My thoughts. First, have you tried running an external input to the non-functional channel? That will tell you if the amp is working or not. If it is, it's a problem with the internal connections. If not, it's the amp.
Next, you may not have a schematic, but you have a fully functional amp in the other channel. A competent technician can take voltage measurements on the good amp and compare them to the same measurements on the bad amp to isolate the problem.
