SpeakerTalk

Dyolf · Posted: Wed Dec 10, 2014 9:57 pm Post subject: Repair or replacement - B250

Hello, my first post here.
Sadly one of my 107.2 woofers (SP1215) have gone frozen. Voice coil stuck on one side of gap, and bottom plate or magnet seriously askew. In a shortlived thread over on DiyAudio, member SpeakerDave said nothing could be done to save the unit.
So, now I'm hunting for alternatives. From what I've read the original drivers is almost unobtainium.
Any advice?
Rgds
Steen

SaSi · Senior Contributor 200+ Joined: 24 Aug 2008 Posts: 256

What I am going to suggest is a long shot but has worked for me...

I came into possession of the original crossovers and mid/high heads of a pair of R107s. The original cabinets along with the woofers were thrown away by the original owner because they "rotted" in a basement flood several years ago.

A few months ago I built bass boxes along the original designs, based on pictures of the outside, the well known cross section plan from KEF and several pictures of the insides as taken by various owners and found in forums.

Tried several different KEF drivers (along my intention to keep the project within the original "family") - all B200 variants - including SP1014, SP1063, SP1022, SP1069, SP1070 and SP1075.

I started with paper coned drivers - to be closer to the B250 manufacturing details - but my best results came with using SP1014s.

I did measurements and also listening tests with a single enclosure and I was more than satisfied with the results the SP1014 gave to me. The low end is pretty extended - rolls of at about 40Hz without a KUBE - but even then they performed much better than my all original R105 do.

Helping the units using a parametric equalizer to boost the low frequency with a rather low Q value gave an even better low frequency cuttoff.

I built two cabinets and am now in the process of veneering them.

If you can obtain a pair of SP1014 (they are rather cheap and easy to find) you could mount them, most likely needing a set of extra wooden flanges to adjust for the slightly smaller diameter. You could try this out and I bet you will not be disappointed.

BTW, have you measured the B250 dc resistance? If so, what is it?

Dyolf · Posted: Wed Dec 10, 2014 10:35 pm Post subject:

Thanks for replying so fast. I have read, I think, most of the forum posts on the net regarding the R107 and R107/2, and almost every one summs up that the driver/crossover design in theese speakers, are dependant on NOT messing up any parameters in the overall assembly. I have even tried to get the Thiele/Small, or any datasheet, from KEF without any success. An easy option would be to find any 10" paper cone woofer with a low FS, and just try to hear what it sounds like. Anything is better than a stuck coil Razz

PS: Coil measures 7.9 on the ohm-meter
Steen

speakerguru · Posted: Wed Dec 10, 2014 11:03 pm Post subject:

It's the moving mass which is the most important driver parameter. As long as the cone area is close the acoustic mass will be there or thereabouts. The rest of the coupled cavity alignment is taken care of by the box volumes and port mass, which are independent of the driver parameters.

If you can manage to source a paper cone 10" unit with a similar size ferrite ring magnet and the same diameter, same resistance voice coil then you won't be far off.

Dyolf · Posted: Wed Dec 10, 2014 11:23 pm Post subject:

Speakerguru, why is there so little, or none actually, info on the net regarding the SP1215 driver? To find a replacement unit, it would help a lot if one knew what to look for. Another thing. If I opt for a different driver, will it be advisory to replace all four, or just the one in question, or maybe both in the same speaker? Also, new drivers mean cancelling (no pun) the rod?
Steen

speakerguru · Posted: Thu Dec 11, 2014 10:04 am Post subject:

The unit was designed and manufactured in-house at KEF for the Model 107 only. There was no need or demand for data to be published to the outside world. If only one of your 4 is bad, could you not measure the others? You can get to the mass by measuring the resonance shift caused by putting a unit into two different box volumes.

Coming up with some way of force cancelling would be a considerable challenge, without the stock rod and fittings. Good luck.

One last thought; If you were really determined and resourceful, ideally having a friend who works in a speaker factory, it could be possible to rebuild a frozen unit. It would first have to have the lower suspension soaked in solvent to remove and save the moving parts (minus foam surround and doughnut). Then the magnet could be removed from the chassis and heated until it became demagnetised and the adhesive destroyed. After that everything could be cleaned up, re-centered, re-magnetised and re-assembled. Access to a magnetiser could be a problem Shocked

darkmatter · Posted: Sat Jan 03, 2015 6:17 pm Post subject:

You could speak to Audio Loudspeakers based in Great Yarmouth. I am sure that they could repair the drive unit as they have extensive experience and excellent repair facilities.

Hope this helps

Simon

speakerguru · Posted: Sat Jan 03, 2015 7:30 pm Post subject:

darkmatter · Posted: Sat Jan 03, 2015 7:36 pm Post subject:

Yes a good point; maybe worth a try but matching it will be difficult at best

Dyolf · Posted: Sat Jan 03, 2015 7:56 pm Post subject:

An update:
I think the coil is actually OK. I have made a contraption that allows me to adjust the rear polepiece, and by doing so the basket/coil is now alligned and moving freely. What has obviously happened, is that the glue is now nonexistant. At the moment I'm constructing a device that allows me to pull off the polepiece in proper alignment, put on some fresh glue, and reinstall the polepiece again, all without removing the spider.
If everything goes wrong, I have obtained two 1994 model Seas papercone drivers, that I plan to experiment with.
I plan to add some pictures in the future.
Steen

speakerguru · Posted: Sat Jan 03, 2015 11:19 pm Post subject:

Dyolf · Posted: Sun Jan 11, 2015 12:55 pm Post subject:

#2 update
Speakergure is right of course. I have made a reviced "clamp" and will use the driver as it is without the glue. Waiting for the foam repair kit, I have now installed "new" woofers in my right speaker. Two prestine 1995 model Seas 10":
http://www.seas.no/images/stories/vintage/pdfdataheet/h108_25f-ewx.pdf
The cancelling rod is obviously not in use, but I have to confess that everything sounds fantastic. The Seas basket is not formpressed iron, but a magnesium mould that seems rather strong.
I have ordered a Umik-1, because I am building a new listening room, and want to confirm what I hear with my own ears. (52 years of age) This will hopefully also register any differences from the left to right speaker.
Steen