Because the amplifier has transistors, diodes, an IC and capacitors as well as resistors, a resistance check is pretty much useless. Add to that, the amps are quite often intermittent. The number one cause of failure is solder cracks around components. Even if you find a good one it should be disassembled and every connection re-soldered with fresh electronic grade solder.

It takes a while to get all the silicone rubber off the board first (trace side only) but it is not that bad with a small jewellers screwdriver and sharp tipped tweezers. Put a favourite radio station on and spend 1/2 hour picking away.

Also change the 100uF 16 volt electrolyic capacitor while you are in there. Use a 25 or 35 volt one instead of 16.

I just repaired 5 that I had accumulated and all were defective due to insuffient solder.