Jbl 375 issue

[script56]

I got a pair of 375 with bad diaphragms and dirty voice coil. I got New diaphragms and cleaned the voice coils. One driver is producing louder output then the other at certain mid range frequencies. I believe the one that is producing lower output is the one that I fully disassembled. I remove the voice coil plate and phase plug from the magnet structure. I did this because I saw companies doing so to fully restore. When I put it back together I've noticed that the phase plug set into the voice coil plate and it looks like I did not set the height correctly. I know this may sound confusing but when I put it all back together it look like the phase plug was not at the correct height above the plate. Does anyone know what the correct height setting is or how to fix?

[grumpy]

Aside from the phase plug issue (which I have no experience with), perhaps someone who -knows-
can suggest whether a remag would be required (or strongly suggested) after such a disassembly... (?)

[script56]

You may be right, but magnet is very powerful. It was very hard to remove plate from magnet structure due to magnetizim. Also the drivers both work at high end of frequency range but one is low output at mid frequency. Does anyone know frequency range for 375?

[Flodstroem]

If you with the word "voice coil plate" mean the top plate then the Alnico has to be remaged because the Alnico has loosen a lot, a LOT of its magnetism.

The whole magnet motor is a very sensitive system: if dis assemble it it looses magnetism and has to be remaged again.

[Earl K]

Like Flodstroem said, don't break ( dis-assemble ) any magnetic circuit ( unless you are prepared to re-magnetize it ).

:)

[script56]

Can anyone recommend good place that would re magnetize and set all measurements correctly? Would I have to re set top plate and phase plug before I send off to re magnetize?

[dezmond]

Are you sure you centered the diaphragm? You have to run a test tone through it and loosen the screws to adjust it and see if it gets louder.

[script56]

i didnt tighten screws. there is no voice coil rub so i figured its centered, but good idea. ill run signal and tighten at same time

[dezmond]

Yes , while the screws are loose run the tone and gently move it around and listen for the tone to get louder. I had to do this for a le85. Had the same issue lower output then normal. Good luck, hope it resolves your issue.

[Flodstroem]

Can anyone recommend good place that would re magnetize and set all measurements correctly? Would I have to re set top plate and phase plug before I send off to re magnetize?

I cant recommend any place for remag because my location are Norway. However I know that the Orange Count Speaker (former) did it but not any more. But give them a cal and ask who is doing it now a days. Normally they demaged, centered everything and then remaged it so you didnt had to do anything your self. But now, I dont know

[script56]

Adjusting screws did not work for me. I think I need to remag. But on HF signal they both sound same level. One 375 is lower only on mid frequency.

[allen mueller]

Not sure of the status of Great Plains Audio these days but a few years back they were able to re magnetize some drivers for me.

Allen

[ivica]

Adjusting screws did not work for me. I think I need to remag. But on HF signal they both sound same level. One 375 is lower only on mid frequency.

Hi script56,
I would expect that that diaphragm has been used under "heavy use" and need replacement. I have no experience with 375 diaphragms, but with old titanium type
I have recognized such experience....

regards
ivica

[Flodstroem]

One 375 is lower only on mid frequency.

If a driver magnet has loosen its full power, then you could expect lower efficiency especially in the lower frequencies which demands the highest power compared with the higher frequencies

But it could also be other faults on the driver but to me it seems to be loss of magnetic force/power

You could do a simple test: swap the diaphragms in the 375ś and see if the fault stays in the same driver or if the fault moves to the other driver. If it does the fault is in the diaphragm (voice coil). If the fault dont move, then the fault must be in the magnet eg. loss of the drivers magnetic power I guess.

[script56]

I sent to Great Plains audio for re mag. Diaphragms are factory new Jbl.

[Doctor_Electron]

I am curious. If 375's or other comp. drivers are used in a stereo configuration of two loudspeaker systems, with a 375 in each cabinet, and one is damaged magnetically should both drivers' magnetic assemblies be re-magnetized to ensure balance? Is there a type of gaussmeter that can be used to check the flux level?

regards, -D_E-

[script56]

I sent both to remag so they both came back recharged. Only $20 each.

[toddalin]

I am curious. If 375's or other comp. drivers are used in a stereo configuration of two loudspeaker systems, with a 375 in each cabinet, and one is damaged magnetically should both drivers' magnetic assemblies be re-magnetized to ensure balance? Is there a type of gaussmeter that can be used to check the flux level?

regards, -D_E-


Typically,no. They just recharge them until they are saturated and call it good.

[Mike Caldwell]

I sent to Great Plains audio for re mag. Diaphragms are factory new Jbl.

As for the new JBL diaphragms, about a month ago I went to replace a diaphragm in one of my 2426 drivers with a new JBL replacement that had been on my parts shelf for about a year. First it would not fit in the milled alignment recessed area with out getting stuck and it had a couple globs of glue on the voice coil that would rub in the gap. You could also clearly see the aluminum ring that holds the diaphragm did not get punched out cleanly leaving large burr along one side.

JBL service would have nothing to do with the diaphragm since it was out of warranty, they did not know how it had been stored and if it had been installed correctly. The guy I talked to gave me the impression he had never changed a diaphragm!

I was able to salvage the diaphragm by filing the aluminum ring smooth and oh so very carefully scraping the globs of glue off the voice with an xacto knife.

So in short, a new diaphragm these days may not be without issues.

[Flodstroem]

Is there a type of gaussmeter that can be used to check the flux level?

regards, -D_E-

Yes, for example F.W. Bell produce and have produced many models that could be used for checking the gauss-flux level. However, the probe must be the transversal type of probe, not thicker than approx 1 mm / 0.039" The gauss meter must have a scale up toat least 10 kGauss (for paper cone models) or at least 20 kGauss for most of the compression drivers.
When magnetizing an Alnico magnet as toddalin mention it have to be enough magnetized so it saturate. Any flux levels higher than that will have no effect as soon as it have ben saturated.
So its relative easy to remag Alnico (5) just zap the magnet with a magnetic pulse larger than than saturation level and all the magnets will become the same as they where when they left the factory.....
The flux level in the magnetic gap then depends on the mass of the Alnico magnet combined with the structure of the magnetic motor, pole pieces, diameter of the center pole etc etc..