I didn't see another place for this, and since it is related to the MC624 I felt this was the most appropriate location for the issue.

On the REM624 everything functions as intended except two functions. The Mono and Diff. Pressing the buttons on the REM624 causes the LEDs to light as expected, but the MC624 does not respond to the presses. While I know that the REM624 is supposed to be used as the master. For troubleshooting I changed the MC624 to master and the REM624 to slave. I figured the micro programming was identical between the two and the REM624 would simply track whatever the MC624 was doing. And that is exactly what happened. Everything functions except when I press Mono or Diff. The MC624 functions as expected, but the REM624 does not respond. However, all other functions work.

Double checked all the solder joints and traced continuity. Not saying it couldn't be something related to that, but seeing as the LEDs turn on? I'm kind of out of ideas for the evening.