Marshall JCM900 losing power

steamabacus

I have a question from a friend of mine which is well beyond my knowledge, I wonder if any of the amp guys here could offer any advice. He has a problem with a Marshall JCM900 which he describes like this...

"I’ve got a query about one of the JCM’s I’ve got.  It’s a JCM 900 , 2 channel with an issue where it loses power after being run for 20 min or so.
I couldn’t replicate the fault, the pots were dodgy so I replaced all of them and the input socket.
Can this be down to the valves ? Other suspects include the 2k2 dropper resistors in the power stage and the dropper resistor that powers the preamp."

Is this a known fault with the model and can anyone offer any advice on what the cause may be?

mudslide73

The effects loop op amps are a common fault on these. Summat to do with having a parallel effects loop iirc. I have a MkIII Dual MV 900 and I've had them repaired twice - it's doing it now but a patch cable in the loop cures the problem  

DJH83004

As per @mudslide73 comments, I have one in at the moment for repair (2101), with low volume due to the send / return sockets, basically they are fitted with stereo (TRS) jacks that rely on the ring connection for the direct path. Try putting a patch lead in, just to see if it restores the output. Other than that the valves would be the next obvious port of call (before the pots I would have said :-) )     

steamabacus

Cheers, chaps - I'll pass on the info.

ICBM

If its a regular thing after about 20 minutes, it sounds like an overheating component issue.

The first thing to check is the 100R 'choke' resistor (R35) which can sometimes desolder its own joints, or may have failed internally. Either problem could be intermittent as well.

I find it best to desolder the joints on this and the screen resistors, clench the wires down tightly onto the PCB, then resolder - that stops them from cracking the joints via vibration as easily.

steamabacus

Cheers, ICBM. I'll pass on the advice to my mate.