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more10
09-22-2010, 03:06 PM
I recently bought a pair of worn 2440 which I intend to refurbish. This is the first time i do this, so some advice will be appreciated.

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more10
09-22-2010, 03:08 PM
I believe the diaphragm is ok, this is a 2441 dia right?

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more10
09-22-2010, 03:10 PM
First thing to do is to remove the diaphragm. For this I need a titanium (non magnetic) screwdriver.

Next thing is to have the drivers demagnitized. Then I can disassemble them. Before buying any spare parts I will check that the magnets are not broken.

HCSGuy
09-22-2010, 04:52 PM
They do look like 2441 diaphragms - definitely not 2240's, which have the half-roll surround. Before you go any farther, do you have a remedy for the cracked throats? I have no experience with these drivers, but I thought a cracked throat meant game over, driver dead. I'm sure the Pro guys here can chime in, but I don't think even the factory would touch these. Hope I'm wrong :)

Robh3606
09-22-2010, 06:36 PM
but I thought a cracked throat meant game over, driver dead. I'm sure the Pro guys here can chime in, but I don't think even the factory would touch these. Hope I'm wrong :)

If you have a shop that can demag them, get the throats from JBL and remag you are back in business. JBL used to fix them. Now?? Do they have throats for them??

Rob:)

richluvsound
09-22-2010, 07:44 PM
More10

I'm pretty sure a good machinist could make new throats if the dimensions are around ... I can't see
as a really expensive thing to do .... If your stuck PM Guido for the name of that company in Berlin that does all this sort of thing ....

Rich

PS stay out of the bar and save for the Be :D

more10
09-23-2010, 06:38 AM
Well, if I canīt get new throats from JBL, a friend of mine can shurely make them. With or without factory dimensions. He has got a CNC lathe.

more10
09-23-2010, 06:40 AM
More10
PS stay out of the bar and save for the Be :D

Going there tonight :o:. Hoping to have a Real Ale or two :).

jerry_rig
09-23-2010, 07:34 PM
I had a pair of 2441s shipped to me and both throats were cracked. But they were made of plastic. Not sure is that would be easier or tougher to fabricate than metal throats. But you have the option to make them from just about anything.

pocketchange
10-01-2010, 06:38 PM
Non-magnetic tools = VERY positive.
Having a cracked throat is not the end of the world and having them demagnetized isn't necessary either.
I've been told that having a cracked throat is a more typical rule of thumb but it's very nice if you can locate a un-cracked pair.
I cut a piece of UL rated aluminum foil (HVAC) tape (the thick stuff) and lined my cracked throat with it after cleaning out the broke plastic fragment pieces.
As for having them re-magnetictized, Great Plains will be happy to help (but most likely it's a wasted effort).
I did make some changes to the diaphragm cap after the typical clean-up and paint. I located some 1/2" wool mating, cut and glued into place it does a great job of whatever that foam material was supposed to accomplish.
After it was all said and done I ended up using a Radian Diaphragm instead of the OEM JBL's in a three way KHorn
crossed over a little higher than typical to unload the 2404H's and I have no complaints with the results. :bouncy:

And please note; I'm not a pro CD repair expert. :eek:
And my 2482's sound just fine. Thank You.
But I did have some nice long telephone conversations with several people that were.

more10
01-29-2011, 05:23 AM
The drivers are almost done now, thanks to my friend Peter who owns the lathe. Some pictures:

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Using a large lathe to center the pole plate.

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Outside was already done when I joined.

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Drilling the inside

more10
01-29-2011, 05:26 AM
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Peter working at the lathe

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Mach3 CNC application

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Inside being machined

more10
01-29-2011, 05:30 AM
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Polishing the inside.

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Throat finished.

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No need for small tolerances :-)

more10
01-29-2011, 05:32 AM
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All parts of a driver (except back chamber)

more10
01-29-2011, 05:33 AM
Details of each piece
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more10
01-29-2011, 05:40 AM
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Using scotch tape as shims. Tolerance is 0.1 mm to allow gluing the throat.

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Throat and magnet in place.

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Phase plug added

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Pole piece added

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Using paper as shims in magnetic gap. There is a small play (0.1 mm?)

more10
01-29-2011, 05:43 AM
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The gaussing machine

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About to magnetize. Lars is operating the machine, which he actually build himself. According to him this is the most powerful machine in Sweden.

