Yup, JapanAxe is correct - that's a 1/8" centre positive adaptor. If you've tried using an adaptor and got the polarity wrong (used a centre negative) then you might have blown the polarity protection diode on the mains socket, which would explain why it's not running on a mains adaptor. It's an analogue unit using a 3007 bucket bridge chip, so I reckon it's well worth keeping hold of.