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Old 06-11-2011, 12:26 PM   #1 (permalink)
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Question CPU Power On Problems

Hello all. First post.

I built this computer a couple years ago, and recently needed to connect another HDD to it, so I had to borrow the SATA cable from my front panel. After doing what I needed and putting it all back together, I began having a strange problem. I couldn't get the computer to boot back up unless I waited about 10 minutes (I assumed there was a mobo grounding problem, and it was over with, but that was not the case.) After another reboot, it did the same thing. Eventually, it stopped powering up all together. The 5V standby indicator is on. I'm able to start the computer, and use it normally, by jumping the PS_ON and COM pins. I've been doing this for a few weeks, and now I want to fix it. Problem is, with my limited understanding, I am not sure whether the problem lies with the PSU or the MOBO. My best guess is the MOBO, but that's why I'm here... looking for advice. Any ideas?
I figured my specific hardware wasn't necessary, but if you need it, let me know.

Thanks!
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Old 06-11-2011, 12:28 PM   #2 (permalink)
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Default Re: CPU Power On Problems

it sounds like your problem is with the on-off switch.

Newegg.com - APEVIA 25" Power switch cable for computer cases that connects to the motherboard Model CVTPWSW
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Old 06-13-2011, 10:35 AM   #3 (permalink)
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Default Re: CPU Power On Problems

I don't think so. I used an on/off switch from another case I have. Nothing.
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Old 06-14-2011, 08:58 AM   #4 (permalink)
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Default Re: CPU Power On Problems

If the system boots when you short the two motherboard pins, then the motherboard is perfectly fine. All the power button does is short those two same pins.

So I agree with Slaymate - your case button appears to be broken, or connected to the motherboard incorrectly.
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Old 06-15-2011, 09:33 AM   #5 (permalink)
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Default Re: CPU Power On Problems

I'm sorry if I didn't make myself clear. I CANNOT boot the computer by jumping the PWR_ON pins on the motherboard, I meant the pins on the 24pin Motherboard PSU connection. While the cable is connected, you can jump them with a paperclip from the back of the connector, as they are open on both sides. I have tried jumping the motherboard pins directly, and it does not work. So, to me, it sounds like the motherboard isn't sending the power on signal to the PSU and would be the culprit.

Again, sorry if I didn't explain it correctly.
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Old 06-15-2011, 11:50 AM   #6 (permalink)
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Default Re: CPU Power On Problems

If me, I would pull all the wires from the motherboard's front panel I/O header. Then, after verifying you have the correct two, short the two power on pins. If the PSU does not come on then, then I agree you have a motherboard issue.

I will note the front panel I/O header connections (and thus front panel power button) use the PSU's +5Vsb standby voltage - which is required by the ATX Form Factor standard. This standby voltage is present, or rather is supposed to be present across several points on the motherboard whenever the power supply is plugged into the wall, and the PSU master power switch (if present) is set to "on" (1). The +5Vsb voltage is used to power such "Wake on..." features as Wake on Mouse, Wake on Keyboard, Wake on Network, AND the front panel "remote" power switch.

So maybe your PSU is having problems when in standby mode. It is normally my troubleshooting routine to ensure I have good power anyway. So if me, I would swap in known good PSU - especially before thinking about replacing a motherboard.
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Old 06-15-2011, 01:41 PM   #7 (permalink)
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Default Re: CPU Power On Problems

When I tried jumping the PWR_ON pins on the Mobo front panel jumpers, I had done exactly as you suggested, and removed all of the wire connections.
I wanted to swap PSUs as well, but recently sold my only compatible PSU in another build, and had to return a friends' before I had time, as he needed it back. I was hoping we could diagnose the problem here without having to do that.
To me, if it were a problem with the PSU's standby power, I'd see more symptoms than this. But I am not certain.

Thanks for your replies! I'll try to get another PSU to test with, in the meantime, keep em coming!
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Old 06-15-2011, 04:42 PM   #8 (permalink)
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Default Re: CPU Power On Problems

Quote:
I wanted to swap PSUs as well, but recently sold my only compatible PSU in another build, and had to return a friends' before I had time, as he needed it back. I was hoping we could diagnose the problem here without having to do that.
I agree with your initial analysis in your opening post that it is either a motherboard or PSU issue. And right now, it is leaning towards the motherboard as the problem.

I forgot you said in your opening post too that the 5Vsb LED was on. That's a good sign, but again, it is that 5Vsb voltage that gets shorted to ground and signals the PSU to fire up. I don't know what other symptoms you would see. It's purpose is solely to power the human interfaces components of the computer - so when you press a button or key or wiggle the mouse, the computer wakes.

Once PSU starts, the +5Vsb voltage is disabled and the regular +5VDC rail takes over. So, I would want to try a different PSU to be sure.
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