Korg Poly800/EX800 Users

Without having seen the video:

Both the Master Tune and the pitch bend go through the KLM601/KLM-601a (I
recently tried to figure out what was wrong with one of my Poly's). First
thing I would look for is loose wires around the KLM-601 or the points
where it connects to the main board.

(One of my Poly's was missing the whole 601/601a board and exhibited also a
very weird problem that was in some ways similar to the one described by
you: no master tune, and pitch bend only in one direction; and either zero
or full).

If that doesn't help we'll look further.