Ron Johnson wrote in message
...
N_Cook wrote:
Ron Johnson wrote in message
...
N_Cook wrote:
Perhaps it is chemistry . Bear in mind that the wire is silver and not
copper and the surface area of a squashed wire is more than when it
was
round and any corrosion on that surface will have proportionally more
effect
on the thin flats than the bulky round.

The voice coil wire looked like copper because it was under a coppery
brown
lacquer. But, before it disintegrated, the ribbon section was darker
brown
than the round section.
Now if air could get under the lacquer and tarnish the silver to black
copper sulphide, or whatever that blackening is, then that would
explain
it.
Unfortunately none of that section remains as it literally turned to
dust
after photographing it, you could not pick it up even with fingers, it
was
little more than a wraithe.
Are you sure that voice coil isn't aluminium? Some aluminuium v/cs are
copper coated btw.

Ron(UK)


The voice coil remains unaffected, do you know of a simple test for
silver v
aluminium ? I would say it looked brighter, more silvery indeed, than
aluminium but I am not familiar with seeing .07 to 0.09 mm diameter Al
wire
or silver wire for that matter.

Are you saying that the part of the coil which failed is a separate wire
from the actual voice coil or in some way 'free floating'?
In most speakers - of the single rear suspension type, the v/c is wound
on the former with the ends coming up the cone on the inside then
soldered to the pigtails where they pass through the cone. The tails of
the v/c and soldered joints are glued to the cone inside and the
pigtails glued on the outside. All this is usually covered on the inside
by the dust dome. There should be little fatigue of the v/c wire
anywhere as it should be secured to the v/c former or cone.
The pigtails and often silvered and do fail either through gross
overexcursion or fatigue leading to overheating. I`ve seen pigtails
melted where the voice coil is intact.

You can sometimes rescue a speaker from this condition with some new
pigtails and a dust dome. At one time you could buy pigtail wire from
Goodmans, you probably still can from Wembley Loudspeakers if you ask
nicely.

A speaker which has been seriously overdriven often shows no voice coil
damage other than an obvious open circuit in the 'straight' part of the
v/c winding. Chances are, your customer was pushing the amps well into
clipping and simply overcooked the voicecoils which melted at the
weakest point.

Wembley Loudspeakers will repair the drivers at a reasonable cost no
doubt.

Ron


This is a high power horn so tinsel ribbon instead of pigtails. Instead of
the voice coil of round wire going from one tinsel to the other there are
short runs of flattened wire. Unnecessary here , assuming it is to save
overlap, so fouling the gap in the magnet, because the magnet is milled out
on both sites of the lead-in and lead out tinsels. In pics of earlier post.
If the wire had not been flattened it would not have failed, at least as how
it was being used, is what I am suggesting.


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/