[GreenKeys] Electrocom FSC-250 + ITTY on RTTY

[Duncan Brown]

I've been working on an Electrocom FSC-250 FSK converter from c1960, 
that was recently donated to the AWA Museum by Ralph, W8ROI (Thanks!, 
Ralph).

The unit is in pretty good shape, considering its age and it has two 
plug-in filters: 1275 & 2125 Hz.  After replacing the filter capacitors 
and line cord, I plugged the unit in and it seemed to work (no smoke)!

I applied audio tones and could see the results on the CRT and loop 
current, but wanted to see it really type something.

I had learned long ago that a 22.7hz audio tone has a period of 44 ms. 
When squared up, you have a pulse train of 22 ms pulses which will 
produce RYs on a (60wpm) TTY (7.0 unit code)  I had made up a cable with 
limiting diodes that I connect from the variable audio output of my HP 
8640 RF signal generator to the FM modulation input, with DC coupling. 
Tuning in the signal generator frequency on my K-3 XCVR and using the 
P-3 spectrum analyzer to adjust the FM modulation gain for 850 hz shift, 
I was able to generate 850 hz RYs on RTTY.

I connected up the K-3 line audio to the FSC-250 input and tuned the 
receiver for a good cross display on the CRT.  Then I plugged in my 
TT-98D to the FSC-250 and printed endless RY's (The TT-98-D has 
automatic end of line CR/LF).  But I soon got tired of just seeing RY's 
typed and wanted to see more.

I normally copy ITTY with an old laptop and a ST-6.  I realized that  
the ST-6 FSK output runs off the same loop as the printer, so it would 
have the ITTY signal on it. So I was just a matter of connecting the 
ST-6 "FSK" output connector to the HP 8640 "FM input" connector and 
adjusting the FM gain again. (The ST-6 "FSK" output is a resistive 
attenuator from the HV loop for +/- 15VDC, but was loaded down some by 
the HP 8640 input).

Then I  was printing ITTY from a HF RTTY signal!

So if you have a RF signal generator that will accept a DC input for 
Frequency Modulation, you to can generate a RTTY signal at any shift.  I 
have another military FSK converter (Harris MIL_3300) that I need to 
pull out now and see if it works!

Have fun,

Duncan
K2OEQ


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