Marshall jtm30 tech advice

ricky1918

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Hello fellas, I got a jtm30 bought for cheap.
when I turn it on, it plays full volume, but after some minutes its volume starts to weaken, to the point it gets mute.
It is also very prone to electricity noises, like if I plug other stuff in nearby outlets it gets noisy.


I know the model has issues with the OT, but I think my problems are linked to the capacitors that may need replacing.

Am I right? How much should the replacement capacitors cost?
 

Cjsinla

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Do you have any way of testing components like caps and resistors or are you going to take it to a tech? The easiest thing to try is switching out tubes if you have any spares on hand.
 

Cjsinla

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Hello fellas, I got a jtm30 bought for cheap.
when I turn it on, it plays full volume, but after some minutes its volume starts to weaken, to the point it gets mute.
It is also very prone to electricity noises, like if I plug other stuff in nearby outlets it gets noisy.


I know the model has issues with the OT, but I think my problems are linked to the capacitors that may need replacing.

Am I right? How much should the replacement capacitors cost?
Ok. I looked up your amp on a forum and your problem is a common one. Here’s what they say:

There are issues of reliablity with these amps; they are biased too hot from the factory. Most of the time this leads to overheating which then causes intermittent volume dropouts.

Apparently the fix may involve adding a pot to the circuit so that the tube bias can be adjusted.
 

Big John

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I have a friend that tried to sell his JTM30 for the longest time on the local CL. It would do the same thing: sound great in the first 20 minutes, and then get volume fluctuations with electrical noise. He went thru a few rounds of new tubes and a full cap job. He's handy with a soldering iron and does all his own maintenance. When I noticed that he had dropped the price to $150 to just get rid of it, I dropped him an e-mail and told him that I used to have a few amps that did this same thing. I told him to replace the diode rectifiers (Marshall uses the cheapest components out there) and to REALLY pay attention to the bias voltage because these amps were always biased hot from the factory. He replaced the diodes and increased the negative bias voltage. Plus, he said he shimmied a little heat sink around the diodes to help dissipate the heat better. The amp was cured and he's been playing 4 hour gigs with it ever since.
 

KSG_Standard

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I have a JTM30, I had the same problem and then it finally died. The problem turned out to be the circuit board heating up, it finally burned a spot in it. I took it to an amp tech familiar with these amps and he took care of it by bypassing the burnt area on the board and adding wires to get around it.

Great f*cking amp when they are running right. Do a search on the forum here and you can find a couple of good threads on this model. Good luck.
 


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