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Has anyone ever tried to repair a deformed Voice Coil?

infullview

infullview

Super Member
I have a 15 inch woofer from a W70E that someone applied to much power to for a sustained amount of time. The speaker isn't burned out, but it's rubbing when you press it in gently. I was able to unbolt the magnet structure to inspect the VC to see what happened. The VC wire got hot enough to melt the epoxy coating and allow a portion of the winding to 'tilt', or pull away on one side of the coil. At the same time, the heated epoxy also deformed the VC form. It now rubs against the pole piece when you push the cone in. Since I have complete access to the VC, I thought I might be able to repair the VC by heating it back up and using a solid piece of aluminum round to reform it.

I located some 2 inch round solid aluminum stock on e-pay. I will polish the end of the stock to a near mirror finish to prevent sticking before attempting this repair. Worst case, if this doesn't work I can always try my hand a winding a new coil using this as a form. The price of admission to try out this hair brained idea was 20 bucks so not a huge loss if it doesn't work.

If I'm successful at making the VC round again, I will add a new coat of high temperature epoxy on the coil to prevent anything from moving again. I will follow up with results pro or con after I have given this a shot.

Please post if you, or anyone you know has attempted any kind of repair like this.
 
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Infullview, if that woofer is one with the sandwich magnet of one iron core plate bolted between two steel plates, check to make sure that the holes in all three plates are lined up properly. I had that kind of a 12" woofer from a W60 with the voice coil rubbing because the bolts had loosened enough for the iron core plate to shift - probably downward due to gravity and the vibration. Once the middle plate is out of alignment the voice coil will rub regardless of the condition of the coil itself or the cone or surrounds. As for reforming the voice coil, if your woofer is the type I'm talking about, you can simply remove the whole magnet structure which gives you wide open access to the voice coil to deal with it however it needs to be if it can be at all. Good luck.

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rumblestrip said:
Infullview, if that woofer is one with the sandwich magnet of one iron core plate bolted between two steel plates, check to make sure that the holes in all three plates are lined up properly. I had that kind of a 12" woofer from a W60 with the voice coil rubbing because the bolts had loosened enough for the iron core plate to shift - probably downward due to gravity and the vibration. Once the middle plate is out of alignment the voice coil will rub regardless of the condition of the coil itself or the cone or surrounds. As for reforming the voice coil, if your woofer is the type I'm talking about, you can simply remove the whole magnet structure which gives you wide open access to the voice coil to deal with it however it needs to be if it can be at all. Good luck.
Click to expand...
Unfortunately not the case. When I removed the magnet structure I found bubbling on the inside of the voice coil form. Using a form and some heat, I think the bubble will flatten out as the heat softens the epoxy and expands the form. Thank you though for the suggestion.
 
Move the dust cap out of the way and insert something stout enough between the former and magnet and in order to reform the former;), enough so it doesn't rub.
I've used a metal finger nail file for instance on other voice coils, sort of like a forceful shim.
Perhaps the same thing on the back side of the woofer for the VC wire, unless it's loose and dangling.
 
tubed said:
Move the dust cap out of the way and insert something stout enough between the former and magnet and in order to reform the former;), enough so it doesn't rub.
I've used a metal finger nail file for instance on other voice coils, sort of like a forceful shim.
Perhaps the same thing on the back side of the woofer for the VC wire, unless it's loose and dangling.
Click to expand...
On any normal speaker that might work, but we're talking Wharfedale here. At the top of the VC, there's a thick aluminum ring that acts to prevent the speaker from damaging the VC. When the cone travels in, it makes contact with the pole piece and makes a loud clacking noise. Nothing gets hurt, but it sure does make you aware that you're over driving it. This ring also makes it impossible to align the VC the normal way by inserting film or paper strips to set the gap around the pole piece. You should be able to see this in the pictures I will upload tomorrow.
 
Remember too that, in addition to flattening the bubble, the coil wraps need to remain insulated from one another. Will a one-coil short cause problems? Likely not. But often coil wire is very, very thin, and chances of coil-to-coil shorts or coil-to-gap shorts are quite high, especially when exposed to heat high enough to disturb the adhesive.

