First thing to check is the preamp output and poweramp in mono jacks. The main instruments signal path passes right through those jacks. Often those jacks get dirty and the signal starts to fade as heads through to the output section of the amp. Easiest way to test the jacks is to just simply connect the two jacks together using an instrument cable. Does plugging this patch cord from preamp out to poweramp in make any difference the volume?
Later you can connect the preamp out (Acoustic 220) to the poweramp of another working amp to test the preamp section. Then take the preamp out of another amp (Guitar signal, iphone preamp works too) and input it into the power amp input of the Acoustic 220 amp. That way you can find out if there is a problem with the preamp or poweramp.
https://elektrotanya.com/acoustic_220_guitar_amplifier_sm.pdf/download.html
Later you can connect the preamp out (Acoustic 220) to the poweramp of another working amp to test the preamp section. Then take the preamp out of another amp (Guitar signal, iphone preamp works too) and input it into the power amp input of the Acoustic 220 amp. That way you can find out if there is a problem with the preamp or poweramp.
https://elektrotanya.com/acoustic_220_guitar_amplifier_sm.pdf/download.html