In 1970, after four years as a technician in the Air Force, I went to work for GenRad, testing, troubleshooting, repairing, and calibrating new scientific instruments, including portable sound-level meters. One day I got a meter that other techs had been unable to fix. It worked fine except that randomly, every few seconds, the analog meter would briefly kick up to full scale, making a tiny "bonk" sound when it hit the mechanical stop. The circuit was all transistorized, (no IC's) mostly direct-coupled, but organized into three blocks with capacitive coupling between them, all powered by an 18V NiCd battery pack.
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