Ron wrote:
On 09/07/2010 16:33, TMI wrote:
When voice coils are wound the former which you refer to as a 2" tube,
is placed an a mandrel with an expanding collet. In better speakers,
it is "wet wound" in glue under precise tension onto the former to
create a mechanically homogeneous coil, using a precision "coil
winder" which advances the width of the wire for each turn of the
mandrel. Without the mandrel and the collet, you cannot apply the
correct tension to the wire, assuming you knew what it was.

Old cones are not good cones and become tired, particularly after an
event like you describe. New cones have a break-in period of several
months until the spider and surround stabilize at their nominal
compliance.

Tannoy speakers are high efficiency with relatively small magnets
which point to a tight gap. This is very unforgiving with respect to
egging the coil and former, extra glue from the repair, misalignment,
debris, nonlinearity of the reworked spider, surround and cone.

You do not mention centering the coil in the excursion, which is
critical for low distortion and is 90 degrees away from the shims.
Since this alignment opposes the spring action of the suspension, the
cone and spider must have no glue grooves to the former.

In short, buy the cone kits or better still, ship the drivers to allow
someone who does vintage Tannoy work to install them and preserve
those $5,000.00 speakers through restoration. A top notch shop may
also have a magnet charger to restore the specified flux density to
the gap.

Tell you friend that there are few "Vintage" cassette decks and for
what this repair is going to cost he should look for something newer.

About glue....a bit of acetone can be used (in areas other than the
coil where it may create a shorted turn) to rewet and bond glue that
has cracked. GC Radio and TV Service Cement was the old fashioned
method used by TV repair shops. Real speaker reconers use the OEM
glues.

Adding epoxy to a solvent based glue joint is a cobblers repair.

Using epoxy in place of a solvent cement is better, but still
different. While tougher, epoxy often lacks the Q of a solvent cement
joint that has aged, particularly when mixed and applied by hand which
yields entrapped air and sloppy nonuniform ratios.

These observations do not apply to modern 1KW drivers used in PA
systems and beaten to death every weekend. Your friend's solution is
perfectly valid in this application and a little DEVCON goes a long
way toward a reasonable operating cost.

Tom Maguire
TMI Engineering


Interestingly, a similar discussion has been going on over in
alt.audio.pro.live-sound, tho mainly concerned with repairing cracked
magnets.

I was surprised (after seeing a program on tv - to find out that
ferrites are magnetised _after_ assembly with the motor and basket assembly

Veejo here

http://www.youtube.com/watch?v=VN0tmyyC0ak

hahaha. Skip to 2:10

they magnetize the ferrite with the operator panel from a reel to reel
tape drive or some washing machine sized hard drive, and by pushing the
Fault button.