sync handling at Ex-SGI Hitachi CM2198MSG monitor

PC cards supply hv sync, SGI supplied sync-on-green. Could we hooke the 2198 at a PC card ?

Answer: At least not by sending it the sync signals directly. Building one of the various sync-on-green multiplexers would likely work, but I doubt the signal quality at 1600x1200x75H . Or get an sync-on-green PC card.

As I already put 2 hours in this and found a dead-end, I may as well go the extra mile and docu it: This Hitachi features a 13W3 connector in SGI style _only_ . If that doesn't mean anything to you, check monitorworld.com or others first.

SGI apparently didn't believe in feeding sync via the "small" pins in the 13W3, so a SUN adapter 13W3->VGA will not work. So why not check whether Hitachi had internally used the same board for both SUN and SGI ? Result: They very likely did, but without docs it'll be a long night to figure that one out. So, to give you an idea what's inside and saving you half an hour dismantling:

The main board is a single layer PCB as shown in the images: The "small" pins are routed via a 13W3 adapter to a field of jumpers (soldered, S302,S303,S304), where they are either forwarded to the (not-connected) output side, connected to gnd or Vcc (the three jumpers at S303 used to code the monitor type for the SGI graphic card) or presumably fed to the sync processing (left hand side of the PCB when looking at the component side).
So it may be possible to reroute the signals and get extra sync input, but without internal Hitachi docs it'll be an all-nighter. Even if you know what signals feed to the sync processing, you may need more active components for signal shaping. There's space left for extra components on the PCB.

Disclaimer: Monitors are not the right place to fiddle without a very high respect for high voltages. Just keep off the internals for your own sake. Your mileage may vary and you have to base any decisions on your (repeat: your) insights.