An attempt was made to put the Astrocade interface into a Space Zap cabinet sporting a monochrome monitor. It didn’t work so I began investigating exactly what the problem was.
It seems I misread the MiSTer IO schematic, and put a GND where a vertical sync should be, so it was not actually passing the sync to the output. I also made a change from the original colour gun used as it meant that it would then be possible to save one IC on the main PCB when using this configuration.
So, lesson learnt. I have now re-created the RGB board so composite output should work correctly with the available signals. I also added a couple of potentiometers to control the levels (since it is way too bright on my Amiga monitor via CVBS)
So, I will get some of these ordered so that I can test it and make sure it works as it should.