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Old 25th January 2010, 06:38 AM   #1
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Default "Magnet slipped" says the Expert; Surround Repair; Poor sound now

Good Afternoon All,

My brother was to be the happy recipient of some hand-me-down JBL loudspeakers which I purchased in 1979 because his stereo is no longer functional, but my poor old things do not work properly any more either. As always there is a story to make things more complicated than they should be, so if you could bear with me please while I tell you what has happened.

Over the years, the drivers have needed the foam surrounds replaced several times. The most recent time (a few years ago), the chap who did the job sent them back and one of the driver cones just did not move: it was jammed out of kilter and clearly the voice coil did not move at all!

I sent it back to him (via my local hifi shop) and they were returned a few days later. I was told that the "magnet had slipped and that it had to be re-glued." Certainly, inspection revealed large amounts of what appeared to be an epoxy glue (Araldite?) joining the speaker magnet to the frame of the speaker. I used it for about one year and the surrounds "went" again, so I replaced them myself by peeling off the old surrounds, removing the dust cap, wedging the gap between coil and pole with thick paper strips, and gluing on butyl rubber surrounds I purchased from Jaycar in a "neutral" position (mid-throw as judged by the way the surround sat on the frame and by the way the spider was nice and flat). I think the result was satisfactory because the repair lasted me a year or two while I set about making my Linkwitz Orions. However, at high sound levels, something was hitting or clunking within the speaker, but the sound was not too bad and the L/R balance seemed satisfactory.

But now, it is clear that this particular driver just doesn't produce anywhere near as much sound as its mate (when listened to in free air) and in the old JBL box it sounds terribly tinny and weak. I notice that when I push just a bit off-centre with my fingers on the cone, I can hear the coil rubbing on the magnet pole.

I have taken off the dust cap again and peeled the surround from the frame again. Pushing the coil backward and forward, if I put sideways pressure on it, still reproduces the scraping sound, but when allowed to centre, there is no noise. The gap all around the coil is very small, and I cannot tell if it is a little eccentric.

1. Could the speaker repair man have done some damage/misalignment when he did the last repair?
2. Is it usual for the speaker bloke to need to re-glue the magnet onto the frame?
3. Could he have damaged it when he separated the magnet from the frame?
4. Could I have done something terrible when I had it apart last time?
5. Should I just try gluing it together making triply sure that everything is in line?
6. While I have the speaker apart, is there anything I can do to ensure that the whole thing goes back together straight or straighter than my last effort? Is my method for re-aligning the speaker coil satisfactory - thick paper strips pushed into the gap?
7. Should I give up? I don't want to because I have a sentimental attachment to these old things (they were my pride and joy for a quarter of a century) and I hate waste.

Any input would be appreciated. (One thing which surprised me is that there were not many web-sites describing how to replace the surround onesself, but that is by the way.)

With thanks and regards,
George
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Last edited by GeorgeBoles; 25th January 2010 at 06:41 AM.
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Old 25th January 2010, 06:56 PM   #2
Bone is offline Bone  United Kingdom
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Some answers to your queries.
1. - Yes
2. - No
3. - Possibly
4. - Yes
5. - Why not you probably can't make it any worse
6. - It should be done on a jig but if the paper/card strips are the same thickness it might be OK.
7. - If all else fails buy a new driver! Recommended

Tony.
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Old 27th January 2010, 08:01 AM   #3
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Thanks, Bone.

I re-glued the surround, and I might have a slightly louder sound, but still not up to speed. At extremes of cone movement (by my gentle hand), the coil still grinds gently against the pole.

I suspect that the magnet is now out of alignment. I wish I knew why he fiddled around with the magnet: baffles me. I wonder if when he removed the magnet he used heat to release whatever glue was holding the magnet cover in place which might have demagnetized the magnet a bit. A veteran now of 5 repairing surrounds, I am pretty sure that my job is at least OK. I am pretty pi*%ed off with him, that is for sure.

By the way, what sort of "jig" is required to do this properly?
Why, oh why, would he want to remove the magnet? I simply cannot understand that.

If I were to buy another pair of drivers, I would be essentially re-designing the whole speaker which is something I really do not have the skill to do. (When I wrote to JBL about getting a new tweeter about 5 years ago, they politely explained that the speaker had been out of production for more than twenty-five years and that they could not help me! I wonder why.)

Regards,
George.
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Last edited by GeorgeBoles; 27th January 2010 at 08:14 AM.
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Old 15th November 2010, 04:20 AM   #4
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Default loose magnet

Hello there. I am replying to an old thread by George Bowles. I am an amatuer when it comes to this kind of thing too but I also have a loose magnet on a cheap set of speakers I scored on ebay. I was shocked to notice that the heavy magnet structure on these cheap speakers was simply glued on with what appears to be wallboard adhesive, 6 of the 8 speakers I have in my shed have shaken the magnet loose and they are now flying on a wing and a prayer.

I just hope some of the manufacturers are listening, because some of them are using the wrong adhesive and although they make a sale with the dodgey speakers the ultimate conclusion is that the customer will work it all out and will black ban the dodgey brand, and train their children to recognise shoddy workmanship when they see it.

If anyone knows of a site that graphically describes the disembowment of the entire speaker for the purpose of re-gluing the magnet, could you kindly let me know.

If it is simply a futile exercise for the 'back yarder' then I will try to recover any parts that I can as I too, just like George very much dislike waste.

I hope you all have a great week at work or in the workshop and I hope to talk again soon.

