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Why doesn't my iMac power on

After six years of normal usage my iMac stopped working. When I press the power button it doesn't turn on any more: the vents don't start spinning, neither does the disk, there is no chime, etc. In short, nothing happens.

When I follow the procedure suggested elsewhere, i. e. when I: i) disconnect the power plug, ii) wait about 10 seconds, iii) press the power button and hold it pressed, iv) connect the power plug, v) release the power button, vi) wait a few seconds, vii) press and release the power button - the vents start spinning, but there are no other signs of activity. I conclude that at least the power button works fine.

Digging further, I went through the "Symptom Charts" in the "Apple Technician Guide for iMac". I verified that the diagnostic LED 1 is ON and 12 V is present between Logic Board Test Point 1 and 4 whenever the appliance is connected to the mains. After pressing the power button, 12 V appears between pin 1 and 11 as expected, but the LED 2 stays OFF and there are no other signs of activity whatsoever. At this point I conclude that the power supply works fine.

To double check I removed the power supply from the case, soldered the power cables directly to the board and connected to mains. 12 V appears between pins 1 and 4 as expected. When I short-circuit pin 6 to ground, thus simulating the "power on request", healthy 12 V appears between pins 1 and 11, which yields the "run mode" power. This seems to confirm that the power supply works fine.

According to the "No Power, Dead Unit" symptom flow of the "Technician Guide" the last suspect is the logic board. My question is, if it could be possible to repair the logic board or at least how to diagnose what got broken? The board does not look suspicious, the capacitors seem intact (they do not seem swollen or unnaturally expanded), there are no signs of overheating, etc. Any suggestion would be very welcome!

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I wouldn't touch that logic board (also you have no idea of what to touch), until I tried a known good power supply. 95% of the time your symptoms and my experience tell me it's the power supply. I have yet to see a logic board failure on this model. Various part of the power supply can fail. I've see power supplies that send power to logic board and not the hard drive and vice versa.

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Thanks for sharing your opinion. However, trying a known good power supply is equivalent to actually buying one, isn't it? In order to spend the money for the right spare part I would like to get a better understanding of what you are saying (not that I am questioning your experience, just trying to get your point). My diagnosis based on the "Symptom Chart" pointed me (possibly wrong) towards the logic board. In contrast, you suggest that it might be still the power supply. Could you please share some more details on your interpretation of my symptoms? Especially disturbing is the fact, that the expected 12 V appears between the Logic Board Test Point 1 and 11 but the diagnostic LED 2 does not turn ON. Doesn't it mean, that the logic board gets the correct voltage, but fails to detect or distribute it correctly?


One more detail, in case it might be relevant. The battery on the logic board was weak (it showed roughly 2.8 V instead of 3 V) so I replaced it with a new one. Before replacing the battery, the voltage on the logic board between pin 1 and 11 went to 12 V after pressing the power button, but dropped to nearly 0 V after some 10-20 seconds. After replacing the battery, the voltage stays stable at 12 V.


I have yet to have to replace a battery since Apple got away from the 3.6 volt 1/2 AA batteries. Compare prices of the power supply with that of the logic board.


Hey Lukasz, did you solve the problem, and what was it ? I have the same issue...


Hi Jurgen, no, I did not solve the problem yet. I got the power supply investigated by two independent electronic professionals with one saying that it's healthy, the other that it's broken. However, having all the information I got by now, my bet is that the power supply needs repair or replacement. The former is currently in progress.


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Lukasz sera éternellement reconnaissant.
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