Seventy-five years ago this month, the January 1945 issue of Popular Science showed this simple tester for troubleshooting radio circuits if problems “can’t be located by guesswork”. The top portion was an audio or radio-frequency circuit tracer. You worked backwards from the speaker to locate the source of the trouble. The bottom section was a continuity tester. If you heard a click in the headphones, then you knew there was continuity. To test a capacitor, you would connect it and hear a click. If you waited a few seconds and repeated the process, a weaker click would tell you that the capacitor was holding its charge.