Replacing foam in 2421A's

[Kay Pirinha]

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:



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!

[macsic]

I used felt and it was perfect.
They used felt before foam replace it.

[Kay Pirinha]

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:



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:



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



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



Then I cut out the foam pads with sharp scissors:



To be continued...

[Kay Pirinha]

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:



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



During this time I disassembled and cleaned the other driver.

Then I glued the new pad into the cap:



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!

[Earl K]

Nice work!

[remusr]

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!

[Kay Pirinha]

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!

[remusr]

Closed cell does not absorb water. It is used for patio furniture cushions among other uses that water absorption would not be acceptable.