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Dislocated Woofer, diy repair?

cosaw

Novice Member
*****The detail (for those interested) - skip this first but come back if you want to learn more.******
I have some approximately 20 year old Technics stereo equipment of my dad's which is good stuff but has never really had much use. The equipment is and has been located in an atmosphere which is subject to temperature and humidity change. Anyway after recently moving the equipment to the front room I have been giving it some more use.

The speakers (Technics SB-3050) are three way and the woofer diameter is 10". The woofers are of paper/card type with foam mounting. While trying to figure methods of insulating them from the floorboards I was keeping an eye on them and driving them quite hard but not overly. I had faded out the left speaker with the balance control (Presumably balance fades out one speaker as opposed to increasing the volume/wattage to the other? Anyway as there was no increase in volume in the right speaker or increased peaking on the power meter I continued.) Noticed a slight buzz/rattle which shouldn't have been there, I thought just a bit of distortion and maybe a flaw in the original speaker design. After listening for moments longer this increased and I quickly turned the volume right down. On closer examination the cone had dislocated itself from it's sponge type mounting. The cone itself is intact but it is dislocated from the foam mount, plus some of the mount is torn. On closer inspection of this spongy material it seems that it has been subject to some perishing over the years. Though its natural colour is a dark grey there is some brown discolouration in places. If you rub the foam gently with a finger it gradually wants to disintegrate, I compared this to a newer cone of similar design where the foam was firm and resilient.
*****END********************************************


**In brief**
While driving some old Technics speakers the 10" paper/card woofer started becoming detached from its foam mounting, the cone itself has not been damaged. There is some perishing of this foam mounting and I believe this to be the cause of the problem rather than me overdriving the speaker.
*********

I'm pretty sure the voice coils aren't damaged. The speakers are my late fathers and hold sentimental value. To this end I would prefer to try a fix before buying a new (different :( ) woofer, even if I make a botch it's worth a try and would be interesting to see what results could be achieved.

First I will attempt: extracting the woofer and gluing the cone back to the mounting from the rear. First question: What sort of glue would anyone recommend? Presumably it would need to be strong yet flexible. Preferably this glue would be easy to remove in case I have to try my second method. I was thinking something like Copydex. Also to reinforce the ageing foam I may think about somehow flexibly securing some gauze type material to the rear of the foam, but this would need some more research.

Second method (if first unsuccessful or until foam mounting fails again): Remove the whole foam mounting ring and replace. Second question: Does such a ring of foam material or equivalent remedy exist for purchase?

When I have time to attempt a repair I will post my results.

Any ideas/experiences welcome.

Simon
 
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STOWITBELOW

Active Member
I once excecuted a similar repair with Evo stick. (without unglueing it all) Seemed to be ok, just fiddley, pulling the cone out to apply glue, & releasing later (contact adhesive) to adhere. It wasn't split, just the edge of the cone had become detached from the frame.
 

davehk

Active Member
If you remove the foam mounting ring, you will almost certainly loose the centering of the voice coil in the magnet gap. It is nigh on IMPOSSIBLE to correct this without specialist skills. It is also exceedingly difficult to fabricate a perfectly circulay replacement and fix it perfectly concentric to the axis of the pole-peice.


If the surround is not to fragile, then just glueing back the bit that has come away might work. However, I would use a latex based adhesive (Copydex) rather than a petroleum based one which is likely to attakc the foam. If the surround has perished, glueing the cone back will, at best, be a very temporary repair.

I suggest you contact Wilmslow Audio (with whom I have no connection) with details of the speaker - they may well be able to re-cone the unit for you, or they can probably supply you with a compatible replacement woofer at a reasonable price.
 

cosaw

Novice Member
Thanks guys. Been doing a bit more research and for people with similar problems here is what I found out.

It seems like it's a common problem for speakers of this vintage. Foam rot is what it's called, a problem that can raise its head after about 10-15 years. While some say that foam is better for sound fewer manufacturers use it these days because it rots, it seems that cloth type surrounds are far more common in recent years. Foam rot is not terminal, as long as only the foam surrounds are involved a re-coning is not required. There are company's that will "re-foam" (I haven't looked into the prices yet, but re-coning is much more expensive apparently. There are also many re-foam kits available for the DIY enthusiast or for people on a budget (I found most from America - as prices are quoted in dollars).

