AR-3a woofer voice coil problem

[bitbandit]

I have just begun restoring a nice pair of 1972 AR-3As (dated from the dates on the drivers).

All the drivers seem to be ok except for one of the woofers. One woofer is a straightforward re-foam job but the other looked a bit odd from the start. Closer inspection revealed that all of the foam surround was missing and the front of the cone was proud of the basket by about ½”. The cone could not be moved in or out. Intrigued, I removed the woofer and sat it on the bench. The cone was rock solid and at a very slight angle (see pic). Shining a very bright light through sideways revealed that the voice coil was actually out of the gap and sitting on the face of the magnet assembly. I was able to carefully move it (now I could see what I was doing) and manoeuvred it into the gap, where it dropped slightly but was still reluctant to move into the pole pieces. I decided the dust cap would have to come off to see what was going on. Horror of horrors was revealed! The coil had completely sheared off at the line where the holes are. I can only assume that someone has driven it really hard for a long time with the surround perished and the coil has been bashing up against the back of the magnet sending shockwaves up and down the coil longitudinally weakening and compressing the “weak” spot by the holes until it was very fragile, and then one almighty push forward has compressed the hole region completely and popped the cone and coil right out. How on earth can people not hear that happening? Unless it was done on purpose of course – “hee-hee, look at this thing flap, let’s see what happens if we turn it right up”. Such a shame.

Anyway, enough of the societal rant. I checked the other woofer out fearing they may both have been treated the same way but although the other surround is shot the voice coil is totally unblemished. Phew. So that leaves me with one seriously stuffed woofer.

So, to the questions for the good people of the forum!

I don’t want to scrap the woofer if I can avoid it, there are few enough originals around as it is.

1. Does anyone know if voice coils/formers are available anywhere for the ar-3a woofer?

2. When I have worked on coils previously, I have removed the spider and cone in one piece then done whatever and refitted it. I can’t see how to remove the spider on this one without damaging it. The spider seems to be in ok condition but it is glued to what looks like a cork ring (see pic). The clear glue is not coming away easily and the alternative might be to go in under the cork ring. Does anyone have any experience of removing the voice coil/cone/spider assembly they could share?

For interest, the stuffing is yellow fibreglass, 360g per cabinet.

Thanks

Dave

[Carlspeak]

Good luck trying to save the spider. If you can get an AR3a VC then you should also be able to get a new spider. The shims under the spider and foam surround are there to properly position the cone and VC centered in the pole plate. So don't destroy either of them.

You may be able to save the cone if you can remove the VC without damaging the cone. The cone is the most valuable part in addition to the frame and magnet.

[KlausDK]

1. Does anyone know if voice coils/formers are available anywhere for the ar-3a woofer?

2. When I have worked on coils previously, I have removed the spider and cone in one piece then done whatever and refitted it. I can’t see how to remove the spider on this one without damaging it. The spider seems to be in ok condition but it is glued to what looks like a cork ring (see pic). The clear glue is not coming away easily and the alternative might be to go in under the cork ring. Does anyone have any experience of removing the voice coil/cone/spider assembly they could share?

Hi Dave

I had the same problem, managed to repair the coil, look at this thread:

http://www.classicspeakerpages.net/IP.Boar...ic=2264&hl=

It still works fine.

BRgds Klaus

[bitbandit]

Hi Dave

I had the same problem, managed to repair the coil, look at this thread:

http://www.classicspeakerpages.net/IP.Boar...ic=2264&hl=

It still works fine.

BRgds Klaus

Hi Klaus,

Thanks for the reply. Very interesting thread and an almost exactly similar failure.

I will give that a go, thank you. As you say, a great way to create at least a temporary or backup woofer, and still preserving the cone and magnet assembly as Carl suggests.

I'm a bit concerned that in my case if I bridge the gap in the coil former where it has ripped, to get a good bond on the lower part of the former, the patch may hit the pole piece of the magnet assembly on large excursions of the cone. I'll have to spend some time to plan just enough contact to be secure while not intruding too far down the former.

Initially the lower part of the former with the coil windings did not move freely in the gap. I think the bottom edge of it had also been damaged against the back of the magnet. I slid a couple of plastic shims down and sure enough it was tight at the bottom. After a bit of judicious fiddling and manouvering of the shims I have managed to push the damaged ends back out and the shims can now be swept right round the coil and it seems to move freely, so there is hope.

If it fails, there is still the option to do battle with the spider, in which case, being able to get at both sides of the former it may be possible to bridge the gap with an epoxy and keep the original profile. I will report back with progress.

Dave

[bitbandit]

Good luck trying to save the spider. If you can get an AR3a VC then you should also be able to get a new spider. The shims under the spider and foam surround are there to properly position the cone and VC centered in the pole plate. So don't destroy either of them.

You may be able to save the cone if you can remove the VC without damaging the cone. The cone is the most valuable part in addition to the frame and magnet.

Thanks for the info Carl.

If I can repair the former as Klaus suggests, do you know what the proper height of the top of the coil windings should be above the magnet assembly at rest?

I can use the "roundness" of the vent holes to give an approximate idea but I'm not sure how much the whole thing has compressed up under impact at the fracture point.

[Carlspeak]

I can't give you a number for the height from memory. However, you simply need to measure the length of the VC windings and the thickness of the pole plate and then calculate where the top of the VC needs to be so the windings are centered on the thickness of the pole plate. The pole plates on a spare 3a ceramic woofer I have are 1/2 inch thick.

The top of the pole piece is level with the top of the pole plate. So, use the top of the pole piece as your reference surface.

Good luck with klaus's idea.

[bitbandit]

Hi Dave

I had the same problem, managed to repair the coil, look at this thread:

http://www.classicspeakerpages.net/IP.Boar...ic=2264&hl=

It still works fine.

BRgds Klaus

Well, I shamelessly stole Klaus's idea and had a go at repairing the damaged coil (pics attached).

It seems to have centred up ok both around the pole piece and in the gap. I'm not 100% sure of the spider, it may have been stretched when the coil was wedged on top of the magnet face - I have no idea how long it was like that before they came into my possesion. As you move the cone from fully in to fully out, there is a sort of springy bit in the middle where the cone wants go either in or out but won't stay put. This may be normal, but I don't remember ever noticing it on other drivers though. I am waiting for the new surrounds to arrive and when they do, I'll do the good one first and check what the cone movement is like compared to the repaired one before I waste a surround on it.

Dave