[GreenKeys] low current keying - 28KTR

[Duncan Brown]

Nick,

In the 1960, the standard US Army TTY Crypto units were the KW-7 & 
KW-26. They were both unipolar.  The keyboard & tape reader Mark & 
Common connections ran directly to the Crypto box through a shielded 
cable and a mil  connector (don't remember the type, but I think it had 
6-7 contacts; Selector magnet had a similar cable.) The Crypto box 
supplied a 50ua TX current loop (don't remember what the open-circuit 
voltage was).  At 50ua, 4.7K would only drop about 1/4 volt, and the two 
resistors & cap looks like a spark suppression circuit.  At 50ua, I 
don't think it would make much of a spark, but maybe they felt it was 
needed!

Or it could have been some weird ham mod!


The AN/GRC-46B RTTY system had a small switch box (SA-1243/GRC) in the 
cable between the TT-76 reperf TX line and the KW-7. It just had one 
(lockable) switch that was labeled "Open on Security Equipment 
Installed."  There was a 27K resistor across the switch which was 
switched out of the line when crypto equipment was removed to allow it 
to operate on a 20ma loop.

This was kind of a kluge and you would think if it was a low-level 
current limiting resistor, it would have been in the KW-7.  But it may 
have been for something like your M28 4.7K resistors.  I do not remember 
an external TX line resistor in other TT-76/KW-7 installations.

Have fun,

Duncan
K2OEQ

On 28-Jan-14 22:39, Nick England wrote:
> In getting a 28KTR working, I replaced some stuff inside the code
> generator - 4.7k ohm resistors in series with both mark and common
> leads and then a 0.01 uf cap across the line.
> The replacement shielded line from the code generator had an MS3101
> military connector on it, but this definitely didn't look like a
> factory or military change - the little circuit board that carried the
> resistors inside the code generator box was a piece of bakelite cut
> from some other terminal board to fit, and the clamp that held cable
> shield was also homebrew. The LPR selector magnets had a similar
> MS3101 military connector on a short length of  shielded cable.
>
> I rewired both code generator and selector magnets back to the
> original configuration, but I was wondering what this mod could be in
> aid of. So, not military low-level which is bipolar +/-6v, because
> only the marking contact was hooked up. And while it wasn't a
> military, factory, or WE modification, I can't figure out who would
> need to do such a thing. I'm puzzled (as usual). Any guesses?
>
> Cheers..
> Nick England K4NYW
> www.navy-radio.com
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