Looks like a pretty uncomplicated amp. If you can't find a schematic you may be able to fix it just by checking the "big stuff" like voltage rails for output devices and preamp ICs. If those look good and you have access to a scope you can often trace the signal through the circuit easy enough. This model has an effects loop, so it is always good to put a patch cord between the send and return to make sure you don't have a dirty switchjack problem. Also inject a signal into the Return (you can just plug a guitar or bass in there) to test the power amp section and cable the Send over to another amp or mixer to see if the preamp is working. This is pretty much my first step with any amp with an effects loop (assuming it turns on). I've also had problems with amps with headphone jacks having contact problems and cutting out the speaker.
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