The fans work but the power led doesn't light up. I'll need to test the board outside of the case, the border of the board touches a couple of metal ledges.
I use a 24 to 20 adapter but i've tried with another 20 pin standard PSU and it's the same. I own a HP PSU but it's more modern and has 24 pins, it has an extra cable for a sensor i think and it works with other boards. But i don't think the connector will fit the board anyway.

Hi.
HP Pavilion, for all the ones I know are using standard ATX PSU, but not very powerful one. Your missing wire is probably the -5V (white wire) that is not required today but used on old hardware, by example for ISA ports. So your board may need to use it. 24-pin PSU have the extras 4-pin power plug made removable precisely to go on older boards and such PSU generally does not have -5V.
So, try to remove all the expansion card and use the original ATX PSU to see if it boot. If not, try the clear the CMOS and test again. HP Pavilion were crap, Everything in the box is cost reduced. I can tell as I have a collection of them with a half not working for a reason or another. ASUS is not a criteria of quality either. At least pre 2005 mainly because of bad capacitors but also sometime because of not very good circuit design.
I don't think the problem come from the side of the motherboard touching the case because if a conductive part can touch it, it can only be a ground plane, but who knows... Try to put a piece of paper and check. Some cases have an extra stand to attach the MD. It is located near the last PCI port or AGP port. If the motherbaord does not have a corresponding hole, the stand must be removed. The P5S-VM seems to have the hole though.

Not much help on the old HP support page for the board...

https://web.archive.org/web/20140906224717/ht … ocname=bph04949

...although it seems to imply both a cpu & chassis fan header - which was the original PSU fan attached to (do you have any clear pics of your actual board layout, as I see some online which don't always have the chassis fan header)

What are the make / model numbers of the original & test PSUs and, as above, the extra cable will probably be the -5v line (for the ISA bus)

I trashed the case and the PSU it was an ugly big HP case. I've tried with an old PSU with -5V and i compared the cables with an old PSU too. There were extra cables (at least one) and it was not -5V. It doesn't boot outside of the case either. If i connect the CPU fan to that PS_FAN connector it doesn't spin, and yeah the PSU was connected there. The board came without CMOS battery and it booted in the original case. Must be a proprietary PSU then. I was aware that ASUS was extremely overrated but i didn't know they manufactured fake microATX mobos, besides the fake power connector it's a bit bigger than a standard microATX board. I shouldn't have picked it, a guy gave it to me
There is a good pic here: https://www.ebay.com/itm/133867373804
I searched for hp pavilion 6425 system number D9309A and i could not find anything.

May be that connector is to monitor the PSU fan and that's why it doesn't boot but i doubt it, how can i fool the board into thinking the fan works? It's a 3 pin header.

dondiego wrote on 2021-10-21, 10:22:

I trashed the case and the PSU it was an ugly big HP case. I've tried with an old PSU with -5V and i compared the cables with an old PSU too. There were extra cables (at least one) and it was not -5V. It doesn't boot outside of the case either. If i connect the CPU fan to that PS_FAN connector it doesn't spin, and yeah the PSU was connected there. The board came without CMOS battery and it booted in the original case. Must be a proprietary PSU then. I was aware that ASUS was extremely overrated but i didn't know they manufactured fake microATX mobos, besides the fake power connector it's a bit bigger than a standard microATX board. I shouldn't have picked it, a guy gave it to me
There is a good pic here: https://www.ebay.com/itm/133867373804
I searched for hp pavilion 6425 system number D9309A and i could not find anything.

What is done, is done.
Your motherboard is ATX and PC’98 compliant as it is quoted in the user manual. ASUS make partnership with OEM and follow their scope statement. If that means not following standards, they do it. But it is not the case with P5S-VM.
Just look the manual [https://dlcdnets.asus.com/pub/ASUS/mb/sock7/s … m/p5svm-103.pdf] @ATX Power Supply Connector section.
And about the -5V I forgot to tell, but some motherboard can produce it by themselves if not supplied by the PSU. It is not the case with this board.
About the PS_FAN, some HP PSU are very small with odd design. So it is possible that to reduce cost it was easier for them to use a 12V line directly on the MB with the benefit to use the tachometer too.
The fans are managed by a controller chip, so if having a problem with that, it should POST only giving a warning. I don't think your problem is coming from here.

dondiego wrote on 2021-10-21, 10:31:

May be that connector is to monitor the PSU fan and that's why it doesn't boot but i doubt it, how can i fool the board into thinking the fan works? It's a 3 pin header.

Sometime high end PSU provide the tachometer-only signal for motherboard to have PWM information. I doubt it is the case here. I'm more thinking of a fully wired fan connector as I said.

That's the regular ASUS board, i don't have the manual for the HP version. I still suspect a proprietary power connector.

dondiego wrote on 2021-10-21, 11:36:

That's the regular ASUS board, i don't have the manual for the HP version. I still suspect a proprietary power connector.

And I say it is not. I have never seen that. From my collection there are Pavilion with such a PSUs.

The motherboard starts without the PS_FAN connected. And starts equally with a common ATX PSU. Apart from the fact that your motherboard is very very special, I doubt the PSU pinout is the problem. Maybe that's more a problem of power line with not enough power.
I have an HP Vectra XU dual Pentium Pro that is using a 100% compatible 24-pin ATX connector but with a different pinout. But the board was made by HP for this very high end workstation.
I repeat, I never see that on a Pavilion from any kind. The idea of using existing motherboard is to reduce R&D cost and I doubt HP or ASUS would have changed the existing design just to fit a non compliant ATX/PC'98 motherboard in their PCs.

