Vintage Synth Repair

Hello, folx!
Got a problem... I have a mixed-up, messed up Korg EX-8000.  The problem?  It refuses to listen to MIDI.  It seems to boot up ok, displaying "8000", then going to the Prog./Param./Value display.  But it refuses to acknowledge any MIDI Note-On and Note-Off messages.
I just changed the back-up battery, installing a new holder in the process.  That seemed to go ok, and it stores and recalls patch information.  But it's not listening to any MIDI inputs; not from my MEX-8000 memory expander, not from my keyboard, nada.
Is there something I'm missing?  Do I need to load the factory programs back to make some checksum agree, like the Poly-61M (or was it the SCI Multitrak?)?  Is there a factory reset?  Frankly, I'm stumped.  Any help greatly appreciated.
 
THANKS in advance.
 
Dave Garf