Hewlett-Packard 521DR Digital Counter from the 1950s

Counting events or precisely measuring frequency with a digital readout may seem pretty simple, but in the 1950s doing it took a lot of vacuum tubes—so many that this vintage Hewlett-Packard laboratory instrument required filtered forced air to keep its cool. Before losing their mojo, Hewlett-Packard was celebrated for “building to last”. Will this gear, more than sixty years old, still work?