I pulled my boards together on the bench mainly to archive two ROM revisions that seem to be undumped in MAME, thanks to
@DogP for pointing that out to me.
I scope the video timings of every board that comes across the bench so I can build a library of timings for the pattern generators on my monitor benches (to perfectly dial-in geomtery before going into a specific game), and I wanted to make a vid on how to do this anyway for a couple friends that want one of these pattern gens, so this all fit together nicely.
I can post the vid here anybody wants to see it, but I'm not sure if anybody would want to sit through 40 minutes of this... In it, I scoped the Ataxx timings and then emulate it on the gen displaying a MAME screencap, showing it displaying on 4 different monitor types using both the comp sync and the direct sep sync, and also quickly go through a ded cab with a 19D7700 in it (digital chassis that works fine with 17khz boards) and archive the ROMs.
But the basics for the sync weirdness are that the PCB without the JAMMA adapter only outputs separate syncs, as H-sync and V-blank. Because of this, you can't just gate them together for Comp-sync because you need the negative H-sync to blank long enough for the monitor to be able to see a V-sync pulse, so you need some logic to do this, hence the PAL.
V-blank is 16 lines (game has 240 lines displayed and 16 lines of blank. 256 total), and the PAL takes this signal and the H-sync and spits out the Csync to JAMMA as 2 lines of V-blank , leaving14 lines of back porch. no front porch. So the logic for the PAL is fairly simple, it just passes the H-sync straight through but goes low for 2 H-sync pulses whenever it sees V-sync (aka Vblank) go low. that's it. No serration in the V-sync.
Obviously, it's still a weird video signal, it's like a Williams timing and has no V-FP at all. But obviously the timings are really strange at almost 17khz and 66hz. Ataxx uses a 7.15mhz dot clock like all the mid-80's Atari stuff (sys1, Tetris, etc.). Game works great if you use a 19K7203, just flip that switch to 17khz!
Green trace is the blue video signal in test mode, so we see 100% of the displayed picture (blue background), and the yellow is C-sync from the JAMMA adapter doing it's thing.
