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Thread: Loose back plate and pole piece

  1. #16
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    I have, over the years, collected quite the seemingly odd collection of hard plastic circles: old "new" plastic pill bottles of every size, the caps from large fluid containers (like laundry detergent/softeners etc) and anything else that might prove useful for restoring even, concentric gaps between two pieces of metal.

    At times (they seldom work right off the shelf) with a little massaging, they have proven indispensable to making this sort of repair successful, especially with tweeters and smaller "mid" and "radio" speakers but have "saved the day" for larger transducers as well.

    I have had good luck using hard plastic and nylon like materials due to the fact that they are perfectly round, non-magnetic and the fact that adhesives are reluctant to bond to them very well if at all (a bonus if any adhesive does somehow manage to creep too far) The height of this type of homemade solution helps too when the two pieces need to be brought togther in a controlled fashion (like the rectangular Peerless tweeters from long ago)

    As a result, I have made and kept a number of these tools, almost to the point now of clutter. Problem is, with most of them, they are a one off, one time solution and most likely will never be used a second time, a mixed blessing of sorts.

  2. #17
    Senior Member Flodstroem's Avatar
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    Thanks guys for your inputs and concern about the magnet gap and the centering of the center pole

    Im going to use som plastic tubes (from earlier JBL re-cone kits ) and brass shims for to center the pole piece exactly in the center of the top plate. But in the first step
    the six screws will be used for to prevent the two poles to come together.
    Also, I know that Alnico magnets eg a speaker magnet will be demagnetized if taken apart, but this is not the case with ferrite magnets.
    I have the proper equipment (F.W.Bell) for to measure the magnetic field before and after doing the job. So Im going to check this strictly.

    I let you know when the job is done.........
    Flodstroem

  3. #18
    Senior Member Flodstroem's Avatar
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    Finally I received the epoxy. Yes, I used the epoxy only for the reason that it gave me the necessary time for adjusting the back plate for to have it exactly centered with shims before the glue was hardening.

    I used raw force to get the two pieces to come loose. There was a lot of rust between the two pieces including som old glue. Locked like the "Elmers" glue or something like that. It was pretty easy to clean the magnet but fare from that concerning the back plate. luckily I have a lathe and that machine helped a lot. The lathes chuck couldnt grip around the back plate but I found a solution (see the pictures)
    After cleaning (sanding) it become nearly as a new. I further cleaned it with compressed air etc...

    Now the two parts was ready for gluing. After the glue had been applied I put the two pieces together. But before doing that I had made a tube of hard paper, only for to center the two pieces when they came together and that was the most scaring moment of all........

    After that and with no fingers in between......... I had to use the center screws mounted in the PVC-tube for to center the pole piece lifting out the paper tube and putting in hard (metal)shims for to lock the center piece exactly in place when the epoxy was hardening.

    I used heat for the hardening period. One benefit among others for to use heat was to making the epoxy more liquid. When the heated epoxy became hot the excess epoxy came out doe to the force of the magnet.

    I used my baking oven and I warmed it up to 70° Celsius /158° F. I left the basket w. the magnet in for approx an hour. After the "baking" it was finished and after some cooling period I then made som checking. I couldnt find any misalignment. If there was any maybe it was down to less than a thousand of an inch or so. I had the equipments for to check the magnetic force in the air gap and it proved to be in order and that also proved the benefit for the ceramic magnets robustness. I checked further with the old cone/voice coil assembly and it looked very good so far

    In the pictures you could see what I had to struggle with.
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    Flodstroem

  4. #19
    Senior Member Flodstroem's Avatar
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    More pictures
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    Flodstroem

  5. #20
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    Lovely work there!

    Congratulations !


  6. #21
    Senior Member Flodstroem's Avatar
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    I dont know if I mention this earlier but I do have two LE14H-1 baskets. One with the magnet now fixed and the other that had a heavily rusty magnet.
    How do you clean a rusty magnet? I had to think twice before I went on with this one.

    I have had the other ones back plate in my lathe and that lead me to the decision for to choose the same procedure for the rusty magnet circuit.
    It was a hell of a job but it went OK. When I was finished with the lathe I used an air gun for to get out most of the dust coming from the sanding.
    Then there was a time boring job taking place: thoroughly cleaning of the air gap. If I may guess, I used approx 2-3 meter of that tape reel for to get all metal (iron) dust out from the air gap that the compressed air coldnt get out. I also used som solvents for to clean the air gap. Finally I made an inspection with a magnifier/loupe for to be 100% sure there was no iron particles left in the air gap.

    The LE14H-1 baskets looked far from nice so I decided to sand them and then spray som mat black paint on the frames(outer rim). They became much nicer after painting them black. After that there was one job left that I had to do and that was the mounting of the cleaned magnet to the basket.
    I used a glue that my brother had told me was very good for metal gluing, the name of it here in Norway is PL400 (by ilbruck)
    The basket holds in place by 6 phillips? screws

    And finally, now Im the owner of two excellent LE14H-1 baskets ready for new cone kits but I really had to look over my finances...............
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    Flodstroem

  7. #22
    Senior Seņor boputnam's Avatar
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    Quote Originally Posted by Earl K View Post
    Lovely work there!

    Congratulations !

    Yeah. This is a very impressive thread...
    bo

    "Indeed, not!!"

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