Exploring what's inside a high voltage insulation tester type XS made by Ganz probably between 1978 and 1981 (The potentiometer is from 1978, the baterries from 1981, but they might not be the first batteries in it). It uses 100V, 250V, 500V or 1000V to measure the insulation resistance in two MΩ ranges and 1.5V to measure in the kΩ range. All the transistors in it seem to be Germanium. It runs on 7 D cells.

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