Hi,  David Evans [in 200408 MMDigest] provides a very good
explanation of contact physics at the point where the individual
contact wires touch the contact roller.  But there is one channel
where the dynamics are different and the contact breaks without
any paper to quench the spark.  That would be the rewind contact,
number 19.

As he says, my contacts were all just fine after years of commercial
use.  All except #19, which failed after a few months of occasional
use in my home.  Sure enough, the #19 contact wires were burned down
to a nub and the roller was rough.

Of course, the first action of #19 is to [cause the roll drive to]
pull the contact wires away from the roller so the paper can run in
reverse (rewind) without getting ripped to shreds.  That means that
there is no paper to quench the spark, just air, and the tips of the
wires vaporize a little more each time.

With reference to Art Reblitz' great book, "The Mills Violano
Virtuoso", I realized the problem and installed a little relay in
series with that contact and have had no problem since.

Craig Smith