i’m having similar difficulties trying to get it to work with a 15khz RGB monitor – the Sony KX14CP1
So i’m taking the signals direct off the MegaPC board RGBHV
My monitor expects TTL level and i suspect this is the issue.
Looking at the MegaPc Service manual the RGBHV goes to the monitor where there is a Toshiba TA8631 RGB Chip but before the inputs there are 75ohm pull down resistors and an ‘in-line’ capacitor 47uf.
on the TA8631 datasheet it recommmends 51ohm pull downs.
I think i need to pull down the lines and also connect the grounds.
This may help you