Marshall JCM 2000 TSL

[jcollins]

I've got a Marshall JCM TSL on my bench. The customer complained that the
output volume was low.

He had gone ahead and purchased a complete set of pre and power amp tubes.

On my bench the output is very low, and the distortion channels don't have much gain. I went ahead and swapped in all his new preamp tubes. This
gave no improvement. I plugged straight into the effect return and got a fairly
strong signal.

I figure the problem in somewhere in the preamp and at a node that is shared by each of the channels.

Found the schematics at Dr. Tubes site. Yikes. It's a bit hard to trace the signal since you can't see the entire signal path on one page with all the connectors. I'll go ahead and do that...

But first, anything that you Marshall experts think I should be looking at?

[jcollins]

Just an update on this amp. I opened up the chassis and checked voltages
at the plates of V1-3. Sure enough. V1 and V2a were low. Also I noticed
that the dropping resistor supplying power to those tubes was discolored.

It was discolored because it was dropping around 200V. Yikes. Went through and disconnected components till I found that one that was shorted to ground. It was C20 a 0.01uF ceramic. Replaced that and the marshall roared back to life.

Not the easiest amp to work on...but, not as bad as I thought it would be.