OK, you have a completely dead one. Try the knob & if no go then it has to be opened up but maybe go a bit further either way from dead center (~1/16 of a turn max maybe) but be conservative with it first. As said make sure the bare metal on the case is grounded then jump fire to the terminal. I'm thinking the knob wont fix a dead one but see what happens. it has restored a good sound on one that the tone is off. Do ya feel lucky? well do ya