Yours sincerely, Phil Elliott
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Old 3rd December 2010, 02:19 PM   #5
JMFahey is offline JMFahey  Argentina
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Hi Phil.
1) It can be done, I do that regularly.
2) To do it properly you will need to leave the basket free, meaning ungluing the "soft/moving" parts: edge, cone, voice coil, suspension, dust cap, flexible wires, so they can be re-glued later (complicated) *or* just cut them out with a box-cutter and recone with a set of new parts , if available (relatively easy) .
3) With frame/magnet clean, you can pry out the magnet with a couple flat head screwdrivers.
Twist it first to see whether the failed joint is either magnet/front plate (the one attached to the frame) or magnet/back plate, the one that carries the pole piece.
4) you will need to improvise some kind of shim to center the polepiece in the gap.
Best option: a thin walled tube with proper dimensions (it should freely slip both around the pole piece and inside the front plate hole, wall thickness will usually be between 1mm and 2 mm).
Best is to have one lathe-turned (it's an easy part, they can't charge you much for it) out of brass/aluminum/nylon in that order of preference.
Plan B , what I often use, is to have a piece of plastic of approppriate thickness bent ino approximately a tube shape (it needs not be 100% circumference, it should have a 5/15mm gap between the ends).
Now you insert your centering tube into the front plate, from the front side, apply a thin layer of epoxy to the front plate, and *carefully* slip the pole piece into the centering tube.
a) use "slow" epoxy , not "10 minutes" or you will not have time to do this.
b) be very carful, because the magnet when close to the front plate will violently pull forward, it may nick a finger, or simplypull forward the centering tube or shim, rendering your job useless.
Get somebody to help you and/or bolt the frame to some plywood, so you worry only about the magnet.
You'll need a 3rd hand anyway, because 2 will be busy with the magnet and the 3rd one is needed to keep the centering tube in its place, avoiding it being pushed out of the gap, so, use 4.
Twist the magnet 1/2 turn as to spread the epoxy evenly and in a thinner layer, some of it should ooze around.
Let it set overnight.
Good luck.
PS: after that you'll still need to recone the speaker, preferably with the original parts.
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Old 2nd March 2011, 12:13 PM   #6
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Hi there 'J'

I have just been looking out the window and there is an attractive woman sitting on her porch, apparently has nothing to do. I think I will ask her to give me a hand.

Repairing these cheap speakers might be good exercise for me to learn a bit more about speaker construction. I have repaired a couple of surrounds before so I know what you mean about the shim/spacer - I used a piece of mylar that I had left over from a job, and it worked a treat. But I used contact adhesive on the surround and I wont do that again, because it is very hard to move once it makes contact! I will try PVA wood glue next time, and just hope that it pulls the edge down as it dries.

Cheers mate and thakyou for the detailed information, this is why I really love this site: Because people give freely and problems are solved. I hope one day that I can answer a few questions, but for now I am doing my 'apprenticeship'. Bye - Phil Elliott
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Old 2nd March 2011, 12:54 PM   #7
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Hello George, that picture of yours makes me laugh! is that some character from Alo Alo?

I am a wood machinist and electronics enthusiast from Tassie so we have a bit in common.

As a wood machinist I can understand your relo's despair. How on earth did the villains get away with that much stuff? You would need a big removals van and half a dozen bludgers to shift it. I guess the people who do see suspicious behaviour, do not report it. It's a sad endictment on our proud heritage and a down-right shame on the dogs who stole it.

I have a crappy memory but the dresser is etched in there now, as I have seen another similar but not the same. It belongs to the friend of a friend. He is a mariner and I believe he brought it back from England about 30 years ago. His piece may have been rosewood from memory but had no glass. Can you tell me if the company that made this item is the same company that made aircraft in the early part of last century? i.e. the Sopwith Camel.

Sincerely, Phil Elliott
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Old 2nd March 2011, 01:02 PM   #8
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by GeorgeBoles View Post
Why, oh why, would he want to remove the magnet? I simply cannot understand that.
The driver was "dropped" either by the Technician or while in transit.
Re-centering the pole piece is critical to a consistent width gap all around. That is not easy!!!!
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Old 2nd March 2011, 05:08 PM   #9
JMFahey is offline JMFahey  Argentina
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I forgot to add an important detail:
Yes, if the magnet slipped (who to blame for it is another different problem) the magnetic system *must* be disassembled, there's no other way.
This particular one is made out of a flat front plate o disk, bolted to the aluminum cast frame, a thick iron "cup" (it can be clearly seen) which can be either glued or bolted to the front plate, and a cylindrical Alnico magnet "slug" inside it.
The gap you see where the voice coil lives is the top of that slug. For precision they glue a disk of iron on top, which can be lathe turned to exact dimensions, the same as the hole in the front plate disk.
Problem is, Alnico *is* one kind of steel, the hardest you can make, so much so that you can't lathe turn nor mill it, only sand cast it slightly larger than needed and grind it with *diamond* wheels. (Not kidding, real diamond, forget Widia or Carborundum).
There is no way you can thread it for mounting, you can't solder it because the heat destroys magnetic properties, so it gets glued , a far weaker solution.
That's why it's relatively easy to whack them out of alignment.
The proper way to repair that is to unmagnetize it; re-glue as needed, with proper glues, using approppriate jigs, and re-magnetize it.
Problem is no regular speaker repair shop has a magnetizer, much less so a de-magnetizer, we are talking tens of thousands of dollars.
If you pull Alnico from its frame, it loses 60 to 90% of its magnetism, that's what happened to you.
Ceramics lose much less, 10 to 20% so it goes almost unnoticed.
In a nutshell? : try to find a used one.
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Old 2nd March 2011, 05:32 PM   #10
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^^+1 JMFahey is right, but when the magnet's broken, don't try to fix it,either way you wont be able to get the factory fit, get a new compatible driver instead.
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