[GreenKeys] TTY Loop Sense and FSK driver CKT

[Eugene Hertz]

Hello Greenkeys and Central Electronics folks

Thought you might be interested in a circuit that can be connected directly to a 60ma loop (loop must be current limited!) and key a transmitter. In my case the transmitter is a CE 100V, but this circuit can probably key lots of other transmitters as long as the Vce and current ratings of the output opto-coupler is not exceeded. 

http://www.hertzmail.com/rtty/loop-sense.pdf

My particular set up is 

[ST-6]----[28ASR]----[CKT]----[CE100V]
|------------------------------------|

Loop starts with the ST-6 with its internal loop supply (current limited) loops through the M28, then through this loop sensing circuit, then into the FSK-1 input of the 100V and then back to the ST6.

My own experience (which seems to be shared by me only) was to be careful with the 1N4733A 5.1V Zeners. Out of a batch of 10, 9 did not work properly direct from the electronics distributor. Anyone ever have problems with zeners before?

But back to the circuit. The bridge DB104 is optional and is used to make the circuit polarity insensitive so that it won't matter how you connect the loop.  This bridge can probably be any number of bridges, I used a W04G. Make sure it can handle at least 200V and 1 or more amps to be safe.

Circuit was designed by Gil Smith (thank you Gil). I can report that I have successfully used this circuit with my CE100V and it now keys like a champ in FSK mode.  Be sure to use the FSK-1 input on the 100V. That input has about 18VDC on it and when shorted to ground, the FSK output goes from space to mark.  The FSK-2 input has 120VDC on it. Don't use that one! 

Good luck!
Eugene