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Using a piece of wood to prevent the driver from escaping from the coil box.

more10
01-29-2011, 05:46 AM
I have tested the drivers with 2445 diaphragms, and whey both work as expected. Now I need to glue the throats, and add new gaskets. And of course new diaphragms.

hjames
01-29-2011, 05:53 AM
Wow - very cool break down/build up!
Thanks for sharing all those pictures - makes more sense now!

Earl K
01-29-2011, 07:13 AM
Beautiful Work !

Thanks for the pictorial !

- Was Lars machine also used to demag the magnet before disassembly of the 2440 ?

- Was there any residual magnetism left in the magnet after demagging ?

<> :)

more10
01-29-2011, 08:57 AM
Beautiful Work !

Thanks :-)


- Was Lars machine also used to demag the magnet before disassembly of the 2440 ?

I don't really know which machine he used. I believe he used the same machine. I think that he magetized with lower and lower energy, turning the driver each time.

- Was there any residual magnetism left in the magnet after demagging ?
<> :)
After first demagging the pole piece could be removed. The magnetism was too strong to allow removal of magnet from case. After removal of the throat the magnet could be moved sideways. Then the maget could be removed using a piece of wood and a club. Lars believe that he needed 100 kg force to remove the magnet from the case. When removed from the case the magent could be completely demagnetized.

Earl K
01-29-2011, 09:10 AM
Thanks :-)

I don't really know which machine he used. I believe he used the same machine. I think that he magetized with lower and lower energy, turning the driver each time.

After first demagging the pole piece could be removed. The magnetism was too strong to allow removal of magnet from case. After removal of the throat the magnet could be moved sideways. Then the maget could be removed using a piece of wood and a club. Lars believe that he needed 100 kg force to remove the magnet from the case. When removed from the case the magent could be completely demagnetized.

Thank-you for your answers :)

<> EarlK

spkrman57
01-29-2011, 09:27 AM
Best one yet that shows all the inner works of the compression driver.

Thanks for sharimg with us!

Regards, Ron:)

Eaulive
01-29-2011, 10:20 AM
Best one yet that shows all the inner works of the compression driver.

Thanks for sharimg with us!

Regards, Ron:)

Yup, I really would like to know more about the procedure of magnetizing/demagnetizing, pictures, explanations, etc.

richluvsound
01-29-2011, 10:45 AM
Hi Morten ,

absolutely brilliant Sir . Thanks for insight . All your need now is the Be dia and you have a world class component that will out live us all .

You have done the research and deserve every reward .

Rich

Steve Schell
01-29-2011, 10:48 PM
Fascinating thread and pictures, more10! You have created and installed the throat inserts that should have been there in the first place had cost not been a consideration. These things have always been a problem, as early D175 drivers had throat inserts cast of pot metal that sometimes deteriorate and increase in size, forcing the top plate up off the pot.

The gap flux density of your drivers should measure 20 kilogauss or more. I have measured this amount on a number of old 2440s and 2441s, some of them in very rough condition.

more10
01-30-2011, 03:04 AM
Thanks Steve

The aluminium alloy used was 6082-T6. Excellent for machining.

I didn't see the measurement myself, but according to Lars it is over 20000 gauss.

Next test will be to compare sound level with my 2445 drivers.

tomee
01-30-2011, 07:48 AM
Thanks Steve

The aluminium alloy used was 6082-T6. Excellent for machining.

I didn't see the measurement myself, but according to Lars it is over 20000 gauss.

Next test will be to compare sound level with my 2445 drivers.

Excellent description - thanks for sharing that!

You should tool up to make as many of those throats as you can and sell them because I think there is a demand for them!

Big question - does the 2445 have these plastic throats?

Eaulive
01-30-2011, 07:53 AM
Yup, I really would like to know more about the procedure of magnetizing/demagnetizing, pictures, explanations, etc.

Now that I see this, I realize that there's no need for the machine to have a direct physical contact with the magnet, am I wrong?

So would it be possible to remagnetize a woofer without reconing it ? (removing the cone):confused:

more10
01-30-2011, 09:17 AM
You need to remove the coil (with the cone) when demagnetizing or magnetizing because the applied magentic force is so huge it would destroy the coil.

Removing the cone is not that difficult I have heard since they use hot melts. Just put the driver in your oven for some time and remove the cone. I have not done this myself, but there should be someone on the forum who knows how to do this.

When putting the cone back you need to shim the coil in order to center it in the magnetic gap.

