VIA chipset? Sounds familiar. No great loss in that case though.
I've also experienced that they may power up if you remove the CMOS battery.

It is a loss for me.

I tried all the basic stuff. What i'm looking for is someone with experience in repairing things, or someone who had the same issue. There's something wrong in the power up sequence, or the board is not receiving the power ok from the psu, or maybe a mosfet, I don't know. I already checked for broken traces but didn't find any.

I only found one similar thread, were people understood what the problem was, but no solution or hint as to what was the problem was given: http://www.techsupportforum.com/hardware-supp … art-up-psu.html

You'd probably have to replace one of the big chips as to me it seems to be a weakness of one of them.

Did you check if it is a proprietary Mobo or PSU? Dell comes to mind.

If I short the green and black cables on the PSU while the ATX connector is connected, they power up fine

It wouldn't do that if the PSU was proprietary.

To clarify, I meant switch the board on with no CMOS battery in it. I think one or two of the three candidates I had (VT82C693A) would power on then. When putting back the battery they'd be dead again.

As the boards do power on with workarounds and the switch works then there is nothing important broken. There are no intelligent looking chips on these boards except the chipset chips, so the power on logic is integrated there. It's also unlikely that always the same external part breaks as it happens with multiple manufacturers that use the VT82C69*.

My conclusion: Chipset broken by design.

Maybe an acceptable solution for you would be to use an additional power switch to bridge green-black or bridge them permanently and use a mains switch, that also saves standby power.

Yeah, I tried without the battery as the batteries were empty so I removed them. And no, the PSU isnt' propietary.

Just to add more info, I kept doing that on the 7VCA one (powering shorting the PSU), and the last time I tried, something blew up. I wasn't paying much attention, so I can't be sure, but something exploded and flew to the air. Not somehing physical, it was like paint or liquid, not sure. The mobo still works, but I checked the area and didn't find anything burned or blowed up. All I noted were some inductors that now are black. It was a loud bang. Anyone knows if inductors can blow like that ?
I mentioned this as I also thought a solution could be the power them on shorting the PSU, but at least the 7VCA didn't liked it very much...

Thanks for the info hal, much appreciated. What do you mean by chipset broken by design ? Does it mean the boards got bad luck and had placed defective chipsets ?

These are the boards in question, for anyone not familiar with them. Last photo are the inductors I suppose burned (?). Don't pay attention to the inflated capacitors. I already replaced them after taking the photos, and prior to doing all this.

Here's another thread with the same problem:
http://www.tomshardware.com/forum/263981-30-m … ack-cable-trick

What do you mean by chipset broken by design ? Does it mean the boards got bad luck and had placed defective chipsets ?

As multiple boards show the same symptom there must be a weak spot somewhere.

As there are actually datasheets available I've found the south bridge chip controls power. Unfortunately we can't get to the pins to make measurements.

http://www.datasheetarchive.com/VT82C596-datasheet.html

In my working experience, there are motherboards which will NOT power up when the battery is discharged.
Try a brand new, 3volt lithium battery. It worked for me on many occasions.

I tried all that. It's not a BASIC problem.

A little off-topic: does anyone know where I can buy dimm memory slots/sockets or if they are for sale ? I cheched eBay and found nothing...

You can get DIMM connectors from Mouser or Digikey... for instance, here. They ain't cheap, though.

Oh well... guess I have to desolder them from a broken mobo... Jameco also have them, but last time I bought from them I got ripped off 😢

Could it be a short?

A suggestion (it may not help but frankly, I don't see a lot of other options):
Remove all slotted hardware (memory, cards) and check inside the PCI, ISA, AGP and DIMM slots to see if any of the little metal connectors inside the slot are bend or deformed.
I've seen 2 or 3 motherboards that turned out to have this problem. One had a malformed ISA slot connector and 2 had this in a memory slot (one was SIMM, one DIMM).

Nope. As I said, I already tried the basic. Also, the mobos are new, which reduces the possibilities of them having bent or shorted pins.
What annoys me the most are those big Soyo stickers claiming "QC OK". Yeah, sure.