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Persistent coil rub

ScooterMcTav

ScooterMcTav

I know less than I think
Run into a situation I haven't run into before.

Refoamed a CV 12" woofer using the D cell centering method which has worked for me 100% over a dozen sets of speakers.

However, the speaker ended up with pronounced coil rub. You could also hear the rub when pushing on one side of the cone.

So I removed the surround, cut open the dust cap, shimmed the voice coil, reglued the surround and dust cap, and no rub at low volumes. Also, no audible rub when pushing on any part of the cone.

However, I'm getting a popping/rattle sound when the woofer has excursion.

At this point, I'm guessing the speaker was driven with a damaged surround, and the sleeve around the coil has been damaged or warped in some way.

Any ideas of other things I could try?
 
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1- as suggested above, check the spider
2- check the coil pole piece glued to spider , look for loose glue or cracks that shift the angle, in particular at excursion extremities. this is usually visible either from the side angle of the basket, or under the dustcap around the pole
3- check inside magnet chamber if possible for foreign objects or corrosion etc.
4- check if the surrounds have been long disintegrated and the speaker has been sitting with no surround for prolonged period of time, which sags the angle, and even with the shims, causes issues, which can be rectified by actually not using shims and setting the stress point in opposite to compensate.

that's all i can think of atm.. if more symptoms and history submitted maybe more to come.
 
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Thanks guys,

1. Spider was good. Thoroughly checked.

2. Did not see any shift off centre at excursion but could be a cause

3. Nothing in the coil gap, but these woofers do have an accessible back - will take a look

4. I'm thinking the most likely case as well, but have found magnetic centering (vs shims) had worked in the past on this issue. Resetting the surround has actually cured the rub, except for at excursion

Although I believe the CV dustcaps allow air rnrough, could this noise also be caused by an unsealed dust cap?
 
i had a polk sub with smilar symptoms in the past.. surrounds were okay, low volume play no issues, but when i turned the volume up, like yours it would present a clack at the excursion , i removed the dustcap check under, there was a barely visible hairline seperation around the cone to coil glue. one side was solid but the other side had weakened by this, so angle was good if you didn't force it, but it shifted once forced out by the magnet but not by hand.. i think speakers can still work fine with some coil rub damage done in the past as long as the circle isn't damaged or chipped and doesn't short circuit too many loops. it was a dodgy fix but i filled the crack area and all round with super glue sparingly, then cemented with starch .. it worked.. even after 3 months it was fine..

i don't think unsealed dustcap would cause clack.. in the end it might cause the woofer run a bit more erratic but angle wouldn't shift.
if this is a cervin vega driver, i am assuming your issue is similar to my polk one.. these are long excursion speakers that many people play with excess bass
my battery method might help you find the issue.. give it a solid dc voltage enough to get to full excursion, not sure maybe 9-10 volts so any hairline crack will open up and can be seen easier.
 
Well, issue solved. Ended up being a two-part fix, though thanks to Glen for the hairline crack idea. Although this wasn't the problem, it helped me find the issue.

So I took the dust cap (and its surround) clean off to check between the coil and cone. Although I did not find any cracks, there was certainly damage around the entire cone - a circular-ish section about 1/8-1/4" outside the dustcap that could only be seen when the cone was flexed far up (or at excursion). So obviously the cone had gone "soft", likely due to being run without a functional surround.

I've fixed a similar (though much more minor) issue with a CV woofer before - I cut Weld-Bond with water until it's quite thin, then brush it over the soft spot to soak in and help re-bind the cone fibre. After about three diluted coats (dry before reapply), I do another two with slightly thicker Weld-Bond, then two final coats with straight glue. I also do two slightly diluted coats on the backside of the woofer.

Since the Weld-Bond leaves a semi gloss finish, you also need to coat the entire woofer with a couple of coats for appearance sake.

After about 5 hours of this (a sunny day speeds up the process), the cone was no longer soft. I was the able to flex it to full excursion to determine if we still had some rub. Sure enough, there was still a little rub - and I determined what way the coil needed to move to avoid it. A double shim to force it in that direction, a couple of coats of slightly diluted Weld-Bond to slightly loosen the previous coats, allow to dry, and presto, no more rub. A final thick coat of undiluted Weld-Bond to ensure we're solid, and we're done.

This set of speakers has been cursed since I got them, and I'm super pleased to have them working properly.
 
glad you had a win on this one mate!
surprising though.. usually partial softening or weakness on the cone presents a visible wrinkle in the area , sometimes it is hidden under the dustcap
back in younger days, i used to spray an acrylic coat over the paper cones to make them stronger :p a bit weight added but they seemed ok.. hadn't done that for decades so i dunno what'd i feel of the results with current ears.
 
Well, it sounds exactly the same as the uncoated one, though now I'll need to coat the other for appearance purposes.

I think the weakness was mainly under the edge of the dustcap, and only really visible with the cap fully removed (vs cut open).

Anyways, gave them a good workout tonight, and everything sounded good. Well, CV good anyways.
 
i'd suggest apply some more of that hardener to the weakened area from under the cone next to spider. and let cure 100% for a week or so before drive them full on..
broken bones take time to heal and break easy again :)
 
Already coated the undersides of the woofers as well, and I don't listen to anything "full on" anymore - LOL. Tinnitus in your 40's has that effect.

Nice part with Weld-Bond is it's hard in a few hours and fully cured in 24, so we should have no issues here.

Thanks again for the idea Glen.
 
ScooterMcTav said:
Already coated the undersides of the woofers as well, and I don't listen to anything "full on" anymore - LOL. Tinnitus in your 40's has that effect.

Nice part with Weld-Bond is it's hard in a few hours and fully cured in 24, so we should have no issues here.

Thanks again for the idea Glen.
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tell me about the f.. in tinitus.. i almost sold all my gear because of it the year before.. it's better now a lot better
i burnt a special tweeter with tone frequency generator trying to offset the tinnitus frequency asking my brain to suppress it :D
 
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