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In Reply to: Magnepan Voice Coil Repair - A Saga posted by Peter Gunn on May 2, 2005 at 09:09:39:
Why do the wires like to curl up at the ends. To me no longer adhering is one thing... but it is pretty clear it is not like they fall off as a "straight sheet" (if you get my point.. not enough sleep and I do not even make sense to myself these days :-).Is there some kind of stress relief (I am an idiot here) that could be done so that the ends do not try to pull away?
Follow Ups:
IMHO, I think it's purely mechanical. If you make a hank from a rope or an electric cord you have to put a small twist with each loop in order for the loops to lie flat. When Magnepan lays the wire down on the mylar the wire is not twisted at the bend and this creates stress at that point making the ends pop up.
I may be an egoist but at least I don't talk about other people.
The Lorenz forces are way too weak. However, my theory is theraml cycling. The U-turn end, by the permanent deformation during manufacture, raises the local resistivity. That's why you never see kinked wires not being crimped and/or soldered. This can be a thermal instability. You see, seeing that these local resistivities are small compared to the overall length, any local increase in resistivity doesn't appreciably change the VC's impedance at all. Thus, the current doesn't change, but the local I²×R heating certainly does. I haven't quite determined if pulling away from the thermal insulating Mylar to a lower (cooler) energy state was sufficient to make the bends "banana", or whether the constant maleting by the excurting Mylar significantly helped while the coil is warm. Once delamination starts, the reciprocating Mylar is only pounding in one direction. The only issue is then initiation. There's no doubt that the U-turns have the greatest delaminating tensile force. As the Mylar excurts away from the magnets, it wants to bow, while the VC wants to keep the area it occupies flat. At the tops & bottoms of the VC, the greatest tensile force is distributed at the appex of the U-turns (over the least contact area). As the Mylar excurts toward the magnets, the delaminated pair re-contacts. Now, repeat several trillons of times over a decade...
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I think it is because in the middle the surface tension of everything around you keeps you stuck. On the edge, you have a side where the surface tension ends, which allows for failure as it's the one weak spot. Stickers never peel from the middle either.As to why they bend, once a wire comes off it is going to move when the speaker plays. At the point where the separation occurs the topside of the wire is under compression, while the underside of the wire by the mylar is getting stretched ever so slightly as the vibrations try to pull it away.
So as more and more comes off, it will have a slight bent put in it from this compression and stretching, which even defies gravity as coils on the bottom bend upwards as well.
At least that would be my guess.
As to why they come off, IMHO it happens because 3M 77 sucks eggs. If you use a REAL adhesive, a mans adhesive mind you, I think that will resolve the issue for good and the ends no longer will be able to pull away.
As far as I know as well, the DAP does not deteriorate over time like the 3m and milloxane apparently do. I know old timers who have used it to veneer with never a failure.
Thanks for a great attitude!Looking forward to reading your adventures in tweaking the crossovers.
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