shull check this as well
No Video, No Boot Chime
The computer will turn on, but there is no boot chime and no video on the display. The faint sound of the fans, hard drive, and optical drive may also be heard.
1. Make sure all software updates have been applied to the computer. Refer to KBase article 303903, Intel-based iMac: Installing available updates , and to Apple Downloads for updates for the iMac (Mid 2007) model of computers.
2. Follow instructions in the “General Information” chapter to reset the SMC. Does the computer display video after successfully resetting the SMC?
Yes: Problem solved.
No: Continue to next step.
3. Verify that only supported SDRAM memory has been installed and that it has been installed correctly. Unsupported and/or defective memory can prevent the computer from booting. It may be necessary to install known good memory for testing purposes and replace any unsupported or defective SO-DIMM’s identified during this process. Does the computer display video after verifying and/or replacing the memory with known good memory?
Yes: Problem solved. Verify full system functionality before return the system to the customer.
No: Replace the logic board. (don’t do that just yet ;-)
Indicates that the trickle voltage from the power supply has been detected by the main logic board. This LED will remain ON whenever the iMac (24-inch) is connected to a working AC power source. The LED will remain on even when the computer has been shut down or put to sleep. The LED will turn off only if the AC power source is disconnected or the power supply is faulty.
Indicates that the main logic board has detected proper power from the power supply when the computer is turned on. This LED will be ON when the computer is turned on and the power supply is working correctly.
Indicates that the computer and the video card are communicating. This LED will be ON when the computer is communicating properly with the video card. If LEDs 1 and 2 are ON and you heard the startup sound, but LED 3 is OFF, then the video card might be installed incorrectly or need replacement.
Indicates that the computer and the LCD display panel are communicating. This LED will be ON when the computer is turned on and video signal is being generated. If the LED is ON and there is no image on the LCD display panel, the LCD display panel or inverter might be installed incorrectly or need replacement.