More harm than good without at least minimal test gear
I order to do competent service work on something as complex as an amp, you need at least some equipment. Here are the basics, an amplified Volt meter, a signal generator, an oscilloscope, preferably two channel. It is also a very good idea to have a Variac. That is a variable voltage transformer, that allows you to bring the voltage up gradually as you observe what happens after you think you have fixed it. It also isolates you from the mains, and significantly reduces the risk of you injuring yourself.
You should always have a circuit of what you are working on. Study it, and get a good idea of how the circuit actually works.
Always inspect boards carefully for signs of burnt out resistors and other components. Remember these passive components burn out for a reason, so you can't just replace them. They need replacing yes, but they often give a clue as to the real culprit of the crime
Now are the fuses to, or on the amp board of the defective channel blown? If so, one or more of the power transistors are likely blown. If that is the case, you need to know why they are blown, and check diodes caps and resistors in the driver and power transistor stages. Also don't forget the protection circuits. I have seen them fail and short the power transistors. Obviously if this is not corrected you will blow another power transistor or two.
