Revised Control Board
I fixed the errors in the old control board but made sure I stayed compatible with the existing daughter boards. I got the PCB back today from the manufacturer.
Assembled the board and fired it up. Partial success. No smoke, the Data and Address Low switches and LEDs work, Address High not (the illuminated LEDs). This means a lot of the hardware and software is working. This revision has a single step mode so tomorrow I will step through the execution and see if I can workout what is still wrong. With luck it may be a software problem but I am not that confident.
Read/Write Register A:
The clock is off, 625kHz rather than 1MHz, but good enough.
Although I said the Data and Address Low switches and LEDs work, they do but the data is not stored. The correct LED turns on if a switch is pressed.