Most of the busted speakers I've seen had mechanical problems.
Banged the volume control and broke it.
Banged the signal conector and broke it.
Dropped and cracked the circuit board.
You need either an oscilloscope or a signal injector.
mike



I don't think I've ever used either to repair speakers, usually the problem
is pretty apparent once you get it apart. A walkman is useful for supplying
a signal though, a DMM is usually sufficient for tracing.