904Ti 4" midrange debris?

[old-fashioned]

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

[Earl K]

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 ) ?

90449

That's the stuff that rots.

http://www.audioheritage.org/vbulletin/attachment.php?attachmentid=3947&stc=1&d=1100459213

[old-fashioned]

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

[old-fashioned]

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

[Earl K]

@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 ).

http://www.audioheritage.org/vbulletin/attachment.php?attachmentid=57488&stc=1&d=1353361536

:)