^What Mr. Tiers said.
However, when You say the amp does work when it's used individually or when patched directly from the pre section, could indicate a wiring harness problem.
Or a mod, someone might have used one side of the amp for something else.
In any case, unless You're familiar with safety procedures around el. appliances and aren't versed with the very basics of amp design and repair, it is probably going to be cheaper and safer in the long run to have a tech look at it.
By no means does this mean that You shouldn't learn the wonderful hobby (or profession) the electronics are, but do learn the safety procedures first.