First step is look for burned components. Once found, replace. Then, with a VOM, measure all semiconductors for shorts and opens. Replace.
Measure with Ohmmeter from all power rails to ground. If there are not too many power supply voltages, apply from external dc supply and see how much current is drawn. Bring up power gradually and watch the current. With no signal the current should not be large enough to allow any components to get very warm.
These are the first steps. You must start with all components good, to the best of your testing ability.
Study the diagram to see if the power output semiconductors have bias circuitry. If so, inspect to verify good components. Look for adjustments in the bias network.