Remember this?
And this...?
Well, it sat on the shelf for a while, after DaveW made a brief but important note about it in IRC... It needed protection. From itself.
Hum is annoying at 60Hz, but at inaudible frequencies, it's downright destructive. The turns expanding and contracting both from thermal cycling and due to the attraction to the core (and each other) will cause insulation breakdown resulting in shorts (lack of air flow was clearly not the only cause for the original failure).
I wasn't about to rewind it again, but I also didn't have the materials and tools to properly "set" a coil either. Dave recommended varnish (if memory serves), submerged, and a vacuum pulled to make sure that it gets between all the nooks and crannies to lock it in place. Good advice, and from a worthy source. So what to do after the fact?
Well, I won't be putting any warranty sticker on it, but in lieu of a can of varnish, vacuum pump, curing oven, and much at all in the experience department, I figured I'd take a stab at least at something logical as the next best thing.
A heat gun, a lot of patience and time, a small fan, too many coats to count, and a little theory, and this is what I came up with...
This was before the final run with the heat... a dozen or so "soaking" applications to get it in the voids, similar again but in the form of thinner misting coats here and there, a lot of twirling and spinning the thing in all directions to minimize waste while the majority of the solvent evaporated, more heat with less fan toward the beginning, and nearly all fan only between coats just before the above pic was taken.
Then one final long, slow heating, continuously twirling and spinning, the solvents bubbled up and made it look really horrific at the peak... but eventually this begins to wane and then it became a super thick goo on the fringe of "smoking temps" (really light, only visible briefly when the hot air gun was removed to check for this specifically).
I held the higher heat for another 5 minutes or so, continuing to twirl and spin, while the surface returned to a smooth shine and the last of the bubbles were coming up here and there. At that point I began backing off the heat, still moving it around constantly, and finally took the heat completely away and let it dry in free air (guess what? Still rotating and flipping! Lol) until it was not much more than "lukewarm".
15 minutes hanging in front of the fan...
This is the end result. The finish is nice and hard and doesn't really mark with a weak-fair fingernail press.
It should continue to harden, and I'm sure that if it overheats like the original, it'll lose its effectiveness. At that point it's probably done for anyway.
Nothing really to lose, and just in case just winding a toroid alone isn't overflowing with enough insanity, this certainly gives me a new appreciation to the efforts I've seen here and elsewhere winding the powerjack transformers and such.
As for this particular case's mentor... Dave, rewinding generators and motors still isn't in my cards, but bet that the next time I see a nicely wound rotor or stator that I'll think of it as the same way I'd describe this if I only could use one word:
Art.
Till next time...
