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Motherboard(/PSU?) issue, PC doesn't start properly

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December 6, 2011 12:43:05 PM

I have a PC I built myself that's been sitting unused for about a year, worked great for another 2 years before that. I've figured out a workaround to the problem, but it's only a temporary solution.

Basically, pressing the power button or jumping the PWR SW leads does nothing. However, the computer starts up just fine after jumping the green and black wires on the 24 pin connector thus forcing the power supply on, then removing the jump and letting the PC shut down. After this, pressing the power button starts the computer just as its supposed to and it stays on. If I shut the PC off, I have to repeat this process to start it back up.

I've tested the PSU with a multimeter, all the voltages are correct. I'm really confused as to why it'll only start after forcing the PSU on once.

Motherboard is ASUS A8N-SLI Premium, PSU is 500w if that helps.
a c 717 V Motherboard
December 6, 2011 12:50:42 PM

Welcome to Tom's Forum! :) 

It seems the MOBO isn't sending the signal to the PSU to turn-on, those (2) wires signal the PSU to turn-on. Therefore, it seems more likely that the MOBO is the root of the problem.

The logic is the PSU can be turned-on manually and works, so the 'weak link' appears to be the MOBO.

-- edit --

Try to breadboard the MOBO to rule-out a short that might be causing the error:

You can try Lazyman's Breadboard -
* Disconnect ALL Front Panel & USB Headers ; keep only the PWR & Ground <best> short w/wire.
* Disconnect ALL peripherals including the Keyboard/Mouse
* (1) Stick of RAM in the second DIMM slot from CPU
* Unscrew both the MOBO and PCIe screws

Pull the MOBO away from anything conductive and supported by a towel ; short PWR & Ground to start.
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December 6, 2011 1:02:47 PM

Thanks

That seems like the most reasonable cause, but what I don't get is, what would cause it to send the signal upon tripping the power switch after being jump started once, but not before? I'm saying jump starting it, then letting it sit for 20-30 seconds till everything completely powers down. Unless once the PSU is turned on once it doesn't really shut off once the jump is removed, it goes more into a standby mode? Because if I flip the on/off switch on the back of the PSU the issue arises again. Not sure exactly how that works.

I'll try the breadboard solution. Although again, wouldn't it still be preventing the PC from starting at all, even after jump starting the PSU?
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December 6, 2011 1:27:54 PM

Now that I think about it, could too much dust buildup underneath the mobo cause a short? It was pretty dusty when I first opened it up, cleaned it up with compressed air but I imagine it's possible there's still some beneath the mobo.
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a c 717 V Motherboard
December 6, 2011 1:37:43 PM

There's a difference between (S0~S4) and S5 (off), the other Sleep States (S2~S4) maintain power and are 'on'.

Ton's of dirt never help, IMO it's a good time for a clean/rebuild, and if the PC is >3~4 years old then probably time for new thermal paste.
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a b V Motherboard
December 6, 2011 4:15:54 PM

A short circuit means that the path of least resistance for power is to go a different direction than you want the power to go.

The shorts we are talking about generally lead to power going from the PSU to the motherboard and then immediately flowing through to the metal in the case. Excessive dirt won't make this happen, only metal touching metal in a way that isn't intended.

If you don't use the separators between the motherboard and the case and the metal on the back of the motherboard physically touches the case, that would be an example of a short circuit.

Again, dirt won't do this. If you can use the computer just fine then there is no short circuit present. Its extremely rare for a short circuit to be "fixed" by anything other than physically moving things around. If it works sometimes and sometimes not then its not a short circuit 99.9% of the time.

Dirt isn't generally good, but I don't think it is your problem either. In any event, I would hit it with some spray air if it were me. I do my own computer every few months usually, but I think I just like using spray air too much.

My gut feeling on this is that it is a PSU problem. What is the exact maker and model of the 500w PSU that you are using?

It sounds to me like your computer is having a problem a lot like one I have seen in the past.

In the past, I encountered a computer that turned off one day and didn't want to come back on.

- Edit - If it isn't clear, the computer was shut down voluntarily through the start menu. It didn't shut itself down. It went down gracefully.

When the power button was pushed, the power would come on for 1 second and then go right back off. If someone pushed the power button every time the power went right back off, then after about 30 times the power would finally stay on.

In that computer, when the power cables for things were removed, it took fewer and fewer "tries" to get the computer to turn on. With only the motherboard power connected it would only take about 4 "tries" (down from about 30) to turn it on.

The PSU was a bad brand and in addition it had been put through a lot of usage too, and it had deteriorated to the point it couldn't provide adequate power anymore.

Replacing the PSU and keeping everything else the same fixed everything immediately.

So my question is, what is the exact maker and model of your 500w PSU?

Just taking a stab in the dark, I would say since you didn't already state the maker and model of the PSU that you probably don't consider the PSU to be hugely important in the functioning of the PC (it is) and you probably bought a cheap one to save money over the better but twice as expensive models, and after a few years your PSU is behaving like the PSU I described before.

I could be off, but it is just a theory. I highly suggest you investigate the PSU as a potential source of the problem. If you have another similar power one laying around it would be a good idea to try it in the not working computer to see if the not working computer will begin functioning normally.
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December 6, 2011 7:57:40 PM

I just didn't mention the name of the PSU because I forgot hah, it's been so long and I didn't think to check. All I can tell you without going home and looking is that it's an Antec 500w, which from what I remember is a quality PSU manufacturer. I haven't kept up on PC hardware since around 2008.

I'm leaning more toward the problem being the PSU rather than the mobo. It would seem that the PSU isn't supplying the little power necessary in it's off state (directly after pressing the on/off switch) to even trigger the pwr switch thus turning itself fully on, yet once the PC is turned on and off it goes into a slightly more functional off(standby?) state in which the necessary power is supplied? But then my theory is ruined by the fact that it still powers the green 5vsb standby power light on the mobo at all times, which is what the pwr sw runs off if I understand right. That means it is supplying the necessary 5vsb power, at least to some of the board, but it doesn't seem to make it to the pwr sw pins? Perhaps the 5vsb (purple) ATX wire isn't making a proper connection? A short doesn't seem to be causing the problem because that power is still traveling along the same circuit no matter what state the PSU is in, so it would malfunction regardless. I may also have no idea what I'm talking about.

For the time being until I buy a new power supply I suppose I can run 2 wires from the green and black ATX wires to a toggle switch and mount it on the front of the case, saves me the trouble of a paper clip every time I want to start my pc haha.
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a b V Motherboard
December 6, 2011 8:04:07 PM

Antec is a good enough brand. If I am not mistaken, they outsource some or all of their PSUs to be made by Seasonic now.

In any event, it could still be the PSU even if it was Seasonic for that matter.

There is a difference between enough +5sb power to turn on an LED and enough power on a +5sb to wake up a computer. It is possible to have a light that is on showing it has some power, but still not have enough juice to pull everything out of sleep mode.

Even Seasonic PSUs get old. Older ones lose their ability to power things over time regardless of what brand they are.

Even though it is an Antec PSU, I would still suggest you try to switch it out with a comparable newer one as the next troubleshooting step.
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