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Thread: 4344MKII and 275ND problem

  1. #16
    Senior Member spirou38's Avatar
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    Yes, of course.

    As soon as I will know a lot more about my problem, I will post. But for the moment, my friend ( the specialist in magnet ) is in Switzerland for about 2 monthes .
    As my problem will not be solved before several monthes , I think that I will order a new 275ND immediatly . Where can I find one at a good price ?:
    And if my deffective one can be repared, I will keep it for spare.

    Thank you for your help.

    Pascal

  2. #17
    Senior Member Figge's Avatar
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    so the driver has somehow demagnitized?

    never heard of before! is this known to happen? (exept for alnico)

    Btw: i could sell u a 2421A
    Last edited by Figge; 09-16-2004 at 06:58 AM.
    Fredrik G.

  3. #18
    Senior Member spirou38's Avatar
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    Hi all,

    I have an answer from Harman France : the 275ND drivers can't be re gaussed as is because of their structure :
    - magnets are just glued
    - there are 2 magnets inside that are assembled North South North South
    - it is almost impossible to dismount the magnets without damage.
    But it seems that they can be rebuilded in JBL US factory but I don't know the cost.

    I will thus consult my friend who is a specialist in magnets and keep you informed.

    Spirou38

  4. #19
    Senior Member spirou38's Avatar
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    Talking

    Hi all,

    Great thanks to Giskard who sent me a new 275ND now my 4344MKII work great . The measurements of the left and right 275ND are now within 1 dB .

    I have not yet met my friend who is always in Switzerland. But I tried to dismount the 275ND, impossible to remove the front cover, I just can remove the back cover to change the diaphragm.

    I will keep you informed of the measurements of the magnet and of the remagnet possibility without removing the back cover

    Pascal

  5. #20
    Senior Member LE15-Thumper's Avatar
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    Very nice of you Giskard ! Once again a real asset to this forum and the L-H site.
    LE15-Thumper
    "Give me JBL, or give me death"

  6. #21
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    Typical way to magnetize a neo driver is to hold it in a non-feromagnetic fixture (like wood or maybe plastic). So when you magnetize, the driver is held in place and does not go flying around. I have heard from some friends that they did not clamp a driver one time and shot it vertically ~30ft. Luckily it didn't land on them.

    If both magnets are magnetized the same direction, I would think remagnetizing would work fine as assembled. If they are opposing to cancel flux or for some other reason, then yes, it would not work. To take them apart you would have to demagnetize them which is hard to do - it's hard to get the material back to zero. Usually, speakers are built with the magnet not charged though - it is difficult and can be dangerous to build a speaker with the magnet hot (although some companies do it that way).

    However, it would be strange for only one of the speakers to be demagnetized. That would most likely be due to not being fully charged to begin with, but that would probably be caught by QC. It could have been caused by overheating, but you would think that both speakers would have overheated together... If due to heat, I am not sure you will be able to remagnetize the speaker.

    Hth,
    John

  7. #22
    Senior Member spirou38's Avatar
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    Hi John,

    Thank you for your reply.

    As the magnets are north south / north south assembled does it mean that they are in the same direction or in opposition .

    And if I understand your explanation, if they are in opposition and if I can't dismount and isolate the 2 magnets, it is impossible to remagnet .

    I will continue to investigate, perhaps it is not a magnet problem .

    Pascal

  8. #23
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    If they are N-S/N-S, they are in the same direction, and the flux will be higher; I am not familiar with Your driver, it has 2 magnets?

  9. #24
    Senior Member spirou38's Avatar
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    Hi Alex,

    Yes it has 2 magnets. And I don't succeed in removing the front cover, I can just remove the back one ( for changing the diaphragm ) .

    Pascal

  10. #25
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    I don't know your particular speaker, but in most speakers, the magnets are glued to the steel parts. This keeps them in alignment before being magnetized. It also helps prevent buzzes and rattles between parts.

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