[sdsw4]

Gotcha. The glue on the back of the boards actually looks like... glue and wasn't conductive. But it's on my books to remove it since my experience with vintage computing tells me if it's not bad now it'll go bad later. And destroy the board in the process.

I also verified voltages at the board where the tubes connect to: they match what the service manual says they should be not only at the connectors but throughout the board, and remain stable when the tubes are connected.

The red used to be the only color that would "come back" "flash" then die, repeating in a rhythm. Now red is completely dead, and it's blue that does it.
If needed I can re encode the videos to something you can view. Surprising to me, mine looks to be in better condition than the one in your pictures. But I suppose condition means nothing if it doesn't work right...