CuriousMarc uploaded a video to Youtube showing the startup procedure for an HP 5061A cesium clock [0]. They switch on the clock and compare it to a GPS reference. Seeing the abstract description realized in concrete terms through hot cesium tubes, phase locked loops, and Lissajous curves was really fascinating.
Quite impressive that a hobbyist got his hands on two working active hydrogen masers. These are still by far the best commercially available frequency standards and cost a few hundred thousand dollar each.
That's the price -- when new. But most "time nuts" buy their atomic clocks well used on eBay or local surplus sales at a massive discount. For example the most recent H-maser on the hobbyist market went for just a few thousand. Granted it was in need of repair, but that journey is part of the fun of an atomic timekeeping at home hobby.
Nowadays there's voltnuts in addition to the older timenuts.
The original volt-nuts mailing list pretty much faded away as the members use the eevblog forum now. See the Metrology section.
The time-nuts mailing list is still very active, now in its 23rd year. More info at the group page: