My MiSTer works great connected to a digital display and I'm trying to get it working on a refurbished BVM which is my only CRT. I'm using these cables to go from HDMI to RGB:
https://www.amazon.com/dp/B075GZ8DX7
https://www.amazon.com/dp/B000067SM6
I've changed to these in MiSTer.ini:
direct_video=1
composite_sync=1
The CRT only displays a horizontal line that bounces around the screen.
There are red, green, and blue sockets on 2 cards on the BVM. One is labeled ANALOG which also has a sync socket. The other is labeled DIGITAL ANALOG and also has a series of other sockets but none of them are labeled sync. My BNC cable has 5 output plugs so I may not be connecting it properly although I've tried several things.