[SiriusHardware]

Julie wants you try applying logic '0' to each of the UF9 output socket pins in turn (with UF9 removed) to make sure that when UF9 puts out different codes different characters are shown. I think we can already say that happens, up to a point, because with the (apparently) faulty original UF9 in place you get different characters generated, and when you have no UF9 in you get sometimes checkerboard, sometimes random, depending on what you poke with your fingers. That is because with UF9 removed the input pins of UF10 are not held in any specific state. However, do try the test Julie suggested.

The characters generated might not be entirely predictable because while you are holding one pin low the other (unconnected) pins will still be free to do as they like, although they seem to prefer to stay in the high state most of the time.

Back to the main plot, am I correct in reading that with the new UF9, UE7, UE8 all fitted you still have a checkerboard pattern on the screen?

If so, then unfortunately the next item of interest is the video RAM itself.