On both boards, u408 (USB) & u1003 (video-in) [the two I2C level-shifting repeaters] behave normally initially, however while the working unit starts to get a lot of comms happening at 25 seconds into the boot cycle... The broken unit never gets to the point where comms start.
There's an empty 10 pin connector spot (next to the main
ICC plug) and this is what I see on the not-working unit (I have not tried the working one yet):
Pin 1 : 13 seconds in goes from low to high
Pin 2,3,6,7,9,10 : Low
Pin 4,5 : Some sort of serial comms happening at 58.82KHz or 117.64KHz
Pin 8 : High.
Comms initialises at 15 secs in (take away 1 or 2 secs for my one handed start-capture press and power plug insert manoeuvre) and starts at 15.6 seconds.
(I know, it's probably 115.2K baud, these things are never accurate. I have not tested the voltages though.)