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theozaurus
Junior Contributor 20+
Joined: 05 Sep 2013
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 Posted: Tue Oct 29, 2013 10:52 pm Post subject: KEF 104/2 B200 woofer sticking |
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Hello
I've finally got my hands on a set of KEF 104/2 in Rosewood. They have suffered the usual foam rot so I've dismantled the speakers and cleaned up the woofers ready to attach the new foam. However one of the woofers is sticking (I suspect some gunk has found it's way onto the voice coil).
I've attempted to shift it with a thin strip of paper and tape sliding along the voice coil, but so far no success.
What are my other options? Will spraying isopropyl in the area? Will I need to recone the entire driver?
Any help would be massively appreciated!
Thanks
Theo |
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theozaurus
Junior Contributor 20+
Joined: 05 Sep 2013
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| Posted: Wed Oct 30, 2013 11:50 pm Post subject: |
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I've managed to sort it out by using some compressed air and a bit of isopropyl alcohol spray directed straight into the gap.
Have now discovered that one of the other woofers is totally blown as the voice coils is now an open circuit. Interestingly if you depress the woofer you can see the base of the plastic(?) part that the rod attaches to has gone white as if it's got very hot.
I guess I've got 2 choices now, recone all of them so they are all matched still, or replace with something modern - which seems a shame, but is probably more sensible.
What do you think? |
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speakerguru
Über Contributor 1000+
Joined: 18 Nov 2005
Posts: 1192
Location: Green Hut, Tovil
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| Posted: Thu Oct 31, 2013 10:21 am Post subject: |
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| theozaurus wrote: | | I've managed to sort it out by using some compressed air and a bit of isopropyl alcohol spray directed straight into the gap. |
Well done. Congratulations for the inventiveness and persistence.
| theozaurus wrote: | | Have now discovered that one of the other woofers is totally blown as the voice coils is now an open circuit. Interestingly if you depress the woofer you can see the base of the plastic(?) part that the rod attaches to has gone white as if it's got very hot. |
I can't think what you mean by this. AFAIK the rod is bolted to both magnet pole pieces. The only plastic part would be to stick the inner part of the doughnut to. Also, in each system, the bass units are in parallel, so, how only one could be overheated and burnt out, I don't know???
| theozaurus wrote: | I guess I've got 2 choices now, recone all of them so they are all matched still, or replace with something modern - which seems a shame, but is probably more sensible.
What do you think? |
Are you in the UK and do KEF still do that sort of repair? If so, then just get the open circuit one re-coned. |
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theozaurus
Junior Contributor 20+
Joined: 05 Sep 2013
Posts: 21
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| Posted: Thu Oct 31, 2013 8:22 pm Post subject: |
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| speakerguru wrote: | | theozaurus wrote: | | I've managed to sort it out by using some compressed air and a bit of isopropyl alcohol spray directed straight into the gap. |
Well done. Congratulations for the inventiveness and persistence. |
Thanks!
| speakerguru wrote: | | theozaurus wrote: | | Have now discovered that one of the other woofers is totally blown as the voice coils is now an open circuit. Interestingly if you depress the woofer you can see the base of the plastic(?) part that the rod attaches to has gone white as if it's got very hot. |
I can't think what you mean by this. AFAIK the rod is bolted to both magnet pole pieces. The only plastic part would be to stick the inner part of the doughnut to. Also, in each system, the bass units are in parallel, so, how only one could be overheated and burnt out, I don't know??? |
I mean the base of the plastic part that the inner part of the doughnut connects to (the base of the rod sits in this plastic part on the bottom woofer). You're right, it's really weird that this has gone, and the opposite one hasn't. Everything else seems perfectly normal, and the opposite one definitely works. I haven't plucked up enough courage to remove the spider and see what the voice coils actually looks like. http://sound.westhost.com/articles/speaker-failure.html has some interesting pictures of different types of failure
| speakerguru wrote: | | theozaurus wrote: | | What do you think? |
Are you in the UK and do KEF still do that sort of repair? If so, then just get the open circuit one re-coned. |
I'm based in the UK (London) so can get to Wembley audio easily enough (KEF recommended them today when I phoned). They have quoted me £59.50 +vat for a recone (it's £49 +vat for a refoam). I've already ordered a kit to refoam all of them elsewhere. Would I ever hear the difference if one was reconed by Wembley and I refoamed the originals my self (assuming I did a decent job)? I've tested briefly one of the other drivers with all of the foam removed and I can hear the voice coil scraping - but I assume this is because there is no foam surround.
I just can't figure out whether it's worth blowing £285 to get it all done properly and the drivers reasonably matched. Alternatively I could do 3 of them myself and hope that the reconed one is not that different in performance. |
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speakerguru
Über Contributor 1000+
Joined: 18 Nov 2005
Posts: 1192
Location: Green Hut, Tovil
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| Posted: Fri Nov 01, 2013 11:12 am Post subject: |
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| theozaurus wrote: | | ....and the drivers reasonably matched.... |
Matching loudspeakers was done at KEF by measurement of the finished units and then matching and selecting from production sized batches (hundreds). Alternatively you could produce matched quartets of units by pre-selecting weighed cones, custom wound voice coils, measured gap flux magnets, etc., etc.
I doubt very much whether either of these options is now available to you (or Wembley Loudspeakers). The bass alignment of a coupled cavity is primarily determined by the cabinet volume, ratio of front and back volumes, woofer moving mass and motor strength. The woofer suspension should be swamped by the box air stiffness. In any case, the output only changes in proportion to the square root of volume and mass so it's not super sentitive to variations. The box won't change much so you only have to concern yourself with the cone surround mass and motor strength. The chassis-magnet assemblies will be re-used so that leaves the voice coil length and d.c. resistance and the weight of the cone/surrounds to worry about.
