Briefly summarizing my original thread, I have been trying to find a way to display (non-Apple) retro video games on the Apple CRT monitors I already have. I started off (successfully) connecting a SNES to my IIgs monitor, and then attempted to (unsuccessfully) connect my Wii to my Macintosh II monitor using a GBS-8200. My current goal has been to find a way to drive my (Sony Trinitron) AppleColor High-Resolution RGB Monitor using modern/alternative hardware.
This evening, I came across a fantastic new development which I think could be a potential solution:
I found
a thread on VintageApple that covers a way to connect a Raspberry Pi to what looks to be another model Apple CRT (that has the same video input requirements as the AppleColor High-Resolution RGB Monitor). I believe that if I can replicate these results, I will be able to use my Raspberry Pi (running EmulationStation or even Basilisk II) with my AppleColor High-Resolution RGB Monitor.
The OP said they first manually modified the video resolution output settings on their Raspberry Pi to match their Apple Monitor. They then attached an HDMI to VGA converter to the Pi's HDMI video output. The VGA Horizontal & Vertical sync signals were then fed into a DIY C-sync converter board.
They didn't say exactly how they did it yet, but presumably, the OP then attached the RGB lines from the RPi VGA connector directly to the CRT's DB15 RGB counterparts, and they also connected the C-sync output to the CRT.
Not only does this hack allow for Raspberry Pi video output on an Apple CRT, but it also allows for the output of an RGBtoHDMI to be fed to the CRT as well. The OP was using this conversion system to connect their Apple IIgs video output to an unsupported Macintosh monitor, with great results!