[Milsurplus] Command Set Transmitter Keying

[J. Forster]

Mike,

Interesting theory, but I'm not so sure a spring would weaken over time.
The material the spring is made out of is likely stresses well below its
elastic limit, otherwise it would fail due to low cycle fatigue. It may
well be stressed below its fatigue limit which is generally even lower.

That leaves creep as a mechanism for the weakening. Some very soft
materials do creep over centuries, but I'd doubt that is the case here
with steel, phosphor bronze, or beryllium copper.

FWIW,
-John

===========


> 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
>
>
>
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