Kef 107/2 - frozen wwofer - diyAudio
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Old 7th December 2014, 08:29 PM   #1
Dyolf is offline Dyolf  Norway
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Default Kef 107/2 - frozen wwofer

I desided to bite the bullet, and opened up my 107/2 to check for any foam issues, a well documented problem with theese speakers. The reason for this was that I felt that there had been a noticable shift in balance in the overall soundstage.
The left speaker looked fine, but the right one had a "frozen" voice coil in the upper unit, and a minor "tear" in the foam on the lower unit.
Reading AndrewJ's many comments of the importance of not trying to fit any replacement units in theese speakers, I am in serious need of advice regarding loosening the coil. I plan to refoam the two units anyway.
Any info would be appreciated.
Regards
Steen
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Old 7th December 2014, 08:47 PM   #2
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On the Swedish forum "faktiskt.se" you can find a thread about the repair of a frozen KEF T33 tweeter. "Diskantreparation Kef T33".
Since I see yuo are from Denmark, maybe it won´t be all "lost in translation, since it is in Swedish.
Good luck!
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Old 7th December 2014, 09:53 PM   #3
Dyolf is offline Dyolf  Norway
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Hi MammaOll
The drivers involved here are the bass units (B250 - SP1215)
Anyway, thanks for the linking, I will be investigating that site
Steen
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Old 9th December 2014, 06:53 PM   #4
Dyolf is offline Dyolf  Norway
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Today I found time to remove the dustcap and the foam surrounds on the jammed woofer. Something is seriously wrong
It looks like the magnet or the rear (bottom)-plate has come askew. The voice coil itself is stuck on one side of the gap, and the basket tilts alarmingly. Putting the whole asssembly on a turntable platter and rotating it, shows that front and rear plate + magnet, all have different centerlines. I will try to make a short film clip of the action.
Is it possible to separate rear plate and magnet easily? I assume they are glued together.
Steen
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Old 9th December 2014, 08:03 PM   #5
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Yes, technically it is possible to separate the rear plate-magnet assembly, but due to the extreme magnetic force present in the skewed gap, it is very likely that you will destroy the voice coil in the process... (if it is not destroyed already )
One possibility however, if you have access to a magnetizer (the one they use to make magnets from ferrite rings) try to put it in "reverse" mode and demagnetize the structure, then it comes apart easy. After repairing- re-centering and re-gluing the assembly- you can then re-magnetize it in normal mode. Do not try the "brute force with screwdriver" method for taking it apart, it is 99% that you will fail... (tried it several times, so I know)
Since you are Danish, there are lots of factories there who use such machines (magnetizers)... at least I guess.

Last edited by dragonweed; 9th December 2014 at 08:07 PM.
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Old 9th December 2014, 08:26 PM   #6
Dyolf is offline Dyolf  Norway
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Quote:
Originally Posted by dragonweed View Post
Since you are Danish, there are lots of factories there who use such machines (magnetizers)... at least I guess.
In Norway however, we only have SEAS, and I dont know if they are still producing driveunits.

Hopefully here is where the video comes

https://www.youtube.com/watch?v=-7Xs0gXwxC8
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Old 10th December 2014, 01:03 AM   #7
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Quote:
Originally Posted by Dyolf View Post
Today I found time to remove the dustcap and the foam surrounds on the jammed woofer. Something is seriously wrong
It looks like the magnet or the rear (bottom)-plate has come askew. The voice coil itself is stuck on one side of the gap, and the basket tilts alarmingly. Putting the whole asssembly on a turntable platter and rotating it, shows that front and rear plate + magnet, all have different centerlines. I will try to make a short film clip of the action.
Is it possible to separate rear plate and magnet easily? I assume they are glued together.
Steen
They are glued together but unfortunately the glue bond has failed. Either the glue never fully cured or the unit was dropped with such force that it fractured the bond or split the magnet.

At this point the voice coil is already pinched and probably distorted enough not to be usable.

Time to start looking for a used woofer.

David
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Old 10th December 2014, 07:18 AM   #8
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KEF woofers seem to be prone to this. They depend on the glue alone and it turns to dust and fails after several years
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Old 10th December 2014, 03:02 PM   #9
Dyolf is offline Dyolf  Norway
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Quote:
Originally Posted by speaker dave View Post
At this point the voice coil is already pinched and probably distorted enough not to be usable.

David
Hi David
I feared that also, but I still read 7.9 on the ohm-meter, and the slightest touch to the diafraghm alter this accordingly. This could suggest that the coil itself is intact??
Steen
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Old 10th December 2014, 06:29 PM   #10
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7.9 ohms means that the coil has continuity, but not that it isn't crimped or warped.

You won't be able to salvage the woofer. The only way it could conceivably be salvaged would be to go to a driver manufacturer, use their demagnetizer to get the unit apart without doing more damage, use a lathe to make a gap gauge to guarantee alignment and concentricity, get the cone out of the gap (to allow using the gap gauge), glue the woofer magnet back together (assuming the magnet isn't badly chipped), get very lucky that the coil isn't warped, glue the cone back in place with proper shims and re-magnetize the woofer.

Sorry, but it isn't going to happen.

David
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