@jacobpem besides all the great point that @mayer is making, try the troubleshooting first. Go through it step by step.
No Video
No Video, No Boot Chime, White LED ON (Symptom 1)
The iMac (17-inch Late 2006) will turn on (indicated by the front LED 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. 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.
2. Verify that only supported SDRAM memory has been installed and that it has been installed
correctly. Unsupported and/or defective memory can prevent the iMac (17-inch Late 2006) 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.
No Video, Boot Chime heard, White LED ON (Symptom 2)
The iMac (17-inch Late 2006) 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 y -- our 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
working correctly.
No: Continue to next step.
4. Make sure the machine is powered off; power off the machine by holding in the Power Button
on the rear of the machine. Turn on the machine. Observe the white LED on the front bezel
during startup; it should go out after a few moments. Did the LED go off?
Yes: If the LED went off, this indicates that the main logic board has communicated with the
LCD display for video signal and your main logic board should be good. You can confirm by
connecting an external monitor and checking to see that you have video out. Go to Step 7.
No: Continue with the next step to reset system power management (SMC).
5. Follow instructions in the “General Information” chapter to reset the SMC. Does the computer
display video after successfully resetting the SMC?
Yes: Press and hold the power button until the computer powers off. Start the computer
again and confirm that front LED is reliably going off. If the computer still has no video but
the LED is reliably going off, continue with step 7.
No: Go to the next step.
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. With the LED going off, you may have a No Backlight condition or an LCD failure. In a dim room,
turn the display at an angle to you and look closely at the display. Is the display completely black
or can you see a dim, purplish light or glow to the display?
-- Completely black: Replace the inverter
-- Dim purplish light or glow: Replace the LCD panel.