Something else I have learned is to use the kind of tape you use for painting to remove dust from the magnetic gap.

I have a pair of 2118 which needs new coils, I will take some pictures of them in the oven in the future.

more10
01-30-2011, 09:25 AM
You should tool up to make as many of those throats as you can and sell them because I think there is a demand for them!


I have the cnc g-code for the throats. I will share when PM:d, but just for non comercial use. Making just a pair is a days work in a hobby CNC lathe. If I get tens of requests maybe I can check the price for professional machining



Big question - does the 2445 have these plastic throats?

The 2445 doesnīt have plastic throats. I don't think the 2441 have them either. I have seen it on 2482 though.

more10
01-30-2011, 02:42 PM
In this JBL drawing of a 2440 (http://www.audioheritage.org/vbulletin/showthread.php?27865-Phase-Plug-Diagram-2425-2426&highlight=playing+with+a+2426#7) it is plain that the throat is exponential. Mine are conical :-(.

Should I make new ones?

Eaulive
01-30-2011, 07:22 PM
You need to remove the coil (with the cone) when demagnetizing or magnetizing because the applied magentic force is so huge it would destroy the coil.

Never thought of that, makes sense.


Removing the cone is not that difficult I have heard since they use hot melts. Just put the driver in your oven for some time and remove the cone. I have not done this myself, but there should be someone on the forum who knows how to do this.

When putting the cone back you need to shim the coil in order to center it in the magnetic gap.

Yep, and for this you need to remove the dust cap, I heard some people do it also but I don't know how well it turns out. Some people do it with thinner.

Last time I did a batch of E140s I managed to remove some assemblies almost intact, mount them and remove them and mount them a couple of times just for fun, but the cone was far from being unharmed and the spider was stretched and cracked in some places. :crying:

1audiohack
01-30-2011, 08:51 PM
I can't think of a reason the cone / coil would need to be removed for de-mag or re-mag. If the coil is unterminated the electrical circuit is open, when the magnetic field changes around the coil it can produce no work, only a temporary voltage potential, the magnitude of which is dependent on many things but potential only non the less.

Steve Schell
01-31-2011, 10:09 AM
I have been told by people who know more about all this than I do that it is not necessary to remove the voice coil from the gap when charging a magnet, as long as the coil is open i.e. not connected to anything. If the coil was part of a circuit then, yes, destructive currents might flow, but with no circuit no current flows and no damage occurs.

Edit: just saw your post, 1audiohack. Great minds, etc...

Eaulive
01-31-2011, 05:39 PM
I can't think of a reason the cone / coil would need to be removed for de-mag or re-mag. If the coil is unterminated the electrical circuit is open, when the magnetic field changes around the coil it can produce no work, only a temporary voltage potential, the magnitude of which is dependent on many things but potential only non the less.

Well, that also makes sense, but maybe the potential would be high enough to cause arcing?

I have a functional 123A that is 3dB lower than my other three, voice coil seems OK, DCR is OK, no rubs, no visual damage, only less efficient especially in the higher part of its range.
If it could be remagged without having to rip it open, that would be nice :D

more10
02-01-2011, 05:27 AM
I have talked to Lars relaying all your questions.

The machine produces 60 kJoule maximum. Capacitors are 12mF. Before magentization the capacitors are charged to 3000 V maximum. The magnetic field is created by releasing the stored energy in the capacitors through the big coil. The cost for the machine was 50000$ 10 years ago.

The same machine is used for both demagnetizing and magnetizing. Demagnitizing is done automatically by the machine. The machine alters polarity for each magnetization, and decreases energy for each turn.

The reason for removing the cone is to be able to measure the flux in the magentic gap after magnetization.

The drivers I have magnetized just reached the specified flux. The conclusion is that you cannot put in too much magnetism into a JBL driver, which makes sense since the magnet would be larger and cost more.

In order to reach specified flux in the driver, you start with a pretty low energy, and increase the enery until you reach the specified flux level in the driver. Increasing the energy even more could cause less flux in the driver.

You can magentize a driver with installed cone, but you cannot be shure it is fully magnetized.

more10
02-01-2011, 05:40 AM
I have been thinking about how I did the magnetization, and I realize that I made 2 mistakes.

The first mistake was to use paper for shims in magnetic gap. A much stiffer material should have been used. When removing the paper shims after magnetization, one shim was clearly squeezed.

