Chosen Solution
Got an i5 iMac 21.5 mid 2010, with power issues. The machine looks completely dead, as when I press power button nothing happens. Took the screen out, checked the diagnostic LED’s, and the one on the right (which should be ON when I plug the power cord), blinks two times fast and then shuts off. I measured the power supply, it gets 230V (Europe), but at the output I don’t get 12V (pin4) or 4.*V (pin6), but instead I get 3,8V and 1,7V. I unplugged it from the board, and measured the voltage again, and I get 12V (not very steady, I mean it goes from 11,3 to 12V), and 4.5, to 4.8V. I get that the supply might be a little iffy, but i guess that the main problem is the main board (?). Checked the board all over with magnifying glass and I could not find anything odd (corrosion or anything blown). Also the power supply is clean (no corrosion or blown parts). Any ideas would be appreciated Update (02/13/2020) So to follow up with the topic, I removed the board and gave it a good inspection with a strong magnifying glass. Nothing bad at all. As my board is the 820-2641, the power connector on the PSU is a one-piece, but for the board is split in two (or three). One is with fat pins, and two with small pins. I saw that when I connect only the one with the fat pins the board is getting power (LED #1 OK). But when I connect the second one which is near by ( the third is the power for the HDD’s) there is a short to ground. Checked with schematics and boardview and saw that all pins was OK, except the pins that power the screen backlight. So I removed the GPU and gave it a try. Normal power again. Cleaned the GPU connector and the connector on the board, and all seems to go well (!). I will try to reassemble the system, and come back with more feedback
So it seems that the problem is the GPU card. Never had this issue before. So this is a new one. Tested the board with no GPU attached and it works fine. I also got a GPU from a friend which is 100% good, tested, and it works fine. Got fanspin and so on… So will try to repair the GPU and see what happens Kudos to everyone!
Messer check pins 3,4 and 11 and let us know what voltages you get. Pin 6 is nothing but the backlight control so for now I would not put to much emphasis on that. When you unplugged the pus from the logic board, did you measure your voltages on the connector or where? If on the connector from the PSU then it does sound like your logic board is shortening out somewhere. Let us know what you find out.
Test Point Function Pin 1 Ground Pin 3 5V power for Hard Drive(s). Note: If present, the 12V power will come directly from the Power Supply to the Hard Drive thru the power harness cable. Pin 4 Standby 12V power (permanent power coming from power supply, present as long as AC cable is connected, even if computer is off); corresponds to LED #1 Pin 6 Backlight Control pulse width modulated signal (from logic board to LED backlight board, to adjust the backlight level setting according to user setup) Pin 11 12V Run-Mode power to logic board (coming from power supply, present as long as system is on or asleep); corresponds to LED #2 Pin12 Power On Request signal (from logic board to power supply when power button is pressed) Pin 13 Backlight Enable (signal from logic board to Backlight Controller board, to enable backlight
