iMac suddenly not turning on!

Hi!

My 2010 27‘ iMac is not turning on anymore.

It‘s completely dead.

Backstory:

I bought this iMac in a dented housing and transplanted all components into a new one.

I plugged it in and it worked!

Then I plugged my Catalina Patcher Installer stick in.

Halfway through installing the iMac just powered off. I powered it back on. And yes, it did turn back on. But this time it turned back off faster.

The last time I got it to power on it beeped quickly 3 times. No, it was NOT the faulty RAM beep. It sounded more like the broken EFI beep. But I turned it back off too soon to hear the three long beeps.

Then I tried to turn it back on. Nothing. No diagnostic lights 2, 3 and 4.

Only diagnostic light 1 blinks once about every second.

Any clue what this could be?

I think it isn’t the power supply as I already tried a different one.