subwoofer dead?

[Chuckman]

so I got a new to me (used) sub yesterday and it acts funny. its a MTX thunder 7500 12" unit. I hooked it up to my existing amp, (this sub was a replacement to an older working one) and it sounds ok, but then it starts moving in and out and making a tapping sound. not the popping that you would hear from over excursion, just starts oscillating and making that sound. what gives? is it DOA or what? could it be that the impendance is wrong? could there be a wire crossed somewhere? doesnt have the odor of being burnt up, Ive done that alot...

 

[jeepn-jason]

sounds like your amplifier is dead. Put a multimeter on the outputs and see what kind of dc voltage is coming out. Shouldnt be any but sounds like there is. Should only be ac voltage. If the old sub was shorted it more than likely caused damage to the amplifier.