Question Motherboard will not turn on and I can't figure out the source of the problem

[Hex Maniac]

So I am having some computer issues.
Story time: 7 or so years ago I custom built my own modest gaming computer. And it has worked beautifully for years. Unfortunately, time marches on, and I felt it was time to upgrade. Of course, upgrading the cpu meant upgrading the motherboard and ram, as well as cooler due to compatibility issues with mounting. So I get the pieces I need wherever the best price is at the time.

So now my build looks like this:
Gigabyte Aorus z790 elite AX (ddr5)
Intel core i5-13600k
G.skill trident z5 rgb ddr5-6000 2x16gb
Coolermaster MasterLiquid ML280 Mirror cooler
Corsair hx850 psu (component from my previous build, purchased as refurbished)
Corsair obsidian 750d airflow edition case (also from my previous build)
All case fans are the stock Corsair case fans
I'm not sure if the rest is relevant, but including to be thorough
Asus strix Nvidia gtx 1080 gpu
Wd 2tb 7200rpm hdd
Wd blue 1tb sata ssd (boot drive)
Inland gaming performance plus m.2 NVMe 2tb ssd
SanDisk 250gb sata ssd
Crutial 500gb sata ssd
Noctua NT-H2 thermal paste
Rgb strip, brand unknown

I read all the manuals and put them together in the old case with old psu still installed. Everything seems to be in order. PSU is plugged in and powered on (psu I/O switch set to I), motherboard light flashes on for less than half a second, then nothing. Power is being received in front and back USB slots. When hitting case power on button, nothing happens. Fans do not start, no lights on motherboard, ram, case, or AIO cooler. The case, case fans, and psu are from a fully functional, tested and working system. All other components are new. Everything should be compatible: z790 ddr5 motherboard, i5-13700k cpu, ddr5-6000 ram (base clock 4800), AIO cpu cooler mounted on cpu. Noctua NT-H2 thermal paste applied in five dot pattern. Documentation on mobo manufacturer's website (Gigabyte) confirms that the ram and cpu are compatible.
So let the troubleshooting begin...
Things I attempted without success or any change at all (one at a time, checking to see if there is any change to attempting to boot):
-ensured motherboard and cpu power cable are correctly inserted
-ensured cpu fan is correctly plugged in to the motherboard cpu fan slot
-tried switching the cable positions for the fan and the cooler part of the AIO
-clear cmos with clear cmos button
-remove, wait, then replace cmos battery
-use different cmos battery
-gpu plugged in
-no gpu plugged in
-use only 1 stick of ram
-ram is correctly installed in slot
-ram is installed in the correct slot (A2) as directed by the motherboard manual
-the plastic guard over the cpu was removed
-removed cpu, completely cleaned paste off of cpu, cooler, and any surrounding parts, reinstalled cpu making sure to follow instructions from both motherboard and cpu manufacturers, applied fresh paste and reinstalled cooler
-ensured as static free environment as possible (wood desk; touch doorknob to ground myself)
-checked for any loose parts
-case should be preventing any wires contacting the board
-front panel plugs should be installed correctly, based on case and motherboard manufacturers
-pressed reset button on motherboard
-double check cpu socket for anything that looks out of place
-only one stick of ram in the A2 slot as recommended by the manufacturer user manual
-switch ram to different slots
-unplugging everything except the 8-pin and 24-pin power cables, power switch, cpu, and AIO cooler, then attempting to power on

Again, all that will ever happen is: when turning on the psu, the purple and blue lights on the motherboard will flash once (I don't think these are troubleshooting lights, just rgb stuff), then no signs of life even when pressing the power switch or reset button.

Again, my build here is:
Gigabyte Aorus z790 elite AX (ddr5)
Intel core i5-13600k
G.skill trident z5 rgb ddr5-6000 2x16gb
Coolermaster MasterLiquid ML280 Mirror
Corsair hx850 psu
Corsair obsidian 750d airflow edition case
All case fans are the stock Corsair case fans
Inland gaming performance plus m.2 NVMe 2tb ssd
-the rest below is unplugged-
Asus strix Nvidia gtx 1080 gpu
Wd 2tb 7200rpm hdd
Wd blue 1tb sata ssd (boot drive)
Inland gaming performance plus m.2 NVMe 2tb ssd
SanDisk 250gb sata ssd
Crutial 500gb sata ssd
Rgb strip, brand unknown

