Maybe someone knows better than me, but I am not aware of a different kind of momentary power switch than what we have today. The most likely scenario is that the motherboard is defective.

You can try bridging the switch with a screw driver. I would look at the caps around the power delivery.

Have you double-checked the pin placement in your motherboard manual? I say this, because sometimes they can be labelled one way, and then appear to be one pin off on the actual board.

Or use a pencil. Use them often to do a hard reset (the graphite in the "lead" is like a 100 ohm resistor/switch.
Couple of things. The Power On uses a momentary contact switch like all ATX powered boards (like a reset switch or door bell push botton <sorry could not resist>: only connects when pushed).
Other thing is that your PSU should be good but some older boards do not like the newer ATX12v standards (that Seasonic is ATX12V V2.3) so it may may not power up when connected to an older motherboard that expects ATX12v 2.0
Have had issues with newer supplies not powering up on an older board, doubt that is the issue but could be...

Welcome to the forum! 😃

The ATX Soft-Power-Switch works the same as on any modern PC. Shorting the contacts briefly will make the onboard circuitry start the PSU. You can also use a screwdriver just like on a modern motherboard.
Have you checked the CMOS jumper? Having it in the wrong position sometimes prevents the system from showing any signs of life.

Other than that you could try to take the mobo out of the case and connect only the PSU to see wether it starts like this. Then add a CPU and try again, then RAM, then an AGP video card and so on until you have all components you want assembled and working. Lastly take it apart and reassemble in the case. If it stops working at any point you know where to investigate.

Is the power supply plugged in and switched on ? Is the extension cord into which the psu is plugged in, plugged in ?

Thank you everyone for your suggestions.
Much like the famous "Have you tried a clean boot?" I should have first attempted to do one component at a time, and I shall try this first (after checking for CMOS jumper positioning)
Fingers crossed the one Slot 1 CPU I have (also untested) works
Thank you for letting me know I could use a screwdriver to jump the pins without worry, and than you for letting me know SPWR works the same as a modern POWER SW header.
And yes, though it does seem obvious, everything is plugged in and switched on as it should be (I work Enterprise IT, and have definitely asked grown adults this question before with a straight face 😀
Sadly I do not have an older ATX PSU to test with, and also sadly, I am, all-too-prepared for the potentiality that this board may be dead.

Thank you all again for your assistance. I shall try it out tonight and report back.

Component by component *outside the case* . Somehow contacting/shorting against the case is a common cause of this sort of issue. With older hardware things can be bent out of shape so this is more likely than with new systems.

Other things to take into account:
- if it looks 'dead', the problem could be a corrupt/misflashed BIOS. I have an EEPROM programmer and if boards look dead with no clear cause, I always re-flash the EEPROM and if that fails substitute a different one. That frequently resurrects the boards (last week saved a K7T Turbo that way)
- The CPU might not be compatible with the board. Exactly which CPU is it? And which revision AX6BX? Early AX6BC boards didn't support Coppermine CPUs (later P3 "E" or "EB" CPUs). Trying to boot with one would make it look dead. Also some mid-revision AX6BC could support Coppermine but only after BIOS update which may not have been performed.

Someone has already suggested checking the clear CMOS jumper, if that is in the clear position, it can prevent the board from starting at all.

Some motherboards also have a jumper for powering the system on by keyboard, so check the manual if perhaps this board has one of those. If set to turn on by keyboard, it will disable turning on through the power switch. I've usually seen it on boards older than this, but it doesn't hurt to double check. On other boards this function could also be controlled through BIOS, so if you haven't yet, try clearing it.

If you have a voltmeter, you can check if the pins of the SWPR header pins even have the correct voltage between them (usually 5V, I've seen 3.3V as well). You can also check if the power supply is sending the power good signal (pin 8 on the ATX connector, grey wire, should go to 5V shortly after the power supply switches on).

