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Old 10th October 2002, 11:45 AM   #21
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Had to detach both spider and surround from basket. There was some pretty obnoxious magnetic debris in the gap. Best way to remove it was with adhesive tape and a small piece of ABS plastic that I used to push the tape into the gap. Best do it in sunlight as there is always bound to be a particle that has escaped the treatment...

Before I did that, by the way, I taped the gap shut and drilled a dozen venting holes into the aluminum basked and covered them with fabric from an old T-shirt.

Straightened the spider outside of the basket by appling hot air only for a few seconds and holding the heated area afterwards.

Reglued the spider with Pattex transparent contact glue (the stuff where you put glue on both sides, let them dry for 5 min, then press hard9, using the paper trick to get the centering of the voice coil right. Once this is fixed, gluing in the surround is not as critical. I tried to get the position right with the paper still in the VC. Fixed the position with lots of tape and then removed tape on segments, glued them with Pattex, redid the tape and moved on to the next segment.

All scratching and ticking was gone last night. However, the speaker had to spend the night on the porch at close to freezing temperatures because of the fumes. This morning, it scratched a little, but I hope that was just due to tempcos...

Eric
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Old 15th October 2002, 09:29 AM   #22
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Default drilling spider vent holes into aluminum and steel baskets

Turns out the scratching was not due to temperatue but because one of the leads going to the terminals was not resoldered properly. Easy fix.

The other Eton driver did not scratch. I drilled a few holes into the basket to vent the spider. I used a sharp 5 mm HSS drill and moderate forward pressure. The big trick was to have the side where I was drilling the hole point downwards so the debris does not enter the gap and use a high power vacuum cleaner afterwards to collect the particles BEFORE turning the speaker.

I had good luck with the debris, no scratching. However, on one hole the drill advanced to quickly one the last bit of aluminum was gone so I perforated the VC former a couple of mm above the VC. This causes a ticking sound when the excursion exceeds +/- 4 mm. Have removed the dustcap for surgery. May need to bend the Kapton back locally. What is a good way? Soldering tip in proximity to Kapton for warm-up and then any kind of nonmagnetic tip for the bending?

Next time I will put a bit of tubing on the drill to limit its effective length!

I also tried to drill holes into a Mivox 10 in. driver with a steel basket. No matter how careful I was, the gap just attracted the debris. So I detached the surround from the basket and cut the spider (flat, but with an initial 5 mm step at the edge which does not move at all) with a sharp knive. Taped the gap shut, drilled all the holes. Lots of particles assembled on the gap. Put a second layer of tape on top and removed the lot.

Then used a thin sheet of plastic and lots of tape to get all the tiny particles out of the gap. Had the impression that the nickel coating on the top surfaces of the pole pieces was not too good so new particles kept coming off and entering the gap. So when I was sure the gap was clean (a halogen lamp is pretty helpful here), I applied a little plastic spray which covered most of the top surfaces but most likely did not enter the gap in large amounts.

Then used Eric's cardboard trick (actually, I used three sheets of CD-R inlay paper)to center the VC. Glued the spider back in along the jagged edges where I had cut it. Next day, reglued the surround. Works perfectly without any ticking even at large excursions. Measurements of TSP to follow...


Eric
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Old 16th October 2002, 11:05 AM   #23
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Hi Eric, I have not had to bend/form kapton , so I am not too sure how, and I don't have any junked kapton drivers at present with which to experiment.
I think kapton is reasonably thermally sensitive, so if you can form an inner sleeve and outer sleeve around it and then heat it real carefully, it may do what you want.
Beware, some plastics will shrink when heated (ever tried putting a Twisties packet in the oven ?), so formers around the kapton before heating is likely mission critical to success.
By your description, I am not sure that you haven't caused a hole in the kapton former with rough/torn edges ?.
If so, maybe a very thin scalpel blade might be used to trim off the burring ?.
The last resort would be to decone the basket (like you did on the other driver) and then you would have full access to the problem.

"Then used Eric's cardboard trick (actually, I used three sheets of CD-R inlay paper)to center the VC."
Thanks for the acknowlegement, but this is how it is done in production, and the dome fitted after, and then the whole assembly is magnetised by placing it over a large solenoid arrangement before hitting the solenoid coil with a controlled current pulse.
Dynaudio woofers with single piece moulded PP cone are centered by piercing the cone and slipping a number of fine wires down between the VC and polepiece.
If you look real close at these drivers you will see a number of fine holes through the cone - I have had a tour of the Dynaudio factory about 6 years ago - very educational.
Some pro EV drivers recone kits come in pieces - voicecoil, cone with surround, spider and dome.
These require usage of a jig to recone them.
A cylindrical slug sits in the polepiece, and all the pieces fitted, and then glued in stages, and with EV supplied glues.
Solvents like tolulene are usefull for releasing some glues, but avoid any vapours like the plague - iow, no glue sniffing ok ?, and leave the driver for a day or two to allow all fumes to dissipate.

