Switching Monitors from CRT to LCD on a GE Stenoscop 2 C-Arm

I have a GE Stenoscop 2 C-Arm, its fully functional but the crt monitors are starting to fail.  I want to change the monitors but don't want to use old CRT's. However when I tried a flat panel it did not work.  I tried with a resolution downscaler (cheap version) and even with a monitor that had the BNC connector factory installed it didn't work either.

Does anyone know why or can guide/help me out to get them to work?

thanks