Issue: The right display is missing two segments on each digit, plus the last digit is missing totally.
As you can see, the last digit is missing (a zero) + the left two segments are missing.
Solution: Lets start with the missing segments - System 11 displays are controlled by strobes (which are all the 16 digits) and segments (eg the -- in a digit). The display tube is being strobed constantly so your eyes don't realize that the digits are not lit at the same time, but serially. For each digit being strobed, the corresponding segments are being triggered.
This problem is unusual; most typically you see segments stuck on and not stuck off. Basically segments are triggered via 4049 Hex inverter IC and an UDN7180A (these fail quite often, but usually cause the "segment stuck on" problem).
As a quick test, i swapped a know good display over from a "Game Show" and the issue remained. Therefore the display itself can be excluded as the source of the error -> we now know that the issue comes from the main CPU board or the display cable. Swapping the cables over yielded the same results - its gotta be CPU.
Using the schematics to trace back the missing segments, the culprit was most likely the 6821 PIA at U51. Using a basic DMM - red lead to ground and black lead to probe the pins - revealed two pins that had different values than the rest. So the 6821 needs to go...
When soldering in a new IC, always use quality sockets. Don't be cheap. Anyway, a new PIA is in place and voila - the missing display segments are back.
Alright, next up - the missing digit. Looking at the schematics, we are looking for last strobe, the last digit of the display. In that case the answer was quite easy - oxidation ate away one of the display connections within the glass tube.
Simple, but expensive fix - replace the display tube. These are really hard to get nowadays and expensive as hell - make sure that the tube is really to blame before doing this. Anyway - tube changed and all was well.