A friend of mine sent me a couple of Vanguard pcbs which didn’t sync at all on all monitors I tested.
The only difference I saw was that his pcbs were Centuri versions while my pcb which synced good was SNK version.
Centuri versions on the video board have a factory mod with some flying wires which are not documented on the schematics and which are related to the sync circuit.
SNK version has no flying wires on the video board:
offcourse undoing the factory mod on the century board fixes the sync problem but I wanted to learn something more.
It seems that Centuri versions were modified to use the positive sync monitor supplied in their cabinets and probably they also improved the sync
signal in comparison to the original circuit.
Anyway , if you want to keep the mod but invert the sync for common monitors or TVs, you have only to solder the wire which goes to pin 3 of the 74LS05 marked in red
to pin 4.