Restoring a Hewlett-Packard 521A Frequency Counter

Understandinging how the decimal counter modules work is worth the time to watch this video. I always assumed they were decimal ring counters, as used in ENIAC. But nooo—that would be too obvious for H-P engineers, who figured out how to derive analogue voltages from a binary counter to illuminate the correct neon bulb for each digit. And wait until you see how they adapted that circuit to drive a NIxie tube!