In the 60 and 70s every transmitter I worked with had a crystal for
either 640 or 1240. As I recall there were serious requirements to be
part of the system. I think they were an underground studio, generator
at both studio and transmitter and a connection to a local, regional
or federal emergency feed to, I think, Civil Defense.
I don't remember what the testing procedure was because everyone would
have had to switch to their assigned emergency frequency. I'm just not
old enough to remember.
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