The high-voltage monitor gives me the willies, so I disconnected the power to the monitor assembly. (First photo.)

The big question for me was whether this was a problem with the PC that generates the video or the monitor itself. I used the oscilloscope to probe the output pins to see if the signal looked right. TLDR, I think it is.

Photo 2 is a pinout I found online and it seems to be correct.

Photo 3 shows how I was testing the outputs.

The rest of the photos show these pins:

• R
• G
• B
• Vsync
• Hsync/Hsync+Vsync

The first three definitely behaved information-bearing signals, and they corresponded plausibly with game state changes. The last two definitely look like sync signals. I didn’t compare against specs, so there could be some subtle failure, but my money’s on the monitor.