Regards, Eric.
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Old 16th October 2002, 12:12 PM   #24
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Yes, I have removed the jagged edges with a scalpel. Local bending with a soldering tip and a thin rod works. I even have access to the other side through the venting hole the drilling of which caused all the trouble. Talk about minimally invasive surgery...

There is still some ticking at large excursions, so minimal bending seems to be critical.


Yes, I have seen these tiny holes in Morel PP membranes.

Do you have any information on demagnetizing and remagnetizing ferrites? This would be a handy way to drill holes into steel baskets without having to take the hole thing apart.

Also, in the thread on electromagnetic speakers, I mused whether it would be possible to hammer the ferrite ring away to replace it with a copper coil. Do you have any idea whether the pole plate assembly will collapse if the magnet is removed in pieces?

Thanks,

Eric
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Old 16th October 2002, 01:28 PM   #25
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Originally posted by capslock
Yes, I have removed the jagged edges with a scalpel. Local bending with a soldering tip and a thin rod works. I even have access to the other side through the venting hole the drilling of which caused all the trouble. Talk about minimally invasive surgery...
"necessity is the mother of invention" (and necessity is a mother)
There is still some ticking at large excursions, so minimal bending seems to be critical.
Sounds like you nearly have it solved.
Variable low frequency and amplitude sinewave is a good way to check operation.
Yes, I have seen these tiny holes in Morel PP membranes.
So, now you know why.
Do you have any information on demagnetizing and remagnetizing ferrites? This would be a handy way to drill holes into steel baskets without having to take the hole thing apart.
I started a thread recently demagnetiser tool
A standard TV workshop tool is a demagnetising wand - this is a big coil (millions of turns of fine wire) around an iron rod enclosed in a plastic tube.
This throws out a strong AC field that swamps the residual field of the item being DMed, and slowly moving the wand far away leaves a zero residual field.(if the coil current is switched off when in close proximity, a residual field may/may not result).
I had a look at the magnetiser at the Scanspeak factory and from memory it was operated by a charged bank of big caps.
If you can provide an AC high level tapering level magnetic field you ought to be able to demagnetise a driver.
Indeed I have thought about demagnetising and reverse remagnetising drivers just for the experiment.
Also, in the thread on electromagnetic speakers, I mused whether it would be possible to hammer the ferrite ring away to replace it with a copper coil. Do you have any idea whether the pole plate assembly will collapse if the magnet is removed in pieces?
All the ferrite magnet assemblies that I have seen are glued together using contact glue.
If such a glue joint is heated high enough, the glue just lets go.
Putting a hammer to it is probably asking for trouble - ferrite shatters like glass.

Q - What benefits are you getting by drilling holes behind the spider ?.(for the trouble and danger involved).

Eric.
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Old 16th October 2002, 01:43 PM   #26
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Lots of answers, lots of questions...

- I have a demagnetizer from the old days of magnetic tape heads but I doubt it would be strong enough for a ferrite magnet.

- The idea behind drilling those holes is as stated in the title of this thread to lower the mechanical losses that occur due to compression of the air trapped behind the spider which will usually only let the air escape slowly.

Bernd Timmermanns, the editor of the German magazine HobbyHifi, has been maintaining for years that low mechanical losses result in a very precise bass. I have a thread on this here but didn't draw much response. He offered the explanation that compression is a nonlinearity causing distortion, and that air streaming through tiny holes is bound to do this nonlinearly and with some time hysteresis from eddies. He also is against aluminum VC formers because they also result in higher R_ms, but I don't see the nonlinearity there.

- regarding the magnet: I want to get rid of the magnet to replace it by a coil. So shattering it is exactly what I have in mind. My question is whether the iron parts are joined in such a way that they won't move if the magnet is shattered. I f I were to heat the glue instead, I might save the ferrite ring, but I'd have to worry about VC alignment once more.

Eric
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Old 16th October 2002, 02:28 PM   #27
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The tape demag is the right principal but much too low power.
You would likely need a coil 6 or 8 inches diameter and quite high power.

You should find a trashed driver and pull it apart and study it
before you do more risky things to your good drivers.
Normal magnet structure is an iron ring attached to the basket, a thicker ferrite ring (flat donut) glued to this, and single piece iron plate with a forged pole piece glued to that.
Variations are a machined end disc and an inteference fitted polepice.
The ferrite ring positions and centers the polepiece, so you cannot remove the ferrite without replacing it - you'll see what I mean.