Edit: my test show me that the PSU fan is working without the cable plug on PS_FAN and the fan have no yellow wire inside, so I think it is just a connector to tell the BIOS the PSU fan is working. Then the POST giving a warning if none is connected and monitoring of it enabled.

That makes sense. The HP PSU was made by Delta BTW. But then i have no explanation i've tried several PSUs one of them with the 20 pin connector giving 250W and the other two with the 24 to 20 adapter giving 300W each one of them by HP. The 250W unit was from another k6-2 tower and it worked. I get no beeps only the fans turn and the power led doesn't light up even changing the orientation of the panel connector. I checked the jumpers too but the board booted before. Putting a coin cell doesn't make a difference wither.
I just took it out and put it in an antistatic bag, i was careful with the board.

I wasn't speaking about the global PSU power, but what each line can output (+5V +12V +3.3V +5VSB etc).
But you say the fan turns on with no power LED active. Does the motherboard starts by itself? I mean if there is no cell battery, the BIOS uses its default configuration which is being halted so it should not start.

The PSU is good and it's just a K6-2. It has -5V too. It booted without the battery before and putting it makes no difference.
Bad quality pics of the board.

Well, I asked that because some power supply cannot supply enough on certain lines. But it is meant to be like this as such lines normally does not drain more power than what the power supply can deliver. But some motherboard needs more power than normally power to even boot. So the voltage can be OK, but not the power delivered by the line, consequently the mother will not start (not just POST).
Visually your caps seems alright.
You haven't asked to my question. Is the motherboard is powering on by itself? Or does it start when you push the power button?

It starts when i press the power button but then it hangs. I need to keep it pressed to turn it off. Either the case button or a switch with a cable on a cardboard box outside.
It's a crappy PSU but it powered another K6-2 (500 MHz too) and i had an athlon xp with the same case model and a similar PSU (those were extremely common, Codegen/Phoenix 250x1), besides i've tried three PSUs and AFAIR the adapter i've used with two of them worked. The board booted a few days ago with the HP PSU.

If the motherboard respond to start and halt signal, it means it is working, which is a good sign. I asked that, because your problem remembered a problem I had with a CUW-AM (An ASUS motherboard from HP Pavilion too). That board does not work anymore for ages now but I didn't want to bring you bad luck. The symptoms is the board starts by itself with no POST, as soon as there is power. And when I say starts, I mean the CPU fan is rotating... Apart from that, nothing gets hot, nothing lights up. I haven't been able to identify the problem then. I have no more clue today. I have a POST card for testing motherboards that does not have a segment display or PC speaker. It works with ISA and PCI ports and give information about the POST but also about the voltages. For this problematic board, I got the voltage reading, but absolutely nothing happens on the POST side. I suspect the ACPI controller to be dead.
But I would be interested to know what your P5S-VM can report, because what is strange, is that the power LED does not light up. That is what surprises me the most, 'cause if it respond to a start signal, the LED should be lit, then it could have been a CPU or RAM detection problem, so I would have given the advice to remove the RAM and the CPU, clean everything properly, plug everything back and test again. But here, I don't see where the problem is coming from... sorry.

The symptoms are pretty much the same as when a board doesn't boot due to a dead battery, this happened on some old pentium boards.
If i had a programmer i could try to flash the ASUS bios, there's something fishy about that fan sensor.

I had the non-OEM version of that board (P5-99VM) until a few months ago. It was exactly the same as yours, and in fact the silkscreened "P5S-VM" label was covered with a "P5-99VM" sticker.

Unfortunately I sold it, otherwise I could have tried to flash the HP BIOS in my programmer to confirm your theory about the fan sensor. I have an Intel 430TX motherboard where the CPU fan is disabled when using the OEM Gateway BIOS but it gets enabled with the Intel BIOS. Onboard video on the other hand is enabled for OEM but disabled on Intel, so it is very likely that in your case something about the OEM BIOS may be preventing the board from starting under certain conditions.

TheMobRules wrote on 2021-10-21, 22:04:

I had the non-OEM version of that board (P5-99VM) until a few months ago. It was exactly the same as yours, and in fact the silkscreened "P5S-VM" label was covered with a "P5-99VM" sticker.

Unfortunately I sold it, otherwise I could have tried to flash the HP BIOS in my programmer to confirm your theory about the fan sensor. I have an Intel 430TX motherboard where the CPU fan is disabled when using the OEM Gateway BIOS but it gets enabled with the Intel BIOS. Onboard video on the other hand is enabled for OEM but disabled on Intel, so it is very likely that in your case something about the OEM BIOS may be preventing the board from starting under certain conditions.

Yup, indeed, I it can be that. But if it is a BIOS problem, why the power led does not light up? Is this LED directly controlled by the BIOS on Pentium/K6 PC? I'm not sure about that. But for what I know, and I may be wrong, its circuit is closed even before the BIOS starts the initialization process of the CPU. The BIOS can control the chip in charge of the power management but they work independently with the second having a default behavior at start and the power LED depending of it.
If I'm not mistaken and if it is a BIOS issue, the LED should be light up. Except if there is a problem during the configuration of the power management during the POST, but the LED should light up briefly... That is in such case a diagnostic card is useful to see if the BIOS is doing something or not.

dondiego wrote on 2021-10-21, 20:05:

there's something fishy about that fan sensor.

What I have found is the pinout of the PS_FAN is GND +12V GND, so just a power line, maybe it can have two purpose, to power a fan or to check if one is connected by adding a resistor in the PSU, as I was thinking before. The wire colors on my PSU match the pinout.