That schematic has some curious idiosyncrasies. The logic is powered by
+5VD, but all the pull ups being used are to +5V. (The ones that are shown,
anyway; there are unused inputs on U420 and U520 that must also be pulled
high for the board to work, but these aren't indicated in the schematic.)
Also, the inputs from the mode switches on A2 are referenced to +5V (not
+5VD). +5V and +5VD are basically two copies of the same supply, powered
from the same output of the switcher, and their domains cross in a few
places within the scope without any real decoupling. That spirit continues
in the 067-0912-00 board.

The parts list and schematic do not appear to line up 1:1 (C621 on the
schematic is very probably C620 on the parts list, but R520 is in the parts
list and so far escapes detection on the schematic.

Another interesting observation is that the _FPR ("Front Panel Read",
active low) signal is left floating when using the 067-0912-00, meaning
that A2U61 and A2U62 are not guaranteed to be tri-stated and could be
fighting U430 and U530 of the 067-0912-00 for D0:D8. I'm sort of surprised
there's no pull-up on the A2 board, but the schematic shows that signal
being driven directly by the MPU board (which would be absent when using
the 067-0912-00).

Enough mysteries for tonight...


Dave Casey