Realistically you will only be able to ensure that the cone and surrounds are made of the same materials and the v.c resistances are within about 5% of each other. As for building yourself, it's no good asking me. I'm an ex-loudspeaker engineer so of course I'd do it. Only you know how good you are but, judging by your compressed air trick, I'd say go ahead. |
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audiolabtower
VIP Contributor 500+
Joined: 06 Jan 2009
Posts: 686
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| Posted: Sat Nov 02, 2013 1:20 am Post subject: |
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| speakerguru wrote: | | Matching loudspeakers was done at KEF by measurement of the finished units and then matching and selecting from production sized batches (hundreds). |
I've often wondered if the production batch was big enough to match ALL units for the reference series, or were the last ones left over then the constructor series or similar? |
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speakerguru
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Joined: 18 Nov 2005
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Location: Green Hut, Tovil
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| Posted: Sun Nov 03, 2013 5:56 pm Post subject: |
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I seem to remember that production batches were between 100 and 200 units. Unmatched units would normally carry over into the next batch for re-matching. Any unit which failed to match with others over more than a few batches would be taken out and re-built. I don't think there were any hard and fast rules about the number of times round a unit could go.
Units built for non-Reference Series would normally not even enter the measurement/matching phase, but it may well be that depending on the sales forecasts, that some units would be transferred both into or out of the Reference Series measurement stage. |
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theozaurus
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Joined: 05 Sep 2013
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| Posted: Mon Nov 04, 2013 12:24 am Post subject: |
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That's really interesting! Now that it has been explained like that I think I'll definitely give it a go doing what I can and report back.
After that I'm going to try and see if I can figure out the transfer functions for the KEF Kube. |
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audiolabtower
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| Posted: Mon Nov 04, 2013 1:40 pm Post subject: |
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Thanks, Speakerguru  |
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speakerguru
Über Contributor 1000+
Joined: 18 Nov 2005
Posts: 1192
Location: Green Hut, Tovil
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| Posted: Mon Nov 04, 2013 11:11 pm Post subject: |
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| theozaurus wrote: |
After that I'm going to try and see if I can figure out the transfer functions for the KEF Kube. |
Which KUBE transfer functions do you want? |
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theozaurus
Junior Contributor 20+
Joined: 05 Sep 2013
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| Posted: Mon Nov 04, 2013 11:35 pm Post subject: |
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Ideally the transfer functions for the KEF 200 which would allow biamping.
The price they are going for at the moment on place like eBay seem very high. I've seen the schematics you've uploaded for the KEF 200 which I'm trying to simulate at the moment.
The www.minidsp.com products look interesting, and I think could do the same thing as the Kube but at a lower cost. Alternatively I think something similar to the Kube could be prototyped with a few opamps (I've seen people replacing the NE5532s with OPA2134s). |
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speakerguru
Über Contributor 1000+
Joined: 18 Nov 2005
Posts: 1192
Location: Green Hut, Tovil
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| Posted: Tue Nov 05, 2013 9:36 am Post subject: |
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| I've already offered a piece of software to this forum, http://www.hifiloudspeakers.info/speakertalk/viewtopic.php?t=1389&highlight=biquad , which allows you to either synthesise a single opamp biquad from the transfer function or the reverse, obtain the transfer function (biquad in S) from a single opamp circuit values. There was almost no interest. I think BIQUAD would do what you want, but in light of the low interest from others, perhaps you'd better PM if you want to discuss this further. |
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speakerguru
Über Contributor 1000+
Joined: 18 Nov 2005
Posts: 1192
Location: Green Hut, Tovil
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| Posted: Mon Nov 11, 2013 2:47 pm Post subject: |
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| theozaurus wrote: | | Ideally the transfer functions for the KEF 200 which would allow biamping. |
You will have to be very specific about what you want if you want me to be of further assistance. i.e. which stages of KUBE200 circuitry do you want to use and therefore analysed?
Please note that I can only provide transfer functions for individual (analogue) op-amp stages in the form of biquads in S, where S is the Laplace Transform variable (S = σ + jω). If the DSP you want to use requires transfer functions in z, you will have to do the Bilinear transformation from S to z yourself. |
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theozaurus
Junior Contributor 20+
Joined: 05 Sep 2013
Posts: 21
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| Posted: Thu Nov 14, 2013 11:51 pm Post subject: |
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Sorry for the delayed reply, work has got in the way of the important business of restoration.
It sounds like we definitely need to have a chat. I notice you've also drawn up a PCB for a Kube. I'm toying with the idea of making my own. On the DSP front I'm confident I can do the transforms if required (although it will involve dusting off a few engineer books!).
I'm really just quite interested in just experimenting to gain a better understanding of how the Kube works and the pairing with the speaker.
The idea of using a little box of tricks to improve the performance of the speaker flies contrary to a lot of hifi doctrine, but intuitively seems like a great idea - we accept that a linear response is hard to get from the transducers alone and filter as necessary.
Hopefully as Christmas draws closer I'll have a bit more time to play.
Thanks for all of the help so far. |
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speakerguru
Über Contributor 1000+
Joined: 18 Nov 2005
Posts: 1192
Location: Green Hut, Tovil
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| Posted: Fri Nov 15, 2013 9:58 am Post subject: |
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| theozaurus wrote: | | ...but intuitively seems like a great idea - we accept that a linear response is hard to get from the transducers alone and filter as necessary... |
Well said. At last someone who understands that there might be more advantageous engineering solutions to sound reproduction. Why have a pair of 200ltr  boxes to go down to 20Hz when that frequency content might only appear in the source material once in a blue moon and at much reduced level compared to the 70-80Hz content of the bass guitar and kick drum. |
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