The schematic is not available.
First, make sure the amplifier MODE slide switch is set properly. Operate the switch back and forth a few times to make sure the contacts inside it have a fresh surface. You could also spray a little contact cleaner into the switch. The switch can be set to L_R, or as Monitor and MAIN mono. make sure both masters are set up.
There is an INSERT jack for each power amp. You can test each power amp by connecting a signal into each jack. Spray some contact cleaner into each insert jack and push a plug in and out a few times for cleaning.
You can connect the MAIN_L and MAIN_R output jacks to some other amplifier to see if they sound good and are operating.
Doing those tests will determine if your problem is in the power amps or in the mixer part.