i have a bondi iMac that starts up with a chime and a green power button, but no picture. the screen is black, but i can tell the the start up process is commencing without any problems. if i then shut the iMac down, unplug it, plug it back in, and power up i will get a picture. this has happened with two different power cords in two different outlets. i have also reset the logic board, zapped PRAM, and replaced the battery. do any of you have any more ideas?