How do I troubleshoot logic board

Thank you oldturkey03! I found the 3 diagnostic LEDs hiding under a piece of black tape, just to the right of LVDS connector. This is a bit different from the graphic you provided, but I expect they provide the same function. My serial number says it is late 2006, though.

Anyway, here are the results:

LED1 comes on when the AC cord is plugged in. (normal)

LED2 comes on when I press the power button. (normal)

LED3 does not come on. (no video signal?)

LED4 comes on with power button, dim at first, then bright, and stays on.

So....does this indicate a logic board problem with the video? Any tips for a reset or workaround there? Your help very much appreciated.

I'd hook up an external monitor. If that shows Video, you know that your video circuitry is functioning and the error is with your LCD. If it does not work, than the issues are with your logic board (video circuitry.)

Again, your advice is appreciated. No video with external or internal monitor. Since the fans all run at high speed, there is no startup sound, and symptoms are the same with and without memory, I suspect it must be logic board or video circuitry related....perhaps something heat related since the fans start up and stay at high speed? Do you know if there is a separate video card, or is it a chip on the motherboard for this model? (Late 2006 iMac a1195) I am more familiar with PCs, and there is a video chip solder reflow issue that can cause similar symptoms on those mother boards.

First, I think I have inadvertently started two questions by opening a post specifically for iMac Intel 17". Sorry, I am a newbie to the forum and will be glad to close one of them as soon as I figure out how to do that.

Again, your advice is appreciated. No video with external or internal monitor. Since the fans all run at high speed, there is no startup sound, and symptoms are the same with and without memory, I suspect it must be logic board or video circuitry related....perhaps something heat related since the fans start up and stay at high speed? Do you know if there is a separate video card, or is it a chip on the motherboard for this model? (Late 2006 iMac a1195) I am more familiar with PCs, and there is a video chip solder reflow issue that can cause similar symptoms on those mother boards

Ron, I think if you click on the blue Actions link on the right side above your question, it may giver you some option s to archive that particular question. Anyhow this is what Apple suggest for symptoms similar to yours:"

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

Now, I realize you already replaced the LCD, so you are coming close to the logic board being at fault. If the video chip would fail/ have failed the behavior is usually somewhat different. More like distorted graphics, plenty of colored boxes on the screen etc. but it is a possibility.