It looks like the 'main' Zaxis signal, which contains both the brightness level and the unblanking pulse, is not getting through to the Zaxis amplifier. The inputs to the Zaxis amplifier from the readout system and from the 'main' signal source are completely symmetric so it looks as though it should be safe to swap the plugs on J2208 and J2203 on the Zaxis board, provided the leads are long enough.
I don't think its particularly safe to leave R2135 where it is, I would restore it to something approximating its original position. Do that first and then reinstate the readout display.
If you then swap the J2203 and J2208 leads and nothing changes it suggests there is no signal on the 'main' Z axis lead where it arrives at the Zaxis board. This could be a plug/cable fault so see if you can trace the cable originally on J2208 from the Zaxis board back to the A6 Logic board at J4408, probably via the main interface board. Check end to end continuity with your multimeter. If the cable continuity looks good then you may well have a faulty U4494. Again, if it is socketed, remove it, clean the pins and reinsert.
If you swap the J2203 / J2208 leads and the main signals work but not the readout then you probably have a fault around Q2216. Is it socketed?
If you or anyone following this thread think this suggestion is too risky (7834s and 7934s are quite rare) then please ignore the suggestion.