2007 iMac power "trips" after attempt to start (video)

I recently dug out an old iMac with the intention of upgrading it for my little sister to play with. It was working fine before going into storage (plain old store room, nothing hot or humid) a couple years ago. This morning I dusted it off and took it apart to install an SSD and add more RAM, but forgot to test it *before* opening it up.

Right now it won't start. I'm confident everything was put together properly and there were no accidents along the way. When first plugged in, one of the logic board LED lights up. When the power button is pushed, 3 of them light up momentarily then it all goes dark. No sound or picture. I’ve put the original RAM back in and still nothing.

Photo: https://imagizer.imageshack.com/v2/732x9...

Video: https://drive.google.com/file/d/1HfcCB03...

I have done a fair amount of searching and a few threads on iFixit come close to what I’m experiencing except:

1. None of my LEDs stay on after power button is pressed, not even trickle power

2. In addition to no chime and no display, disc/hard drive and fans do not spin too

Any advice will be greatly appreciated.