Damaged magnet on JBL 2447

[BullFire]

Hi, I have got my self three JBL 2447 with 2352 horns that I'm going to use together with low frequency system from 4675C.
One of the 2447 fell from the 4675C low frequency cabinets during transport. The bracket took most of the damage and the first hit was half the hight of the speaker. Sadly the maget took some damage too. I have attached a picture showing the damage. What do you think, will this damage effect the preformance of the compression driver? I have a spare JBL 2446, can I modify it to be a 1,5" and use it together with 2352 horns? how different will it sound compared to 2447?

[ratitifb]

only superficial ferrite issues apparently without any crack ... that does probably not affect the sounding performance of the driver. But checking all the parts of the magnet circuit would be strongly suggested

[BullFire]

Ok, how whould you have checked it?

[4313B]

Those little flakes off the magnet shouldn't be a problem.

Pull the back cap and make sure the top plate didn't shift pinching the voice coil in the gap. You'll have to make sure that the diaphragm is easily removed. From the photo it still looks viable.

One can see why JBL put rubber tires around their ferrite magnets.

[BullFire]

Ok, thank you: I will do the check later today or tomorrow. On the compression drivers I have seen in the same series, it seems like JBL put rubber only on drivers for bigger horns like the 2446 used in 4675C.

[maxwedge]

I think JBL only put the rubber doughnut on drivers that were sold separate (as components).

[subwoof]

You should be fine - just make sure the little magnetized chips don't fall where they can cause issues. I use the 2" wide Scotch 88 electrical tape ( available in the high-end electrical supply houses here in the US ) to cover the tire-less magnets. Otherwise use thinner tape and multiple passes.

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[BullFire]

I have now removed the back cover and inspected the diaphragm. The diaphragm was easily removed and I could not see any damage to the voicecoil or diaphragm it self. Is there any special method of inserting the diaphragm back in again without the voicecoil touching the side walls or is that not a problem as long as I'm carefull?

[louped garouv]

i believe that most folks will use a test tone generator, and
slowly/carefully/deliberately tighten the screws down a little at a time
while playing the test tone to ensure no rubbing....


someone (think it was actually member Subwoof (above)) mentioned mounting the driver to a horn to make it easier to tell if the driver is rubbing VC or merely resonating...

he also mentioned attenuating the sound level by placing carpet at the horn mouth....
(maybe a keyword search on 'carpet,' and "subwoof" as the user will retrieve the post?)


hope this helps....

[subwoof]

The newer large-format ferrites are well machined and most of the time a dia can be removed and replaced back in the *same* driver without much of a problem. make note of the rotation - there are machined larger area's for the lead wire points and the dia can only go in one way and have the cap go back on correctly.

When the driver is face down, one back cap mounting screw is at the 6 o'clock position and the (+) terminal of the dia is on the RIGHT ( think right on red ).

One of the test jigs I have used is a pair of wood blocks on top of super-thick carpet scraps. When the driver is mounted to the horn, and face down, much of the forward sound is absorbed into the carpet BUT it is STILL loud.

One issues that will fool ANYONE attempting to align a diaphram is that unless the driver sees an air mass / load on it's exit it WILL resonate and sound like a mis-aligned dia at certain frequencies. This is really an issue around 1.2 K

You should be able to sweep the 4" coils from 500 to 2K without hearing any harmonics due to rubbing. You can go lower on the test bench ( easier on the ears actually ) but the driver will actually JUMP around unless the screws are tightened since you are getting closer to the mechanical resonance.

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