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!