HELP!! Marshall TSL100 Hum Clean channel only

[kladmin]

I have a Marshall JCM2000 TSL100 in my shop, there is an extremely loud buzz on the clean channel that goes almost completely away when I switch to the crunch or lead channels. It goes basically from deafening to not even noticable. Its very steady, like a or bflat in pitch (low) like typical AC lighting hum.

I noticed from the schem there are some diodes and a couple fet transistors in the clean channel but I'm not really familiar with how to test transistors or diodes - wouldn't I have to take them out of the circuit? Seems like a ground loop but I don't see how that is possible, I haven't modified it in any way. Tried some different tubes, none made a difference.

At first I thought it must be power filter caps, but surely it would stay through all 3 channels if that were the case? Basically means I've got something wrong in the clean channel before I even get to the gain channels or power section, right?

Any help would be greatly appreciated!!!

Kurt

[Enzo]

DOn't make it more complicated than it needs to be. Look for a bad tube. Swap a known good one into V1,2, and 3, one at a time and see. That only takes a couple minutes.

Isolate the problem. DO any of the controls affect the loud hum? Any that do are either at the source or after the source of the hum. Any cintrols that do not affect it are begore the hum source.

The JFETs act like switches. If the gate shorts to the source/drain part, then noises can result. I'd just look to see if any gates are shorted to the source or drain at each. In circuit readings will do here, if it reads shorted, it probably is. Pull it then and test for sure. In the case of the two on the clean channel board, if the gain and volume controls are at zero, that shorts across the FET, but not to the gate.

Note: in the non-powered condition, the JFETs are ON until turned odd at their gate. SO it will be normal to read a low resistance between source and drain legs of a JFET. Oh I don't know, 200 ohms maybe? The gate lead will not show continuity to the others, but it will act like a diode to either one of them.

There are FET 1, 2 on the Clean channel board, and there are TR1,2,3 on the main board. Just quick check them all.

The channel switching stuff runs on +24VDC. if that supply loses filtration it coul;d wind up trying to modulate anything the FETs are connected to. But I suspect that would affect all channels, but maybe not. That supply is on the rear jack board.