On the Thunder IV, the final output is coming from the ADV473, which I think was a cheaper, drop-in replacement for the Bt473 -- 24-bit CLUT/DAC. The video signal should be sync on green (for the old trinitrons and hitachi clones), so if there is no green output, then there won't be any image. Check the output on the ADV473...and that is something you may be able to replace. The DAC may be blown/ESD'd from the connector side.
Thanks for the pointer on ADV473, I'll take a more detailed look around that chip.
I think the sync is fine as I'm seeing an image and it's not distorted. The only issue is that the color is bluish. I have a picture of it higher up in this thread (the picture of a very blue Share window). I do see output from the green pin but the voltage seems very low. And nothing on red (which explains the blue).