The line driver ICs are known to give out sometimes from static discharge. There isn't much in between transmitter board and receiver boards for the buttons so I would guess a bad IC. The control line of the display which controls the buttons and remote run through pin 6 of the display connectors. On the transmitter board, the control signal goes through pin 17 and 12 of IC34, through filter C107 pins 6 and 15 to DB-25 pin 13. On the receiver it goes through DB-25 pin 13 to filter C116 pins 15 and 6 to IC35 pins 8 and 15 and back to display connector 6.

Check all these connections again, and see if you have access to a scope see if you can find where the signal stops.

Stu