JBL E-120: magnet resetting

[Rhodesplyr]

I was given a non-working JBL E-120 which turns out to have a slipped magnet from failure of the adhesive between magnet and top plate.

My brother used to be a speaker repair professional, and, between the two of us, with pieces of wood and a large pry bar, we managed to remove the magnet from the basket with no damage either to the magnet or our fingers.

He tells me that, years ago, he managed to reset a magnet by completely shimming the voice coil gap to keep the magnet centered during reassembly with some hard plastic tubes. But it was tricky, and he no longer has these shims.

Most sources say that the ideal way to do this is to degauss the magnet, reassemble, and remagnetize, but, other than heating it to its Curie point, I don't know how the few speaker repair places that claim to be able to do this go about degaussing a large ceramic magnet.

At a certain point, it costs more to fix the speaker than it's worth, so I'm wondering if there are practical ways to degauss the magnet. I have a couple of friends who make ceramic art, and they must have access to kilns. I also have a ~8" diameter toroidal degaussing coil that plugs into the wall -- inherited from my grandfather, an engineering professor. I have no idea if that would be sufficient.

A local speaker repair shop does have a remagnetizer and will do that part of it for me. In fact, they were the ones who gave me the speaker, not wanting to mess with it themselves.

Why so much trouble over an E-120? I've got a client with a Leslie Model 18/Vibratone that needs a speaker, and there isn't a lot of clearance between the baffle and the back of the cabinet. JBL speakers of the D/K/E-120 series are very shallow.

[J M Fahey]

I have never taken that road, but if the datasheets say so, it must be true.
Unglue the ring magnet from the base plate which is still attached to it (I understand it separated only from the one that's fixed to the frame), and heat it slowly (to avoid cracks) to te Curie Point.
It *should* work and demagnetize it much better than any other method.
Eager to hear the results from you.

[jrfrond]

The ONLY way to do that is to degauss, repair and regauss. Unless you know someone who has built a degausser of this magnitude (I've heard of a few) the only COMMERCIAL place I know of to get it done is Orange County Speaker in CA. This is where I send speaker frames to when I am in the process of reconing and discover a gap shift.

[Rhodesplyr]

John,

Thanks for the information. I'm checking around to see if anyone in my area has a degausser that could handle something like this. I'm in the Research Triangle area of North Carolina, so there are quite a few home-basement mad scientists who've bought surplussed equipment from university labs over the years.

[J M Fahey]

Hi rhodesplayr.
Fact is , degaussers are magnetizers which have a capacitor (in fact a huge bank of them, and non-polarized, which excludes electrolytics) which are connected to the coil by a non polarity sensitive switch (a "contactor", relay's *big* brother).
The resulting tank resonates back and forth, with an ever decreasing and polarity changing field, until it eventually reaches zero, and the remaining magnetism too.
Now you know why they are so hard to find.
If possible, follow the ceramic oven road, it *should* work, although I don't know anybody who did it.
Of course, if "Orange county" can solve your problem, fine.

[Mark Black]

I've fixed a few JBL E-series over the years by doing just what your brother did. No de-mag/re-mag, just a built-up shim equal to the voice coil gap, some JB-weld, and KEEP YOUR FINGERS AWAY FROM THE GAP BETWEEN THE TWO ASSEMBLIES AS THEY COME TOGETHER!

I must admit I did have a ready supply of shim stock at the time, that being the plastic shims that come with the JBL recone kits. I just cut a bunch of them lengthwise and using a micrometer built a stack of suitable thickness with the joints spaced around the circumference.

[Diablo]

I seem to recall that Weber speakers in Kokomo IN degausses speaker magnets and recharges them when they do a recone. Maybe you could have them degauss it for you?

[Rhodesplyr]

Mark -- Yes, that's *precisely* what my brother did. But, as you say, it's impossible to avoid a fairly violent collision between magnet and top plate. To get the magnet off, I had to hold it down with my feet while my brother pulled with both arms.

Diablo -- I know about Weber's service, and I like Weber speakers. What I'm trying to do is to find the closest place, to save on shipping, since the magnet is pretty heavy. It would be nice to know of a local resource in case I ever needed to do something like this again. There may be a place in Charlotte, NC if I can't find one in the immediate area.

