MarkS, I'm looking to go the other way around from the direction you're hacking towards. That is to say, I want to convert the existing Mac CRT (and, if possible, analogue board) into a compatible display for later computers (or indeed PAL video sources).
EvilTim's MPxPlayer has already accomplished this, albeit using software to force a PC video card to lock to the Mac's scan rates. I am investigating the possibility of hardware modding the CRT/ab to avoid this step, for use with systems where this is not an option (due to unavailable software or hardware limitations). I chose VGA as a kind of lowest common denominator for newer systems.
The MPxPlayer and the PMGS demonstrate that a 640x480 dot pitch and greyscale are no problem: the next step is to create standard scan rates.
Apart from scan and sync, the issue here is not converting the motherboard TTL video signal to analogue, it's piping the analogue signal into the Mac CRT in place of the TTL signal.
EvilTim's ingenious approach to this seems the most promising:
In this case, of course, there is only one gain (or brightness) pot to bypass, and the R-G-B signals will have to be summed into it.Remove the red, blue and green gain pots, the video signal goes in where the wiper (middle leg) went
Yes, that seems likely. That is of course the approach the Poor Man's Greyscale takes - I might end up cloning the board he used. I'm sure I have one in the teetering stack o' monitors in the lab.
How is the electron gun (as opposed to the deflection plates/amps) affected by frequency? /ETA/ the PMGS seems to show that it's not an issue.