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computer won't turn on

By thepubear ·
I went away for a few days, and when we did, i turned off all power strips going to both computers. However when I came back home, I went to turn on both computers.

One computer had no problems and is the one I'm posting this on. All accessories to the other one did also. Looking on the mobo, there is a little green light that comes on. However when I push the button on the case, it does make the normal click sound, but nothing inside the case turns on. There is no activity, and except for the small light you would think there was no power going to the case, and taking out the power cord produces no more difference than the light goes out.

So at this point, I'm trying to figure out whats going on. Before i left, everything worked fine. All strips were off. We did have evidence of a power failure because of blinking clocks. There are no scorch marks or blown capacitors on the mobo, that i can see. I also made sure all plugs were well pushed in.

At this point, i'm leaning towards the power supply taking a hit. So Im asking if this behavior indicates a bad power supply. I keep thinking it should turn on, then turn off because of a bad power supply. I've never had it work enough to turn on a light and nothing else.

for a while i was thinking that somehow turning it off for 4 days would set off some sleep mode.

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All Answers

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It takes two to tango

by robo_dev In reply to computer won't turn on

While the power supply provides the current for the little green LED, and obviously also the power to run the computer, the motherboard has to send a 'power good' signal back to the power supply so it can switch on.

Therefore, it's either the mobo OR the PSU.

If the case is open, unplug EVERYTHING from the mobo, including the drives, memory, cards and see if it acts any different. Press down on any socketed chips to make sure they are seated, and unplug/re-seat the psu connector to the mobo.

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In addition to the above

by OH Smeg In reply to computer won't turn on

You need to understand that a Power Supply doesn't just produce 1 Voltage there are in fact at least 3 separate Voltages produced by any Power Supply.

A 3 V DC a 5 V DC and 12 V DC.

The 3 V DC provides power tot eh Logic Circuits and things like LED's on the M'Board. The 5 V DC provides power for the CPU which runs at around the 3 V DC Mark and the Logic Circuits in things like HDD and Optical Drives. The 12 V DC provides power to run things like HDD and Optical Drive Motors.

So just because you have 3 V DC in no way implies that you have either 5 V DC or 12 V DC going to the M'Board and even if you have the actual Voltages it in no way implies that you have enough Current to provide the necessary power to run the computer. The current required to light a LED is negotiable where as the current required to start a computer is considerable more so it's possible that the PS has died.

It's also possible that the PS passed any Spikes straight through it to the M'Board, HDD and whatever else is inside this case depending on who made the PS to begin with.

Ideally when you go away you should Unplug all Electronic Devices including Computers as large spikes are not stopped by switches which have a very small air gap of often less than a few millimeters. This gap does get jumped when large spikes come down the mains.

Here I would first suggest a Known Good Power Supply to test with but you need to understand that if the M'Board has taken a hit it could destroy the Known Good PS into the bargain.

I've had a few cases where a New PS didn't work out of an unopened box which is more to do with the way it was stored in Transit than a Bad Quality PS or lack of Quality Control. So it's possible that a New PS may be faulty and give a misleading diagnosis if it doesn't work.

On a side note here I've found that SIS, NVidia and AMD Chip Set M'Boards are far less reliable than Intel Chip Set M'Boards when Poor Quality Power Supplies are used. Many of the better Power Supplies have a dubious reputation by many End Users because they fail more often than the cheaper ones and this is because they are designed to stop Spikes, Surges and Sacrifice Themselves rather than allow the Spike through into the M'Board and associated Electronics.

Many people mistakenly think that the Expensive Power Supply that has just failed is not much good as they break much more often than the cheap no name ones which unfortunately do not stop the Spikes and contribute to destroyed M'Boards when they get an Over Voltage Event.

Col

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also try

by JC66 In reply to In addition to the above

before buying any new part try an ATX reset, unplug the power from the socket, take battery out press and hold down the power button on the pc for at least 30 seconds then try to reboot again.

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Marginal CMOS battery, HDD

by oldbaritone In reply to computer won't turn on

Before spending a bunch of money on parts you may not need, try this simple technique:

Remove the CMOS battery, and unplug everything that isn't absolutely necessary for POST. Certainly, unplug the HDD(s) and CD/DVD(s) and anything else (audio, etc) that you can. (Of course, make notes so you know where to plug things back in!) If the NIC is a separate card, remove it. Unplug other cables like Network, Speakers, Microphone, etc. Be sure to unplug the power from drives and anything powered by the system PS too. A bad drive can pull down a PS line and prevent the system from booting.

Reinstall the CMOS battery after a minute or two, and try to boot with just MOBO, Keyboard, Mouse and Video. Yes, you should get an error like "No boot device found" - but then you would know that the PS, video and MOBO are OK. Of course you'll also get a CMOS error; just setup with factory defaults for now.

Sometimes, just unplugging and re-plugging will fix the problem. For example, some NICs will allow enough back-fed power to keep the system in an unknown state. Same with some audio systems. Once the system goes to a "completely down" state, it might recover and restart.

If it does, great. And if it doesn't, you have eliminated many possible causes of the problem.

If it boots to the "no boot device" error, shut down and start plugging devices in, one at a time. Reboot again after each device is reconnected. When you plug one in and the system stops booting, you'll know which device is causing the problem. Don't be surprised if it's the HDD. I've had that happen more than once. It was OK while the system was running continuously, but it was a problem for a reboot.

And if there's any chance the CMOS battery is weak, just replace it. They're not expensive, and they don't last forever.

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