Motherboard powers on itself when plugged to AC

[JorgeHernan]

Hi guys.Do you have any idea?What appears in the topic title is what is happening to my motherboard.Power button doesn't work.I kept this motherboard in storage for sometime but the last time i use it was working ok.A few days ago i use it to rebuilt a system using a good cpu good memory but when i plug the cord to the AC socket the motherboard powers on itself and obviously does not boot.It looks ok with no bulging caps, no burned mosfets.Is an Asus a7v8x MX SE.

[Tom41]

Re: Motherboard powers on itself when plugged to AC

Some boards can be set to automatically power up after a power outage without having to press the power button - that is, after power is removed from the PSU and reapplied. My P3 machine does that when you put the mains cord in, or switch the PSU isolator switch off and on.

You say it does not boot. I don't quite understand; is this part of the problem, or it just doesn't boot because there's no HDD/floppy/CD connceted?

Check the BIOS settings for an option, usually called something like "Auto power on after power failiure", and set it to off. Also, check that the power button is actually connected to the motherboard properly. If so, use a multimeter or continuity checker to ensure that the button is actually shorting the connectors when pressed.

[arneson]

Re: Motherboard powers on itself when plugged to AC

The bios may have the ability to power the board immediatly, if it lets you into the bios thats a good sign. Turn off the power feature that repowers after failure.
The not posting could be many possible issues but may also be bios setting related, or connections.

[JorgeHernan]

Re: Motherboard powers on itself when plugged to AC

No, no.You got me wrong.The motherboard does not boot, i do not get any video signal.Cpu fan spins normally but nothing happens, no beeps nothing.Its BIOS is good condition(not corrupted).The problem is that the motherboard POWERS ON ITSELF when plugged to AC socket.It powers itself on EVEN if i TAKE the BIOS OUT from its socket.(Sorry about my english).

[Rainbow]

Re: Motherboard powers on itself when plugged to AC

Did you check the battery?

[willawake]

Re: Motherboard powers on itself when plugged to AC

does it do this outside the case?
with no front panel wires connected?
what is the cpu?
does the psu test fine on another pc?

[JorgeHernan]

Re: Motherboard powers on itself when plugged to AC

does it do this outside the case?
with no front panel wires connected?
what is the cpu?
does the psu test fine on another pc?

Exactly you described my problem perfectly
I tried with durons and athlons and get the same results
The psu works fine with the others motherboards i have
It powers itself on even without cpu, without memory, without front panel wires connected !!

[hkivan]

Re: Motherboard powers on itself when plugged to AC

Try clear the CMOS !!

[JorgeHernan]

Re: Motherboard powers on itself when plugged to AC

Try clear the CMOS !!

That's the first thing i did

[Tom41]

Re: Motherboard powers on itself when plugged to AC

Perhaps the caps are bad even without showing any physical signs. Teapo, for example, are notorious for failing without any sign of a bulge or leak. Also check to see if any traces on the motherboard look like they're broken. You don't need to test every single trace with a multimeter, just see if there's any damage on the reverse side of the board.

[JorgeHernan]

Re: Motherboard powers on itself when plugged to AC

Perhaps the caps are bad even without showing any physical signs. Teapo, for example, are notorious for failing without any sign of a bulge or leak. Also check to see if any traces on the motherboard look like they're broken. You don't need to test every single trace with a multimeter, just see if there's any damage on the reverse side of the board.

I changed 3 large green 1500uf 6.3 "KZE" capacitors located near mosfets already.Things stay the same.There are 4 blue small 1500uf 6.3 "OST" capacitors left to change.I am going to check the traces on the back of PCB before any capacitor changing.

[willawake]

Re: Motherboard powers on itself when plugged to AC

checking for a damaged trace is a good idea (on both sides of board), maybe the board get damaged when you remove it for storage. although powering on by itself indicate a short maybe so check also for a very small piece of metal shorting something. check the board carefully for any missing small component.

although i am not optimistic about this board

[gonzo0815]

Re: Motherboard powers on itself when plugged to AC

May be a short on any trace carying the PS_on signal. Try it outside the case on the plain dektop or may be isolated with a sheet of paper. If that cures it, you should take carefully remount it.

[linuxguru]

Re: Motherboard powers on itself when plugged to AC

Probably bad PS_ON circuit - can be hard to trace. Also check the caps on the +5VSB rail in the PSU - if they're marginal, the PSU may work fine on some boards, but show this kind of symptom on other boards. Easiest way to check is with a known good, premium brand PSU.

[kc8adu]

Re: Motherboard powers on itself when plugged to AC

some boards power up to test themselves and validate config each time power is turned on or psu plugged in.thats why some systems come on several seconds after they are plugged in then turn off till you hit the on button.if cpu bad some stay on.

[willawake]

Re: Motherboard powers on itself when plugged to AC

that is true with my intel d875pbzlk

[Rainbow]

Re: Motherboard powers on itself when plugged to AC

It's common for Intel boards.