I am mostly finished with this project so this log will catch up on the story from the beginning.
I got a broken cable tester from work for free intending to repair it. After troubleshooting I found that the microcontroller was dead. Of course, the microcontroller had no markings and a non-standard pinout so a direct replacement was out of the question. I decided to take the opportunity to re-engineer the cable tester to learn about PCB design and SMD assembly.
The original blue PCBs were the first PCBs I designed and ordered myself. Assembly with the SMD components was easier than expected and after fixing a few mistakes I was able to get the board running. Unfortunately, I had confused the drain and source on the source MOSFETs in the design so they could not turn off (the internal diode runs the opposite way in P-Channel MOSFETs compared to N-channel). This caused excessive current consumption (I could watch the battery voltage sag in real time!).
I decided rather than cutting tracks and bodge-wiring all nine MOSFETs I would just order another board, and, since another board revision was needed anyway, I would go ahead and design a new board for the remote end. This also means that my design can be used stand alone without needing the remote end of the original tester. Fortunately the assembly of the second set of boards went well and I now have a functional cable tester. I just finished with some clean up of the design and at this point most of what's left is documentation and writing up the project for others.