I have a Betamax tension cartridge which I want to hang onto, but
don't overlook using a fish gauge on a dismantled tape. This can give
pretty good results though you have too fool the microswitches and for
some tests the rotation sensor into thinking a tape is present and
spooling. Actually, holding the microswitch and spinning the unloaded
reel by hand will do this while feeding or pulling the tape and fish
gauge to/from the reel under test.