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ATX Power Supply Failure Diagnostics

The first step in power supply diagnostics is determining whether or not the power comes on. How can you tell if the power is on? Can you hear fans turning and drives motors spinning up, see little lights on the front of the CPU case or hear any beeps? If the system case is hot (if you get a shock) pull the plug immediately - you have a ground failure and a short. If your hearing isn't good, you can always check to see if the power supply fan is creating a breeze. Monitors are powered independently, so unless you're looking at a notebook PC, a live screen doesn't indicate a working power supply.

If the power doesn't come on, the first thing to check is that you have a live power source. You don't need a DVM (Digital Volt Meter) to check if your power outlet is live. Just unplug the power supply cord and plug in a lamp or a radio. If you are using a power strip, don't assume the socket you are using is good because the other outlets are working and the power strip status light is on. Many power strips I've encountered in the field have at least one bad outlet, and working outlets have been known to fail for no particularly good reason. Power supply cords very rarely fail, but it's possible for the female connector on the power supply end to back out of the socket. Make sure that both ends of the power supply cord are fully seated in the outlet and the power supply, respectively.

Check to make sure the correct voltage (110V/ 220V) is selected on the power supply. While this should never come up with a PC that's just been sitting on the desk, if you've replaced the power supply or moved the PC, it's always a possibility. This small red slide switch is located on power supply, usually between the power cord and the on/off override switch on the back of the case. Unplug the supply and select the proper voltage for your country. If you tried to power up with the switch set to 220V in a country using 110V, the system should be OK when you correct the voltage. If you tried running on 110V in a 220V country, you've at probably blown a fuse in the supply (at the least), or damaged the supply and possibly other components.

If pressing your power switch doesn't immediately shut down the PC, that's normal for ATX systems. The action of the power switch is programmable and is controlled through CMOS Setup. The default operation for most power switches requires you to hold the switch in for three to five seconds before the system will power down. This allows use of the power switch to wake the PC from some power saving "sleep" or "stand-by" modes, depending on CMOS Setup power management settings. If the problem is that the operating system can't turn off the PC when you shut down, it's likely a bad setting in power management or a corrupted file in the operating system.

One very good reason for the power supply to fail is an unconnected power switch. The power switch lead on ATX PCs, often labeled PW or PW-ON, runs from the front panel of the case to a connector block on the motherboard. This issue should only come up if you've been working in the case (the leads can pull off the connection block very easily), or if you've replaced the motherboard. The switch isn't polarized so it doesn't matter which way it goes on the motherboard posts, but it must be on the correct two posts. The proper location is usually printed right on the motherboard next to the connector block, and you can also consult the motherboard documentation. In cases where the documentation provided with the system and the information printed on the motherboard don't agree, I go with the motherboard.

Check the switch operation with a Digital Volt Meter on the continuity or resistance setting. On all ATX systems, the power switch is really just a logic switch that tells the motherboard, to which a trickle of live power is always supplied, to instruct the power supply to come fully alive. If your power supply features a heavy cord running forward to a large switch on the case front, with four connections, you have an old AT style supply and potentially deadly live line voltage is present at the switch. These procedures do not apply to the obsolete AT power supplies. When I'm working on an ATX system and don't have a tester handy, I short the two pins on the motherboard with a screwdriver, where the logic switch from front panel should be attached, and see if the system starts. This is a "live power" test. Don't do it if you may get startled and bash the screwdriver into something should the system power up, because there's no way to repair short-circuit or gouging damage once it's done. If the switch is bad and you don't have a replacement switch, check for a reset button on the front panel. You can usually get away with using the reset button for the PW-ON logic switch, and live without the hard reset.

The power supply will to fail to operate if the power to motherboard isn't connected. Check that the 20 pin ATX power connector and any additional motherboard power connections, such as the 12V supply for P4 systems, are properly connected and seated. The latching mechanism for the standard ATX connector is counter-intuitive. You have to push in on the latch at the top to release it at the bottom, at which point the connector should pull off with almost no force required. You should hear or feel the latch click in when the connector is seated.

