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Posted by Jim Haynes on September 5, 2008, 1:09 am
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(The picture that Jim sent is online at
http://www.telecom-digest.org/mystery.html - bh)
If you were asking for a picture for the Western Union transmission
question, here is the portion of the schematic that deals with connection
to the signal line.
+D.P. and -D.P. are the outputs of a double-pole rectifier. Looks like
about 120 volts each way, and used only for the polar transmission. +4
is the output of a single-pole rectifier used for internal purposes in
the set, and also about 120 volts. The polar relays (W.U. type 202)
have four windings. U-D and U-circle-D-circle are about 150 ohms each.
A-A' and O-O' are about 110 ohms each. + into A', + into O', + into U,
+ into circle-U all move the relay to marking.
You see the sending relay has a bias to spacing using the circle-D
circle-U winding. The D-U winding is in series with the keyboard
or tape transmitter and has current in the marking direction approximately
twice the bias current. This is a pretty standard way to operate a
polar relay in a make-and-break circuit. So with the toggle switch in
the Terminal position it sends positive for mark and negative for space
out the tongue.
Then the receiving relay is biased spacing using the A-A' and O-O'
windings. When the sending relay is marking, which is the way it will
be when the main office is transmitting, it is putting a current in the
marking direction through the U and circle-U terminals, hence presumably
overcoming the spacing bias and moving the receiving relay to marking.
So I'm assuming the Line In from the main office just goes through a
polar relay winding to ground at the main office. And simply opening and
closing the line at the main office will cause the receiving relay to
go marking (closed circuit) or spacing (open circuit).
Turning the toggle switch to the Intermediate position simply extends
the circuit through the Line Out jack to the next station down the line.
The drawing title says "Class C Way Operation" and then there are some
notes about changing the strapping for Class CT which seems to be the
same general idea but operating over telephone company wires. I haven't
figured out exactly what the changes are, but probably some additional
wave shaping and perhaps reducing the current.
jhhaynes at earthlink dot net
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