[volt-nuts] Update on 720A

[David C. Partridge]

> The bridge balance pot may not be so lucky, though.  I don't think it needs to be anything very special.

As far as I can tell it is a very special pot wired with two resistance tracks.  The coarse track has with two wipers connecting to the fine track which has just one wiper, all driven by a clever mechanism to provide both coarse and fine adjustments with only one shaft.

In line art like this:

----
   x
   x
   x<-----
   x      x
   x      x<---
   x      x
   x<-----
   x
   x
----

Dave