I've managed to plug a PS1 into an Atari ST monitor. The picture's very good, but it's kind of dark. Is there a way to fix this?
Some ST monitors were just a bit dark, but you might try to remove the caps (or just short them) in the PS RGB cable.
I've played with an Atari ST and the picture on the monitor certainly wasn't dark. I'd say it's just a case of the Atari monitor wanting non-standard video (amplitude wise).
Most RGB standards (VGA, SCART) use have a 0.7V p-p while the Atari ST (and hence SC1224) use 1V p-p. That's why standard RGB looks dark on a monitor designed for an ST.