Is there supposed to be a mechanical stop to keep one from turning the
band change dial too far and screwing it up, or from turning it the wrong
way??

Yah, there definitely is a stop at both ends. That is, if you try to go
below the lowest band, or above the highest one, you hit a stop. My second
set is in pieces right now, so I can't investigate.

There is a detent mechanism at the rear end of the bandshaft which might
have a hard stop. Again, this thing is all over the workbench, so I can't
say for sure. But that's where I would put a stop if I were designing a
multi-band radio.

Below is a diagram for stringing the tuners. It mentions a blocking washer,
and also says that you may break the cord if tension and knot position in
the tuning is not correct.

http://antiqueradio.org/art/temp/SX28DialStringing.jpg

I can't testify that this information will be identical for SX-28 and
SX-28A. Your mileage may vary :-)

From using my first SX-28, I have to say that the stop feels more mechanical
(i.e., look at your detent assembly), not like hitting the end of a string.
If your bandswitch turns smoothly from one band to the next, is it possible
that somebody disassembled the bandshaft and left out some pieces, or forgot
to tighten things down?

Regards,

Phil Nelson