[julie_m]

There was a circuit in Television magazine for a cassette speed tester. This was very simple indeed, feeding the audio into the gate of a thyristor and switching a filament light bulb fed from a suitable transformer. There was also an attenuated output from the transformer for recording the 50Hz mains on another, known good deck. The idea being that you played the recorded 50Hz signal into the thyristor, and adjusted the speed until the brightness of the lamp was steady, meaning the pulses from the tape are coming at the same interval as the mains. Too slow and the pulses arrive later and later in each successive cycle, meaning the lamp dims gradually and then suddenly goes bright again; too fast and the pulses arrive earlier in successive cycles, causing the lamp to swell to full brightness and then dim suddenly. The closer the speed to the correct value, the slower the variations.