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Old 24th Nov 2020, 4:07 pm   #1
dlv1uk1978
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Post How to restore Keesonic KBMs

Hi

Hoping for some advice if possible please.

My Dad was a broadcaster and sound engineer and as long as I can remember he had his Keesonic KBMs hooked up to his hi fi (which was a very good set up in the 1990s and his pride and joy.)

When he died (in 1995) my brothers and I inherited his whole set up including the speakers. Very long story short, one brother has the turntable and I've inherited the speakers, amp and CD player which I've already started adding to with my own kit to make a set up I'm pretty pleased with (I got the best end of the deal, but one brother is an audiophile who's as slavish as Dad was so had no need of them, the other lacks the space or the interest so inherited some other non-hi fi stuff!)

Unfortunately the speakers have been in storage a couple of years and the foam has deteriorated and I suspect more needs to be done (they sound almost "fine" to my untrained ears, but I know they're not good and definitely not as great as they once were and obviously know the foam needs replacing.)

I'd like to return these speakers to their former glory. I've read a lot about Keesonic and the KBMs and knowing my Dad's history in sound and also supporting small business, I'm not surprised at his choice. So, in part because I'm a music buff (but a relative hi fi novice) and in part in tribute to the old man, I want to renovate these speakers.

Any tips, websites, books or general information would be great. I don't think I'd have a problem with doing any of the work and I know enough about the more obvious things like the cones and foam etc. to be confident. But I know next to nothing about wiring, whether this would need doing and how I would go about it?
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Old 24th Nov 2020, 4:24 pm   #2
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Default Re: How to restore Keesonic KBMs

Given the quality of these speakers and the additional sentimental value, it may be better to have them restored professionally. I'm not suggesting that you can't do this yourself, but an experienced professional is likely to do a better job. They will also be worth more in cash terms if restored by a reputable specialist, should you ever want to sell them.
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Old 24th Nov 2020, 4:32 pm   #3
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Default Re: How to restore Keesonic KBMs

Thanks.

To be honest I had thought that as I do want them to be as good as they can be. I suppose I wondered if it was something I could have a good go at. Ultimately I wouldn't sell (but my kids may want to one day when I'm gone!)

It's a project I'd love to undertake (and I have a vintage radio I picked up at a flea market I'll be asking questions about at a later date) but I'd be crestfallen if I ruined these.

I had seen some Kubs on Gumtree in poorer condition that I might want to have a go at - as I say, I'm definitely a novice but it's something that interests me more and more.

Appreciate your honesty. I'll start looking at suitable places in the north west.
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Old 24th Nov 2020, 6:03 pm   #4
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Default Re: How to restore Keesonic KBMs

If you have a deft hand and are mechanically savvy you can replace the foam surrounds on speakers. There's loads of information on the internet. Here's some for sale, but there's lots of other sources:

https://www.ebay.co.uk/itm/4-10-Inch...Cclp%3A2334524
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Old 24th Nov 2020, 7:08 pm   #5
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Default Re: How to restore Keesonic KBMs

The crossovers will probably need attention too. It really depends on how keen you are to do the work yourself. I'm not suggesting you're likely to wreck them, but it's a fair bit of work for a novice and a pro will probably make a neater job of it (and give some sort of warranty). If these were just a pair of old Wharfdale Lintons I'd make a different call.
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Old 24th Nov 2020, 7:35 pm   #6
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Default Re: How to restore Keesonic KBMs

One very important step - if the foam in any of the surrounds has deteriorated, then please stop using them! Even though they may sound fine at low volumes it won’t take long for a rubbing voice coil to be damaged.
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Old 24th Nov 2020, 8:22 pm   #7
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Default Re: How to restore Keesonic KBMs

By 'foam' I assume you mean the grill material?

I think the KBM had a Dalesford derivative of the KEF B200 with a synthetic rubber surround, which seems to last well, so just check it's intact and pliable. The other three drivers are all domes where the surround material is part of the dome. If I'm right there are no foam surrounds in any of the drivers.

It would be good practice to remove the bass drivers and refit them rotated 180 degrees to combat droop, but otherwise leave them alone.

Other than that, check the crossovers for non-polarised electrolytic capacitors and replace any you find. I think you'll have to take the bass driver out to access the crossover anyway, so just mark them so you can do the rotation at the same time.

