[jpl56]

Bought a Technics M12 at a garage sale. When I plugged it in, it had an horrible hiss when playing, even with no tape in it.

Solution was simple : put a blank tape then record, play, record, play short chunks of tape in order to clean the record/play connections in the mechanism. Problem solved.

[trinix912]

Oftentimes it's just a dirty head, careful cleaning with isopropyl alcohol fixes it.