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Thread: 4344MKII and 275ND problem

  1. #1
    Senior Member spirou38's Avatar
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    4344MKII and 275ND problem

    Hello,

    I am a happy owner of a pair of JBL 4344MKII ... but I have a big problem : the level of one of the 275ND compression drivers is 6 dB under the normal level. Of course, I reversed the two 275ND to be shure that it was not a filter problem. Then I changed the diaphram with a new one and the problem remains. So it seems obvious that it is a compression driver problem, perhaps a magnet problem ?

    My préamp is a TACT RCS2.2x and the amps are TACT 2150. As the preamp is a room correction préamp, it succeeds in giving a flat response curve by applying corrections and amplifying the frequencies between 1.3 and 8 kHz.

    I mailed to Harman USA but never got an answer and Harman France told me that is was not possible to re magnetize the magnets of a 275ND.

    Have you ever heard of such speakers problems ?

    Excuse my very poor english and thank you for your help.

    Pascal

  2. #2
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    Re: 4344MKII and 275ND problem

    Originally posted by spirou38
    ... Of course, I reversed the two 275ND to be shure that it was not a filter problem. Then I changed the diaphram with a new one and the problem remains. So it seems obvious that it is a compression driver problem, perhaps a magnet problem ?...

    Pascal
    Pascal,

    Just try the following:

    Test the new diaphragm in the good 275ND as you may have a new defective diaphragm.

    If the diaphragm is fine, try the bad one in place of the good one.

    Thus will let you know who is responsible!

    Bonjour,

    Voici une méthode d'investigation:

    Placer ne nouveau diaphragme dans le bon moteur pour vérifier un éventuel défaut du diaphragme neuf.

    Placer le "mauvais" diaphragme dan le moteur qui est bon pour déterminer si c'est un problème de diaphragme ou d'aimant!

  3. #3
    Senior Member Flodstroem's Avatar
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    Neodymium can be re magnetized

    Hi Pascal

    If you find out that the magnet in the bad driver is weaker than that of the good ones
    ( you have to get it confirmed by a serious measurement at some factory or university or at the JBL/Harman-France) then you can have the 275ND re magnetized

    The only thing is that Neodymium is much harder to re magnetiz than other magnetic materials (eg. Alnico, Cobolt, Ferrite etc), and one must use a lot more of
    power/energy to magnetize ND-materials. Because of that there is not all of the magnetizing factories that have the necessarily equipment to do such a work.

    Hope its not a magnetic problem with your 275ND-driver.

    How old are these drivers ?

    Best Regards
    Flodstroem

  4. #4
    Senior Member spirou38's Avatar
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    4344MKII and 275ND problem

    Hi Yuri,

    Here is what I did :
    - I reversed the two 275ND in the 4344MKII -> the probem that was on the left channel is transfered to the right channel ;
    - I tried the new diaphragm in the good 275ND ( to be sure that it was a good diaphragm ) -> OK ;
    - I tried the old diaphragm that I suspected in the good 275ND -> also OK ;
    - I tried the new diaphragm in the bad 275ND -> more than 6 dB under the normal level ;

    So I'm sure that the problem comes from the driver, not from the diaphragm ; I'm also sure that the diaphram is correctly centered. It's why I suspect the magnets. Do you think it is possible to re magnetize the neodynium magnets ? Or where could the problem come from ?

    Pascal

  5. #5
    Senior Member spirou38's Avatar
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    Hi Flodstroem,

    The drivers are almost new. I bought those 4344MKII 6 monthes ago. They were stored and unused in France for about 2 years. But unluckyly the retailer has now disappeared, so there is no possibility of guarantee.

    I hope it can be a magnet problem because I have a friend that could do that job, he works for atomic energy committee in Grenoble and he is a specialist in magnets. So I will contact him for that problem and first make measurements. But if it is a magnet problem, the difficulty is that it is not possible to dismount the 275ND to re magnetize the magnet ( I was told of that by the JBL service station in France ; perhaps it is sealed ? ). But my friend could perhaps find a solution.

