375 construction / assembly / parts

I had a 375 with the phase plug sitting high. After reading a few threads here about the dos and don'ts of taking a 375 apart, I decided to tackle it myself. Yes, taking it apart without degaussing was a task and I was very aware of where my fingers were at every moment as well as told my wife I was going to be doing something stupid. I got it apart and back together with the phase plug in the right location. Not really sure how it got out of alignment as it all seemed to just go back where it was suppose to when reassebling. While apart I noted all the parts. I found a cross section and labeled the ones I knew (or think I know) and put question marks for the ones I didn't. Could someone take a look at the attached picture and fill in the blanks of 1??, 2?? and 3?? I wanted to start a discussion about the assemble and servicing of the 375 but wasn't sure of all the names of the parts. I'm curious to discuss if the 375, LE85, 175, etc. can be serviced by us and not necessarily sent to JBL or other professionals when needing more than new diaphragms. There seems to be a lot of back and forth about this in the posts I read. Maybe more detail can be provided for those wanting to tackle it themselves or advise to those wondering if it is something they can do or should refer to a pro.

Thanks,

Eric M.

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Originally Posted by Eric M.

I had a 375 with the phase plug sitting high. After reading a few threads here about the dos and don'ts of taking a 375 apart, I decided to tackle it myself. Yes, taking it apart without degaussing was a task and I was very aware of where my fingers were at every moment as well as told my wife I was going to be doing something stupid. I got it apart and back together with the phase plug in the right location. Not really sure how it got out of alignment as it all seemed to just go back where it was suppose to when reassebling. While apart I noted all the parts. I found a cross section and labeled the ones I knew (or think I know) and put question marks for the ones I didn't. Could someone take a look at the attached picture and fill in the blanks of 1??, 2?? and 3?? I wanted to start a discussion about the assemble and servicing of the 375 but wasn't sure of all the names of the parts. I'm curious to discuss if the 375, LE85, 175, etc. can be serviced by us and not necessarily sent to JBL or other professionals when needing more than new diaphragms. There seems to be a lot of back and forth about this in the posts I read. Maybe more detail can be provided for those wanting to tackle it themselves or advise to those wondering if it is something they can do or should refer to a pro.

Thanks,

Eric M.

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Hi Eric,

I think (not sure):

1xx=Magnetic Circuit (return path)
2xx=Aluminum short ring for HF impedance control (as on bass driver 'Flux stabilizing ring )
3xx=Top Plate

regards
ivica

Quote:

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Originally Posted by ivica

Hi Eric,

I think (not sure):

1xx=Magnetic Circuit (return path)
2xx=Aluminum short ring for HF impedance control (as on bass driver 'Flux stabilizing ring )
3xx=Top Plate

regards
ivica

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Thanks,

I'll update the photo.

Updated cross-section. Does this look correct to all?

Thanks,

Eric M.

From what I can tell from disassembling the 375 is that the only crucial assembly is between the Aluminum Short Ring (2) and the Top Plate (1). This sets the VC gap. The Magnetic Circuit (1) and the Phase Plug appear to be factory (and permanently) attached to the Aluminum Short Rig (2). Everything else is held together by magnetic force or screws where alignment is not an option or not crucial. The throat, dust screen, magnet and Magnet Circuit (1) / Top Plate (3) assemble are all held in place by magnetic force and then screwed together with the back cover when that is installed. Their location or placement does not change the VC gap which is set between the Aluminum Short Ring (2) and the Top Plate (1) before the final assemble begins. Getting them in the correct location is not easy due to the extreme magnetic force but can be done and does not require the use of precision measuring tools. You either have it in the right location or you need to keep trying. It really can't be assembled wrong, just not assembled at all if things do not line up.

Do other agree with this? Am I missing something? Again, I don't want to down play the extreme magnetic force and the danger that you could get a finger caught when doing this. I shrugged off the comments I read about this. Once I took the 375 apart, I realized it was no joke.

I do want to talk about the disassembly process (which could cause damage just by itself) but wanted to make sure we are all on the same page as far as assembly is concerned.


Thanks,

Eric M.

