GE Adventurer II / 15SF not locking sync

[ieure]

Hi folks. I have a Computer Space arcade game which uses a modified GE Adventurer II (model SF3101BK) television to display the game. These were inexpensive hybrid sets, mostly tubes, with a handful of solid state components. They were modified to inject signal directly into the video amp, by connecting it to pin 11 of V2.

My symptom is that vsync doesn't lock, so the image rolls. You can get it to roll very slowly by adjusting the vertical hold/size/linearity pots, but it never locks, and will begin creeping up (or down, depending on the pot position) after a few seconds. This set has very low hours on it, maybe 100 total over 50 years, so the failures are more likely to be from age than wear.

I replaced C402, which is a multivalued electrolytic cap, and looks to be the only lytic in the set. This maybe helped a little, but didn't solve the problem.

I kind of suspect the vertical tube, a 17JZ8 @V4, but it seems unlikely that it would have gone bad over time.

Am I off base? What else should I be looking at?

The schematic is in the back of the manual for the game: https://www.computerspacefan.com/ComputerSpace.pdf

Note that some copies of this game came with an S3 chassis set, and others with an SF, so the manual has both drawings. Mine has the SF chassis.