Thanks everyone for the continued support! It's really fantastic to get this kind of assistance and I appreciate it very much.
I think we were clear that the first PSU had blown. The paper clip test showed that it didn’t boot up. I know that we could get a more precise test in store but wouldn’t something have happened with the paper clip test if it worked? So I’m not sure we would have done anything differently here, but I acknowledge that I still should have done it to rule it out! There is no horrible smell, I can double check if there is a lingering smell if I really sniff in there. My only sign was that it booted up fine before, and then with the graphics card changes, it tried to boot up and failed and now no longer boots up at all.
I will get the new PS checked out at the store and see what I can do about a possible refund. I’ll see if they can test it also.
Happy to take a closer look at the motherboard as instructed, I will report back later tonight once I have looked over the points you all suggested.
I don’t have another PC to test the video card in unfortunately.
As for my steps with the new PS, I'll go over them again, let me know if anything is unclear.
First I started the computer WITH thread-on video card, screen plugged into the slot on the card. The PS booted up fine, nothing on the display (and beep code as explained).
Since that didn’t work, I plugged the screen directly to the other screen slot, the one that is on the mother board. PS booted up fine, nothing on the display.
Since that didn’t work, I removed the add on graphics card, and booted the comp up (screen was still plugged into the integrated one). That’s when things turned maybe once, and stopped.
Tried to boot it again, same thing (I think).
Then, put the add-on card back in, booted the computer up, and nothing happened anymore. I think I was still plugged into the on-board card when I did this final boot up.
IMPORTANT – with the first PS, I did the exact steps as above, but I DID NOT HAVE A SCREEN PLUGGED IN.
So I: booted the computer. Booted fine, wouldn’t turn over. Didn’t bother with a screen because I knew nothing appeared on it (this is how it was at my parents’ when it was set up).
As instructed, I removed the add-on graphics to see if it would boot. No screen ever connected. Fans did one turn then stopped.
Re-added the extra graphics card, and nothing booted up. No screen ever connected.
So I’m not sure the screen is a factor at all here, since the same thing happened to the other one?