Dead iMac, pinout PSU and hardware troubleshooting

first thing to try would be trying to put the old CPU back in place. regarding PSU- it gives voltage to some pins, but not all of them (there should be at least 5v standby voltage to the logic board somewhere, even in off mode)

Sorry to let you know it this way, but 2010 i3 iMacs are only compatible with dual core i5s: the fastest CPU you can install is the 680. That's because your iMac's socket has slightly different electrical connections (FCLGA1156) compared to the one used on the quad core iMacs (LGA1156). Check ark.intel.com for more informations about the CPUs.

I don't know if this could have caused a shortcircuit to the logic board, but PSUs usually "snap" when have a failure, so if you didn't hear anything, maybe the problem lies somewhere else.

No my friend i just working today in my iMac, i replace the entire PSU but my mac stay dead. I´m searching info about the pinout of the PSU and i can´t found yet. You know some about this? (sorry, i´m argentinian people!) Best regards!

Hello guys i have a similar problem but i didn't change any hardware, my imac a1312 was full of dust so i just cleaned it and removed the old heat conducting material between the cpu and the aluminium thing between the fan but when i got everything in place it wasn't booting, a small fan activity take place every 5 seconds same as the mouse light.

Please help me with this problem

Posting this after a VERY frustrating week on a customer's dead 2011 iMac 27".......

Hope it helps someone.....

Symptoms: iMac 'dead'...... no power, no fans, no chimes, nothing.

Try with new mains lead - nothing.

Open it up: Mains power tested & present at the PSU but no DC volt output & no diag' LED number 1 ( no standby trickle)........ bad PSU?.......

Tested start button was functional with multimeter.....

Tried shorting the two start button pins directly on the MLB socket........... Nothing.

Try machine in minimum configuration (i.e. disconnect the various peripherals in case a short is preventing the MLB starting up ) - NO difference.

Disconnect output multi-pin connector from PSU and test for DC outputs directly on the PSU output pins......... all pins at 0V........... bad PSU?.........

Changed power supply: - NO difference.

According to Apple diagnostic trouble shooting - now change logic board - not convinced this is an entirely logical step but had one here so did so - NO difference........... Aaaarrrggghhh !!

Out of frustration, resorted to opening up a working 27" iMac and testing the Power supply and MLB in that - where both work fine !!??

How can this be? - doesn't make any sense ??!!

Re-assemble machine's 'guts' and walk away from it until the next day........

Next day, after an overnight think, I disconnect mains connector from the PSU and temporarily solder a mains lead directly onto the PSU itself ( care !!), & make sure metal case has a solid earth connection ......... Result?........ Diag LED number 1 lit up (trickle voltage present) and machine fires up straight away !!

Diagnosis?......... faulty mains socket/filter........ it IS actually showing 240v at the PSU input pins ( measured via a multimeter) but presumably with no real power (multimeter possibly only seeing a misleading 240v induced voltage via a fried filter???), but hence PSU cannot actually operate.

Changed the filter/socket and machine is now fine...... all that and it was a £10 item !!

Moral of the story ?........ don't assume the mains filter must be OK because you can see mains voltage at the PSU pins........ also look for the required 12v and 5v DC being present on the output pins with the output connector disconnected; ( please be careful NOT to accidentally give yourself a 'belt' from the exposed 240v on the primary side) !

Repaired 100s of these iMacs but never had one with a bad mains socket filter.......... I presume this machine had seen a nasty spike / surge at some point before it came to me. At least the filter did its job and protected the Mac.....

You never stop learning in this game !