My buddy and I have spent the afternoon generating horrible noises with a waveform generator and a NAD 216THX amplifier, which we thought was faulty. turns out its the speakers, a pair of 8 Ohm Mordaunt Short MS45TI. We had set the speakers up on the deck, as we were working down in the back paddock, and the sound faded away....it wasnt *that* loud though.
Its the crossover network (MSP1B), which has PTC thermistors in series with the mid/bass and tweeters. They both have a (measured) PTC characteristic, bass/mid measures about 0.3 Ohms cold, the tweeter about
51 Ohms - which seems pretty high to me. Especially as its in series with a 3.3Ohm 5W W/W resistor....The bipolar caps measure about right, with a Fluke 12. The voice coils look good, we've re-done any dodgy solder joints, and the choke windings look OK.
my guess is that the PTC characteristic has changed, such that its resistance increases before it ought to. But I am not familiar with this sort of failure mode with PTCs - is it possible? common? It also seems a bit odd to slap a 50R ptc in series with a 3R3 WW resistor....
we were sticking 17.3Vrms up its bum, at 100Hz (while wearing grade 4 earmuffs).
Any thoughts?
Cheers Terry