You can do it yourself. Overview: unplug, remove wires, label them, unscrew. Screw unit in, attach wires. Check and set voltage switch correctly. Plug in.
Tho you may not need them, I'd get out 2 Phillips head screwdrivers, a #2 and a #1, and a slotted screwdriver 1 size smaller than standard size. I think they are called 3/16 inch slotted. You may want a needle nose plier to pull off stubborn connectors that are hard to get off with your fingers, or where you don't have room to put your fingers. Get out your static-discharging wrist band. If the fasteners have hex head in addition to phillips holes or slots, you omay want to get matching hex drivers. A hex driver makes unscrewing and screwing the fasteners, easier than using a Phillips driver or slotted driver.
Unplug the power supply wire at the computer. If you are going to connect your wrist band to the power supply wire ground hole, you may want to leave the power supply wire connected to the electricity supply socket. Otherwise, ground yourself by using my gadget described below. Open the case. Put on your wrist band and, after you test the supply socket in the wall for proper wiring, clip your wrist band to the ground wire hole in the electricity supply socket, or to a conductor that is connected to it (and only it). You should probably test the supply socket first, with a voltmeter or simple voltage-no-voltage probe that cost $1.79 in the hardware store, to make sure no-one has wired in the socket incorrectly, and caused voltage to appear at the ground hole. To test the socket at the wall vor correct wiring, in the United States you should get voltage across the large slot on the left (neutral), and small slot on the right (hot); it should be 120 volts in the US; you should get the same voltage across the ground hole (the round hole in the middle, below the 2 slots) and the small slot on the left (hot), but you should NOT get voltage across the ground hole and large slot (neutral). Ground and neutral should be at the same potential, and the voltage measured across them should be zero. If you get a voltage of more than about 1/2 a volt (a teensy deflecton on the meter) there is a potentially dangerous wiring problem, somewhere, and you should not try and ground yourself to the ground hole. You'd get a small shock or possibly a nasty, even fatal shock.
Then check the power supply wire similarly (or with an ohmeter) to make sure you know which hole at the end of the wire, is connected to which hole in the wall socket. Do NOT plug your wrist band into the hot slot or hole!
An easy way to clip your wrist band to the electric supply socket ground hole, is to get a 3-prong replacement plug from the hardware store, and connect a wire only to the ground terminal, then plug it in. Make sure that wire is properly connected to the ground terminal (for round plug in the middle), and not to the hot terminal (the small spade plug that plugs into small slot on the right)! Then strip the end of the wire, and clip your wrist band's alligator clip, to the stripped end of the wire. Since there is only one wire, you can't connect the alligator clip incorrectly. But make absolutely sure that one wire is connected correctly, at the wall outlet.
If it is an unusual case opening it will probably be documented by the manufacturer, on their web site, as will removing the power supply, if it is installed unusually. If everything is generic, you shouldn't need any documentation.
Look over the power supply and detach all the wires from where they attach to the computer and MARK THEM with where a label describing where they were attached. Then when you install the new power supply, you will know where the wires go.
After you remove the wires, remove the screws that hold the power supply in place, and remove the power supply. You may have anywhere from 2 screws to about 8 screws. For a Compaq or an HP, you may be able to remove the power supply by pressing spring clips, instead of removing screws. You'll need the Compaq or HP instructions, from their web site.
Then install the new power supply, the same way the old one was installed. You will be glad you labeled the wires on the old one.
That is pretty much all there is to it. You may have to use a switch or something to select the correct voltage, 120v for United States, other voltages for other countries. There should be instructions for how to do this, with a new power supply. You should do this before installation, and definitly before plug in, or perhaps you might fry your system board.
Oh yea, it is a real good idea to be very careful you don't drop any screws into the case or onto any of the circuit boards. There is a tiny risk that the collison could damage something. Mostly: they may be hard to retrieve and there is a chance they may short something out if left there. It may fall somewhere where it it irretrievable. If you drop one, it is handy to have 2 items, a claw-type screw-retriever and a magnetic-end-type screw retriever. The claw type screw retriever doubles as a screw starter. You may be able to get a magnetic type with a screw starter on the other end.
The best thing to do is to work on prevention. Never drop a screw in the first place. Put your fingers around the screw before it is fully loosened, and get a good grip on it before it is out of its hole. Or if your fingers don't fit, don't unscrew all the way with a screw driver. Do the last part with the claw screw-retriever-starter.
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