I’ve wired the usual RGB and composite sync, but then got confused as to why it wasn’t switching to RGB, then found the blanking signal for selecting RGB was more or less zero. The resistor divider I added from 12v should have given about 2v… then I spotted 12v goes through a 10k resistor and there’s no other power on the monitor port.
Is the best option here to use vsync with 75R (ish) resistor as voltage divider and possibly a smoothing cap to remove the low sync pulse? I think my old CRT isn’t going to care too much about the low sync pulses, but I could be wrong.