Older X68000 models have a sync signal which is open collector, generated with a LS06 with 1000Ω pullups for the hsync/vsync signals. This doesn’t play very nice with devices such as Extron converters, and probably a bunch of other hardware.
Later models instead use a LS04 which outputs clean TTL sync, which Extrons will happily eat.
Since these are mostly pin compatible, I assume it would be fine to remove the pull-ups resistors (R26, R27) and swap in a LS04 (IC3) into my CZ-600C, so I tried it out. The only 04 IC I had at home was a SN74HC04N, but I assumed it couldn’t really hurt to use that, and it did indeed work great, and my X68000 now syncs with my Extron perfectly.
On my model, the inverters of the IC are connected to a pin of the stereoscopic 3D port, and to the “Hig Res” pin on the LED board connector. The LED works fine after the swap, and I assume the 3D port should as well, although I don’t see myself using that.
Overall I can very much recommend this mod.