IMac mid 2011 model with possible damaged PSU

[Drassh]

I have this IMac as a personal collection and started to notice that it was making a noice during daily use. So I decided to give its first ever maintenance.
Once everything is clean and reassembled, I power it on but there's no reaction. After a bit of teardowns and reassemblies I give up to Google the issue. From there I learned that there are 4 diagnostic LEDs marked LED501-504. With the screen disconnected, I get the first led with a solid on, Led 502 turns on, blinks a few times then turns off. This repeats until I pull the power cord. Now to make sure the machine is receiving the proper voltage, I check a few points ( I'll edit it in once I figure how to do this on a phone) from where the PSU connects to the motherboard. Pins 1-3 get the 12v and 4-6 go to ground as they should but every couple of minutes pins 1-3 go to ground which I figure is a short. In one of the of pins 1-3, I disconnect the PSU from the motherboard and the short disappears altogether on the motherboard. Time to check the PSU, I poke around a few capacitors and find a few of them are shorted.... Here are some pictures of the PSU unmarked if the phone let's me.....

[RukyCon]

Re: IMac mid 2011 model with possible damaged PSU

Did you test the capacitors on board or off board? Because if they were tested on board then chances are something other then the capacitors could be shorted as electrolyte capacitors don't short that often. Also, what are the brands of the capacitors? From the vents i can only guess it's a mix of UCC and Teapo. UCC [United Chemi-Con] is a really good brand however, Teapo is not as good.

[Behemot]

Re: IMac mid 2011 model with possible damaged PSU

Check especially the two smallish caps by the middle…

[Drassh]

Re: IMac mid 2011 model with possible damaged PSU

here are a bit more pictures. the big capacitor under the vents is hard to read the markings and another is a square ceramic.

[koolcat6666]

Re: IMac mid 2011 model with possible damaged PSU

Hi,
i have a similar problem on my iMac 21.5 mid 2011.
I'm Italian and im sorry in advance for my bad english.
i will describe the behavior of my mac.
The computer is turning on when it is cold, and it stays on with no problem. if you will shut it off it will not come on until it will cool off.
diagnosing the problem i found this:
led1: Always on
led2: when pushing power button led 2 comes on for a second, fan start spinning, but immediatly goes off, no sound no nothing.
when computer is cold normal post:
led1: Always on
led2: when pushing power button led 2 comes on and stays on, after 2 or 3 seconds led 3 comes on.
led3: after led 2, led 3 comes on and stays on, sound (bong) and video comes on.
led4: comes on after led 3 when computer is on.

can you help me to solve my problem?
thank you very much

Nicola