I'm not sure. I helped with the testing for the 15/31 switching on x68k. If you can tell me how to boot a few of the 24k games in 24k mode I'll give them a shot.What about 24 khz? Not many games use it but I'm curious.
Try without the SyncSlayer - you don't need it, and the sync will be double-processed which may cause issues. The A1 has an onboard sync processor like the SyncSlayerI received my Retro Scaler A1, but I’m experiencing an odd quirk with it.
I’m using the Chipnetics SyncSlayer II to convert the SCART signal from my retro consoles and HAS SuperGun to VGA, which I then feed into the Retro Scaler. The SyncSlayer has several jumper options for choosing Csync vs true horizontal sync, as well as TTL vs 75 ohm for h-sync, v-sync and c-sync.