7854: XY display mode bit stuck at 0?


Hi folks,
I have a 7854 which works in analog mode but has some troubles in storage and readout mode. One possible symptom I seem to have discovered is that the XY signal (display mode bit 5) in schematic "32: Vertical Display and control" measurable at TP400-3 of board A29 seems to be stuck at zero when I switch on the scope with no plugins, press STORE and switch off readout. I believe it should be a steady logic HI. Can someone corroborate this?
I changed U630 but the symptom remains so I am either mistaken about XY or there is an upstream IC that is not transmitting this mode bit correctly via the data bus.
Any pointers would be greatly appreciated - TIA!
Best - Ram