Very good thread that was really missing from the forum! Accidentally i just today posted a video of 2441 disassembly that might be helpful.



Here is the video:
https://www.youtube.com/watch?v=wvWSg2tpFLk

Excellent video. This was what I wanted to address. You took your driver apart very much like I did mine. It seems as though the magnetic force of the 2441 is less than the 375. I needed wedges and shims to get about a 3/4" gap before I could get my 375 separated as the force was too great. I see you got a pinch putting them back. I was worried about a crushed finger when doing mine! There isn't any tight tolerances or measurements that need to be observed when reassemble, just lining things up. As you showed while inserting your throat, it either fits or you need to adjust and try again. Not too difficult, you got yours apart and together in 10 minutes.

I'm curious how much you can reduce the magnetic force of the magnet while striking your driver to separate as some have mentioned on this forum. The magnet is actually inside the housing. When striking an object like you did with a knife and I did with a chisle, we are actually striking the housing and top plate. Would this have the same affect of reducing the magnetic force of the magnet as if you were to hit the magnet directly?

I like your idea of using a knife. Much less impact to get it between the top plate and housing. After that it is all about prying and inserting shims / spacers until the magnetic force has decreased enough to remove. I'm not sure if I could get away with doing this on our kitchen counter or with my wifes favorite carving knife though!


Thanks,

Eric

Dear Eric, the video is not mine so I have no idea about it. Point is that the gagnetic strength is not only decreased due to the striking with the hammer, I believe its the magnetic strength is decresed because of interuption of the magnetic circuit.

I was reading the other post you were commenting on about the same subject. I have a shop that does all my JBL reconing. It's a one man shop and I've know the guy for 30 plus years, he is very knowledgeable about JBL products. I need to pick up a pair of LE15a's he's working on for me and will run this question by him. Perhaps we'll get to the bottom of this. I was aware of the "striking of the magnet" reducing its magnetic force, but was not aware that pulling something that was "stuck" to it could do the same thing.

Thanks,

Eric

Quote:

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Originally Posted by Eric M.

I was reading the other post you were commenting on about the same subject. I have a shop that does all my JBL reconing. It's a one man shop and I've know the guy for 30 plus years, he is very knowledgeable about JBL products. I need to pick up a pair of LE15a's he's working on for me and will run this question by him. Perhaps we'll get to the bottom of this. I was aware of the "striking of the magnet" reducing its magnetic force, but was not aware that pulling something that was "stuck" to it could do the same thing.

Thanks,

Eric

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That would be so very nice of you!

Thank you!

I talked with Dan at Audivex Speakers In Berkeley CA. I've known him for 30 years. He's been an authorized JBL service center for years and is my go-to guy for anything JBL.

You were right. Taking apart an Alnico speaker does reduce the magnetic force significantly, unlike a Ceramic which is very minimal. It sounds like the disassemble does more damage than any sort of banging on the unit that might demagnetise. Apparently the way JBL made these speakers (375, LE85, 175, etc.) was to completely assemble them and then magnetize the entire unit at once. He says he does not know anyone who remagnetizes speakers. JBL no longer does it, Orange County is gone. Possible Great Plains Audio might still do it.

I wonder what it would take to build a magnetizer? Anyone have specs of a design large enough to do a 375 (or even a 15" or 18" driver). It could be a fun project. I'm guessing it would be a Big transformer and maybe a 220 volt input?

Thanks,

Eric

While Orange County may be gone, one of the guys who worked there forever bouhgt the speaker stuff and continues operation under a new shop name. He/they repaired a LE15 with a dragging coil for a friend recently and did a perfect job in a two day turnaround.

I will ask him for their contact information and post it here later today.

I have never seen their mag charger but have seen pictures of other units and they would not be something one builds in a day at the shop although I agree it would be super fun to have/make one!

More central in the US would be the guys at Great Plains who do all the Altec stuff.

Narry.

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Originally Posted by 1audiohack

I have never seen their mag charger but have seen pictures of other units and they would not be something one builds in a day at the shop although I agree it would be super fun to have/make one!

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I think you might be right. After my last post I did a little searching. Not as simple or basic as I was hoping. And, lots of power needed for big speaker magnets.