Eric.
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Old 16th October 2002, 04:17 PM   #28
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Quote:
Originally posted by capslock
Do you have any information on demagnetizing and remagnetizing ferrites? This would be a handy way to drill holes into steel baskets without having to take the hole thing apart.
I don't know about demagnetizing (probably just heat would do it), but a magnetizer is a serious piece of kit and VERY DANGEROUS so if you are going to persue this please proceed with extreme care.

dave
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Old 16th October 2002, 04:38 PM   #29
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Quote:
Originally posted by planet10
magnetizer
Further on this. I have extracted a few posts from a Joelist thread:

Note: These are serious & dangerous & should be approached with caution.

from Bill Perkins/PEARL
---------------------------
I just get twitchy
when I hear people talking about building discharge maggers because

THEY ARE NOT TOYS ! ! ! YOU CAN GET KILLED ! ! !

Anders, the following links will give all the info you might want - and
almost certainly more - on the subjects of magnets and magnetization.

http://www.magnetsales.com/Design/DesignG_2.htm
http://www.magnet.thomasregister.com...t/dsgn_gde.htm

while the text that follows will give you and idea of just how seriously the
people at Delco Remy take the matter of building your own PD magger.
Although these papers relate to the truly high-energy devices needed to
thump the rare-earths into performance, their cautions apply to all such
devices.
While the coils and fixtures need for the magnetization Alnico and
ceramic magnet structures will run for hours on end without overheating
precautions should still be observed.
These things don't groan and cry before they let go . . . they just go
BANG ! ! !
---------------------------

Dan Marshall (please note the exploding wire bit -- these can be deadly)
---------------------------
I just as well throw in my two cents worth. It is an approach that is
simple, works well and is relatively cheap to implement. Many decades
ago, at the Magnavox research labs, there was a home-brew magnetizer
consisting of a bank of 450 volt computer-grade caps mounted in a simple
wooden box. I don't recall the exact values, but there was something
like 20 caps each about 3 1/8" in diameter and 4 1/2", or so, tall, all
connected in parallel. The contactor consisted of a copper plate and a
copper hammer made from something like 1/4 inch thick copper plate. The
hammer head was weighted to make the contact more positive.

With it they were achieving around 8000 gauss in a particular
special-purpose Alnico impact driver they manufactured. I had occasion
to use the device on a special project I was working on and reckoned
that it could be improved. Out of the clear blue sky, which is were
most of my ideas come from, I reasoned that the circuit was ringing down
and the current reversal was causing a partial demagnetization. So, I
mounted a couple bolts with wing nuts on the side of the box, in circuit
with the magnetizing current, and experimented with various gauge wires
until I found the largest gauge that would explode when the circuit was
energized. That was something like common 24 gauge buss wire. When the
hammer whacked the plate, the wire would explode on the current upswing
sounding like a firecracker. This interrupted the circuit before the
current could reverse. All of a sudden the little impact drivers were
being magnetized to 11,000 gauss, measured in the air-gap. It is a good
idea to keep your eyes away from the exploding wire, as well as your
hands away from any exposed 450 volts.

The inductance of such a circuit is important, so the bank of caps
should be connected in a manner to minimize both inductance and
resistance, preferably using heavy copper strap wired in a grid, rather
than in a long line.

Some folks back then used a 10 uf cap charged to 3,000 volts, coaxially
mounted in an aluminum, or copper tube, to minimize inductance. It
takes a special sort of cap to provide the peak currents though and it
would be easy to fry yourself, so the electrolytics and exploding wire
is probably the more economical and practical approach for the DIYer.
The contactor for the HV approach was a spark gap with a little tickler
electrode along the side of one electrode. When the trigger electrode
was sparked, the 3 kV would arc across main gap conducting the charging
current through the magnetizing coil. When the voltage drops, the gap
stops conducting thus preventing demagnetization. Yes, a spark gap is
extremely low impedance, essentially a dead short.
---------------------------

Nicholas McKinney
Lambda Acoustics Inc.
---------------------------
I spent the better part of 2 years working up the courage to build my unit, and
I was lucky enough to have worked around them before.

Here is a page showing some of the pictures of the unit I built, and some of
the stuff inside the shop here.

http://www.lambdacoustics.com/librar.../shoptour.html
---------------------------

Hopefully this gets the idea across. These are serious & dangerous & should be approached with caution. If anyone wants to persue it, i can collect the entire JoeList thread together and mail it to them.

dave
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