Marshall JCM 900 in the shop

This is a 50W JCM 900 combo amp, a 2×12 cab. Two Celestion speakers of course!

The owner reports that the volume is dropping in and out…

All four volume pots in this amp are bad. Cleaning them did not help. Going to put in 4 new volume pots. The amp is about 12 years old, so we’re going to replace the power supply capacitors as well as the output-tube bias capacitors. The large blue cans on the underside of the chassis house 4 of the main power supply filter capacitors. The top photo shows why replacing the volume pots is somewhat time-consuming: The pots are soldered to the preamp circuit board. In order to de-solder the pots (so you can remove them) you have to access the underside of the preamp board.


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