I'm trying to get an old 305mhz SL iMac board working minus the iMac... and I've run in to a problem with the video.
I've made an adapter to connect from the video port on the board (the one that normally goes to the analog board) to a VGA socket, in several different ways (crimping, soldering) and both times I get a rolling picture that my monitor can't sync to...
so the question is: does anyone know what the sync frequencies are that the iMac is putting out? And whether I can change this to allow my monitor to work 
When i used to run external monitors on mine, i found that the 1024 x 768 setting was the only one my monitor could use because it would slow the freq down to i think 75hz or 85hz, anything lower for res has freq up over 100hz AFAIK. if your already running it that high and your monitor cant sync then there may be a problem with wiring, i never did the atx hack so i dont know much about that.
(A bit of googling to remind me...)
The iMac's internal monitor supports these resolutions:
640x480 at 117Hz refresh rate
800x600 at 95 Hz refresh rate
1024x768 at 75Hz refresh rate
You actually need a fairly capable multisync monitor to handle these. Something too cheap or too old won't do. If you're not already using one try hooking the machine up to a more expensive recent-vintage CRT. (Any 19" or bigger CRT less then seven years old should do, while a bottom-of-the-barrel 17" or smaller might not be capable enough.)
As for changing the refresh rate, I think you might need a software hack like SuperRes/SwitchRes/DisplayConfigX on a slot-loader motherboad. With the tray-loaders just booting it once with an iMac-frequency-capable monitor was enough. (Having a VGA adapter plugged in will render it "unlocked". I don't know what the sense line arragement is, if any exists at all, on the slot loaders.)
--Peace
It's also been noted by people doing *slot* loader conversions that the signal strength from the internal video connector is fairly weak. Ex:
http://www.cryogenius.com/hardware/imac/
I suppose it's possible that even if your monitor is "capable" of displaying iMac video modes it might have a problem with the signal level getting to it.
--Peace
So I found an old 14" monitor that'll sync to it... better than nothing and it fits quite well on my desk....