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Jan Rowland

So, I use a cumbersome vacuum box I made of 1/2" Lexan plastic, with an O-ring set into a groove routed along the bottom rim, and I can then place this airtight onto a sheet of Lexan or thick glass, onto which I have first placed a container large enough to accept the transformer, and into which has been poured fresh Formvar, to about 1/4" over the highest point on the lamination-stack (or coil), and generally, with the lead-wires protruding upwards, out of the goo.  Then, I turn on a small vacuum pump which is pneumatically connected to this box.  In several seconds, a vacuum of better than 27" Hg is measurable, and bubbles come out of the coil area. Rather violently at first, then "tiny" and slowly, after a moment.  I leave the pump and vacuum on for a few minutes, this withdrawing all the air from the transformer parts.  I then pull the hose off, allowing the atmosphere back into the box.  This is when the Formvar "gets in there".  So, one must leave it alone for several minutes after the vacuum is off.  After you remember where you were, lift up the "soak", and hang it over the container of varnish by the leads for a while.

Let it drip! Then, "cook" the transformer in the family cooking oven! You can do this either by hanging it from the leads from a grill placed as high as you can, and put a scrap of aluminum foil under the transformer to catch drippings (it will drip a bit more for a few seconds, as it heats up!), or, you can set it on a small piece of hardboard covered with first a scrap of aluminum foil, then waxed paper (so you can get that off the transformer when cured!) Do not set the heat beyond, say, 375°.

What I have gotten seems to cure in a 350° oven in 30 min. It's nasty and sticky when still hot, so, set the transformer down on Teflon or Delrin wedges, over newspaper, etc., and wait four hours before handling. If you undercook it, it will be sticky-soft in places, once cooled to room temperature. If you overcook it, the color of the insulation and varnish will have darkened noticeably, and/or, the PVC insulation on some/all of the soldered on lead wires will have obviously melted off! Bad!