all in one PC not powering on

Without knowing too much about the exact unit I can say in my experience processors (especially now-a-days since they have thermal shutoff) don't go bad to often. I'd look more at the power supply or power switch itself.

When did it stop working, did it just not boot one day? Can you access the start switch and try pushing on the circuit board switch and not the plastic piece?

I really doubt it's a CPU, even if its seated wrong it'd power on and do something (flash, spin a fan, etc.) at least in my experience. Also check and make sure the power supply plugs that you are trying actually fit into the computer (the barrel piece in the middle) if the piton in the computer port is smaller and the power supply has a larger barrel it may just be a contact issue.

Can you run a voltmeter on the original AC adapter and see what you get for Voltage?

Just some ideas.

In cases like this it's best to start with the simplest solutions and work your way up. Try a different power cable and outlet, for starters. I have seen those go out. It's possible that the CPU doesn't have enough heat dissipation and is preventing power up, but usually you get some kind of response before that happens.

Do you get any lights that come on at all? Or is it just no display? It could be a bad display.

I agree that it may be a bad display.  When you say "it won't power on" exactly what does that mean?  Are you seeing any lights, anything on the display, any fans running?

I'd connect a wired keyboard and see what happens with the CapsLock and NumLock LEDs.  Tap the appropriate keys for those LEDs and see what happens with the LEDs.

Also, I'm assuming that it has an external video connector.  Connecting an appropriate monitor may tell you a lot.

i agree a bit more info on the boot up is needed here
you can also try a diagnostic card :  https://www.ebay.com/b/Computer-Motherboard-Diagnostic-Cards/168060/bn_1637411

If the supply has a standard ATX connector, install the supply into the system and then plug the supply into an ATX power supply tester.  That will point at or rule out the power supply.

it probably has an Ac adapter, laptop style - 19.5 V

This is an all-in-one PC, try with just board+ cpu+ ram+screen + keyboard and see if it boots up.
I would make sure you inserted the CPU+fan correctly and make sure ram sticks are inserted and locked correctly and yes sometimes CPU can go dead even if it's rare.

Hey, With all the power supplies used i am getting a light on the motherboard (old and replacement) as well as the power button. however fan nor hard drive is spinning up. Again i've tested this with multiple adapters with the same power output and a new motherboard.

With Only CPU,RAM,screen, and keyboard i am getting the same results as well

Are you sure you connected the power button cable correctly to the motherboard?
Do you hear any beeps coming from the built in speaker if you have one.

Yes I'm 100% positive the power button is connected correctly. I'm not getting any sound through the speakers. From what i've tested its as if the 19.5v 6.15 A power supplies recommended by hp aren't enough to power it for some reason. Power is going through the machine from what i see by the LEDs on the board as well as power button. I know its a rare possibility that a motherboard  may be trying to draw more power than it needs. But I wouldn't think it would do that with two separate motherboards. But I also wouldn't want to pump more ampage through the machine and put it beyond repair.

Just tested the same power supplies on a similar AIO and they all powered up fine.

As a quick test, remove the RAM, HD, and the keyboard and try again.  The fan(s) should spin and the system should beep at you because of no RAM.  If not, considering what you've already replaced, I'd be looking at something overloading the power supply.  The display panel is a possibility considering that there's not much else left in the system.  Can you easily disconnect that?

Another possibility: I don't see any mention of having tried a different power switch.  That could certainly be the culprit of these symptoms.  If you can figure out which pins get shorted together on the motherboard with the power switch, try shorting them manually.

That was my first thing to look at however the Power button is a ribbon cable and doesn't have any pins for me to short. So the only options are the power button and what i stated in the previous comment. If you know of a way to power it on even with this type of connector id be happy to learn how.

Forgot to mention that I just made sure and unplug the Screen/LCD completley again and tried to boot and it did not work.

Where the power switch ribbon cable connects to the motherboard, is it removable?  If so, you should be able to disconnect it and short the pins on the motherboard.  If not, you may be able to access the pins on the other side of the board (though they may be difficult to reach).

Where that same ribbon cable connects to the power switch, are there any pins exposed?  You should see a voltage across them (probably around 5VDC) that goes to 0 when the button is pressed.  That's a fairly good test.

if i want to test a power button, i take my ohm meter, and measure the resistance when open and close

Take some photos of the motherboard and upload here for us to look at, something is clearly shorting the system causing it not to startup.
Was this new motherboard working before?