The second mistake was to screw the pole plate hard to the case. The pole plate should have been loosely fastened to the case.

When magnetizing the driver, the magnetic force will draw the maget to the case and the phase plug piece to the magnet. This can cause the phase plug to move sideways. This is why you want very stiff shims and a loose pole plate.

Eaulive
02-01-2011, 05:49 AM
In order to reach specified flux you start with a pretty low energy, and increase the enery until you reach the specified level. Increasing even more could cause less flux in the driver.

My understanding is that this goes against your previous statement where you say:


The drivers I have magnetized just reached the specified flux. The conclusion is that you cannot put in too much magnetism into a JBL driver, which makes sense since the magnet would be larger and cost more.

Or maybe I didn't understand, which is possible since I really don't know too much about those procedures.
I'm sorry to insist and question, but I'm really interested :)

more10
02-01-2011, 06:08 AM
Ok, I will try to explain in other words.

It is impossible to overmagnetize a driver, that is to go beyond the specified flux in the magnetic gap.

However, in order to achieve the specificed magnetic flux in the driver, you must give an exact dose of energy when magentizing. If you give too less or too much energy, you will not reach the specified magentic flux in the driver.

So turning the volume to 11 :-) (3000V) will not magnetize the driver optimally.

Also you may need to adjust the position of the driver in the magnetizing coil, and use iron pieces close to the case in order to direct the magnetic field into the driver.

My impression is that it takes skill and experience to magnetize a driver.

more10
02-01-2011, 06:12 AM
I can't think of a reason the cone / coil would need to be removed for de-mag or re-mag. If the coil is unterminated the electrical circuit is open, when the magnetic field changes around the coil it can produce no work, only a temporary voltage potential, the magnitude of which is dependent on many things but potential only non the less.

You are absolutely right.

Eaulive
02-01-2011, 08:56 AM
Ok, I will try to explain in other words.

It is impossible to overmagnetize a driver, that is to go beyond the specified flux in the magnetic gap.

However, in order to achieve the specificed magnetic flux in the driver, you must give an exact dose of energy when magentizing. If you give too less or too much energy, you will not reach the specified magentic flux in the driver.

So turning the volume to 11 :-) (3000V) will not magnetize the driver optimally.

Ok right, now it makes sense.
But on the other hand, why would JBL install magnets in their drivers that are bigger than necessary and then not magnetize them at their maximum?

I was under the impression that they were always trying to get the most of the magnet, meaning that if they needed 10 000 gauss for example, they were picking the magnet size according to the gap to arrive exactly at this level when fully magnetized.

Unless there is some reason NOT to magnetize a magnet to its full potential, maybe... :dont-know:

more10
02-01-2011, 10:03 AM
This is exactly what I have tried to explain. They have put in a magnet which will deliver the magnetic flux they want when fully magnetisized.

grumpy
02-01-2011, 10:44 AM
I think what more10 is trying to say is that the magnet can be "filled" only so far, but
further charging is counter-productive... i.e., there is a charge vs. flux curve that
has a maximum flux value that can not be exceeded, but the flux value may get somewhat
less after the peak has been reached and the charge level continues to rise.

I'm just trying to paraphrase,... not claiming to be an expert on this.

based on Figure 1.16 (bottom of page) http://cnx.org/content/m28345/latest/
I'm not sure I agree that overcharging is really an issue, but I would certainly defer
to the reality of actual field experience.

John
03-14-2011, 04:22 PM
From my understanding of the subject of charging a magnet is you can not overcharge it. And you cannot remove charge or guass by giving it multiple applications of a charge.:blah:


I have had 2215A woofers re-gaussed with the cone and coils in place as well. They work like the day they were built at JBL.:bouncy:

more10
03-15-2011, 02:24 PM
You cannot make the magnet stronger than its specification. However if the charging pulse is too weak or too strong (overcharging) the magnet will not reach the specified magnetic flux.

Because of this one should measure the gap flux after charging. This is why the cone (or diaphragm) should be removed during charging. You cannot measure with the cone still mounted.

If the guy doing the charging is experienced, he will probably put just the right amount of charge into the magnet with the cone still mounted.

