[GreenKeys] model 15 20/60 switch

[Jim Haynes]

At the risk of an overly long post...the issues are how to build the
machine, and how to transmit signals to it.  If you're transmitting a
short distance, such as from a radio demodulator to a nearby machine,
you can get by with the standard 60 ma 120 V current loop. But the
machine might have been built with a holding magnet selector so that
20 ma loop is another possibility.  And the signal source might be
nearby, or might be a long wire line, or a carrier system channel,
and might be operated neutral or polar or something else.  So the
simplest circuit migh not do.  If it's one sender to a lot of receivers,
especially on a long wire circuit, polar operation is preferred.  But
it's hard to use polar if there are several senders in the net.
Or if the machine is connected to a switchboard you need to have
everying appearing on the switchboard be the same so that you can
connect any station to any other without worrying about whether
it is polar or neutral or something more exotic.  In some systems
line current is on all the time, so if you have a big installation
it is advantageous to run 20 ma loops just to save power.  Recall that
a lot of Model 15 sets in the military were not connected directly
to a line, but used that BE-77 (?) line unit between the machine and
the signal line.

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 		Meredith Willson, The Music Man