As stated it would seem that one of the main issues in re-foaming involves getting correct voice coil alignment. The most accurate (and some would say the only reliable way) to re-foam involves cutting away the central dust cap of the woofer and using shims to centre the voice coil while gluing the surround in place. The shims are then removed and the dust cap is re-glued/replaced to finish. Problems associated with removing a dust cap include cutting the voice coil wires or the cone itself. Also it would seem that some dust caps can be more difficult to remove than others. You may have to use the original dust cap (which for me would be preferable) but re-gluing may lead to a less than aesthetically pleasing result. The dust caps on my speakers are concave (see pic - "for extended frequency response" says the brochure) and look more tricky to remove or find a suitable replacement for.

There are methods to re-foam without removing the dust caps. There is some debate as to how reliable these methods are, the most basic involves pushing the cone backwards with your fingers and then releasing it while the setting glue is in place, it is hoped that the coil will then find its natural resting point. During this procedure you can listen for scraping of the voice coil. If the coil needs further centring additional manipulation of the cone and foam can be applied.

A method which sounds more promising involves first gluing the foam to the cone and then to get centring sending a current to the speaker. Using a signal generator or test tone and an integrated amp you send a low amplitude low frequency (60-80Hz) sine wave to the cone, enough to see the speaker vibrate. As this happens the magnetic force will centre the voice coil in turn centring the loose cone/surround within the speaker basket. While the signal is still applied clips are then used to clip the outside foam edge to the speaker basket. Once clipped firmly the signal can be discontinued. You then go through the steps of removing a clip gluing the section to the basket re-testing with signal for voice coil rubbing, repeat etc.

The more crude method uses a 1.5-volt battery jumped across the cone terminals, but I don't fully understand the procedure to explain properly.

What I have explained is only what I've found out. If people want more information then I should be able to point you in the right direction. Do a google search for "foam surrounds" and other keywords as above.

The research continues, though I am looking into the signal generation method without removing the dust caps. If I botch it I can always try again or let a professional have a go although they will probably insist on removing the dust caps.

Hope this helps anyone in a similar situation.

Regards.

Simon
 

golem

Standard Member
Simon,

I have the exact same speakers as you, with the same problem. I'd be very interested in how you get on with refurbishing them, especially if you have good results. The trick with the battery is very simple, by the way. There is a separate restraining collar at the coil end of the cone. Applying a 9v bias will either pull the cone in or push it out, putting the collar under tension, which will automatically centre it. This will flatten a battery very quickly, though.

Tony.
 

cosaw

Novice Member
Hello Tony,

Can't beleive I didn't update this thread with this link: Refoam

These have been done a while now. Read the above link thoroughly and it should allow us to talk more about the best way to go about things.

How much do you know about speakers and speaker building? Have you done any refoams before? Since this job I have learnt alot and now can appreciate how the refoam may have affected the character of the speakers negatively. I have no real way of measuring as don't have orriginal woofers to compare to. I don't rely on memory or subjective listenings unless I can have units side by side.

I can direct you to the exact foams I used and help you with the methods I used and even suggest other maybe better ways of doing things. What made this refoam difficult was the small area which was available to work in - you may understand more when you read the link above.

Thats it for now,

Simon

P.S. Have updated the picture links in this thread.
 
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nunkest

Standard Member
Hi Simon,
I'm facing the same exact problem you faced some time ago: a pair of Technics SB - 3050 speakers to refoam.
I read all the threads and the pages you wrote, but I have some questions to make you, if you have enough time to answer me.
You said you used a pair of foam surrounds for each woofer: did you join them together with glue before join each pair to the cone?
At the end wich kind of glue, did you use exactly? the Copydex?
And more important: which side of the surrounds did you join for first to the cone: the inner one or the outer one?
And ... I swear this is the last one...which technique did you follow to join the inner side of the surround to the back of the cone?

Thank you very much, Alessandro.
 

cosaw

Novice Member
Hi Alessandro,

Firstly, did you check the link in the thread (as above) for more details? In case you didn't here it is again:

Refoam

Here is the kit I used used, you'll need two for each woofer. They include the correct glue - I ordered them from this site:

Parts-Express.com: Surround Kit For JBL LE10 | Surrounds surround repair surround kit surround replacement repair kit repair parts kits kit jbl drivers driver SURROUND KIT JBL LE10 Parts Express Speakers Speaker Surround Kits

Let me tell you - I did many months of extensive searching and research before I found any refoam kit that came close to a good fit. Most were just too large in the radius of the actual roll. You've probably got your work cut out if you want to find a foam kit that will work better and most measurement data held is inaccurate and unreliable. You will save yourself a lot of hardwork if you go with my suggestion. It is still a compromise but then again the original foam is not available.

nunkest said:
Don't worry Simon!
Take your time...in the meantime I'm removing the old foams and of course... thank a lot ;)

You'll find removing the old foam and cleaning up the cone and gasket a large part of the job. If you haven't already done so the rubber gasket can be removed by removing the "half roll" part of the foam first (with scissors or a sharp knife). Then you can use something more blunt to insert between the gasket and the basket to lift the gasket away from the basket without damaging the gasket. You will re-use the gasket latter.

