You will need only a few basic tools to remove and replace your Power Supply Unit (PSU): a Phillips screwdriver with a No. 2 head (preferably magnetic), a pair of diagonal cutters, and several small or medium-size cable ties.
First, unplug all of the cables attached to the back panel of your computer, then move it to a work area with adequate room to lay it down. Before you remove the back-panel cables, especially the power cord, toggle the PSU's power switch to off as this will cycle down any auxiliary/standby power.
Now, to start installing the new PSU you will need to open up your computers case. The method for opening this will vary depending upon your computers design and layout. Most new cases use either a panel or door, while older systems require the whole cover be removed. Be sure to remove any screws fastening the cover to the case and set them aside in a safe place. You will need them later.
Carefully disconnect all the various power connectors from the components, i.e. the various hard drives and CD/DVD drives. If some of the connectors are hard to unplug try loosening them by very gently moving them from side to side. You must also disconnect both the 20 pin or 24-pin motherboard power connector and the 4-pin or 8-pin 12-volt connector. These power connectors usually have clips on one side that you must depress while pulling the connector away from the motherboard.
Tip: If you feel that you will not know what plugs into where after you have fitted the new PSU then tag each cable with a name tag (perhaps using a post it note to record where the cable fits - try numbering 1,2, 3 etc both the cable ends and the slots on the motherboard and other components, so you can immediately see what goes where).
Finally, unscrew the four screws on the back of the case that are holding the old PSU in place and remove this unit from your PC.
Now insert the new PSU into the case (sometimes this can take a little manoeuvring to get it into position). Align the new PSU so that the four mounting holes are properly aligned. Also make sure that any air intake fan on the power supply is facing towards the centre of the case and not towards the case cover. This job can be fairly difficult to do on your own as the PSU has to be held in place while it is fastened to the PC's case with the screws. If the case has a shelf ledge that the PSU sits on, it will make the job far easier to accomplish.
Make sure that the voltage switch on the back of the power supply is set to the proper voltage level for your country and region. Europe and many other countries use 220v/230v but North America and Japan use 110v/115v. In most cases you may be able to avoid this step as the switch will come preset to the voltage settings for your particular locality.
If your computer already has a motherboard installed into it, the power leads from the power supply need to be plugged in. Most modern motherboard use the large ATX power connector that gets plugged into the socket on the motherboard.
A number of items reside within a computer case that require power from the power supply. The most common devices are the various hard drives and CD/DVD drives. Typically these use a 4-pin white molex style connector. Locate the appropriate sized power leads and plug them into any devices that require power.
It may be that your PC already has SATA drives installed which require different power and connector cables. The process is the same however and these have to reconnected.
Make sure the PSU's motherboard and 12-volt power connectors are oriented correctly so their clips engage the ledges on the sides of the motherboard's sockets. The connectors are keyed and will fit only one way - do not force them. SATA hard drive power connectors are also keyed, but not as clearly as the older four-pin Molex connectors. Observe the correct orientation and do not force them into place.
Before you close up the case tie-wrap all of the cables neatly out of the way, then clip off any cable-tie excess with diagonal cutters.
When you have completed the installation, carefully inspect all of the cabling to make sure it's not touching any heat sinks or cooling-fan blades.
Neatly routed cables allow for far better airflow through your PC's case to help prevent any overheating problems due to cable clutter.
At this point, all of the installation and wiring should be completed so replace your computer's cover or panel and fasten it with the screws that were previously removed to open the case.
Now all that is left is to provide the power to the computer. Plug in your AC power cord to the power supply and turn the switch on the power supply to the ON position. Your PC should now have more than enough power to run successfully.
Warning: Power Supply Units contain various capacitors, inside of the box, that retain power even after the PSU is turned off. Be warned - never open up or insert any metal objects into the vents of the PSU as you can risk a serious electrical shock.
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