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Dead psu? paper clip test

Last response: in Components
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August 23, 2009 2:26:47 PM

before i charge ahead i thought i would check with the experts...

The other day my wife's dell failed to turn on. She leaves it on for days/weeks at a time and the fan noise was driving me crazy so I turned it off. When she went to boot it the next day we got no response from it at all. The start button is doing nothing, no fans spinning, no noises, no nothing. the computer is about 5 years old, is a basic dell model with an intel p4 2.6 or 2.8 gig, windows xp os, and a 250 watt psu.

in looking through the first couple of pages here, i read about the paper clip test... should that be my first step? if i do it do i need to unseat the psu and take it out of the case? or can i simply unplug all the wires from the hdd, fan, optical etc... and give it a try?

If you think i should do something else first, please let me know!!

thanks!
a c 144 ) Power supply
August 24, 2009 4:05:54 PM

Here's something that I cut out from another post.

Try to verify (as well as you can) that the PSU works. If you have a multimeter, you can do a rough checkout of a PSU using the "paper clip trick". You plug the bare PSU into the wall. Insert a paper clip into the green wire pin and one of the black wire pins beside it. That's how the case power switch works. It applies a ground to the green wire. Turn on the PSU and the fan should spin up. If it doesn't, the PSU is dead. If you have a multimeter, you can check all the outputs. Yellow wires should be 12 volts, red 5 volts, orange 3.3 volts, blue wire -12 volts, purple wire is the 5 volt standby. The gray wire is really important. It sends a control signal called something like "PowerOK" from the PSU to the motherboard. It should go from 0 volts to about 5 volts within a half second of pressing the case power switch. If you do not have this signal, your computer will not boot. The tolerances should be +/- 5%. If not, the PSU is bad.

Unfortunately (yes, there's a "gotcha"), passing all the above does not mean that the PSU is good. It's not being tested under any kind of load. But if the fan doesn't turn on, the PSU is dead.


You do not need to remove the PSU from the case. You can just unplug all the PSU cables.

Having said that, if you are not really familiar with insides of a computer, I recommend that you take notes covering where you unplug everything.
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a b ) Power supply
August 25, 2009 2:17:33 PM

Before you start the procedure jsc showed (and it IS a good way to do it - just remember as it says the test is not under real load) try two simple things.

1. On my machine and some others, it seems to get "confused" occasionally and just won't start - looks absolutely dead when you push the front button. But if I unplug it and leave it with no power (or even, switch off the switch on the back in the PSU itself) for at least 5 minutes, then plug in and push the front button, it works again! I suspect it has to do with a hang-up in the BIOS loop that waits for a signal to re-start.

2. If that does not work, another simple thing is that the front panel switch may have developed a fault, or its associated wiring. It is just a momentary-contact switch connected by two wires to a pair of pins in the mobo connector for the front panel. (That same connector has other pins for hard drive activity LED, speaker, etc.) If there is one multi-pin connector there, just pull it off. If the connections are made with separate plugs, pull off only the one for the front power switch. Use a small screwdriver or something and touch both pins from that front switch (DO NOT short out other pins!) for a second or so; shorting together those pins is exactly the same as pushing the front panel switch button. If the system starts up when you do this, then it is fine and the problem is either the switch or its wiring. If it still does not start up, you've finished the simple stuff and should proceed to check out the PSU.

If your PSU seems to be OK, it also is possible there is a problem on the mobo in the components that regulate voltages for the board. Unless you are skilled in electronics, this is something you probably can't fix without a mobo replacement. But here's something you can look for as a clue. Several years ago there was a rash of problems that were traced down to poor electrolytic capacitors that seemed OK but developed flaws after a year or two of use. These devices are used both in PSU's and in mobo voltage regulators. They look like little metal cylinders standing on end, ¼ to 1" diameter, and about ½ to 1½" high. Many have a plastic sleeve on the outside. Their tops are supposed to be flat, but defective ones have tops bulged up in a curve. Look for them in a group, often near the CPU or the rear connector panel. If you see one or more with bulged tops, you can be suspicious that this is your problem. If the simple checks and the PSU tests say they are OK but you have bulged capacitors, see a service shop.
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a c 144 ) Power supply
August 27, 2009 5:45:30 AM

Paperdoc said:

2. If that does not work, another simple thing is that the front panel switch may have developed a fault, ...

Or simply swap the power and reset switches and try to boot with the reset switch. The odds of both switches being bad is pretty small.
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a b ) Power supply
August 27, 2009 1:47:11 PM

jsc said:
Or simply swap the power and reset switches and try to boot with the reset switch. The odds of both switches being bad is pretty small.


Great idea!
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