Kef 107/2 - frozen wwofer

Dyolf

Member
2009-04-09 12:42 am
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
 

Dyolf

Member
2009-04-09 12:42 am
Today I found time to remove the dustcap and the foam surrounds on the jammed woofer. Something is seriously wrong:confused:
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
 
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 :Ouch: )
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.
 
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Today I found time to remove the dustcap and the foam surrounds on the jammed woofer. Something is seriously wrong:confused:
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
 
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
 

afa

Member
2008-07-31 12:57 pm
Hi, it still might be repairable, if you have the skills, I have just repaired a B&W driver with a similar problem, box was dropped, magnet assembly shifted, cone was frozen, got driver out of box, clamped basket between my legs, using fingers, gently(but forcefully) removed magnet (and bottom plate) from top plate, voice coil (kapton former) was still intact no damage, cleaned glue off mating parts, "unglued" cone/surround/spider, measured up pole piece / top plate gap thickness, worked out shims required for correct realignment, using shims (for voice coil gap)and cut off nylon cable ties as spacers for top plate and magnet assembly, just drop (yes drop) magnet and bottom plate assembly on to the top plate (with cut off cable ties on top), then put shock proof super glue between mating parts, pull out cut off cable ties, let dry, pull out shims, refit cone/surround/spider and driver fixed.