[Milsurplus] Command Set Transmitter Keying

[Mike Morrow]

Mike wrote:

>Steve, KB4DMF, was preparing a complete SCR-274N set ... he ran into an 
>interesting chirp problem with keying the entire set as it was 
>originally meant to be.  We commiserated at some length and wrote a 
>review of the problem with scope photos, presented here:  
>http://aafradio.org/docs/Relay_timing_issues_-_SCR-274N.html  The final 
>transmitter note was pretty clean, but it took a lot of thought to make 
>it so with the original keying schema.

Steve's write up is an interesting discussion of what happens when (and
why) the HV keying relay K52 stays energized longer than the antenna
transfer relay K55, which results in the antenna being disconnected
before the transmitter stops RF generation.

Steve shows that it's due to the interaction of the magnetic field
collapse transient of the parallel coils of all six relays K52, K51, K55,
K1, K2, K3 that drop out when the CW key opens.  Apparently this results
the HV keying relay K52 *not* dropping out before the antenna relay K55
has dropped out.

The real problem, it appears to me, is the delayed dropout of K52.  Steve's
installation of a DC blocking diode in the K52 coil circuit seems like
a good way to eliminate dropout delay.  But it seems to me that installing
similar diodes in the other relay paths would be unnecessary.  I would
have tried installing a resistor of relatively low value at K52's coil
terminals at the easily accessible top of the modulator, which could help
with K52 field collapse without using a diode.

I'd also agree that this would seem to be an inherent characteristic of
the design, with only one observation.  If the relay opening spring
force of K52 has weaked over the last 65 years, that would also increase
the time it takes for K52 contacts to open.  For K52, that opening "spring"
is provided only by the contact leaf, and there's only one such leaf.  It
would be interesting to be able to compare the drop out time of a new K52
versus that of all the 65-year-old examples that exist today...but that's
obviously not possible.  I suppose one could disassemble K52 and bend the
contact leaf so that it exerts greater opening force and see if that takes
care of the issue.  If new-made K52s had stronger opening spring force,
then this problem may not have shown itself in the original era of set usage.

In any event, Steve's and Mike's discussion and experiments are interesting
and seldom discussed aspects of vintage technology usage, stuff you don't find
in old CQ or 73 magazines, or surplus conversion handbooks.

Mike / KK5F