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When an AC system turns on or off a relay opens and closes power to 2 big fan motors...When that happens the relay contacts spark and sparks generate pop noises on all radio frequencies.
Big loads like AC systems, electric clothes dryers and ovens can sag line voltage... measure line voltage with the AC system off and on and use a variac with AC off to simulate the line voltage with the AC on and if volume drops the same amount you'll know it's the radio being sensitive to line voltage... Sometimes when tubes get old they'll be strong at rated heater voltage but loose emission strangely fast as heater voltage is decreased below it's rating.
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