> I am having a problem in that it does not seem to respond to the
> new-convention MIDI "note off," with zero velocity being "note off."

That's the way the original system worked (note off, not velocity 0).
There are upgrades for the PDS-128 and later available at the PianoDisc
web site,  http://www.pianodisc.com/ but I don't know if the upgrade
fixes that problem.

> I am an electrical engineer and manufacture a MIDI converter for
> older Yamaha organs; so I am familiar with MIDI encoding.

Well, you could always build a MIDI repeater that turns velocity=0 into
"note off" commands.

Marc Kaufman