HP 5245L Nixie Counter
The HP 5245L was the top-of-the-line digital counter when it was introduced in 1963. You can see it used in the Honeysuckle Apollo tracking station for example, just below the operators' eye level. Although digital, it uses no ICs whatsoever. Everything is done with discrete transistors. Repairing it feels like repairing a giant IC where you have access to every transistor.
The base instrument is quite impressive on its own, with an oven stabilized quartz oscillator, 50 MHz direct bandwidth, and direct access to both the main signal prescaler dividers and a set of pre-divided reference frequencies derived from the oven quartz oscillator. There is even a super high stability version, the HP 5245M, with a double ovenized oscillator.
But this counter really sings when used in conjunction with one of its many plug ins. The HP 5257A plugin, for example, will extend the frequency counting capability up to 18 GHz! I amassed quite a collection of plugins. I eventually got all of them to work in a repair-a-thon series of videos.
HP 5245L Repair-a-thon Playlist
HP 5245L Counter and Plugin Repair-a-thon Playlist.