Replaced power supply and logic board - still only diagnostic LED #1?

After some power hiccups (various outages + voltage fluctuations) with our utility company, our mid-2011 iMac 27” won’t boot. I’ve taken it apart, done some research here, and I have the dreaded issue that LED #1 is the only diagnostic LED that comes on.

I found a replacement power supply on eBay - switched that out, same symptoms. I persevered and bought a replacement logic board on eBay (and an AC inlet cable, for good measure), but after replacing both of those I still see the same issues. No full boot, LED #1 only.

I should note that beyond the “LED 1 is the only one coming on”, pressing the power button causes some very faint noises coming from the power supply. It’s a very faint high-pitched type noise, and it largely, but not completely, goes away if I hold the power supply longer than ~10s (to “turn the computer off”). I have this same symptom with either power supply (the existing one + the used eBay replacement).

I’m not sure if I just got extremely unlucky with the used parts on eBay (since I’m truly not sure if I should blame the power supply or the logic board), or if it’s something else entirely.

I’m ordering a replacement DC cable (goes from power supply to logic board) but also tearing my hair out a bit. If that doesn’t fix it, what do I try next? I’ll have replaced every component coming from the wall to the logic board, including the logic board.

I’m only recently finding out about the graphics card issue with this model of iMac - but a power surge wouldn’t have taken that out, would it? And you’d still see LED #2 (with a bad graphics card) I suspect.

I’d appreciate any and all advice (especially places to look on the logic board pinout and/or power supply pinout).

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You’ll need to check the voltages on the logic board at the power supply connector. Do you have a DVM?

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Yes. I just can’t find the pinouts for this particular year.

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@thomato - You'll need to scare up the schematics and board view drawings to get to all of the voltage buses. Google 820-2641 (board) or 051-8115 (drawing set)

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