    And if it is not a magnet problem where does it comme from ?

    Pascal

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    Hi Pascal,

    I'd have the gauss level within the gap of the suspect driver checked by your friend in Grenoble.

    Ask him if neodymium magnets can be regaussed. I don't know. Also, ask him whether they can be regaussed while still encased within the 275nd shell. I've been told ( so this is just hear-say ) that depending on the type of the gaussing machine, the presence of aluminum parts makes it very dangerous to regauss. It apparently has to do wih how quickly the magnetic charge is applied. A "too fast" charge ( as I've been told ) turns any aluminum part into a moving "projectile" . This works along the electromechanical principal of the spaceage "rail-gun" weapon ( see Regans/Bushes STAR-WARS ) . Again, this is all "hear-say" .

    I've heard that a 275nd diaphragm can be fitted into a 2425/6 driver. You might want to verify this information with your local JBL repsresentative. If correct, this could represent at least a temporary fix.

    <. Earl K
    Last edited by Earl K; 09-13-2004 at 11:40 AM.

  7. #7
    Senior Member Guido's Avatar
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    Hi Pascal! Welcome

    Although I'm no magnet specialist at all I do not believe in a weak ND magnet. It would never have passed the JBL test.

    I had a similar problem with a 2426 driver and the problem was a loose/corroded connection between Diaphragm connection cable and the terminal.

    Another possibility is a partial blocked phase plug.

    Hope you get this beautiful speakers back into operation
    BLS Reference on Passlabs XA 30.5 + Everest DD66000 clone on Pass XA 100.5

  8. #8
    Senior Member spirou38's Avatar
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    Hi Guido,

    I will try to test the connection between the driver's plugs and the diaphragm. When measuring the DC resistance from the 275ND plugs, I get 3.2 Ohms which is exactly the same on the good 275ND but I will check again. What was exactly your corrosion problem ?

    Sorry, my english is very bad and I don't understant what you mean when you say "Another possibility is a partial blocked phase plug".

    Thank you for your help.

    Pascal

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    Originally posted by spirou38
    ... Sorry, my english is very bad and I don't understant what you mean when you say "Another possibility is a partial blocked phase plug".
    Bonsoir,

    Cela veut dire que la pièce servant à la mise en phase peut bloquer le déplacement du diaphragme ce qui diminuerait de ce fait le rendement global du moteur!

  10. #10
    Senior Member spirou38's Avatar
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    Thank you Yuri,

    So,
    1/ I should see some marks on the back of the diaphragm ?
    2/ I should hear it if the diaphram bumps on the phase plug ?

    Pascal

  11. #11
    Administrator Robh3606's Avatar
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    If you have the diaphram off the driver shine small flashlight from the horn side to see if the phase plug slots are all clear. Easy way to look for a partial obstruction.

    Rob

  12. #12
    Senior Member spirou38's Avatar
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    Hi Rob,

    I just checked, no obstruction .

    Thank you for your help.

    Pascal

  13. #13
    Senior Member Guido's Avatar
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    Huh!

    This is going to get difficult

    Did you check all connections for corrosion?
    How do you measure the 6 dB loss?
    BLS Reference on Passlabs XA 30.5 + Everest DD66000 clone on Pass XA 100.5

  14. #14
    Senior Member spirou38's Avatar
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    Hi Guido,

    Yes, I cheched all connections .

    The measurements were made by my TACT RCS2.2x with a measurement microphone. When listening in bypass mode I can notice that one of the two 275ND is weaker than the other. Of course, I have tried with exchanging the 2 drivers.

    Pascal

  15. #15
    Senior Member Guido's Avatar
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    OK, now we will need the help of your atomic friend

    Please keep us posted.
    BLS Reference on Passlabs XA 30.5 + Everest DD66000 clone on Pass XA 100.5

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