And then it gets worse.
I decide to rma my motherboard, and while I wait for new parts to come in, I'll put my old system back together but with the new cooler to make sure that functions. Everything back as it was, save the new cooler (to hopefully test if that's working):

Corsair Obsidian 750D Airflow edition case
Asus ROG Maximus VI Hero motherboard
Intel core i7-4770k
Coolermaster MasterLiquid ML280 Mirror (was originally using Corsair h115)
Patriot Viper Xtreme DDR3-1600 8gb x2
SanDisk 250gb sata ssd
Crucial 500gb sata ssd
Asus strix Nvidia gtx 1080 gpu (left this out for the time being)
Wd 2tb 7200rpm hdd
Wd blue 1tb sata ssd (boot drive)

Before I began upgrading, the above system was to the best of my knowledge, fully operational - no issues to report anywhere. But when I plug in the power and flip the switch to I on the PSU, there is no effect from anything. Case fans don't start, AIO no lights or fan movement, no lights on motherboard. This model has a q-code display and start and reset buttons on the board. Neither the case nor motherboard start or reset buttons have any effect.
With the sole exception of a different CPU cooler, this system is identical to what I was using before, and it was 100% functional.
So I tried several of the above tests to no avail.
I got a Thermaltake "Dr. Power" tool and tested my 24-pin cable on my psu. It only had one of the six lights it was supposed to. This said to me: ok, somehow, my psu didn't survive the upgrade process (somehow??)
So I grab a spare 750w psu to compare. Plug it into the wall and Dr. Power, and I get the full 6 lights I'm supposed to.
But when I try to use this instead, there is no change: no lights on mobo, psu and case fans don't turn on, no effect from start/power switches.
Re-checking the original 850w cooler with Dr power now gives 6 lights. Re testing that psu on the motherboard, no change.
So I got some cardboard and a low static workplace and breadboarded it.
Only motherboard, cpu, cpu cooler,1 stick of ram in recommended single stick location, tried each of the power supplies.
So far as I could tell, the motherboard is now dead, I suppose???. Yet somehow a USB flash drive plugged in the back panel still lights up as though all is well (it is not).
I admit that I am far from being an expert, but I do my research, follow instructions, read the manual several times; not to mention that this is not my first rodeo (although you could make a case that it is my second). I've scoured q&a's, forum posts, videos, and articles. Somehow, the root of the problem eludes me, terrifying me from proceeding any further for lack of knowing the problem.
At this point, the only theory I have left is: one of the new components (my money's on the motherboard) somehow damaged my psu in such a way that attempting to use it on the old system damaged THAT. My knowledge is limited, and I don't know if that is possible. I am at wit's end here, and any help, any insight, or any suggestions that could be provided would be helpful.

 

[Ralston18]

This PSU?

Corsair HX850

Refurbished - correct? Where purchased?

Did you use connection cables from any other modular PSU?

 

[Hex Maniac]

This one, purchased refurbished from Micro Center.
Only used the modular cables that came with it.

 

[Vic 40]

That is quite an old psu and though in the time a very good one, probably time to retire and test with a new good quality psu.

 

[Grobe]

-ensured cpu fan is correctly plugged in to the motherboard cpu fan slot

This one typically gives a warning, but shuldn't prevent further loading - i.e. you should still be able to enter bios menu in.

The case, case fans, and psu are from a fully functional, tested and working system.

This question - you have ability to re-test that PSU on the old system ?

-use only 1 stick of ram
-ram is correctly installed in slot
-ram is installed in the correct slot (A2) as directed by the motherboard manual

Have you tested without any RAM sticks at all, just to see if it power on ?


Also, there is one more thin not tested - try this:

• Check if there is an on/off switch next by the main power cable on the PSU, and also check if there is a voltage leveler (typically to switch between 110/240V) that it is set to the correct voltage according to your country/location.

And then

• Unplug main wall supply.
• Hit the PWR button (this should drain leftover charge from internal capacitors on PSU - nb: dangerous to open). A couple of LED may lit for a split second and some fans may seems to failed an attempt to start rotating, this is normal.
• repeat once (just to be sure)
• Then plug in wall adapter and try to power ON.