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spirou38
09-13-2004, 03:30 AM
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

PSS AUDIO
09-13-2004, 04:01 AM
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!

Flodstroem
09-13-2004, 05:29 AM
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

spirou38
09-13-2004, 05:48 AM
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 :confused: ;

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

spirou38
09-13-2004, 06:10 AM
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

Earl K
09-13-2004, 08:27 AM
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

Guido
09-13-2004, 09:28 AM
Hi Pascal! Welcome :wave:

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 :)

spirou38
09-13-2004, 11:06 AM
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

PSS AUDIO
09-13-2004, 01:43 PM
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!

spirou38
09-13-2004, 02:00 PM
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

Robh3606
09-13-2004, 02:23 PM
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:)

spirou38
09-14-2004, 01:00 AM
Hi Rob,

I just checked, no obstruction :( .

Thank you for your help.

Pascal

Guido
09-14-2004, 03:24 AM
Huh!

This is going to get difficult :hmm:

Did you check all connections for corrosion?
How do you measure the 6 dB loss?

spirou38
09-14-2004, 03:35 AM
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

Guido
09-14-2004, 02:13 PM
OK, now we will need the help of your atomic friend :p

Please keep us posted.

spirou38
09-16-2004, 05:33 AM
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 :banghead: , 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

Figge
09-16-2004, 05:52 AM
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 :)

spirou38
09-22-2004, 07:28 AM
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

spirou38
11-13-2004, 11:50 AM
Hi all,

Great thanks to Giskard who sent me a new 275ND :bouncy: 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 :mad: 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 :confused:

Pascal

LE15-Thumper
11-13-2004, 05:37 PM
Very nice of you Giskard ! Once again a real asset to this forum and the L-H site. :cool:

John Sheerin
11-13-2004, 10:37 PM
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

spirou38
11-14-2004, 01:22 AM
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 :confused: .

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 :banghead: .

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

Pascal

Alex Lancaster
11-14-2004, 04:43 AM
:) 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?

spirou38
11-14-2004, 08:26 AM
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 ) :mad: .

Pascal

John Sheerin
11-20-2004, 04:12 PM
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.