[GreenKeys] Trepac 538-A Telegraph Relay For Sale

[Jim Haynes]

Oh, that takes me back.  (Not that I want it, but it takes me back)
to 1958 when I first had a summer job at Teletype.  My boss had a
number of little projects that a summer student could work on, and one
he handed me was to do a solid state line relay.  He had one, I think
it was Trepac but maybe a different model, that had been taken apart
to see how it worked.  At the time there were not many high voltage
transistors, so the relay used two of them in series with suitable
monkey business to get them to turn on and off together.

I couldn't think of much of anything to do along that line, but had
a different idea.  At the time there were power transistors for auto
radios, so I said why don't we wind the selector coils for 12 volts,
600 ma and use a power transistor to drive them.  So I got some coils
made and tested it - also turns out my boss had already thought of that.
So it didn't solve the line relay problem, but the idea did take hold
by the time of the Model 32/33/35 and later equipment.  Whomever
designed the selector magnet driver didn't like my simple saturated
transistor model and changed to a constant-current driver, which probably
did give faster operation as needed for 110 baud.  Also unknown to me
there was a transistor driver developed for 60ma selectors, as well as
a vacuum tube driver somewhere back in the woods.

The sad part of this is that I failed to sell what I still consider a
better concept.  That was to have every selector magnet equipped with
a transistor driver, and every transmitting contact equipped with a low
voltage circuit, and run the whole set internally on low voltage along
the lines of RS-232 polar signals.  (RS-232 didn't come along until 
several years later, and for a different purpose)  This would get rid
of all the current loops inside the set and greatly simplify the
internal switching.  No more having to open a loop to insert something
and keep the loop closed when something was not in use.  And would easily
accomodate other transistorized accessories such as a regenerative
repeater.  The interface between the set internals and the outside world
could be a line relay, or an electronic line relay, or the output of a
modem, or anything else you might happen to want.  But for some reason,
I'm guessing the old geezers at Bell Labs, the Teletype selector magnet
drivers were designed to be in current loops, even though it was at low
voltage.

Jim W6JVE

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 	"Ya can argue all ya wanna, but it's dif'rent than it was."
 	"No it ain't! No it ain't!  But ya gotta know the territory."
 		Meredith Willson, The Music Man