I did some investigation of the schematics and identified some possible locations where there may be an issue. I opened it up and found that the first 3 pins on the main CPU board's ribbon header connector going to the JTAG board were bridged at the back of the PCB where they were soldered. Some quick work with a soldering iron to resolder them everything was fixed. My console output (channel B) now works.
So, what was broken? The soldered points on the board?