[GreenKeys] [AWA] We have this Model 31 Teletype and need the control box as showen...

[Jim Haynes]

Note that the original Model 14 and Model 15 equipment with pulling
magnet selector has the selector coils in series for 60MA operation;
20MA is not an option.  Model 14, 15 and later equipment with the
holding magnet selector allows the coils to be in series for 20MA or
in parallel for 60MA.  I've seen military Model 15s with a decal on
the copyholder saying they have been refitted with different magnets
that can go either series or parallel, and the picture on the decal
shows the cylindrical magnet coils that belong to the pulling magnet
selector.  But I've never encountered a machine that actually has the
special coils, in spite of the decal.

The original purpose of the holding magnet selector was to allow the
voltage on the cord circuit of the TWX manual switchboard to be limited
to 48V.  Otherwise they had 120V, which could be deadly and definitely
shocking.  This I heard from the late Walt Zenner who invented the
holding magnet selector, so I take it to be truthful; but I've never seen
this mentioned in any of the literature about the selectors.

I read somewhere that the Bell System  went to 20MA operation just to
save electricity in situations where the TTY machine is not very far
from the telephone office.  An office with lots of TTY loops consumes a
lot of power just keeping them all turned on all the time.  7.2 watts
per loop, versus 2.6 watts for 20 MA loops.

I don't know that the use of 60MA in ham installations was so much of a
conscious choice as it was a matter of accomodating whatever equipment
might come along, including Model 15s with pulling magnet selectors.

jhhaynes at earthlink dot net