I have an SM58 "made in U.S.A." "Dual impedance 38 & 150 ohms" and does NOT say "Unidyne III".
1) In what instance would I use the 38 ohm impedance and how do I change it from 150? I don't see any references to dual impedance on this site.
2) Could you tell me the aproximate vintage?
3) The foam on the top of the capsule is crumbling. What should I do?
1) Remove the XLR connector. The impedance is changed at the back of the XLR connector. Red wire is 150 ohms/Orange wire is 38 ohms. In 2010, the 38 ohm setting has little use. You would notice a decrease in the output level if changed to 38 ohms.
2) 1966 to 1983.
3) No need to be concerned. The amount of P-pop protection provided is minimal. Add an external windscreen to compensate if P-pop is a problem.