[GreenKeys] MODEL 15

[WA5CAB at cs.com]

The M14/15/19 sets were available with two types of selector assemblies, 
pulling and holding. Pulling magnet selectors only operate on 60 ma (magnet coilss 
are round or cylindrical). Holding magnet assemblies can operate on either 20 
or 60 ma (magnet coils are smaller than pulling magnet coils and are square 
with radiused corners). Most machines were equipped with pulling magnet 
selectors. Some of the Bell or WU system refugees could probably shed light on what 
service usually used holding magnet machines. I know that they are much rarer 
than the pulling magnet ones. And I don't recall ever seeing holding magnets on 
a military set.

Other than the coils, there are mechanical differences . If a machine with a 
pulling magnet selector runs open, you will see that the selector armature 
doesn't move. If one with a holding magnet selector runs open, the armature moves 
rapidly one cycle per bit. With current flowing through the holding magnet, 
the armature is held against the magnet poles during the bit set period and a 
MARK gets set. With no current, it isn't held and a SPACE gets set. The pulling 
magnet has to pull the armature against its return spring across an air gap, 
which takes a significantly stronger magnet field.

There are hundreds of M15 variants and I haven't worked on any since 1973 so 
I couldn't hazard a guess as to what the resistor might be for.

In a message dated 11/20/2007 9:08:37 PM Central Standard Time, 
RLAAG at PACBELL.NET writes: 
> ONE MORE THING, THE SELECTOR MAGNET CURRENT... I SUPPOSE THE MAGNETS 
> WIRE IN SERIES FOR 20 MA LIKE THE 28 AND PARALLEL FOR 60 MA... RIGHT 
> NOW THERE IS ALSO A STRANGE RESISTOR IN THAT AREA ON A BRACKET... IN MY 
> APPLICATION I DOUBT IF ONE IS NEEDED THERE BUT AM NOT SURE WHAT IT WAS 
> FOR... I WAS WONDERING IF THE SEL MAGNETS NEED TO BE IN PARALLEL LIKE 
> IN THE 28 OR IF THEY JUST RAN NORMALLY ON ONLY 60 MA???
 

Robert & Susan Downs - Houston
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