[ozbluefish]

Hi
I am trying to sort through some computer problems in the hope of learning a thing or two about repairs.
I have a machine here that was doing all sorts of things including shutting down and restarting at random.

I have a Power Supply Tester here, and after plugging in the 20pin+ 4pin (2 seperate leads)into the tester
all but the -5V LED lit up. Is this an indication that the power supply unit is faulty?

Any help and extra advice much welcome

Regards
oz

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Is this an indication that the power supply unit is faulty?

Depending on the psu tester that you are using, it is definitely an indication of a problem. But after many years of using psu testers, I've come to use them as a first indicator, and not a final authority. I suggest when you get information that indicates 'possible' problems you learn to confirm it.
In this case you can do this with a volt meter and a little guidance.
First, you will need a digital multimeter, a paperclip, and a known good hard drive.
I say a known good hard drive, because a faulty drive can cause a psu to over or under supply power.
You need the hard drive to put a load on the psu while you are testing it.
Straighten the paper clip and form a horse shoe shape with it. You are going to use it to turn the power supply on.
Now with the psu connected to your hard drive, unplug the main plug from the motherboard, the same plug you used to test it in your psu tester. Now use the paperclip to short the green wire and one of the black wires. Don't be alarmed, this will cause your psu to turn on, and the hard drive will spin up... this is good.
Now you are going to use the multimeter to test the different voltages of the following wires. You want the results to be within the parameters listed for each of the colors, any variation above or below these ranges indicates a failing psu.
Red, purple, and grey: 4.75 to 5.25 VDC
Yellow: 11.4 to 12.6 VDC
Orange: 3.17 to 3.4 VDC
White: -5.5 to -4.5 VDC (negative)
Blue: -13.2 to -10.8 VDC (negative)

You may find that you do not have a purple or white wires, this is nothing to be alarmed about, just check the other colors. Unless a wire is broken, all the oranges will be same voltages, same with all reds, and so on...

[Cotutor]

Is this an indication that the power supply unit is faulty?

Depending on the psu tester that you are using, it is definitely an indication of a problem. But after many years of using psu testers, I've come to use them as a first indicator, and not a final authority. I suggest when you get information that indicates 'possible' problems you learn to confirm it.
In this case you can do this with a volt meter and a little guidance.
First, you will need a digital multimeter, a paperclip, and a known good hard drive.
I say a known good hard drive, because a faulty drive can cause a psu to over or under supply power.
You need the hard drive to put a load on the psu while you are testing it.
Straighten the paper clip and form a horse shoe shape with it. You are going to use it to turn the power supply on.
Now with the psu connected to your hard drive, unplug the main plug from the motherboard, the same plug you used to test it in your psu tester. Now use the paperclip to short the green wire and one of the black wires. Don't be alarmed, this will cause your psu to turn on, and the hard drive will spin up... this is good.
Now you are going to use the multimeter to test the different voltages of the following wires. You want the results to be within the parameters listed for each of the colors, any variation above or below these ranges indicates a failing psu.
Red, purple, and grey: 4.75 to 5.25 VDC
Yellow: 11.4 to 12.6 VDC
Orange: 3.17 to 3.4 VDC
White: -5.5 to -4.5 VDC (negative)
Blue: -13.2 to -10.8 VDC (negative)

You may find that you do not have a purple or white wires, this is nothing to be alarmed about, just check the other colors. Unless a wire is broken, all the oranges will be same voltages, same with all reds, and so on...

[123Runner]

You can also leave everything plugged in and probe with a meter directly on to the 20 pin connector that goes to the board. You do need to be very careful due to the voltages present.

Your psu could be under powered for the computer. Just because you have the voltages does not mean the psu is up to snuff.

PSU wiring

[ozbluefish]

Thanks to both of you for the replies.
I am keen to give it a go, but my first concern is how dangerous is this?
I obviously have to leave the PSU plugged into our 240V power outlet, so what does

You do need to be very careful due to the voltages present.

mean exactly? Wear rubber soles? don't touch any other wire when testing another?
I am not sure how to go about this in a 100% safe manner.

Any help appreciated

[Cotutor]

Ok, obviously any time you are messing with electronics, err on the side of caution.
With that said, the caution by 123Runner were in reference to checking the voltages with the psu still connected to the mainboard. This can cause shorting and damage your components.
If you are dealing with the internal end of your psu, we are talking dc voltages in the 12volts and below range. Now that might cause some problems if you short it out on your pacemaker, but you are relatively safe otherwise.
The business end coming from the wall would require a lot more caution on the other hand.
If you remove the psu plug from the board and short the green and black pins, this is a 12 volt wiring harness, with the same voltage as standard car batteries, but a lot less amperage.
The biggest precaution inside of any computer, is static, I recommend unless you are familiar with grounding and neutralizing procedures that you use a static wrist strap. This keeps you from frying mainboards and other sensitive equipment with the static electricity that you build up in your body.

