I had two 7175 PE receivers that did exactly what you are experiencing. Disconnect everything, take the cover off, spray contact cleaner in the volume pot and turn knob back and forth. Spray cleaner in all the switches; input selector, tone controls, and balance control. Work those back and forth, push input switch buttons on and off. Should work fine after that.
I still have a 7020 down in the basement. Great amps for that era.
I still have a 7020 down in the basement. Great amps for that era.