Hey, I have a Carvin SX-200 solid state head, and it just stopped working. Before, every once in awhile, the volume would turn down until it was completely silent, but if I turned it off and back on, it would work fine.

The red light turns on and it creates a little hum, but no matter what, it won't work. It's not my guitar; I have other amps where they work just fine.

So, any ideas on how to fix this?
It could be heaps of things. Diagnosing this based on the info given would require an exhaustive testing process. It could be any number of things from dirt to power transistors. You'd have to get inside with a multimeter and maybe a scope and know what you should be seeing.
Basically, you'd have to learn to be a tech. Way beyond the scope of a forum thread.
Gilchrist custom
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Agree with above comment. With solid state you generally would be better off just buying another used head that looking to get it repaired. Try ebaying it for spares & repairs to get some money for it.