[GreenKeys] More on Model 15 Wiring

[ad7i]

Thanks to all who responded to my request for info about Model 15 wiring.

Jerry Murphy and Nick England suggested that the power relay is likely
related to TWX service and that the relay may have been activated by 48 VDC
or 120 AC with a suitable current limiting resistor.  My M15 is a Bell
System TTY with a faceplate that looks like TXW. See photo below.  So that
may explain the inclusion of the power relay.  This TTY came to me with a
wire strap across the relay contacts, so the motor runs whenever AC power
is supplied to the TTY.

[image: m15 twx plate.jpg]
John Koster reminded me about the selector magnet switch that is sometimes
included with Model 15s.  My M15 does have this switch, a switch I had
totally overlooked when I was measuring the selector magnet resistance.
This switch is used to put the two magnets into a parallel or series
configuration.  A photo of a schematic snipped (pasted below) shows the
switching circuit.

[image: m15-magnets.jpg]


Unfortunately, on my M15, when the switch is in the series position, I
measure 5K ohms between terminals 45 and 46, which means that something is
open (other than the resistor).  I was hoping it was a switch problem, but
investigating further, it looks like the inner coil is open.  I was
originally planning to leave the problem alone, since the M15 prints fine.
But in thinking more about it, given that power is proportional to the
square of current, and with one of the two parallel coils being open, that
means that the working coil carries twice the normal current and thus is
dissipating 4x the normal power.  Not having experience with TTYs, I'd
expect that coils go open either because of excessive current (heat) or
that the magnet's internal wire breaks where it connects to the terminal
because of vibration when the wire it too taut.  Who knows how long the
inner magnet has been open on this M15, but I'm thinking that I should
either repair or replace the inner coil.   Do others agree that it needs to
be repaired?  Or do you think I can safely leave it "as is"?

thanks,

Paul, ad7i
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