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The local electricity supply went off and when power was restored the computer would not come back on again. What has happened?

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Could have been a voltage spike. When the utility company turns the juice back on, there is a rush of power. This is needed to meet the power demands for all the users, who are on the line.

Problem is the voltage may be higher than what is needed, and a spike may occur. (Spike: Temporary voltage increase )

No sir. The utility company is not held responsible. (Yes, that bites!)

I have found that around 80 percent of the time, the problem is a bad Power On switch, or a bad Power Supply, when this occurs.

{The rest of the time the power went further than the Power Supply, and burned out the motherboard, Processor, Ram Memory, etc}

Power Supplies used in desktop computers, are generally a generic low quality unit.
Low quality Electrolytic Capacitors, Rectifier Bridge, MOSFET's, small gauge wiring, etc.

Couple that with some age on the Power Supply, and it doesn't take much to put it in an 'early grave'. (Plus being dirty inside)

I suggest you test to see if the problem is a $5 ATX power on switch, or the Power Supply.
I can only give you generic information, as the computer manufacturer name, and model number were not given.

(It's on the back of the computer tower next to the Windows product key, or up on the side/top of the tower, or behind a hinged panel in the front of the computer )

The test is to use a jumper wire, and bypass the Power On switch.
This has Nothing to do with the switch itself, however.

The main power cable coming from the Power Supply, and plugging into the motherboard, is either a 20-pin ATX main power cable, or a 24-pin ATX main power cable.
(Unless the desktop computer is Real old)

1) Basic example of a 20-pin ATX main power cable, and it's respective connector on the motherboard,

http://www.playtool.com/pages/psuconnectors/connectors.html#atxmain20

{NOTE* Color of connectors does Not matter }

2) Basic example of a 24-pin ATX power cable, and it's respective connector on the motherboard,

http://www.playtool.com/pages/psuconnectors/connectors.html#atxmain24

(Same thing about color)

Here's the way it works when you press the Power On button, and the computer is plugged into power;

The Power Supply, when plugged into power, has a constant 5 Volt standby power present. (DC)

Pressing the Momentary Contact Switch of a Power On switch, momentarily routes the 5 volt power present in the Power Supply, back to another circuit in the Power Supply.

The Soft Power On circuit.
This small voltage 'excites' the Power Supply, and turns it on.
(No pun intended)

As stated, the Standby power is 5 Volts.
It is 5 Volts DC. In comparison two D cell flashlight batteries store 3 Volts DC.
Stated in case you are worried about electrical shock.

(The Power Supply in your computer is a SMPS.
Switched-Mode Power Supply.

It converts the AC power from your home, or business, into three main low DC voltages.
3.3 Volts, 5 Volts, and 12 Volts.

No matter if your country uses 100 Volts AC, or 120 Volts AC, or 220 Volts AC )

Look at either ATX main power cable. Note in the photo to the far right, the power cable is shown plugged into the motherboard. This is how it should be for the test.

Note also that the wires go in the BACK of the connector, and there is one Green wire.
This is the Soft Power On wire. (Can be abbreviated as PS_ON)

ALL of the Black wires you see are Ground wires.

The preferred jumper wire is a paper clip. It is straightened out, then bent into a U-shape.

The U is wrapped a few times, with black plastic electrical tape
for your fingers, and thumb to hold onto.

The 'legs' go down into the socket holes, in the BACK of the ATX power cable's connector.

The jumper 'legs' go down in the socket holes, RIGHT NEXT TO the existing wires already in the socket holes.

Right next to the GREEN wire, and ANY Black wire.

At the end of every wire going down into the back of the connector, (ATX main power cable connector) is a metal terminal.

This metal terminal is pretty far down in the socket hole. (1/2 inch?)
The jumper wire MUST touch the metal terminals.

The contact made is temporary. No longer than 2 seconds.
(Power Supply plugged into power)

IF, the Power Supply comes on, you have a bad Power On switch.

IF, the Power Supply does Not come on, you have a bad Power Supply.

The Power On switch, is located inside the plastic Power On button assembly.
It is an ATX Power On switch.

This is one example,

http://www.directron.com/atxswitch.html

The plastic of the Power On button assembly gets hard over time, and with constant heat from the computer.
The best method I have found, is to remove the Front Panel, and the Power On button assembly.

[ NOTE* Make notes as to which pins on the motherboard, in the Front panel header, the wires of the Power On switch go to.

I may Not have access to the Front Panel header pinout, to tell you where those wires went.

{The plastic front of your desktop computer is the Front Panel.
The area of pins on the motherboard, that the wires from the Front Panel go to, is the Front Panel header ]

I then use a hairdryer, or a bowl of very warm water, to soften the plastic.
(The ATX power on switch inside is going to be thrown away. Doesn't matter if it gets wet )

Then carefully ease the case of the Power On button assembly out, and ease the ATX power on switch out.

There MAY be a spark as you connect the jumper wire. Letting you know in advance. The tape is to protect your fingers.
You may feel safer using a glove on that hand.

The voltage however, is 5 Volts DC. As previously stated two D cell flashlight batteries store 3 Volts DC.

Would appreciate it if you would post back in a Comment, as to your findings.
With the computer manufacturer name, and model number, I may be able to show you direct information.

I can also help you decide on a Power Supply to buy, should it be the problem.

Regards,
joecoolvette

Posted on Feb 12, 2012

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Try unplugging the power supply from the wall socket and leave it unplugged for at least 5 minutes. Then plug it in and see if your computer will turn on. If it will not, you require the assistance of a PC technician to test your power supply and computer.

You should connect a surge protector or UPS (Uninteruptable Power Supply) to your computer to prevent this from happening again.

If you have any additional questions contact me by clicking on the "Add a comment" link located under your problem description on FixYa. Also, if you believe that this information was helpful to you, please return to the FixYa website and rate my response to your question.
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The power supply or mother board probably went out. Did any other items in the house stop working? This happened to me years ago and the electric company reimbursed me for damages.
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If al has failed your power supply is no longer working and will need to be replaced.

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1.-Your CPU fan is working?

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Check the first to points above and if 1 and 2 are not working

Replace your power supply.

Let me know how it is going!

Thank you for using FIXYA!
Don't forget to rate my help
Pablo :-)

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