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Power Supply

By tnemeth ·
I am having trouble with a clients machine. It's an ATX machine that has an intermittent problem with power up. When the power button is depressed, the indicator LED comes on, but the system does not initialize (the HDD's and fans stay dormant, and there is no output to the screen). If the power button is depressed a second time the system will power up and initialize like it's supposed to. I suspect the power supply, but due to financial constraints, I cannot swap it with a known good one. Has anyone experienced this phenomenon before? What would you reccomend I do?

Trevor Nemeth A+

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by TheChas In reply to Power Supply

Start with the power switch.

Connect an ohm-meter to the switch leads, and make sure that the meter reads nearly zero ohms when the switch is pressed, and greater than 10,000 ohms when the switch is not pressed.

Next, check that the ATX power connector is firmly seated to the motherboard.

On an ATX system, there is a 5 volt standby line that powers the power control circuit on the motherboard.

I would check that the 5 volts is good, and that the power signal line switches properly when you press the power switch.

If the 5 volts is low, then you likely have a power supply problem.

If the power control line does not switch properly, then the problem may be in the motherboard.

1 more thought, if this is a socket A system, make sure that the CPU front side buss and multiplier are set properly. I have had several socket A systems where they would not turn on properly if the CPU speed was set wrong.


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by TheChas In reply to

There are a few articles here at TR that cover power supply testing and using multi-meters.

Just search on either term and you will get further details.

Remove any spaces from the pasted links.

Also, take the time to read through the discussions of each article for more tips.


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by stroke311 In reply to Power Supply

Take off your case panel and find a 4 wire connector. The connector should have 1 yellow wire which is 12volt, one red wire which is 5 volt and 2 black wires that are ground. Take a voltmeter and check voltage to the yellow wire while you try to power up. If you don't have voltage on that wire I would suspect a bad power supply. If you have 12 volts check the red one for 5 volts. If no voltage is found, you have a power supply problem. Hopes this helps, Frank

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by joenmr In reply to Power Supply

The problem could be the motherboard. Motherboards that have the video onboard could show the problem that you described.

Therefore check the output voltages from the power supply to confirm or reject that problem (power supply).

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by csmith In reply to Power Supply

The other problems occur, but 65% of the time the problem is the switch, not the power supply.
Regards, Chris

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by donmars In reply to Power Supply

Interesting problem. Does this sequence reproduce itself everytime with no variation in the process? 2 pushes of the button to get things going with the first push lighting the indicator. It seems too neat to be a problem but who knows. What I am thinking is that in bios setup there may be a selection for power on that designates 2 presses to boot the computer with the one press to set it for some later remote activity or timed turn-on. I know that later versions of AMIBIOS has settings for the power switch that designates 1 or 2 presses for hibernate (sleep mode) or some other power down function I cannot remember right now. Go into BIOS setup and check it out under power management setup if you are running AMIBIOS, or look around. You can also try "load defaults" in setup.

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