[GreenKeys] Model 28 signal lines

[Jim Haynes]

On Wed, 16 Oct 2019, Ralph Mowery wrote:
> 
> For the original poster, you can set the supply to give you about 20 ma. 
> You may have to try both ways with the plus and minus wires .  If the
> machine quiets running open and quitens down at 20 ma it is probably set for
> 20 ma.  If it still runs open, then do the same with more current and see if
> it quiets down at about 50 ma.  If so, then it is a 60 ma machine.
> 
There are probably easier ways to figure out if the coils are in series
or parallel.

I've never seen this in the books, but I was told by the guy who invented
it that the holding magnet selector of Model 14/15 vintage was intended
to avoid having to use 120V on the loop.  And the reason was that the loop
voltage appears on the cords of the TWX switchboards and you can get a
nasty shock from 120V if you accidentally touch the tip of the plug.
So that suggests that 48V might be enough for a holding magnet selector.

With that kind of selector the rise time of current is less critical
because a cam pushes the armature into the attracted position and then
the line current just holds it or lets it drop back.