Issue solved!

Did:
- Confirmed resistor on TX board was in R51 not R52 (it was already good).
- Reflow solder on all ribbon connector pins on TX board.
- Reflow solder on all chip pins on TX board, including MAX3237EAI.

Did not solve problem.

Then Did:
- Reflow solder on all ribbon connector pins for the small ribbon connector on the RX board.
- Reflow solder on pins for IC38, IC39, IC35 (MAX3241ECAI) and 4420P-601 on the RX board.

It works correctly now!

Thanks, guys!