A noble effort, but one fraught with peril.
 
Chip Chester said:
Remember too that, in addition to flattening the bubble, the coil wraps need to remain insulated from one another. Will a one-coil short cause problems? Likely not. But often coil wire is very, very thin, and chances of coil-to-coil shorts or coil-to-gap shorts are quite high, especially when exposed to heat high enough to disturb the adhesive.

A noble effort, but one fraught with peril.
Click to expand...
The enamel on the wire doesn't seem to have been burned. I will Ohm it to rule out any shorts. As I said, I will add pictures to this thread so people can see for themselves. Coil is not discolored. I think it got just hot enough to melt just the epoxy.
 
I hope your repair turns out well.

I once repaired a whisker to a vintage Gibson voice coil. Since the field-coil speaker serves as a choke I, really , really deeded to preserve the speaker. That was just a small part, as I also had to re-glue the field coil magnet, and, of course, re-attach the voice coil to the spider and cone.

Repaired Coil Lead.JPG
My EH125.jpg
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maxhifi said:
No experience but I have an idea. There is a tool you can buy for flaring car exhaust pipe. You tighten a bolt, and the diameter of a cylindrical piece of metal expands. It seems like it would be ideal for this work.

example: https://www.princessauto.com/en/detail/large-tail-pipe-expander/A-p8104424e
Click to expand...

I do have experience with that type of tool (I'm a mechanic),and I wouldn't recommend it for this job. It's just not "precise" enough, and would likely do more harm than good...
 
As promised here are some pictures of the VC I'm trying to repair. The arrow is pointing to what I called bubbling, but is more like ripples in the paper.
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infullview said:
As promised here are some pictures of the VC I'm trying to repair. The arrow is pointing to what I called bubbling, but is more like ripples in the paper.
View attachment 1432648 View attachment 1432649 View attachment 1432650 View attachment 1432651 View attachment 1432652
Click to expand...

It does not look like the former is out-of-round. If the rubbing is solely because of the bubbling/rippling epoxy on the inside, I suggest using some fine sandpaper (220 or 320 grit wet/dry) to remove any protrusions on the inside of the former.
 
RTally said:
It does not look like the former is out-of-round. If the rubbing is solely because of the bubbling/rippling epoxy on the inside, I suggest using some fine sandpaper (220 or 320 grit wet/dry) to remove any protrusions on the inside of the former.
Click to expand...
Thought about using abrasive to remove the ripple. I am worried that if I remove to much material I could damage the coil or weaken the form. My biggest concern isn't even the ripple -it's returning the wire on form that has slid down on one side to its original location. I don't own a heat gun at the moment so I haven't tried realigning the wire. Do you think it's even possible to do this? What are the implications of leaving to wire the way it is? Will this impact the woofers performance?
 
infullview said:
Thought about using abrasive to remove the ripple. I am worried that if I remove to much material I could damage the coil or weaken the form. My biggest concern isn't even the ripple -it's returning the wire on form that has slid down on one side to its original location. I don't own a heat gun at the moment so I haven't tried realigning the wire. Do you think it's even possible to do this? What are the implications of leaving to wire the way it is? Will this impact the woofers performance?
Click to expand...

It is going to be extremely difficult to heat up the epoxy inside the former and redistribute it evenly inside. Your best bet is to mechanically make the inside smooth. That means sanding the high points down - work slowly and check often. All you want to do is remove the high points where the epoxy layer got thicker than the original.

If you want to close the gap in the coil, you will need a controllable heat source like a heat gun. The epoxy needs to be heated only to the point where it becomes soft enough to move the coil. The former looks like paper, so you need to be careful not to use too much heat. If you do decide to try to move the coil, do it before working on the inside of the former.

If the coil is still closely wound on the former and there is no part protruding away from the former, I would likely leave it alone. I would make the inside of the former smooth. If there is no more rubbing when assembled, I would call it good.
 
I'm concerned about the gap in the voice coil wires. Are some of them loose?
I'm also noticing that the distance between magnet pole and former seems unequal. Is it possible that the woofer needs to be rotated, and that sagging is the problem, not other stuff (though smoothing out the ripples seems like a good idea).
 
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