Same '75 D300 I mentioned earlier had (actually still has) an old Pioneer Super Tuner Cassette player that had a loose connection. Sometimes it would quit playing any sound (but was still turned on and working) all you had to do was tap the face of the radio and the sound would kick back in.
I found the best way was to tap with your knuckles (not punch it, just tap). One day it quit when I was driving and I tapped the radio, nothing... I went to tap it again, just as I was going over some train tracks, lunged forward when the truck crossed the tracks and ended up punching the radio by accident. It's been sunk 4" deeper into the dash ever since.
