[ David Evans wrote in 200329 MMD --

> The difficulty about diodes would be that the original DC
> motor-generator could start up in either polarity, so that the
> magnets would not get permanently magnetised.

Hello everyone,  About that polarity switch on the Violano.  When
I got my Violano many years ago, I decided to add a solid state
DC power supply.  As luck would have it, I restored the Violano back
in the middle 1980s, just _before_ Harvey Roehl published "The Mills
Violano-Virtuoso".[1]  I bought one of the first copies of the book
and got it just in time to use the schematic to make the full wave
DC power supply.

In checking the voltage before installation, I discovered that the DC
voltage produced was about 20 volts too high, so Harvey sent along an
addendum correction with each copy.  (I just added a Variac before the
transformer input so I could adjust the voltage.)

Anyway, the power supply schematic included a double-pole double-throw
(DPDT) switch to reverse the voltage polarity now and then, to keep
from magnetizing the solenoids.  Sounded like a good idea to me.

In the first few months, I'd flip the switch now and then as
a good owner should.[2]  Then, one day I flipped the switch while it
the instrument was running; that was definitely a _bad_ idea.  Back
to the shop for a new set of diodes.

Also, since the switch is between the power supply and the solenoids,
shunt diodes would either block the current or blow the diodes.

Regards,

Craig Smith

[1]  "The Mills Violano Virtuoso", by Mike Kitner and Art Reblitz,
Vestal Press, 1984, with errata sheet.

[2] I haven't thrown that DPDT switch in 25 years and the Violano
doesn't seem to care in the slightest.

 [ I tried to make the power supply match the design of the Violano
 [ roll frame -- notice the antique volt meter.
 [ http://www.mmdigest.com/Attachments/20/04/01/200401_133837_Violano%20001.jpg 

 [ Extra diodes and fuses, just in case.  Hard to find nowadays.
 [ http://www.mmdigest.com/Attachments/20/04/01/200401_133837_Violano%20002.jpg