You wanted a picture? [Telecom]

(The picture that Jim sent is online at

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- 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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Jim Haynes
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