[GreenKeys] model 15 20/60 switch

[Duncan Brown]

The 60 ma loop comes from the DC telegraph circuits which could be miles 
long.  In the 1940-50s, when more TTYs were used with voice circuits 
(telephone modems & radios), the distance to the modem or radio 
converter (TU) was measured in feet, not miles, and a lower loop current 
could be used.

All the Kleinschmidt models (designed for the military, starting in the 
late 1940s) had easily accessible 20/60 ma switches. The switch on the 
TT-4 was labeled "DC LINE / VOICE FREQ LINE"

In the M28 line, there was typically an "Electrical Service Unit" (ESU) 
which contained a polar relay that isolated the selector magnet from the 
external line.  Since the SM was on an internal loop, it did not need to 
easily switched between 20 & 60 ma. I'm not as familiar with the M28 
line, but I think there as probably a wiring choice on the terminal 
strip on the rear wall for the 20/60 ma choice.

Have fun,

Duncan
K2OEQ



On 11/15/2024 12:07 PM, Tom Hunter wrote:
> W9DDD,
>
> I have a holding magnet 15 with a 20/60 mil switch. So they did indeed 
> put them on some 15s.
>
>
> Tom N3CRK
>
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