Re: Variac brush repair

• To: tesla-at-pupman-dot-com
• Subject: Re: Variac brush repair
• From: Scott_L_Hanson-at-notes.seagate-dot-com (by way of Terry Fritz <twf-at-verinet-dot-com>)
• Date: Tue, 15 Dec 1998 17:11:01 -0700
• Approved: twf-at-verinet-dot-com

I'm facing the same problem with fabricating a replacement brush for an
older Variac.

As far as I can tell (at least on the lower-power Variacs) the brush
material is identical with the graphite brushes used in
fractional-horsepower motors (i.e. not a graphite/powdered-copper composite
like is used on higher-power DC motors).  I will de-solder the remaining
brush stub out of its holder and try to confirm if it was copper-plated
(almost 99% sure it was). In any case, I'll fabricate a replacement brush
out of a similar-sized motor brush and set it upside-down 50% submerged in
a strong solution of copper sulfate in a glass container; within 1/2 hour
there should be a deposit of metallic copper plated on the brush. If
nothing happens, I'll add a copper wire to the container and impose a few
volts across the electrodes to speed up the plating. This should make the
brush easily solderable. I also have access to very low resistance
silver-filled epoxy used for PCBA repair and "cold" IC mounting; because
the bond line is soo thin, this might be acceptable for low power/low
temperature applications.

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