I have read somewhere that low frequencies will discharge alnico magents. So cone speakers might need recharging. Compression drivers will be less affected. I had two tweeters, four 1" drivers and a 2220 checked. Only the 2220 needed recharge.

herki the cat
01-17-2012, 10:57 PM
....http://cnx.org/content/m28345/latest/ (http://cnx.org/content/m28345/latest/) ....I'm not sure I agree that overcharging is really an issue....

grumpy,You are absolutely correct. No one has yet presented proof...."that overcharging is really an issue".... Very likely the problem has been "Cock Pit Errors, Operator Care-Lessens, or Charger Hardware Failure if not a "Defective Work Piece Assembly."

Three Magnet Charging items were always observed in the RCA Camden factory:
#1) Always begin with a completely demagnetized Magnet Structure.
#2) Ramping up the charging current very slowly, & especially ramping down the current to zero. A failure mode causing "Instant switching of the charge current to zero with out instant disconnection from the current source" will result in a demagnetizing event due to the severe current transient in the charging winding L/C components.
#3) Always provide an air gap, like a 2 inch thick wooden block, between the Magnet Work Piece and the vertical Charger Component snugging the work piece in place, other wise, removing the charged magnet structure from intimate clamping between the top & bottom poles of the charger would severely de-magnetize the work piece.

At page bottom, See figure 1.15 "Magnetic circuit including both a permanent magnet and an excitation winding."

grumpy, thank you for presenting the link: http://cnx.org/content/m28345/latest/ (http://cnx.org/content/m28345/latest/) of The Textbook [# 1 Electric Machinery - A.E. Fitzgerald, Charles Kingsley, Jr., Stephen D. Umans- 6th edition- Mc Graw Hill series in Electrical Engineering. Power and Energy] which describes with "Maxwell's Equations" a leveling off of flux value due to saturation of the magnetic iron."

Assuming the hard magnetic material is initially unmagnetized, corresponding to point (a) of the figure 1.16, consider what happens as current is applied to the excitation winding. With a core of infinite permeability, the horizontal axis can be considered a measure of the applied current & a measure of H, the magnetic field intensity, in the magnetic material.

As the current i is increased to its maximum value, the B-H trajectory rises from point (a) toward its maximum value at point (b). To fully magnetize the material, assume the current has been increased to a value/max sufficiently large that the material has been driven well into saturation at point (b). When the current is then decreased to zero, the B-H characteristic will begin to form a hysteresis loop arriving at point (c) at zero current. At point (c), notice that H in the material is zero, but B, the magnetic flux density is at its remnant value Br.

As the current then goes negative, the B-H characteristic continues to trace out a hysteresis loop. In Fig. 1.16, this is seen as the trajectory between points (c) and (d). If the current is then maintained at the value H to the (d) power, the operating point of the magnet will be that of point (d). Note, this same operating point would be reached if the material were to start at point (c) and with the excitation held at zero, with an air gap inserted in the core.

Should the current then be made more negative, the trajectory would continue tracing out the hysteresis loop toward point (e).

However, if instead the current is returned to zero, the trajectory does not in general retrace the hysteresis loop toward point (c).

It then traces a minor hysteresis loop between point (d) & point (f) at zero current. If negative polarity current is then varied between zero Br point and point (d), the B-H characteristic will trace out the minor loop as shown.

This happens when the voice coil current is driven into the negative polarity region from "zero drive" Br point toward point (d), creating the minor loop/(d to f) recoil line in Alnico alloys.
herki[Quote/]

54496

more10
01-19-2012, 05:39 AM
Well. I was only quoting the guy who did my remagnetization. He has been working with magnetization for at least 20 years. It could be something with their machine that made the result less optimal if "overcharging".

Anyways, you should be able to measure BL after recharging, so there is no need to remove the cone for measurements.

DingDing
09-14-2015, 01:36 AM
Would it be possible to see if the optimum charge has been applied to the magnet by a proxy such as measuring the impedance curve of the driver?

more10
09-14-2015, 04:02 AM
For a cone driver you can measure BL, for a compression driver I donīt know.

more10
11-14-2017, 12:12 PM
Forum member Charles0322 (http://www.audioheritage.org/vbulletin/member.php?9143-Charles0322) asked if I could send the drawings for the throats. Unfortunately I had lost the original drawings, so I had to make a new improved version. This drawing has an exponential flare, mine are straight.

I have used a cheap caliper measuring both the original throat and the one I made previously. This drawing should be 0.05 mm accurate.

My throat has more material compared to the original. It should fit snugly in the casing and magnet, so it can be used to center the magnet when remagnetizing.

I have also calculated fc of the throat, it is 186,5 Hz