[Macboatmaster]

I would urge caution
Using a multi meter is the most unsatisfactory of testing.

Firstly, one slip and you would NOT be the first to make a mistake and it may very well be goodbye power supply -

If you try it with power connected to the motherboard, rather than shorting power on - green and black -COM earth and you make a mistake, it may very well be goodbye motherboard

Also, the multimeter needs to be of a certain standard, before the results are reliable

Additionally pin outs are different on the ATX 24 pin v2 and the ATX w20 pin.
So if you ignore my advice and go ahead be extermely cautious about following any online guide that suggests what voltage you should see on what pin

See this
http://www.hardwares...Supplies/181/14

If your PSU tester is of a known make - rather than the PSU tester made in ? brand, then that is far more reliable than any multi meter tester.

See this as well, by one of our staff members Digerati (Retired Staff) as to why you should NOT consider using a multi-meter
To properly and conclusively test a power supply unit (PSU), it must be tested under various realistic "loads" then analyzed for excessive ripple and other anomalies. This is done by a qualified technician using an oscilloscope or power analyzer - sophisticated (and expensive) electronic test equipment requiring special training to operate, and a basic knowledge of electronics theory to understand the results. Therefore, conclusively testing a power supply is done in properly equipped electronic repair facilities.

Fortunately, there are other options that are almost as good. I keep a FrozenCPU Ultimate PSU Tester in my tool bag when I am "in the field" and don't have a good spare power supply to swap in. While not a certain test, they are better than nothing. The advantage of this model is that it has an LCD readout of the voltage. With an actual voltage readout, you have a better chance of detecting a "failing" PSU, or one barely within specified ATX Form Factor Standard tolerances. Lesser models use LEDs to indicate the voltage is just within some "range". These are less informative, considerably cheaper, but still useful for detecting PSUs that have already "failed". Newegg has several testers to choose from. All these testers contain a "dummy load" to fool the PSU into thinking it is connected to a motherboard, and therefore allows the PSU to power on, if able, without being attached to a motherboard - great for testing fans, but again, it is not a true load or suitable for conclusive testing.

Note the required voltage tolerance ranges:
NOTE: Disregard the -5VDC reading. It is no longer used.

Swapping in a known good supply is a tried and true method of troubleshooting used for years, even by pros. If you have access to a suitably sized, spare power supply, carefully remove the suspect supply and replace it with the known good one, and see if the problem goes away.

I do not recommend using a multimeter to test power supplies. To do it properly, that is, under a realistic load, the voltages on all the pins must be measured while the PSU is attached to the motherboard and the computer powered on. This requires poking (with some considerable force) two hard and sharp, highly conductive meter probes into the main power connector, deep in the heart of the computer. One tiny slip can destroy the motherboard, and everything plugged into it. It is not worth the risk considering most multimeters, like plug-in testers, do not measure, or reveal any unwanted and potentially disruptive AC components to the DC voltages.

Note the ATX Form Factor standard does not "require" specific color coding for power supply connector wiring. It has recommendations but manufacturers often do not follow them. Sadly, many testing guides or tutorials will refer to wire color only and that can lead to improper testing.

The voltages can be checked in the BIOS Setup Menus of most motherboards but they do not reveal ripple or other anomalies either. And of course, booting into the BIOS Setup Menu requires a working PSU.

As always, before working in the interior of the computer case, take necessary ESD precautions to ensure static buildup in your body does not discharge through and destroy any sensitive devices. Unplug from the wall and touch bare metal of the case before reaching in. And remember, anything that plugs into the wall can kill. Do not open the power supply's case unless you are a qualified electronics technician. There are NO user-serviceable parts inside a power supply. If you do not have a tester or a suitable spare to swap in, take the PSU to a qualified technician for testing.

PLEASE NOTE
I am not suggesting that any advice you have been given is INCORRECT I am simply saying that my recommendation is DO NOT attempt the multi meter test.

Doing so with the PSU connected to the motherboard and of course the CPU power on is very risky.
Doing so without it connected and shorting power on - green and earth black is unreliable as the PSU is not then under load, it is simply turned on.

[ozbluefish]

Thank you all so much for the valuable and detailed advice.
I have a anti-static wrist strap on the way, as well as a multimeter, and a new PSU.
I would like to try all methods mentioned, as I mentioned I have taken a real interest in trying to understand PC's
and trying to trouble shoot them, so I would like to see the results of all of the testing methods.

Will report back with all of the results, but it will probably take a week to get all of the parts.

Thanks again for the help - have no idea where to look if the power supply unit is not to blame
for the computer problems, but will cross that bridge when I come to it.

Regards
oz

[Cotutor]

Oz,
If the psu proves not be the cause, let us know, and we'll give you some other things to test in order to nail the bandit.

[Macboatmaster]

oxbluefish

Will report back with all of the results, but it will probably take a week to get all of the parts.

Just wondered how you went on.