"Because replacement parts are no longer available, an "out-of-band signaling" system was installed in 1998 for the transmitters located in the U.S. This system uses a continuous tone that is below the normal audio frequency range. When the tone is present, the transmitters are keyed. When the tone is not present, the transmitters are unkeyed. It worked fine, but the Astronaut Office complained about the lack of tones which everyone had become accustomed to as an alert that a transmission was about to start. So, the Quindar tone generator, which was still installed in case it was necessary to key the transmitters at an overseas site, was re-enabled. "

Wonderful link, thanks. The quote above almost rivals Primo Levi's story about the paint recipe in The Periodic Table.