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Model A1225 / Mid 2007 and Early 2008 / 2.4, 2.8, or 3.06 GHz Core 2 Duo processor

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PSU or logic board fault?

I cannot work out if I should replace the Power Supply Unit or the Logic Board in my iMac, mid 2007, EMC 2134 model. I have tested the power switch which works fine. The Diagnostic LED 1 illuminates when power lead is plugged in, then every second LED 2 flickers on for a few milliseconds and both fans turn 1-2 revolutions - 1 second later this repeats itself. Pushing the power button doesn't have any effect. I have reset the Pram/Nvram, replaced watch battery and tried one RAM, 2 RAMS and both swapped around to no avail. Perhaps someone can tell me how to check the PSU voltage outputs on the 12 main prongs (which are in 2 x rows of 6 and numbered) of the PSU. Or any other suggestions to the brick wall I've hit would be much appreciated. The PSU and Logic Board both visually look OK with no brown out activity etc.

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Mat, just a quick thought. If LED#2 only comes on briefly I would start with the PSU. Reason for that is 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." So if it is NOT on, the PSU is not working correctly. Here is what the guide recommends:"

Power Issues

No Power

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.

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Thank you for the very informative & timely response.

LED 2 is literally blinking quickly as the larger fan spins half a revolution and the smaller fan about 2 revolutions, then a second later the cycle repeats itself etc etc etc. So my feelings are it could be the PSU but it could also be the logic board... Therefore I'd like to test the PSU if possible...

por

I did get hold of a multimeter and took the following readings from the 12 terminals of the PSU:

Pin #1 Ground

Pin #2 +11.97v

Pin #3 +0.4mv then spikes (as fans spin 1-2 revolutions and LED 2 blinks once)

Pin #4 +2.61v then 2.58v (as fans spin 1-2 revolutions and LED 2 blinks once)

Pin #5 0v then 0.1 (as fans spin 1-2 revolutions and LED 2 blinks once)

Pin #6 Ground

Pin #7 Ground

Pin #8 +0.8v to 8v (as fans spin 1-2 revolutions and LED 2 blinks once)

Pin #9 +0.82v to 7v (as fans spin 1-2 revolutions and LED 2 blinks once)

Pin #10 +0.86 to 4.33v (as fans spin 1-2 revolutions and LED 2 blinks once)

Pin #11 +0v to 0.5mv (as fans spin 1-2 revolutions and LED 2 blinks once)

Pin #12 Ground

por

Not sure if you (or anyone else reading) can confirm the above multimeter readings mean the PSU is faulty.

As it's a 7 y.o. iMac working off an external start-up HD (ever since the internal one packed up a year ago) a new PSU is about all I want to invest in repairing it.

Many thanks in advance...

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Mat será eternamente grato(a).
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