I just acquired this A4000 along with the ZZ9000 card. RTG modes work fine, but with passthrough video, white is usually displayed as yellow. This is not a problem with the analog video output, it’s only seen in the passthrough. Additionally, it will sometimes flicker to the correct color, and it might even stay there for a few seconds, before flicking back.
This is more of a question, but I also noticed that I get no image when using 31kHz modes. Should those modes also be visible with HDMI passthrough?
It seems that RED analog signal is not picked up/sampled continously. I suggest you inspect IDC ribbon cable and IDC sockets for bent pins or loose wires. Trying another ribbon cable is 2nd best bet before diving further.
I did try a different ribbon cable. The behavior is similar, but if anything it flickers to “correct” color more often. I used the Amiga floppy connector, which gave me the opportunity to attach a scope to the third connector. With a full-screen white image, all the red bits look correct (high during the entire active horizontal scan time).
I suppose the next logical place to probe would be at the 74245’s, but being on the back side of the card, that’d be quite annoying…
I was hoping this might be an issue with something (timing?) in the FPGA, but it sounds like you’re leaning towards a hardware fault?
One more interesting tidbit is that the behavior is stranger with the longer ribbon cable. Moving the ribbon cable around seems to have some effect on the flickering and whether the color appears as red or yellow, which implies that it could be related to noise?
For the record, this turned out to be a bad trace on the motherboard. Between the 74245 and resistor pack to ground for the B7 trace (high bit of blue)