In case anyone cares, I ended up getting working that
Videotrax card I wrote about a day ago. What might be useful
to know is that I followed Dwight's advice and re-seated the
only socketed chip on the board (that customy Motorola chip).
After doing this everything seems to be working peachy. I'm
more than halfway through dumping another 10 megabytes of
data from one of the VHS data tapes I have; so far so good.
Long ago I had one of these brand new. I never got it to reliably
restore. It might have been the quality of my tape deck or the tapes,
but I gave up.