Subwoofer stopped working - Help Wanted

I have a Martin Logan Dynamo Sub that stopped working a couple weeks ago. When the gain is turned all the way up there is still no sound and if you place your fingers on the cone you can feel it twitching ever so slightly. Turning the gain down even a little, there is no cone movement whatsoever.

Well at first I thought the amplifier was probably at fault, because the woofer *seemed* to pass the basic tests of: 1) Manually exercising the cone with my fingers, 2) Connecting a 9V battery to the woofer, and 3) Checking the DC resistance of the voice coil.

So.......I went and connected a Dayton 200W Plate Amplifier to the sub. I even went to the trouble of fabricating a back plate that seals, with a pass through seal for a wiring harness to the externally located Dayton Amp. Well would you believe it still didn't work?? Exactly the same behavior. This amp tests out fine on a Mordaunt Short subwoofer that is also 4 Ohm impedance rated.

So obviously something is actually wrong with the woofer driver. Here's how the tests pan out when I repeated them:


1) Manually exercising the cone results in pretty good range of motion with no sensation of any voice coil rubbing whatsoever. I'd say the woofer is a bit stiff but it has a very substantial surround on it so I think it is normal.

2) Connecting a 9V battery (measuring 9.5V) to it with the proper polarity results in click or snapping sound and the cone rapidly moves outwardly about 1/8" and then stops abruptly, which is quite a bit less than the full range of movement I got when manually pushing on the cone. Reversing the polarity gives the exact opposite result. When I repeat the test with a more worn out 9V battery (measuring 8.8V) I get a clicking noise but the cone will barely move at all, with just the slightest amount of visible motion, regardless of the polarity of the connection.

3) Checking the DC resistance of the voice coil gives a fluctuating reading between 2.9 and 3.7 Ohms which doesn't quite seem right to me, that it should be so unstable. I mean it never settles at all.

So, just wondering if anyone has any helpful suggestions on what could be the problem or any remedies to the situation. Martin Logan says it is very unusual and they are checking upon driver availability but the subwoofer has been out of production for quite some time so they may not have any at all and I don't know how repairable this driver will turn out to be, even if a replacement voice coil is to be had any where.

Thanks!
George / NJ

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Custom Vibration Isolation Speaker Stands and Sub Platform

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