Restoration of frequency counter hindered by kittens


We think of digital screens as something you see on relatively modern equipment. But some old gear had things like nixies or numitrons to get cool retro digital displays. The HP 521A Frequency Counter, however, uses four columns of ten discrete neon bulbs to create a decidedly low-tech but effective digital display. [Usagi Electric] has been restoring one of them for a while, but there was a gap between the second and third video as his studio became a kitten nursery. You can see the latest video below.

In previous videos, most of the device was working, but there were still some weird behaviors. This video shows the final steps to success. One interesting thing is that since each of the four columns is identical, it was possible to compare the readings from decade to decade.

However, in the end, the neon bulbs were found to be badly corroded, and replacing all the neon bulbs improved operation. However, the self-check that should read the line frequency of 60Hz read 72Hz, so a realignment was needed. But it was relatively easy with a pot accessible from the rear panel. If you want to see more details about the repair, be sure to watch the previous videos.

We love this old gear and how clever designers have done so much with what we consider so little. We hate to encourage your potential addiction, but we’ve already given advice on how to acquire old hardware. If you want to see what was possible before WS2812 panels, you can build this neon bulb contraption.


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