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Help! My PC won't turn on!

by Tom Cumming


Due to popular request, this FAQ is the first one in what will become series of FAQs offering advice on how to recover from PC disasters. As this is a very large topic, I am splitting it up into various aspects, which I will attack individually.

This FAQ focuses purely on the completely dead PC. You turn it on, and absolutely nothing happens. No sounds, no lights, nothing on the screen, nothing at all. If this is your situation, this indicates that either the computer is not receiving power, or there is a problem with the on/off switching. Here are some things you can check:

Check that the mains power outlet you are using is actually working, by plugging something else that you know is working, like a lamp or radio, into it.

Check that all the power cables have not worked loose round the back of the PC.

Try changing the fuse in the plug. Take a fuse of the correct size (A red 3A or black 5A normally) out of another appliance that is working, and put it in the plug for your computer. Try turning your computer on again. If it still does not work, then take the new fuse out again and put it back in the appliance you took it out of. If this appliance still works, then this shows that the problem is not a problem with the fuse. If this appliance now does not work (ie, computer has blown a second fuse), then this indicates the computer has a fault which could have safety implications, so take it to be repaired professionally. Do *not* try putting a bigger fuse in it!

Whilst you've got the plugs open, check all the screws and the cord grip are nice and tight, if they are hand-wired plugs.

If the computer is plugged in via any kind of surge protector, extention cord or mains adaptor, then also check the fuses in these using the same procedure as above. These should all have brown 13A fuses in them. Again, if it blows a second fuse, this is time to get someone qualified to look at it.

If all of the above has not helped, I am afraid it is time to open up the inside of the computer! Turn the PC off at the mains before doing so, but leave the power lead plugged in, so that the earth is still connected. When you are working inside the computer, touch the outer casing every few minutes to earth yourself.

Once you have got the case opened, look for where the power supply unit is. The power supply unit is a big metal box that sits in the corner of your computer's case, where the power lead plugs into, and coming out of it will be lots of smaller wires running to supply power to the various parts of the computer. Check that none of these cables have come out of their sockets - their should be a power lead running into:

- The hard disk drive
- The floppy disk drive
- All other storage devices
- The motherboard
- All cooling fans (Which may be powered via the motherboard)

Check that all of these things have a power lead running into them, and if one has worked loose, then reconnect it, being careful to put it in the correct way round. If you connect it the wrong way round, the plug will not quite fit properly.

The other thing you could check is the power switch. Depending on how old your PC is, you may have one of two different types of power switch:

- One which the big mains cables run into, which actually turns the power on and off;
- A small switch that connects to the motherboard, which in turn turns on the computer.

If you have the latter type, then the best way to test it is to disconnect the lead which connects it to the motherboard, turn the mains on, and then *very carefully* touch the two terminals on the motherboard together with a screwdriver, and see if your computer turns on. If this does turn your computer on, then your power switch is broken, so you need to order a replacement one from your PC manufacturer.

If you have the older type of switch, which does actually turn the power on and off directly, these are a bit more difficult to test. The best way I know is to connect your monitor to the power supply's "auxilary" power outputm and turn it on. This is the other power connector above or below where you connect the mains lead, assuming you have one. Once you have done this, connect the PC back up to the mains, and you should find that when you press the power button on the computer to the "on" position, the monitor should turn on. If this happens correctly, then there is nothing wrong with your power switch. Unfortunately, if the monitor does not turn on succesfully, this does *not necessarily* mean the switch is broken - it might alternatively mean that the power supply is broken. So I would advise in this situation to carry on through and if you get to the end of this FAQ and you still cannot get it to work, then go back to suspecting the power switch.

If you have exhausted all the possibilities so far, then your power supply unit has probably failed, and you need to replace it. Do *not* even attempt to try and repair a failed power supply unit - they are extremely dangerous items unless you know what you are doing. They are however quite safe to take out and handle, provided you do not start taking them apart.

Before buying a new power supply unit, I would suggest removing one from another PC to make sure this is where the fault lies, so that you do not end up wasting your money. Actually replacing the unit is fairly straightforward: you simply disconnect all the leads running out of it, making a note on a piece of paper of where they all came from, and unscrew it from the inside of the computer. Then do the reverse process with the replacement power supply.

When shopping for a new power supply, make sure that:
- The new one has the same wattage or higher as the old one;
- It has enough leads coming out of it for all your internal connectors;
- It is the correct type: AT or ATX. Most PCs made from about 1998 onwards are ATX. You can tell by the size: ATX power supplies are approx 86 x 150 x 140 mm. If you have an AT power supply, but cannot find someone who will sell you a replacement (as they are getting obselete now), then you can buy adaptors for ATX power supplies that will allow you to use them in an AT system.

The last possibility I can think of would be a failed motherboard. This is unlikely, but if you have checked everything else so far then it is the only thing left to check. Again, the only totally reliable way to check is to replace the motherboard with a known working one. This is quite a big job, so I would advise calling in the PC repair people at this point.

Copyright © 2001-2006 © Copyright Karl Davis.

No part of this site may be reproduced in any format.All documents author acknowledged are copyright retained by the author.

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