Remove the power leads to the drives to ensure that you aren't trying to power up into a short. The motherboard power must remain connected to activate the ATX power supply. If you have a DVM (Digital Volt Meter) and experience working around live circuitry, you can try checking the DC voltages at the connectors to see if they are live, or within 5% of the rated voltage. I'm not advising you do this live testing, as it's much easier and safer to try swapping in a new power supply. Unless you have a special testing fixture, you must leave the connectors attached while checking, which requires an exposed connector surface or a cheater lead. This is necessary because switching power supplies won't operate properly without a load, either failing to come on or even self-destructing (in extreme, low quality, instances). I just poke the DVM probes into the top of the 20 wire connector at the motherboard, since there's usually room next to the wire to get down to the conductor.

ATX Version 1.2 - 20 wire motherboard connector

Pin 1	Pin 2	Pin 3	Pin 4	Pin 5	Pin 6	Pin 7	Pin 8	Pin 9	Pin 10
3.3V	3.3V	Gnd	5V	Gnd	5V	Gnd	P_OK	5VSB	12V
Oran	Oran	Blk	Red	Blk	Red	Blk	Gray	Purp	Yell
Oran	Blue	Blk	Green	Blk	Blk	Blk	White	Red	Red
3.3V	-12V	Gnd	P_ON	Gnd	Gnd	Gnd	-5V	5V	5V
Pin 11	Pin 12	Pin 13	Pin 14	Pin 15	Pin 16	Pin 17	Pin 18	Pin 19	Pin 20

The color scheme used for the voltages in the 20 pin connector holds for the other ATX standard power supply connectors. However, brand name manufacturers often build proprietary power supplies or make up their own color coding, so I wouldn't throw out a power supply that supplies 5V where you think it should supply 3.3V. It's more likely a proprietary design than a failure.

The 5V on Pin 9 is always present when the power supply is plugged in. This connection supplies power to the various PC circuits that operate even when the PC is turned off, such as "Wake on Modem" or "Wake on LAN." It's also the reason you should never work in the PC with the power supply plugged in, unless you can remember to turn off the override switch every time. This live power is supplied to the adapter slots, so replacing adapters with the power cord plugged in may damage the motherboard or adapters. Even though the drive leads aren't powered with the system turned off, you might drop a screw while working on a drive. If that screw lands in just the wrong place, like an open bus slot, it could create a short and damage the motherboard.

Assuming your PC is connected to a monitor, the next question is, do you have a live screen? Does text or a splash screen appear? A message saying "Please connect monitor" or "No video signal detected" counts as a "No" answer in this case. If the screen is live, but you see multiple images or endless scrolling, the video adapter is providing signals that cannot be interpreted by the monitor. This usually occurs when you attach an old monitor to a new PC and the monitor doesn't support the refresh rate at the screen resolution selected in the Windows settings.

If the power supply comes on but you don't get a live screen, switch off and try again. You may have to hold the power switch in for five or more seconds before the system powers back down. If it fails to power down, you can turn off the switch on the back of the power supply, turn off your power strip, or unplug the cord. A PC that boots on the second or third try is most likely suffering from a quick power_ok (or power_good) signal, coming on before the power supply has stabilized. The presence of the power_ok signal tells the motherboard that the power supply is stable, while its absence tells the motherboard to stay off to protect itself. It's possible the power supply isn't quite up to the current ATX standard or the motherboard is a little too demanding about timing. Booting twice every time you want to turn on the PC isn't an ideal situation, so unless you leave it on all the time, look into buying a higher quality power supply, ideally one recommended by the motherboard manufacturer.

Beep codes are part of the PC's Power On Self Test (POST) routine. One beep means the system has passed the test and the BIOS believes that the CPU and memory and video are functioning properly. All other beep codes vary by BIOS supplier and system brand, but endlessly repeating slow beeps often indicate RAM failure, so shut down and try reseating the memory module(s). A repeated string of beeps, either 3 or 9 beeps long, is frequently video failure, so unplug the power and try reseating the video adapter. If you are getting beeps with a live screen, the problem is unlikely to be power supply related. Proceed to the Motherboard, CPU and RAM Failure diagnostics.

