Marshall JCM 900 schematic or service manual?

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Bricolo

Member
2002-11-11 6:52 am
Hi,

I'm looking for a service manual for this guitar amp. The full reference is JCM 900 4200.

A friend has one, and it doesn't work properly.
When cold, there's no problem. But after a few minutes, the sound stops, comes back, re stops...

Sounds like a faulting solder join. What do you guys think?
 
You want a schematic for a Marshall 4200 then, JCM900 doesn't specify an amp model.

Look at the tubes, in particular the first preamp tube. Does the heater go dark when the sound goes away? If so, there is a square black bridge rectifier in the midle of the board, and it probably needs to be resoldered.

Otherwise use an insulated probe - fancy words for a chopstick or something non conductive - and push on things to try to isolate what has an intermittent connection.

Do not forget the pots - they can crack or the solder under them can fail.
 
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