I have tried several vga second monitors with my coreduo(1) 2.0ghz iMac, all of which have the same problem: the screen is very dim. So far i have tried a mitsubishi 930SB and an apple studio display 21. These displays look great on on my windows junkbox and my ibook g4 with the display hack but they look as if they are broken on my iMac. I notice the same problem under boot camp and thus i suspect the problem to be rooted in either hardware or firmware. It would be helpful if someone could perform a similar test on their iMac as i suspect this is a widespread problem.
Top