[GreenKeys] Old War Stories, related to snubbing

[jhhaynes at earthlink.net]

When I had a summer job at Teletype in 1958 or 1959 my boss asked
me to look at a solid state replacement for the polar relay.  There
were no decent high voltage transistors in those days, and it occurred
to me that we had nice power transistors and we ought to just wind
the selector coils for 12 volts and 600 ma. and switch with a power
transistor.  He had already thought of that too, as he showed me his
notebook.  Anyway I got some selector coils wound and of course it
worked fine - I don't remember now if I did anything about snubbing
at the time.

Later on someone else took the idea and ran with it and that is why
the Model 32, 33 and 35 machines have low voltage selectors.  The
person who did this also decided to use a complicated constant current
driver circuit instead of my simple saturated transistor.  Maybe this
was necessary for operation at 110 baud ASCII - I had only tested with
75 baud Baudot.

We could put Model 35 selector coils in Model 28 equipment and run it
at low voltage (20V, 500ma.)  There is something in a book put out by
Teletypewriters for the Deaf many years ago that suggests Western Union
had some low voltage selector coils made for some of their Model 15
machines; but these would be hard to find today.

What I had in mind at the time was a scheme in which all the signal
paths inside a set would be low voltage, and run as voltages rather
than as current loops.  This would allow building in various gadgets
such as a solid state regenerative repeater, and would also simplify
switching in a complicated set like an ASR.  But I wasn't around long
enough to promote this concept, so for some weird reason even the
Model 3x sets with selector magnet drivers were designed for current
loop operation rather than voltage.



jhhaynes at earthlink dot net