[D_S_J_R]

Showing my age, the first thing I thought of when 'P40' was mentioned was the slimline 1970 original (I use a T55 tuner daily in my workroom). I believe the later model from the 80's was a goodie though and worth sorting out. the fact that the amp works both channels in 'mono' suggests to me the power amp sections and power supply components are fine (the blowing fuse may not be a problem although a component in the preamp going short momentarily may have caused the blow, but I'm guessing obviously here).

I don't know if anyone feels this is a possible course of action, but trace and check from the input to mono switch. It could be a shorting cap or blown transistor?