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Basics first check voltage on main dc rails on pcb you can do a cross check from the good Chanel be careful from electric shock left hand always in pocket if voltage ok then you need a signal injector giving high impedance output and series capacitor to block dc it will needs a 400 to 3500 hz. Frequency. There are plenty of circuit diagrams on the net for self build or use the line out from a PC with music playing remember the series capacitor and 20k resistor. Now you have to put signal into audio path through amp starting at the jack socket and moving to various parts until the sound is heard when this happens it is the previous parts faulty.
Power output amplifier circuit should be checked. If there is any delay relay fitted, its drive circuit too should check. Check all the components at the main power amp output section. Refer its schematic diagram for more details. If you wish to get some details; check the site linked here. Viewing it in "Mosaic" will make surf easy. Surf the site with patience. Pull up older posts. Surf the site with patience. http://electronicshelponline.blogspot.com/
Hum or buzz is often caused by the mains filter capacitor letting the 50hz mains hum into the system. Regardless of voltages you just need to try new capacitors. If that doesn't cure it, the problem is elsewhere. If it's a stereo amp and the buzz is on both channels then it's something common to both channels.