Quote from: gtrguy on December 02, 2022, 10:12:46 AMI doubt anyone at GC knows how to work one.

^^^This was how I stumbled upon mine ~25 years ago...shopping at the Guitar Center in Boston (back when it was on Comm. Ave. across from the BU central campus) I saw an SF-2 in the clearance rack and the sales dweeb literally said "If you can figure out what it does you can have it for what we paid for it"

If the wrong polarity power supply is used the green light indicator will turn on but there will be no sound. There's a post in the FAQ about the choke (inductor) that blows when you use the wrong polarity for the unit.

Hum is a different (or one of several different) but repairable issue(s).