Stiles Gowland, check on this:"
No Video, Boot Chime heard, White LED ON (Symptom 2)
The computer will turn on, the boot chime can be heard, a white LED appears on the front bezel and sounds from the fan or drive activity can be heard, but the display has no picture or color.
1. Check if the computer is sleeping. Press the space bar to wake the computer from sleep mode.
Did the computer wake from sleep?
Yes: Put the computer to sleep from the Apple menu and wake the computer again to test.
Check Energy Saver setting to see when the computer has been designated to sleep.
No: Continue to next step.
2. Reset your computer’s PRAM (Parameter RAM):
If your iMac is on, turn it off by holding the power button -- until the unit powers off. You should hear the fans go quiet.
-- While the computer is off, with your left hand, hold down the Apple key, the Option key next to it, and the letter “R” key on the keyboard. When you have these keys all pressed down, push the power button with your right hand and then quickly move that hand to hold down the letter “P” on the keyboard. Keep these four keys pressed while the computer starts up.
-- Make sure you hold down the four keys (P - R - Apple - Option) while the computer is
starting up. When you hear the computer’s start up chime for the second time, you can
let go of the keys on the keyboard.
-- If you didn’t hear at least two startup chimes, go to the top of this section and begin
again. It’s important that you hear two startup chimes when performing this procedure.
This indicates that you have successfully reset the computer’s PRAM (Parameter RAM).
3. Does the computer display video after successfully resetting the computer’s PRAM?
Yes: Restart the computer from the Apple menu and make sure the computer is now
No: Continue to next step.
4. Make sure the machine is powered off, and remove the access door and front bezel to access the diagnostic LEDs. Plug in and start up the computer to observe the diagnostic LEDs. At this point in the Power On process, you should hear a boot chime, and LED #1 and #2 are On. Is LED #3 On or Off?
LED # 3 is On: The video card and logic board are communicating properly. Go to the next step.
LED # 3 is Off: The video card and logic board are not communicating. Replace the video card. If the issue persists, replace the main logic board.
5. LED #4 should be On when the video card and LCD display have communicated properly to produce a video image. Is LED #4 On or Off?
LED # 4 is On: The video card and LCD display have communicated to LCD video. In this case, there is a problem with the backlights in the panel or the inverter. Replace the LCD display which replaces both the backlights and the inverter.
LED # 4 is Off: The video card and logic board are unable to communicate to generate video.
Replace the video card. If the issue persists, replace the LCD display.
6. Unplug the iMac and remove the SDRAM. Replace with known good SDRAM. Plug the iMac back in and power the unit on. Does the front LED go off after a few moments now?
Yes: Replace the original SDRAM and test again. If the front LED does not go off with the only the original SDRAM installed, replace the SDRAM. If the LED is now reliably going off after a few moments, but yet you still do not have any video, go to step 7.
No: If the LED remained on, the main logic board is not communicating with the LCD panel to generate video. Replace the main logic board.
7. If video is displayed normally on an external display but not on the iMac internal display panel, replace the LCD display.