Off centre voice coil gap in compression driver - diyAudio
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Old 2nd April 2013, 08:55 AM   #1
3GGG is offline 3GGG  Australia
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Default Off centre voice coil gap in compression driver

I bought some vintage Yamaha JA4281B drivers on ebay. Apparently they were good. I tested them and got that horrible silence, on inspection the diaphragms were broken. On removal of the diaphragms both drivers have vastly incorrect VC gaps. I cannot see any way to fix this, looks like an expensive lesson.
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Old 2nd April 2013, 10:10 AM   #2
afa is offline afa  Australia
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Hi 3GGG, you can get the gaps realigned by these guys, SPEAKER REPAIR MAGNET RECENTRE SPEAKER REPAIRS REPLACEMENT DIAPHRAGMS etone PROFESSIONAL SOUND
they have done some JBL drivers for me in the past.
good luck, afa
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Old 2nd April 2013, 10:34 AM   #3
3GGG is offline 3GGG  Australia
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Hi afa
Thanks for the link, an aussie dealer may be worth exploring. At least I have two NOS diaphragms.
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Old 2nd April 2013, 12:08 PM   #4
JMFahey is offline JMFahey  Argentina
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Ugh !!!
Expensive mistake, those magnets unglued and shifted, grabbing the VC as in a vice (literally).
Sorry but you don't
Quote:
have two NOS diaphragms.
, you have *nothing*.
You destroyed them making them unusable when you pryed them out from the damaged driver.
Bits and pieces of the torn VC are still visible, tightly grabbed between polepiece and front plate.

As a side comment, the EBay seller is a SOB for selling stuff in that condition.

The Company mentioned above looks like it could do the job, you need to DE-magnetize first to be able to do any work there, and it ain't easy.

And then they will need to get a new diaphragm.
Send them the remains of the old one, will help anyway.
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Old 2nd April 2013, 12:28 PM   #5
3GGG is offline 3GGG  Australia
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no, i do have two NOS diaphragms, in a foam box hidden away The yamaha diaphragms are pretty much unobtainable.
I also have two very broken old diaphragms
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Old 2nd April 2013, 02:59 PM   #6
JMFahey is offline JMFahey  Argentina
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Oh, that's better, the torn bits visible in the picture pointed straight to
Quote:
two very broken old diaphragms
Ok, good luck.
I think you still have the black plastic or aluminum mounting rings from the broken ones .
Keep them, maybe the reconers can glue a new diaphragm and VC there, so you get a new spare pair, just in case.
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Old 2nd April 2013, 03:39 PM   #7
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Quote:
Originally Posted by 3GGG View Post
I bought some vintage Yamaha JA4281B drivers on ebay. Apparently they were good. I tested them and got that horrible silence, on inspection the diaphragms were broken. On removal of the diaphragms both drivers have vastly incorrect VC gaps. I cannot see any way to fix this, looks like an expensive lesson.
I have fixed a few drivers using a vise or a Jorgensen clamp to move the top plate back in to place, (confirmed with feeler gauges or card stock) then externally epoxy the plate in place.
OK for static installations, but may not stand up to road abuse.

Between the fall that shifted the drivers, and sitting in a pinched position, the magnet may not have the force it used to have.
It's still worth a try, if they sound weak than you can then get them re-magnetized, if you think it worthwhile.

Last edited by weltersys; 2nd April 2013 at 03:43 PM.
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Old 2nd April 2013, 08:51 PM   #8
3GGG is offline 3GGG  Australia
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I was thinking that a crude method may work. If I have a solid bar that can go it the centre it could move it for me. I will await a quote first before getting out the tools.
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Old 3rd April 2013, 06:54 AM   #9
3GGG is offline 3GGG  Australia
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Looks like for $180 AUD i can get them demagnetised, re aligned, cleaned and refitted with my NOS diaphragms. Seems ok, plus postage.
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Old 3rd April 2013, 10:07 AM   #10
afa is offline afa  Australia
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Hello 3GGG, that's not to bad, I think $90 each must be his standard charge for the work, I have realigned lots of magnet assemblies of the small variety (<3" diameter magnets) for dome tweeters and small woofers and the like, but once you get to big diameter magnets and pa type stuff you can still do it, but it takes a lot of care and one off jig set ups, but some time its better to let a guy with the gear to do it, cos if you stuff it up and break the brittle ferrite magnet then you are sort of stuffed, but if you are short of the cash and and are manually dextrous and willing to have a go yourself I can give you guidance, cheers afa
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