One of the transistors in the early stages of the amplifier (or effects pedal) acted as a detector for the AM signal probably being picked up by a cable that wo

One of the transistors in the early stages of the amplifier (or effects pedal) acted as a detector for the AM signal probably being picked up by a cable that worked as an antenna. It worked essentially as a untuned crystal radio followed by a powerful audio amplifier. The problem usually is a result of bad grounding or screening.