Hardened "goop" (ferro-fluid) on midrange/tweeter speakers?

[Bill Webb]

I have come across several sets of speakers where one or more drivers
(midrange/tweeter) have siezed up. Opening them has revealed a black,
sticky tar-like (or harder) substance between the coil and magnet,
restricting motion of the coil. I assume this is (or was at one time)
ferro-fluid coolant. Is this correct? What causes it to harden this way?
Finally, is there any proper way to clear out this gunk or otherwise
restore the driver to normal?

Just recently I have picked up what would otherwise be a *beautiful* set
of speakers, except for a siezed-up midrange driver as described above.
While waiting for a response from the manufacturer about a replacement,
I figured I would see if it could be temporarily rejuvinated until the
replacement arrived - I put in a drop of nail polish remover to loosen
the gunk, removed as much as I could without making a project of it,
then fed in a couple drops of light machine oil (to roughly approximate
the original composition of the ferrofluid) and let the driver exercise
overnight on a signal generator. To my utter amazement, this hamhanded
ghetto-rigging actually worked! This driver sounds about the same (a
little louder, in fact) as the other. I don't know for how long, though,
but now it looks like I will be finding out: the maufacturer replies
that the part has been discontinued

 

[Jim Adney]

While waiting for a response from the manufacturer about a replacement,
I figured I would see if it could be temporarily rejuvinated until the
replacement arrived - I put in a drop of nail polish remover to loosen
the gunk, removed as much as I could without making a project of it,
then fed in a couple drops of light machine oil (to roughly approximate
the original composition of the ferrofluid) and let the driver exercise
overnight on a signal generator. To my utter amazement, this hamhanded
ghetto-rigging actually worked! This driver sounds about the same (a
little louder, in fact) as the other. I don't know for how long, though,

I believe ferro-fluid is just a mixture of oil and finely ground iron
(or some other magnetic material.) Your description sounds like the
oil has gotten overheated and either slowly evaporated or oxidized and
thickened. My first thought was to just try adding some heavy (low
vapor pressure, so it won't evaporate quickly) oil and see if it mixes
with what is in there and loosens up the speaker.

A better way to proceed might be to dig out a bit of the thick stuff
and try adding various oils to it to see what it seems to be happy
with.

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