JVC TT 71: buy now for spare for my or TT81?

Elliot, I am not sure what you meant, but if your interest in a TT71 is based on its fitting your TT81 plinth, I do believe a TT101 would also fit a plinth made for TT81.  Of course, the factory plinth for TT101 (to be found included with a TT101, along with a Victor tonearm, in the package known as QL10) might be a bit better than an OEM TT81 plinth, but I really don't know that for sure.  Anyway, if what you want is an inexpensive back-up turntable for your TT81, sure, the TT71 would be fine.  You either want it or you don't.  It's not a matter of anyone's opinion.

And I forgot to mention that because of that issue with the swelling of the printed circuit boards, I suggest using the Victor turn tables in a low humidity environment or at least in an environment where the temperature fluctuations are minimal.

Elliot, why do you expect your TT81 to fail eventually? I recognize the favorable cost factor with the TT71, but I always prefer buying upscale, rather than down. Also issues with the TT101 are a bit overstated here, and that is partly my fault because I described my problems in detail on this website. In the end, my unit had a single crack across one tracing on one of its PCBs which was very difficult to find both because the malfunction was intermittent and because the crack was covered over with solder. Turns out the Victor PCBs are hygroscopic and their freedom to swell and contract is restricted by fixed mounts bolted to the chassis. This can lead to cracking. But there was nothing wrong with the complex circuit itself. A working TT101 with fresh electrolytic capacitors installed is nothing to be afraid of and is a great turntable. However, I’d stay away from the TT801, especially one with no vacuum motor and no mat. That’s ridiculous.