[Milsurplus] Command Set Transmitter Keying

[Mike Hanz]

To elaborate on Mike's excellent analysis - when Steve, KB4DMF, was 
preparing a complete SCR-274N set for the 2005 Collins Amateur Radio 
Club event in Cedar Rapids 
(http://www.collinsclubs.com/carc/b-29/index.html ), 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.

73 and happy new year,
Mike  KC4TOS

Mike Morrow wrote:
>> David Stinson ... has written several times about running the
>> transmitters as is without any extra regulation and without chirp,
>> click or other unpleasantness.
>>     
>
> The remark that follows is not germaine to the set in the
> earlier thread (a command set transmitter with significant
> modifications).
>
> Often those wanting operate an ATA, SCR-274-N, or AN/ARC-5 transmitter
> in stock original condition will try to key the transmitter by
> keying the K53 and K54 relays in the transmitter.  But that is not
> how the system was designed.  K53 (Oscillator HV and PA cathode) and
> K54 (antenna connection) relays are NOT keying relays.  They are
> transmitter *select* relays.  They are energized ONLY, but CONTINUOUSLY,
> when that particular transmitter in its rack has been selected at the
> control box.
>
> The only relay that operates on key-down in the original system design
> is K52 in the modulator unit.  This relay supplies ALL B+ (Oscillator,
> PA screen, and PA plate HV) to all transmitters in the rack.  But the
> only transmitter that will function will be the one that has its K53
> and K54 select relays energized.
>
> So, for the ATA, SCR-274-N, and AN/ARC-5 transmitters, if you want to
> duplicate the methods used in the original system, you must energize
> K53 and K54 continuously, and key the three HV sources going to the
> transmitter.  Using K53 and K54 to key the transmitter is mis-using
> select relays as keying relays, and thus it is not operating the
> transmitter as it was designed to be. 
>
> I'm not suggesting that will eliminate CW chirp, only that this is
> the way the system was designed to function.  As Dave Stinson has
> pointed out, attempting to key a transmitter by keying K53 can cause
> problems if the sequence of contact closures on the K53 relay doesn't
> cause oscillator HV to be applied before the PA cathodes are grounded
> by the same relay.  This isn't a problem in the original system,
> since K53 would be continuously energized in the selected transmitter.