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?
Comments
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.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
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