It took a while, but eventually, I tracked the problem down to this 3.3M resistor that had gone open. I took a while because I tried measuring it in circuit and by coincidence it measured about 3.3M across it.
After exhausting other possible bad resistors, I came back to this one and disconnected one end. Sure enough - infinite resistance. That got me good sound, now to try for some video.
First though I had to replace a busted brightness pot
Then using an 8XP4 test CRT only produced a faint, ghostly image.
Next I'll try the 14XP4.