The 16550 chips have both active-low and active-high read and write signals.
This board using the active-low signals.
The active-high high signals should be tied to the inactive level (0V).
The !ADS pin also needs tying low.
If the sockets have been fitted, the chip pins must be bent out of the socket and tied low.
I had not fitted the sockets yet, so I drilled out the erroneous PCB holes (for pins 19, 22 and 25) with a 1.3 mm drill to avoid neighbouring tracks. I then soldered solder wires from the ground pin (20) to the socket pins before soldering the sockets to the board.
Both UART sockets need correcting, I have drilled both sets of holes but only fitted one socket.
For a 4 MHz CPU, the timing controller U10 requires a 12 MHz crystal.
The UART chips external clock can be up to 24 MHz. I have some 14.7456 MHz crystals that would work but I need to check what Vince's software expects.