If you look around the internet you'll see people have replaced all the wiring in many types of classic speakers with stuff compliant with their religion - several times the diameter of the original wire and marinaded in holy water for an amount of time handed down by some great prophet. You have to be a fully paid up member of their church to be able to hear the differences they claim. Simple answer: Don't bother

And welcome to the forum!

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Old 24th Nov 2020, 9:20 pm   #8
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Default Re: How to restore Keesonic KBMs

Thanks for the replies all.

Some food for thought. I'm going to either get some quotes for what I think might need looking at or take it very, very slowly and see what I can do.

When I say "foam", unfortunately I mean the rubber surround of the Dalesford, not the grill material. I'm not entirely sure how, but this appears to have perished.

The speakers haven't been in use at all for a year (they're in a dry, secure lock up) so I'll definitely bear in mind not using them to prevent damage. I'd be fairly comfortable removing the bass driver and checking, I'd be more cautious (though not against) actually replacing any wiring. I didn't have a clue about the rotation, which makes perfect sense now you've said it, so that's useful knowledge whether I do any refurbishment myself or not.

As I say, I'm less concerned about resale value - I doubt the speakers will leave the family for some time (my kids are all under twelve so they're not interested in my vinyl or CDs...which suits me as I'm not looking for Little Mix or Taylor Swift to be played on my system - my daughter's learning, but one step at a time!)

Funnily enough, my brother said the same thing about the wiring: unless you know there to be something wrong don't bother changing. Though he recommended the capacitors coming out as they deteriorate over time (which I've read elsewhere on these forums and other places too.)

Definitely a lot to think about and I really appreciate the welcome too, thanks.

I'm a bit of a newbie, but I'm much more interested in giving new life to quality equipment than simply replacing.
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Old 24th Nov 2020, 10:53 pm   #9
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Default Re: How to restore Keesonic KBMs

I agree with David that Dalesford drivers normally don't have foam surrounds and don't physically deteriorate. Some pictures would help us establish what's going on.
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Old 24th Nov 2020, 11:27 pm   #10
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Default Re: How to restore Keesonic KBMs

OK, this doesn't sound too bad if they do need new surrounds.

The surrounds can be replaced at home. You need a scalpel, a steady hand, peace and quiet and good self-confidence. No electronic experience needed, this is arts and crafts technology.

First, have a look around to see if you can buy replacement surrounds. Good sites give proper dimensions and you can measure up your existing units to check.

If you decide to go ahead, the first step is to use a fine blade to cut into the glue between the surround and the cone. If in doubt err on the side of cutting into the surround, not the cone. Once you've gone all the way round and freed the surround from the cone you cn now get a bit less delicate in removing the surround from the basket (the stationary frame of the speaker)

The next job is to clean up the face of the basket that the surround will glue to, then you clean up the cone. Don't glue the surround in just yet, there's a bit of alignment yo do first.

The next job is to remove the dust cap at the centre of the cone. You can buy replacements in various sizes, so sacrifice the old cap, cut the cap not the cone, then gently cut and scrape to remove the old stull to leave a pristine cone.

You'll see the tube shaped former which carries the voice coil on its outside. Inside this tube is a cylindrical pole piece, part of the magnet structure. There needs to be a uniform gap between the pole piece and the coil former. So you shim it with some paper. You want a strip of paper whose width is just a bit less than the circumference of the gap you roll it into a cylinder shape and feed it into the gap. Don't worry that the paper forms less than a full circle, you want a smallish gap just to be sure you don't have an overlap. Is the paper a snug fit? If not maybe find some thicker paper, maybe add another of the same stuff. You need to arrive at a snug fit with maybe an inch of engagement into the pole piece's gap.

This now holds the cone structure centred for you. Place the new surround in position to see where your glue needs to go down from the cone edge. Glue the surround to the cone, check it's sitting evenly. Go have a well earned drink, and leave it for tomorrow.

Tomorrow arrives, lift the edge of the surround above the basket and apply glue as per glue instructions and let the surround fall naturally onto the basket. You may need to pinch them together, rather like finishing a pie or I get a few packets of small nuts or washers and use them as lots of little weights around the surround-basket glue joint to get them together. Give it another day to cure.

Next, the paper shimming comes out. Check the gap is uniform and glue in the replacement dust cap.

Beware of stray iron steel dust, filings or rubbish. The field in the gap is very strong and if any magnetic muck gets into the gap it can be impossible to clean out. Compressed air lines aren't enough. Vacuum cleaners have no chance. Move the cone a bit with your fingers, trying to move it evenly, and check for no scraping noises.

