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Thread: Refurbishing a pair of 2440

  1. #31
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    Quote Originally Posted by tomee View Post
    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

    Quote Originally Posted by tomee View Post
    Big question - does the 2445 have these plastic throats?
    The 2445 doesn´t have plastic throats. I don't think the 2441 have them either. I have seen it on 2482 though.

  2. #32
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    In this JBL drawing of a 2440 it is plain that the throat is exponential. Mine are conical :-(.

    Should I make new ones?

  3. #33
    Senior Member Eaulive's Avatar
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    Quote Originally Posted by more10 View Post
    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.

    Quote Originally Posted by more10 View Post
    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.

  4. #34
    Senior Member 1audiohack's Avatar
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    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.
    If we knew what the hell we were doing, we wouldn't call it research would we.

  5. #35
    Senior Member Steve Schell's Avatar
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    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...

  6. #36
    Senior Member Eaulive's Avatar
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    Quote Originally Posted by 1audiohack View Post
    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

  7. #37
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    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.
    Last edited by more10; 02-01-2011 at 07:15 AM. Reason: Clarified

  8. #38
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    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.

  9. #39
    Senior Member Eaulive's Avatar
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    Quote Originally Posted by more10 View Post
    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:

    Quote Originally Posted by more10 View Post
    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

  10. #40
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    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.

  11. #41
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    Quote Originally Posted by 1audiohack View Post
    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.

  12. #42
    Senior Member Eaulive's Avatar
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    Quote Originally Posted by more10 View Post
    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...

  13. #43
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    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.

  14. #44
    Senior Member grumpy's Avatar
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    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.

  15. #45
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    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.


    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.

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