Rounding a speaker voice coil?

[Axtman]

I have a Peavey 12 inch Black Widow speaker with coil rub. I think the voice coil is slightly out of round. Has anyone tried to re-round a speaker voice coil? Thanks.

[The Dude]

This is sort of a crap shoot. Sometimes it works- sometimes not. If the coil form has been hot and is deformed, reshaping it is usually temporary. That said, some of the BW baskets have aluminum coil forms and I would think, once reshaped, it would be more likely to stay. The method that has brought me most success is:

1) Remove the dust cap.
2) Find the area of the gap where the coil is rubbing on the inside of the coil form.
3) Install shims tightly to that area ("over shim" if you will) to stretch the coil in that direction.
4) Work the coil/cone assembly up and down carefully with shim(s) in place several times to work top and bottom of coil/coil form.
5) Return to resting position with spider level.
6) Let it sit overnight with shim in place.
7) Remove shim(s) and test.
8) If all is good, glue on a new dust cap. If not repeat until satisfied.

I have an assortment of various thickness shims. If you don't, use business cards or similar.

[Jon Snell]

Have you considered the probable cause is overloading the speaker. If that is the case, it requires a new basket because the coils are loose.
Personally, as this is my profession, I would not entertain the thought that the coil has somehow moved. They don't.

[Mick Bailey]

I've tried many times to rescue speakers with coil rub and only a few times have I had any level of long-lasting success. Probably the worst have been the Fender branded 10" Eminence blue Alnico speakers; the early ones had paper voice coil formers that deformed badly even when run well within their rating. I removed the dust cap and found the coils had gone to a flattened oval shape. The only fix was a re-cone, but uneconomical to do over here. Eminence recognized this and changed the design to use Nomex instead - no more problems with those.

Another situation I've come across is rub due to the cone shifting after it's been damp, causing coil rub - especially where the speaker has been soaked on one side. The fibres swell and change the tension across the cone so that it pulls to one side or tilts the axis of the coil. Sometimes the gap is corroded too. Black Widows have impregnated cones, so this is unlikely with those.

Coil rub can also be caused by a hot spot in one area that causes the former to char and swell in one location. Usually caused by overloading, but it can be caused by initial rubbing that removes insulation from the coil and an intermittent short develops.

Yet another issue is if the magnet assembly has shifted. This isn't a problem where it's riveted, but bonded magnets can suffer (though rarely) if the bond gets broken and the magnet's own pull is the only thing holding it in place. I've completely dismantled many speakers to strip out the spider for use as lamp shades. Usually a bonded ceramic magnet is a challenge to remove and has to be chiselled off, breaking up like glass. Other times a moderate tap is all that's needed to break the bond and in those speakers I can see that a heavy drop or bang to the cabinet would be enough to loosen the magnet.

With a Black Widow with its bolted-on magnet, I would take off the magnet and take measurements with a vernier or micrometer. Whilst there's the possibility of shifting the magnet slightly to improve the alignment, only a measurement will really tell you where the problem lies. It's more difficult though to determine if the coil is circular but has tilted.

[g1]

rub due to the cone shifting after it's been damp, causing coil rub - especially where the speaker has been soaked on one side. The fibres swell and change the tension across the cone so that it pulls to one side or tilts the axis of the coil. Sometimes the gap is corroded too.

Sometimes this also happens when the cone is just old and fatigued. Then if you're real lucky rotating the speaker 180 degrees (so top is now bottom) will cure it.