp Repair]

Hemholtz has a good question above.
Just to get myself oriented, is it IC2b section you're referring to? Seems it would be as that is right where all what appear to be the send/return jacks are.
Geeze, it's really difficult to follow the schemo w/o a interconnection diagram. I wonder if the original full service lit had such a diagram.
Also noticed that there are multiple Same designated CON connectors with the same numeral designation on each page of the schemo. Nutz!

[xtian]

Also noticed that there are multiple Same designated CON connectors with the same numeral designation on each page of the schemo. Nutz!

I know! You have to have the amp in front of you, so you can trace the jumpers and see what the CON numbers are on each board.

Anyway, client took the amp back and is considering their options.

[Mars Amp Repair]

I know! You have to have the amp in front of you, so you can trace the jumpers and see what the CON numbers are on each board.

Anyway, client took the amp back and is considering their options.

Yeah, sometimes because of crappy svc lit you spend a ridiculous amount of time just trying to ferret out nonsense like tracing where wires go! Thanx for response.