Chosen Solution

After a couple of power bumps to our home’s system, and power was restored, all home electrical panel breakers were still in the on or untripped position. The 6 outlet power strip, my iMac was plugged into, was still on, again, untripped, but my iMac will not turn on. I have tested the power cable from the power strip to the iMac, it’s OK. My first thought is blown power supply. Second, damaged motherboard. Need info. on diagnosis procedures, repair, ect. Thanks! JMP

John Poen, here is the trouble shooting guide from Apple. It should help to narrow it down. Diagnostic LEDs The iMac (20-inch 2007) has four built-in diagnostic LEDs on the main logic board that can help you to troubleshoot the computer. The four LEDs are located to the right of the memory slot, under the hard drive data cable.

It may be necessary to carefully remove the hard drive data cable to view the LEDs.

LED #1 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 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. LED #2 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. LED #3 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. LED #4 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. The Computer Will Not Turn On. The Display Remains Black and There Are No Sounds From the Fans or Drives.

  1. Verify the power outlet is good. Plug a different device into the socket to ensure there is power, or plug the iMac into another outlet. Does the iMac power on now? Yes: Resolved. Bad outlet. No: Go on to the next step.
  2. Check the power cord. Use a known good power cord. Does the iMac power on now? Yes: Your power cord has failed. Replace the AC power cord. No: Go on to the next step.
  3. Check the connection of the power cord on both ends. Verify that the power cord is securely plugged into both the AC outlet and back of the computer. Does the iMac power on now? Yes: You may have a loose fit to your power cord. Replace the AC power cord and test. No: Go on to the next step.
  4. Follow instructions in the General Information chapter to reset the SMC. Does the iMac power on now? Yes: Issue resolved. No: Go on to the next step.
  5. Remove and reinstall the SO-DIMM memory modules, located in the bottom of the computer. Does the iMac power on now? Yes: Issue resolved. No: Go on to the next step.
  6. Remove the memory access door and front bezel to gain access to the four diagnostic LEDs. See “Diagnostic LEDs” in the General Information chapter for complete instructions on this procedure.
  7. Plug the power cord into the iMac and the power cord into the AC outlet. Check to see if LED #1 is On or Off. LED #1 is On: This indicates that the power supply is getting good power from the AC outlet. Go on to the next step. LED #1 is Off: This indicates that the computer is not detecting AC power. If both the AC outlet and the power cord are good, replace the AC/DC power supply. If the issue persists, replace the AC line filter.
  8. Press the power button. Check to see if LED # 2 comes On, comes on momentarily, or stays Off. LED # 2 is On: The Power Supply is functioning. Go on to the next step. LED # 2 comes on momentarily or stays Off: Replace the Power Supply.
  9. At this point in the Power On process, you should hear a boot chime and see that LED #1 and #2 are On. Do you hear a boot chime? Yes: The power systems of the computer are working correctly. See “Troubleshooting No Video” in this chapter. No: The logic board is not passing the Power-On Self Test (POST). Replace the SDRAM with known-good memory and test. If the issue persists with known-good SDRAM, replace the logic board.

Hi John ! There are several things that you may try to fix your problem. At 1st, try to plug it somewhere else. Then, try resetting your SMC. To do that : Unplug your MacPress the power for (at least) 5 seconds while unpluggedPlug it in again then wait approx 10s before trying to start it. You may know that power supplies on Macs are pretty strong. I’d say that your MotherBoard is dead if the previous reset does not work. You may test each of these components before replacing one (of mabe them both). Hoping to have helped you, Don’t forget to give us some feedback ! Vincent

Here’s a diagnostics manual. Very comprehensive. Sorry if it’s a couple years late, but maybe someone else has the same problem. iMac diagnostics manual link: http://tim.id.au/laptops/apple/imac/imac…

I ended up replacing the power supply. Cost about $150

Just a thought after repairing my via card (thanks to youtube, the comp booted up then shut down , this happened a few times, after finally double checking all connectors again was fin for a week then on a reboot /start up all the fans kicked in , so let it load and did a shut down, the fault was the power lead , believe it or not, due to the initial problem heat was a problem, the main s lead had suffered , after replacing with my guitar amp plug line (common Kettle lead as i call it) fired straight up , the other issue was slow performance this is now gone, if you have poor power its obvious the rest will suffer, for under a fiver worth replacing , before panicking, seams to be quite common if you have had a previous problem , hope this helps, some one Update (02/26/2020) iMac 27" Vga card with lines and shuts down, look at the youtube oven trick it works!!!!, secondly this problem is over heating and can cause or the cause can be the power cable, i did the via fix and the screen was fine but had some shut downs, reboot fine and the again after checking everything again week later it shut down and was dead , the fault was the main power lead ( ie Kettle lead) had suffered heat inside , i tried my guitar amp power lead and fired straight up, only by chance did i come to this as had this with a marshall amp kept shutting down and over heating due to poor power supply conductivity hope this helps some one the cost under a fiver worth a try first.