You said you used a pair of foam surrounds for each woofer: did you join them together with glue before join each pair to the cone?
No, I cut a break in a single foam and glued this to the rear of the cone. I then carefully, and over a long period, cut a smaller section of foam from one of the spares to fit the gap left in the circumfrence. You want to cut it on the big size and then trim it down bit by bit with a sharp knife or scissors until you get a fit you are happy with. It may take a few attempts but you should have plenty of spare foam to work with. I then glued the final piece to the rear of the cone. I did not overlap the foams but just cut them to meet tightly and then painted a small amount of glue over the foam at the rear (I also did a bit of gluing at the front - but that is up to you).

And more important: which side of the surrounds did you join for first to the cone: the inner one or the outer one?
Join the inner to the rear side of the cone first. The cone rear should be extensively cleaned so the old glue and foam is removed as best as possible - this will assure a good seal with the new foam and the same overal cone weight when finished. I made some basic scraping tools out of an old credit card (or similar) to cut into suitable shapes to reach between the basket and the cone and to the rear side of the cone. Most of the cleaning of the metal basket I did with white spirit - don't use any cleaners on the cone - just scrape it while gently but firmly holding the cone central with the other hand.

And ... I swear this is the last one...which technique did you follow to join the inner side of the surround to the back of the cone?
Well because you've already cut into the foam's circumfrence it's fairly easy to just slide it under the back of the cone (and I seem to remember letting the otherside of the foam rest on the opposing basket side while it dried). Start from the opposite side to where you made the cut in the circumfrence and work from both sides squeezing gently with your finger tips and taking out any slack. Leave this to set before you put in the final small joining piece of foam. When all the foam is on (including on the basket side with the gasket) you wan't to press gently on the foam itself where it meets the cone and look for any places where it hasn't stuck completely (they'll causes buzzes even if there isn't an air leak). I then made a thick sryinge end out of some tiny brass pipe I got from a modeling shop and used it on the end of the squeezy pot with the glue in to gently push into any places that needed extra glue.

You'll need to importantly experiment with how you're going to cetre the woofers well after you've glued the inner foam to the cone. Remember the last part you should glue is the outside of the foam to the gasket and it should be centred some how before you do this. The new foam (if you use the one I suggest) is slightly narrower than the old so will allow more room for correction. I created a test tone on cd - can't remeber what it was but I'm guessing somewhere between 60 and 100hz sine wave will do. You'll find more info on this in one of the links above. It works and cone cetres itself but only while the tone is playing - at this point if you give a light tug on one side of the foam to de-centre the cone you may hear the voice coil catch the pole piece. On some types of speaker I think there may be the potetntial for a short if the voice coil and pole piece touch so while doing this centring I'd suggest you try and use an amp with short circuit protection on the outputs.

I found with one of my woofers it had a bit of a centering error when no signal was applied so I just pulled it radially by less than about 1mm in the direction opposite to that which was creating the voice coil rub at the time - I then glued this bit first, just a spot, let it dry and then tested it and the rub had gone. Once there was no rub I glued the whole thing - I then put a layer of glue on the original gasket, fortunately I had a dinner plate with a rim the same size of the gasket so I used it with rim side down (and the speaker lying on its back) to hold the gasket in place while it dried - it worked a treat.

Hope all this helps.

Feel free to get back to me and let me know how you're getting on,

Simon
 

BlueWizard

Distinguished Member
In various do-it-yourself forums, the glue that is most often mentioned is -

Aleene fabric glue

-which is available in the UK.

aleene's glue, Crafts, Home Garden, Glue items at low prices on eBay.co.uk

Aleene's Tacky Glue

This is typically found in Fabric and Sewing stores.


As to maintaining the alignment while repairing the Surround, the SPIDER will do that. The Spider is the part of the suspension that is mounted to the magnet structure down at the base of the cone near the voice coil.

If the surround or cone are damaged, they can still be repaired but it becomes much more difficult.

If the surround or cone are not torn, and have merely separated, a little dab of Aleene's glue applied to the area with an artist's paint brush should work fine.

You should not have to support the cone if only one small section has separated.

Steve/bluewizard
 

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