Alright, so I made a bunch of measurements!
Resistance from U3 pin 1 to 0V: 6.50kΩ
U3 pin 1: 5V with Display A selected, 0.9V with Display B selected, 2.9V with ALT, 1.1V with CHOP
U3 pin 2: 0.1V with Display A selected, 4.4V with Display B selected, 2.2V with ALT, 2V with CHOP
U3 pin 3: 4.9V with Display A selected, 4.9V with Display B selected, 4.9V with ALT, 4.8V with CHOP
U1 pin 1: 5V with Display A selected, 5V with Display B selected, 5V with ALT, 0V with CHOP
U1 pin 2: 5V with Display A selected, 0.4V with Display B selected, 2.7V with ALT, 0.9V with CHOP
CR20 anode: 5V with Display A selected, 0.9V with Display B selected, 2.9V with ALT, 1.1V with CHOP
CR20 cathode: 4.9V with Display A selected, 4.9V with Display B selected, 4.9V with ALT, 4.9V with CHOP
CR19 anode: 5V with Display A selected, 0.4V with Display B selected, 2.7V with ALT, 0.9V with CHOP
CR19 cathode: 5V with Display A selected, 0.9V with Display B selected, 2.9V with ALT, 1.1V with CHOP
A vs B is OFF. Only MAIN is ON.
R108: 6.23kΩ one way, 6.40kΩ the other way
R97: 6.33 kΩ, 5.73kΩ the other way
Could these wrong resistor values be causing the problems? Also, out of curiosity, are those fluctuations in resistance values reading one way vs the other way normal? I know that theoretically they should be the exact same, but of course, that's only theory... And about the diodes, which show a reading in both directions, does that mean they're Zener diodes? We never actually touched a circuit board in school, just books! So it's pretty cool seeing these principles in action.