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Thread: 904Ti 4" midrange debris?

  1. #1
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    904Ti 4" midrange debris?

    Hi,

    I have bought an used Synthesis Four FCR set.
    2 of the 3 904Ti midrange drivers produce audible distortion and the moving assembly is somehow stuck in the magnetic air gap.
    It doesn't sound like loose voicecoil windings. It sounds öike rust particles. A very "fine crispy" sound.
    Is it possible that they used the older Neo magnet generation at that time with disassembling Chrome surface?
    I am really sursprised and shocked...

    Cheers,

    Andreas

  2. #2
    Senior Member
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    GTA, Ont.
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    Hi Andreas,

    My guess is that those mids were built with an internal foam filter ( found over the inside of the "Blow-hole" ) .

    That foam rots, turns into granular particles and will definitely get into the VC gap and then produce a unique type of rubbing sound.

    Turn the driver(s) face down, put a LF wave into the driver ( so the voice-coil pumps back & forth ) to see if you can get the particles to drop out onto the inside of the cone.

    If they drop out, you then might be able to vacuum out the derbis//particles . Go easy with any suction // you don't want to collapse the cone.



    PS; See the foam plug at the bottom of the heat-vent ( blow-hole ) ?

    Name:  JBL 908Ti backside_.jpg
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    That's the stuff that rots.


  3. #3
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    Dear Earl,

    thank you very much.
    I tried it. One of the two defect units became better.
    Depending on the driver tilt and position I can still hear noise.
    The worst unit now has got back a Fs, but still the moving assembly is stuck and produces distorted sound.

    Can I buy replacement or recone kits somewhere?
    :-(

    Cheers,

    Andreas

  4. #4
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    Hi,

    I sent the worst unit to a repair shop.
    They disassembled the cone and found a deteriorated Neo magnet with loose particles.

    Anyone having at least one silver 904Ti-001 for me?

    Cheers

    Andreas

  5. #5
    Senior Member
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    @Andreas,

    Have you contacted JBL to see if replacements//new-ones are still available?

    That's what I would do ( also, I wouldn't be too choosy on colour ).



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