I don't deal with Orange County Speaker anymore. They screwed up a recone of a Jensen F15 woofer for me and gave me a very hard time about correcting their mistake. They reconed the speaker with an 8 Ohm voice coil, when it was supposed to be 16, and, as I know now, it was not remotely the correct voice coil for this speaker (much shorter winding height, etc...). In addition, when my brother dissected the speaker to recone it (OCS was going to make me pay for shipping both ways for them to fix it), he found that the work was sloppy overall: parts not precisely positioned or well-glued. The speaker never sounded right with the OCS recone; after my brother did it, it was fine.

Your mileage may vary. I know lots of people use OCS and swear by them, but I also know other people who've had similar experiences to mine. I understand the problem: it's hard to find good reconers. Thus, if I have to send a speaker out for reconing, I try to send it to someone who does a smaller volume business, but does all the work himself. A recone is only as good as the skills of the person doing it, and it's harder to do oversight in larger volume operations.

[jrfrond]

I seem to recall that Weber speakers in Kokomo IN degausses speaker magnets and recharges them when they do a recone. Maybe you could have them degauss it for you?

The main reason for this is that, when reconing stamped-steel frame woofers e.g. Jensen, Celestion, Weber, etc., you create shards of steel when scraping the frames clean. Demagging makes it easier to clean later. I just protected the gap very well when reconing these type of speakers. Cast-frames are aluminum alloy, therefore there's no issue with magnetism.

I've had no issue with the people or service at OCS, but I've only used their degaussing service when needed. I do all of the recones myself. As a professional, the act of repairing the motor structure without degaussing first is a cowboy maneuver that I just won't attempt. Too easy to damage things that way.

[Rhodesplyr]

The main reason for this is that, when reconing stamped-steel frame woofers e.g. Jensen, Celestion, Weber, etc., you create shards of steel when scraping the frames clean. Demagging makes it easier to clean later.

I'm also working on a restoration of another Jensen F15 field-coil woofer that had a rusty basket I had to sand and repaint. The field coil makes that a very simple job; you just un-bolt the field coil from the basket. No power = no field, no ferrous junk in the gap. In fact, the pole piece comes out with the field coil. I don't know of any speaker that's much easier to peform major surgery on.

When OCS recones Jensen F15 speakers, they use a voice coil that turns it into an underhung motor -- less than half the height of the original, and they don't sound the same.

How much does OCS charge just to degauss a magnet?

[Mark Black]

As a professional, the act of repairing the motor structure without degaussing first is a cowboy maneuver that I just won't attempt. Too easy to damage things that way.

LOL - I totally agree it is a cowboy maneuver. The very few I did that way were for people (usually friends) who were unwilling or unable to spend the money or downtime for a proper repair, and I told them up-front it was a "one-shot" attempt that could result in a permanently ruined speaker. That being said all the ones I did are still in service as far as I know (and I have had periodic feedback on most of them). But you are certainly correct - it's not the right way to do things...

I don't "cowboy" things anymore - I have a new job helping to build airplanes and I get to do jobs the right way all the time now. Feels pretty good too!

[jrfrond]

LOL - I totally agree it is a cowboy maneuver. The very few I did that way were for people (usually friends) who were unwilling or unable to spend the money or downtime for a proper repair, and I told them up-front it was a "one-shot" attempt that could result in a permanently ruined speaker. That being said all the ones I did are still in service as far as I know (and I have had periodic feedback on most of them). But you are certainly correct - it's not the right way to do things...

I don't "cowboy" things anymore - I have a new job helping to build airplanes and I get to do jobs the right way all the time now. Feels pretty good too!

Unless I am damn sure of the outcome, I refuse to do any "improper" repair work. There are certain valid shortcuts you can take, and then there's bonehead maneuvers. For me, the attempt of magnet resetting on-the-fly (quite literally!) falls into the latter group.

I think OCS charges $55 for a degauss/reset.

[Diablo]

I'm also working on a restoration of another Jensen F15 field-coil woofer that had a rusty basket I had to sand and repaint. The field coil makes that a very simple job; you just un-bolt the field coil from the basket. No power = no field, no ferrous junk in the gap. In fact, the pole piece comes out with the field coil. I don't know of any speaker that's much easier to peform major surgery on.

Could you do me a big favor and post some photos of the F15? I'm interested in the details of the field coil assembly, bobbin, plate structure and pole piece. I'd like to make my own field coil speaker and the F15 should point me in the right direction for ease of manufacture and ease of assembly.