Burned-out voice coil (amplifier failed and 'gone DC')
We've had this (pair) of drive units returned because the customer's amplifier failed. This has destroyed the voice coil which has become so hot that the varnish on the enamelled copper wire has boiled and bubbled
Beware of using an old unserviced amplifier bought second hand at a flea market - it can be a very expensive mistake. Amplifiers, just like cars and -shock, horror, people - do age. My advice is, if the urge to buy is irresistible do at least test the amplifier with some old speakers. But that will not insulate you from instantaneous failure which does happen.
I urge you to ask yourself this essential question before buying second hand "who can I turn to who has the specialist knowledge and source of parts that can restore it to 'as new' and put it right when it fails at a reasonable price in a reasonable timescale?". If you can not satisfy yourself about this do not buy.
Alan A. Shaw
Harbeth Audio UK