Once the dust cap glue is cured, it's time to screw them back in and have a listen.

So, care and cleanliness and not rushing will get you there. You can make a fully professional grade job of it, and anything you do yourself sounds 314.159% better than if anyone did it for you.

(110.823% of all statistics are made up on the spot)

David
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Old 25th Nov 2020, 12:05 am   #11
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Default Re: How to restore Keesonic KBMs

A very good writeup from David, but you will now understand why I suggested you might want to pay an expert to do it.
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Old 25th Nov 2020, 1:19 am   #12
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Default Re: How to restore Keesonic KBMs

I thought I'd write that so he could see the scale of the job and make a decision.

I must remember where it is so I can use pointers in future.

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Old 25th Nov 2020, 8:39 am   #13
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Default Re: How to restore Keesonic KBMs

Great write up David. A couple of points from my own experience. I can't see how a roll of paper will 'shim' the voice coil equidistantly around the magnet, it's more normal to use about four or more stiffish paper or thin card (larger speakers use thin plastic) shims in the form of long narrow strips placed equidistantly around the aperture. Whatever you use has to be in contact with the voice coil and the magnet to keep them apart. But anyway, that part of the structure, the spider, is normally ok in which case I generally don't cut away the dome. Ok, 'replacement' cones are available, but it's doubtful if you'll get the exact size and material, and it will affect the frequency response. Spring pegs can be used to hold the new surround to the basket frame.
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Old 25th Nov 2020, 10:17 am   #14
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Default Re: How to restore Keesonic KBMs

That's a really good write up, thank you.

Actually that part is something that, though quite involved, I'd feel quite confident doing and happy to take my time doing so. Having read through a couple of times, the biggest barrier to me doing this would be finding the time! Really appreciate the detail.

It's really replacing the capacitors internally that I've never done before (or anything similar.)

In terms of the surround, I can't take a picture at the moment (as they're in storage) and perhaps "foam" was the wrong term to use - but I can confirm it is the surround of the cone of the Dalesford drivers. I said foam because it feels to the touch like brittle foam (but it could be very old, dried rubber) that has now perished. I'll try to find a picture online of something similar (or get out to the lock up.)

There is a chance that my Dad or someone else replaced this years ago incorrectly (though this seems unlikely knowing him) or even that my Mum asked someone to do so - I'll check with her.

Much as I'd like to have a go at this myself, I'm leaning towards getting a professional to do it. I think what I might do though is wait until I have them in front of me and post again another time. Definitely appreciate the input - it's actually great to have comments from people who clearly know what they're talking about with no judgement on my naivete!

Plus, outside of my family and one friend who was a sound engineer, I've actually never met anyone who's even heard of Keesonic.
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Old 25th Nov 2020, 10:34 am   #15
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Default Re: How to restore Keesonic KBMs

Quote:
Originally Posted by stevehertz View Post
But anyway, that part of the structure, the spider, is normally ok in which case I generally don't cut away the dome.
I agree that it's not usually necessary to cut away the dust cover. This instruction sheet produced by Good Hi-Fi describes the whole process and gives three alternative centering methods:

https://speakerrepairshop.nl/en/inst...oam-manual/c-7

I've successfully used the first two methods on different bass drivers.

Alan
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Old 25th Nov 2020, 11:28 am   #16
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Default Re: How to restore Keesonic KBMs

I'm still puzzled that these drivers have deteriorated like this. I've never known this happen with a neoprene/bextrene driver.
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Old 25th Nov 2020, 11:31 am   #17
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Default Re: How to restore Keesonic KBMs

I've avoided removing dust covers by making three or four little holes in it, aiming inwards at the gap between the centre pole piece and the inside of the voice coil former, then inserting lengths of copper wire to centre the former around the pole piece, and to align the former axis with the axis of the pole piece. It's more fiddly.

Doing alignment against the pole piece avoids the risk of damaging the winding if trying to shim on the outside.

Using bits of wire for shimming means a new dust cap isn't needed and a few dods of glue will seal the holes afterwards. This is easiest with drivers having their dust cap inside the former.

By roll of paper, I meant a large diameter roll, about the size of the pole piece.

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Old 25th Nov 2020, 11:34 am   #18
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Default Re: How to restore Keesonic KBMs

I've seen a couple of neoprene surrounds that had gone hard, but they'd been exposed to organic solvent fumes.

If there'd been a general problem with Dalesford drivers, we'd have heard about it.

My KEF B139s seem fine after 45 years.

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