JCM 2000 DSL100 problems

[Prairie Dawg]

A guy brought me a Marshall JCM2000 DSL 100w head thinking it had a bad output transformer. Of course that wasn't the problem. The power section's working fine as I plugged a guitar in the effects return and got plenty of volume. The green channel works sort of (low volume, cuts in and out with scratchy distortion) and although the LED changes when the switch is pushed the ultra channel is inoperative. The first relay clicks when I'm doing this but there's nothing else that'll make any of the other relays click.

I'm about to do some voltage measurements and such, but in the meantime,

Any ideas?

Thanks folks.

[g1]

The 5201 switching IC's are worth a look.

[Amp Kat]

The 5201 switching IC's are worth a look.

Check the bridge rectifier from the power tube heaters to the preamp tubes. If it has a burnt area underneath that is likely the problem and increase the size of the new rectifier. Also do a search on this amp here I think there is a whole section on it. Switching could be a either a footswitch jack problem or bad connection.

[Prairie Dawg]

The 5201 switching IC's are worth a look.

Right you were, sir. My hat's off to you. That nailed it.

Of course all that let me do was work my way from the channel switching board to the main board, wherein I found three open resistors that completely shut the amp down, no particular reason why they went open (1 screen 1k 5w and 2 plate loads) which got me to the reverb not working because there were no springs in the tank and three broken wires to boot.