Ok, I think I may have a clue to this mystery. I still don’t have a schematic for my set, but I was able to find one for the KV-1311CR, a contemporary set that also has the 34-pin RGB connector. Interestingly, the 1311 also has a separate 8-pin digital RGB connector, which is selectable by a switch. Here is the schematic for the blue analog input:
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This input has 75 ohm termination, as expected. Now, here’s the schematic for the RGB digital inputs:
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These have a higher termination value, 470 ohms. In a post I found describing how to connect the KV-25XBR to an Amiga computer, they state that pin 21 of the RGB connector selects between digital RGB and analog RGB (no mechanical switch as on the 1311). I’m guessing it has a pull-up resistor on it to select analog RGB by default, so that it worked perfectly to display the signal coming from the Jaguar. However, when the TV was off and I measured the RGB resistance, the pull-up resistor had no effect and I was seeing the resistance of whatever electronically controlled switch they’re using to select between digital and analog RGB.
Now I’m wondering if I can just hook the 3.3V output of the FPGA directly to the monitor with no dropping resistor and pull pin 21 of the connector low in order to get a CGA display... The only issue is that it’s expecting 5V inputs, so the question is whether there will be enough voltage to drive the logic inputs (the AND gates).