I should have specified: *outside* the case, component by component
The CPU is a Pentium II 233, which is the "default" CPU for this board, per the manual (There's a section in there somewhere about modifying the multiplier and bus frequency(?) depending on which CPU you have, and the 1st one listed was 3.5x and (some freq I can't remember) for the P II 233

This may sound bad (cuz it is) but I did actually drop this CPU once, from about 4 feet up, onto my concrete basement floor. There is an external cooler (fins and fans) on the outside of it and I think they took the brunt of the shock. As silly as this sounds typing it out: would anyone hazard any kind of guess as to how shock resistant the CPU might be?

Looks like I might need to get a voltmeter :p

I'm also going to scour every jumper setting on the board as it currently is and compare against what the manual suggests it be set to

Definitely going to go clear the CMOS. I believe the procedure for it was also in the manual (unplug PSU, switch it off, unplug the ATX power cable, set the jumper, unset the jumper, put the ATX connector back, plug the machine back in and turn it on)

Ah, the famous drop test 😜

Tbh it's tricky. If you have a minimal set of known-good components you can locate issues very quickly. If not it's more challenging. Know anyone in the vicinity who might have a slot 1 system?

I have another Slot 1 board on the way from fleabay as we speak. (Had ordered it a day before I stumbled across this one on CL)
Still only have the one Slot 1 CPU to test with (And I really don't want to buy another one)
I will presume that that one is functional and if it does the same thing as this one (nothing, not even fan activity) then I will consider that the CPU is toast and drop a few more dollars on a replacement CPU.
My pile of computer crap is growing ever larger and my wife's patience ever thinner 😀

I'm not sure what the question is. If the newly build PC won't POST, then it's time to start the troubleshooting. This isn't much different from modern systems imo, since it's also ATX.

Take it out of the case and try to start her with minimal parts added. You mention being an IT techy so you should know the drill 😜
I'd first try out a different PSU (one which you can plug into the 20p power connector on your board).

Regarding the dropped Pentium 2 CPU, if you mention it dropped onto the fan part then the CPU itself may have been undamaged. But the HSF might have been damaged (should be able to see if there is any sign of obvious damage. Especially look for cracks or parts that have dislodged due to the impact force).
One thing you might want to try is to remove the heatsink (can be difficult especially if the CPU is boxed), then look for any (obvious) signs of damage and then reinstall the HSF.

GooseNipples wrote on 2021-03-08, 18:17:

I have another Slot 1 board on the way from fleabay as we speak. (Had ordered it a day before I stumbled across this one on CL) […]
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I have another Slot 1 board on the way from fleabay as we speak. (Had ordered it a day before I stumbled across this one on CL)
Still only have the one Slot 1 CPU to test with (And I really don't want to buy another one)
I will presume that that one is functional and if it does the same thing as this one (nothing, not even fan activity) then I will consider that the CPU is toast and drop a few more dollars on a replacement CPU.
My pile of computer crap is growing ever larger and my wife's patience ever thinner 😀

I'd still recommend that you reconsider. Having this one CPU be the bottleneck will put your entire project here on hold anyway and that might be more of a waste.
Could opt to get a s370 CPU and a slotket. Even if it's 'just' a s370 Mendocino with a more simple slotket, the CPU could then also be used in most s370 motherboards, if you ever decide to go down that part.

I almost forgot, but you could also try to clean the contacts of the Slot 1 CPU with some rubbing alcohol. I've resurrected seemingly dead SLot 1 CPUs multiple times this way and this is an easy 'fix' so I'd suggest you try this first before taking stuff apart 😜

You bold, beautiful bastards.
I don't know if it was pulling it out of the case or clearing the CMOS, but she's up and running with all hardware plugged in, happily installing Windows 95.
Thanks everyone!

GooseNipples wrote on 2021-03-09, 03:29:

You bold, beautiful bastards.
I don't know if it was pulling it out of the case or clearing the CMOS, but she's up and running with all hardware plugged in, happily installing Windows 95.
Thanks everyone!

Nice *thumbsup!*

Congrats! 😃
Thanks for letting us know!👍