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Thread: Replacing foam in 2421A's

  1. #1
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    Replacing foam in 2421A's

    Hi,

    out of curiosity I've removed the back of one of my 2421A compression drivers yesterday in the afternoon. What should I say? As expected, I've found the foam in the inside being degenerated, right before falling to dust:

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    You see the imprints of both leads and two imprints that I did with my fingers to see if the foam would come out again immerdiately. It didn't .

    Now, what should I do? Ist this foam still available from JBL/Harman? Or will any polyurethane foam of appropriate thickness do? Is it even better to replace it by felt?

    Best regards!

  2. #2
    Junior Member macsic's Avatar
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    I used felt and it was perfect.
    They used felt before foam replace it.
    Music is emotion.

  3. #3
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    Well, as these drivers might be well over 30 years old, and as I had two sheets of 10 mm thick foam laying around, I decided to replace foam by foam. I think the new foam pads might survive me if they last another 30 years . This is the procedure:

    After I removed the four screws and lifted the back cover, the drivers presented like this:

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    Then I removed the black and red wires and the terminals. As my flat bed scanner makes scans in true dimensions, which a camera usually doesn't, I scanned the back cover:

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    I printed the file, glued the print onto cardboard and cut out the black area:

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    Using this template I drew the contours onto the foam sheets using a sharpie:

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    Then I cut out the foam pads with sharp scissors:

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    To be continued...

  4. #4
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    Removing the old foam goop and cleaning the cap's inside from contact cement turned out to be the most tedious part of the procedure. I used a spatula, a small brass wire brush and lots of ethylic. I decided that the job was done at this state:

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    I applied new contact cement to either the cap's inside and the foam pads and let it dry for 15 minutes:

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    During this time I disassembled and cleaned the other driver.

    Then I glued the new pad into the cap:

    Name:  20190331_143652_kl.jpg
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    Snug fit, doesn't it ?

    I reassembled the terminals, the wires and screwed the back to the driver again. Job successfully done!

    I apologize to all of you who have done procedures like this one several times and feel bored .

    Best regards!

  5. #5
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    Nice work!


  6. #6
    Senior Member remusr's Avatar
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    That foam looks pretty stiff. I think the original is a polyurethane closed-cell foam, usually dark grey/green about 10mm. There is some discussion of open or porous cell vs closed or impermeable. Literature indicates for 1/4-1/2 inch foam of the same pore density (PPI), closed cell attenuates better below 500Hz, both are similar above 500. A higher PPI (Pores Per Inch, linear inch that is) absorbs lower frequencies, acoustical is usually 60+. Internet search expert!

  7. #7
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    No, this white foam isn't stiff. It's also polyurethane foam, not expanded polystyrene. I can't tell, though, whether it is open or closed cell. Right, it's pore densitity might not be that high as in the original foam, but as I plan to xover at 1 kHz, not lower, I don't mind.

    Best regards!

  8. #8
    Senior Member remusr's Avatar
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    Closed cell does not absorb water. It is used for patio furniture cushions among other uses that water absorption would not be acceptable.

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