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So far as I know Conelrad was to avoid enemies locating the exact point where the radio transmission was coming from. Each AM transmitter had 2 crystals: the normal one for the station's frequency, and the Conelrad crystal which was either 640 or 1240 khz. During an attack, stations would switch to the Conelrad crystal and operate for a short period of time, then switch off, then come on again later. I don't know what the period of on to off was. Since all stations were operating at 640 or 1240, listeners would hear continuous information, even though different stations were providing it over a period of time on the same frequencies.
The Zenith AM/FM sets with selenium rectifier used a neon indicator lamp (just an indicator, did not light up dial) in earlier models, and a 28 volt lamp in the dial with a dropping resistor directly off the powerline in later models.
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