Broken AR 12" woofer coil

[KlausDK]

Recently I had one of my LST's woofer stopped sounding with a strange crack at moderate listening level, no smoke or other signs of the coil being fried. The cone moved freely. I removed the woofer, measured DC resistance, it was 2.8 Ohm. The battery test showed that the cone protruded when plus connected to plus, but did not retract when plus was connected to "minus".

After some time of thinking, I figured that the most likely reason for the woofer's behaviour, was a cracked woofer coil. TT confirmed that this do in fact happen. So Yesterday I disected of the dust cap and there was a broken voice coil. I was somewhat surprised that the coil is wound around what seems to be a thin piece of carton. The are some wholes in the carton just beneath where it is glued to the cone, and this was where it broke.

I shimmed up the coil and positionned it correctly to the remains on the cone. I glued another piece of carton over the "crack" on the inside of the whole circumference. Tested now the glue has dried, it centers well, it moves freely both ways tested with a battery, but it remains to be tested in an enclosure with music. I'm afraid that it it won't hold up for real heavy bass at high levels, but lets see.

BRgds Klaus

[roundhome]

I also have two of my LST woofers like this that makes three total of these sitting here not sure the fix . find a new foarmer and wind the coils again then glue it back on the cone?

I thought of what you have done but the added weight and covering the vent holes might get hot

[KlausDK]

Hi Roundsound

I think the added weight is negligible, but you might have a point about the ventilation. However, my biggest concern lies with the strength of the glue, if it can stand the tension during operation. No doubt the best thing to do would be to dismantle the driver and get it made properly. My problem is that nobody does that around here anmymore, and shipping the 12" woofer abroad adds considerably to the cost of repair.

BRgds Klaus

[roundhome]

If you could seperate the spider from tha basket and the foam then it comes out as one unit . Then you could send just that vary light it would be just a matter of shimming and gluing down the spiner and the foam ring again. I have had the spider comes lose all by its self but when i need to take one apart its not so easy.

I have a friend that does a lot of reconing but I have not had time to talk to him about these. will check with him soon.

Jim

[KlausDK]

Hi Jim

Thanks, that is a good idea, if it becomes necessary. Today I mounted the woofer in my 10Pi, since I was changing a capacitor anyway, and it sounds just great, no noises, only sound!. So far it lasts, and I have given it what I consider to be the ultimate woofer stress test, "Wishing Well" by Michale Ruff (Sheffield Lab CD) at high levels.

BRgds Klaus

[tysontom]

>Recently I had one of my LST's woofer stopped sounding with a

>strange crack at moderate listening level, no smoke or other

>signs of the coil being fried. The cone moved freely. I

>removed the woofer, measured DC resistance, it was 2.8 Ohm.

>The battery test showed that the cone protruded when plus

>connected to plus, but did not retract when plus was connected

>to "minus".

>

>After some time of thinking, I figured that the most likely

>reason for the woofer's behaviour, was a cracked woofer coil.

>TT confirmed that this do in fact happen. So Yesterday I

>disected of the dust cap and there was a broken voice coil. I

>was somewhat surprised that the coil is wound around what

>seems to be a thin piece of carton. The are some wholes in the

>carton just beneath where it is glued to the cone, and this

>was where it broke.

>

>I shimmed up the coil and positionned it correctly to the

>remains on the cone. I glued another piece of carton over the

>"crack" on the inside of the whole circumference.

>Tested now the glue has dried, it centers well, it moves

>freely both ways tested with a battery, but it remains to be

>tested in an enclosure with music. I'm afraid that it it won't

>hold up for real heavy bass at high levels, but lets see.

>

>BRgds Klaus

>

Dear Klaus,

That repair looks to be good. I believe I see what happened now: the voice-coil former "bobbin" in the 200003-0 woofer is made of a Nomex-type fireproof treated paper, and it is more than strong enough for normal operation of this woofer; i.e., excursions that don't exceed approximately 1.125 inches peak-to-peak. When the woofer does bottom, it makes solid and very raucous contact with the steel back plate (which is not recessed in this woofer), and inertia takes care of the rest! The mass of the woofer cone, voice coil and assembly results in a lot of force to stop suddenly, and the result is a flattening of the coil former at the weakest part along the ventilation holes. Once bottoming has occurred the former is compromised, and subsequent high energy in the voice coil can cause it to tear -- which is what appears to have happened in this case.

If the coil doesn't hit bottom again or rub against the pole piece, I would think that the repair will probably hold up for a pretty good period of time.

--Tom Tyson

[dynaco_dan]

Hi Tom;

Excellent photos, Tom.

Was the repair done with high temperature material, Klaus?

[KlausDK]

Hi Vern

No the repair was done with a normal off the (office) shelf carton, since the repair is done just "above" the areas with the coil on the outside. I used a glue that retains some elasticity when dry.

BRgds Klaus

[KlausDK]

Thanks a lot Tom for that explanation and the excellent pictures. That makes perfect sense to what has happened, and might I add, before I acquired the speakers, of course (I hope). I have listened very carefully for rubbbing sounds during operation, and haven't been able hear any. I wouldn't rely on this woofer for the Paris-Dakkar rally, so to speak, but if it will last for normal use it is nice to have as a spare woofer, just in case...

BRgds Klaus