Lame question but, have you connected the VGA cable? 😀 Since it starts, you connected the 'start' contact good so if you also connect PC speaker (red and black cable) to the right pins (check the MoBo manual) put an ear to the speaker and look for beeps. Also check if the video card sits ok in the slot.

The motherboard pics aren't clear itself... so I don't even know if it has onboard video. If it does, plug in the monitor to the onboard VGA out first, then hop into the BIOS, set it to initialize the AGP card, then save/quit then move monitor cable over

Best trick to try out is this:

1) Remove all the expansion cards including the graphics card etc...

2) Remove all the RAM/Memory modules....

3) Leave only the PS Unit and CPU installed/connected....

Now power on and see if you get an POST beep errors.

Could be a faulty memory stick or something on the motherboard got zapped by that previous PS Unit you said was faulty.

Also make sure that if that P4 board has a 4-pin AUX power connector to use it, some PS Units do not have this cable so you have to use MOLEX to AUX power adapter cables instead 😉

Best thing to do in cases like these is to put the board on a flat cardboard box and start testing with the basic minimum.

Here's a question. Are the motherboard or the case proprietary? If you're using a Compaq, Dell, HP, etc motherboard or case then the voltage from the power supply may not be compatible. There was a period of time where they used unique voltages that don't work with standard ATX power supplies or motherboards that work with standard ATX supplies.

sliderider does have a point worth investigating. I know Dell uses proprietary PSU's in their P3 ATX boards. Can't comment on their pre-ATX stuff but I reckon those use normal PSU's.
I know that at least "some" compaq P3 boards use the standard ATX PSU wiring (or I would've seen smoke 🤣).

I don't know about HP, so far I never had one with non-standard wiring.

Will go look up some stuff and see what I find 😉

Edit: Lots of info here: http://pinouts.ru/pin_Power.shtml

sliderider has a point, but so does Tetrium.

If you know you've got a compatible power supply, and you're not having a conflict with onboard video, and everything is hooked up okay yet it still won't power on, then you've probably got a short 😀. It can happen if the stand offs aren't tall enough and/or the board isn't in the case right.

I'd check for the power supply and video problems first though, and then I'd reset the cmos. If that doesn't work then make sure the board is in right.

I did some experimenting, turned out the video card wasn't seated properly. I fixed it by loosening the motherboard screws, re-inserting the graphics card and then tightening the mobo screws. The computer works perfectly, besides that the CMOS gets reset if the PSU is disconnected. I'll have to get a new battery it seems. Thanks a lot for the help guys!