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MMD > Archives > July 2002 > 2002.07.22 > 07Prev  Next

Substitute for Open Cup Mercury Switch
By Miguel de Mattos

Most of the DEA and Welte early 'reproducing' player pianos (excluding
the late model Triphonolas that used very sensible mechanical switches)
used this kind of open cup mercury switch.  Usually those instruments
work with DC current, and this is the only problem.

As you know, DC current produces a continuous voltaic arc during the
fraction of a millimeter before contact across the switch is made or
broken.  This arc produces heat, and this is responsible for the 'fried'
mercury and liberated toxic gas [mercury vapor].  With AC current this
doesn't happen, or it happens in very, very small and tolerable
quantities, because there's much less voltaic arc energy to boil the

I had the same problem when I restored a Hupfeld DEA.  What I did was
to feed the player with AC current, including the mercury switch board,
and then before the DC motor I installed a rectifier device to convert
AC to DC.  Problem solved -- no smoking or gases.

The DEA electrical board is a very nice and clever device, that's why
I didn't change it, but if it's against your environment ideas then
"No problema!"  There are dozens of very soft mechanical switches that
can perform the same or better then the old ones, and of course making
your air much less polluted.

Kind regards
Miguel de Mattos
Lisboa, Portugal

 [ Thanks for reminding us, Miguel, of the extra problems encountered
 [ when switching direct current.  Your solution is ingenious!  :-)
 [ -- Robbie

(Message sent Mon 22 Jul 2002, 20:39:59 GMT, from time zone GMT.)

Key Words in Subject:  Cup, Mercury, Open, Substitute, Switch

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