JamesF wrote:Thank you paulbnl, it works great!
Seems like the SOG resistor+diode injection of csync to the Y (luma), it signal pulls is slightly below 0v so the black shades are clipped when using the VGA adapter with this resistor+diode SOG mod.
The IO boards has a buffer transistor for SOG and does not change the Y signal, but this quick resistor+diode mod does.
Both circuits doing the same. Y signal is pulled up at some level (depending on resistor) during active area (which must be treated as black level by display) and remove pull during sync signal and let the Y drop bellow the black level. This is how sync is injected.
Depending on display and its ability to auto-adjust the black level independently on each component you may or may not see slight green component discolouration. This is why I/O board has SOG switch which you set to OFF position if SOG is not required, so it won't affect TV/Monitor expecting normal VGA signal without negative sync on green.
Original circuit with transistor doing 2 functions: 1) mixes hsync+vsync 2) inject the sync to Y/G. Since MiSTer has option to provide CSync (over HSync) from all cores, only HSync is required, so diode is enough for MiSTer.
You may increase the resistor to reduce the pulling level. For me 2K works fine too. This will decrease level of sync pulses as well. So it will depend on your display ability to catch the sync pulses.
More complex SOG circuit needs negative power supply (or power inverter) to inject the syncs as true negative pulses without pulling Y/G.