[R-390] How to repair your Variac

[Bill Hawkins]

Coupl'a things -

The wire diameter doesn't have to be exact, like to .001", if you're not
rewinding the whole thing.

What it needs to do is fit in the space occupied by the burnt and
removed wire. You don't want the brush dropping into an empty space. If
you have to add or subtract a few turns, that's no big deal.

When you fuse it, put the fuse in series with the brush. You can put
another fuse in series with the hot like for electrical safety
(something shorts to case ground - you do have a three wire cord,
right?).

Say you have a 1 amp line fuse. If the dial is set to 10% of line, you
can draw 10 amps (by transformer action) through the brush without
blowing the line fuse. Perhaps this explains why the low end of the
variac gets burned. That, or there's no fuse at all.

If the winding can get hot, because it isn't fused, then don't depend on
solder to hold the splice together. Make a mechanical connection first.
Use some transformer paper to keep the splice from rubbing on the
insulated turns of wire. The brush has to short turns as it moves, so
it's made of carbon. You don't want a copper shorted turn from the
splice.

Bill Hawkins