Unable to Power On but Able to Power On sometimes

I think my iMac Power problem is bit different on what I'd seen on here.

About 2 weeks ago I unable to turn on my iMac, so I thought maybe it's in an cold & high humidity environment (air-con overnight at room), normally I unplug all AC power & disconnect the power cord, wait about 30 mins, then I can turn it on after re-plug them. It's working fine by then except you going to shut down the Mac. (which I try never do it).

Then last week due to power shortage in a house, need to turn it off, after that I unable to turn it back on since then. So I guess it should be a problem to PSU. I disassembled my iMac and checked the diagnostic LEDs, LED no.1 just lit while I turn my AC power on then went off almost instantly ( about 0.5 second), no 2,3 & 4 LED light up at all. I think maybe the Pram problem, then I take out the Pram battery and put it back, it able to turn on!

Few days ago it's unable to power on after sleep mode. So I redo the pram battery procedure again, but no luck, swap a new pram battery, still out of luck. No any sign of LED1 at all. I guess perhaps my PSU unit is broken.

I continue my investigation, took down the PSU unit and discovered these 'bomb' has bit burst at the top.

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The day after I ordered a new 2nd hand PSU unit online and then I swap the new PSU in. What more worst it's no any LED lilt at all! Just nothing happen. So I thought WTF must be the seller give me a broken PSU unit huh? I took off the Mac's LCD and the Pram battery, then I try turn on the AC and suddenly it turn on! Unfortunately I'm unable to use it due to no LCD connected and the Mac keep beeping while boot. So I thought this time I'm able to fix it. Right after I assemble back the LCD, it's unable to turn on again. So I take off LCD and it still unable to power on. Swap on and off the pram battery, still no LED 1 lit up at all. Just nothing happen.

Last night, I fully disassemble the whole iMac, take out the Logic Board and the Graphic Card, and check see any sign of broken/fry 'bomb' like the old PSU, but no, it's look clean, even look like a new thing to me. I even try just plug the Logic Board power without assembly back all those cable & connection to check will the LED1 light up but no luck. :(

After this massive take apart and reassemble back all the connector and cables, it still unable to power on. I need to know what caused my iMac won't light up the LED1 because I'm sure the newer PSU is working fine, I get shocked last night while try to assemble the LCD back while the AC is ON. (lucky me I'm still alive :P).

Can iMac boot up without pram battery? I accidenlty broken the plastic clip to hold the pram battery, but I use a paper clip to hold it now instead. Should it work this way?

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UPDATE 8 Dec 2015:

- Tried 2nd power cord from another working iMac, still no luck.


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