If you don't get any beeps, make sure the case speaker is connected and check for beeps again. If the motherboard lacks a connection point for a case speaker near the power switch and LED block, it probably employs an onboard piezoelectric speaker. If you have recently added any new components to the system, they may be overtaxing the power supply or causing a short circuit. This includes both adapters and drives. The first step in any failure situation is to try undoing the last change you made. I recently encountered a system which powered up but failed to initialize the video adapter when the secondary IDE ribbon cable was connected backwards to a CD drive! Although the component you just added may have worked in another system, it doesn't mean you hooked it up properly, that it's compatible with the current PC, or that it didn't fail in the interim.

Common power supply problems unrelated to the boot process are noisy operation and unstable voltages, both of which are a reason to replace the supply. There are two common noise problems associated with power supplies, noisy fans and whistling capacitors. Noisy fans can be replaced, but only if you're a reasonably competent technician because you can really get a nasty zap from the stored energy in the capacitors even when the power supply is unplugged. Make sure your noisy fan problem isn't due to something silly like a piece of paper poking in through the fan grille before you rush out and buy a replacement. If your dog won't stay in the room when the computer is turned on or if your kids hear a high pitched whistling that you don't, it's probably a capacitor. To determine whether the capacitor is in the power supply or elsewhere in the system will require a process of elimination or some parts swapping.

Unstable voltage problems are real ghosts in the machine, and can mimic all sorts of other problems. If you get into a flaky failure situation that you can't diagnose and you've already started troubleshooting (i.e. swapping parts), you may as well try a new power supply as well. I've seen power supplies produce some really bizarre failures, like a PC that reboots when you set your coffee cup down too hard on the table. The most pervasive of the unstable power supply problems are random lockups or spontaneous reboots. Modern motherboards have some ability to regulate the power they receive, but it's got to be within a reasonable range. When it starts overshooting the limits, the system may freeze or shutdown the motherboard to protect itself.

As soon as the PC powers up, you should be able to hear the hard drive motor spin the drive (like a very, very, quiet jet taking off) and the read/write head seeking (a gentle clunking sound). If you're absolutely baffled as to whether or not the drive is spinning up, due to background noise or hearing problems, you can resort to feeling the drive cover. If that still doesn't do it, I power down, remove the drive cage or the drive itself, and hold it firmly by the edges (not touching any exposed wires or the circuit board on the bottom) while powering up. The drive resists twisting movements like a gyroscope if it's spun up. Don't play with it. If you move too fast or touch the circuit board to something that can cause a short, you'll damage the drive. Just power down, reinstall, and continue with the diagnostics.

If system power is coming on but the drive still isn't spinning up, make sure that your power lead is seated in the drive power socket. It doesn't need to go all the way to the ridge stop on the connector, but it should go in a good half inch or so. It does take a good deal of force to seat the cheaper leads in some drives. Try another lead, even if you have to disconnect another drive to get it. Try another drive. At this point it's still quite possible that the power supply is defective, but if you have a drive that you know spins up, it's a good way to eliminate one possibility. As long as you don't smell smoke coming out of the drive you can test the drive in another system. If you are using SCSI rather than IDE hard drives, check the documentation for a jumper that suppresses spin up on boot. SCSI drives offer this option because you can install as many as 15 in a single system, and if they all tried to spin up at once it would swamp any power supply. Normally, the SCSI host adapter will spin them up in order of their SCSI ID.

If system power isn't coming on, disconnect all drives, one at a time, and try powering up after each change. If the system powers up, you've found a faulty drive or a faulty lead from the power supply. If the system won't power up with all drives disconnected, start removing adapters, one at a time, leaving the video for last. Unplug power cord before removing each adapter, then reconnect to power up. If the system powers up, replace all adapters except the last one removed before power came on. If power still comes on, try the last adapter you removed in different slot before giving up on it..

If you find an adapter that actually prevents the system from powering up, it must be replaced. If this is your video adapter and it is an AGP type, for which the motherboard only has one slot, it could be that the AGP slot is faulty. Another possibility is that the adapter is keyed as universal but is installed on a new motherboard that expects low voltage AGP adapters (AGP 4X or 8X).

Once you've eliminated the drives and the adapters, one of the few remaining possibilities is a motherboard short. Remove the motherboard and check for a standoff or screw located in the wrong place or rolling around loose. I often build out systems on the bench without a case, supporting the motherboard on a static proof bag over a cardboard box or some similar arrangement to give the adapters room to seat. This method eliminates any case mounting issues from the diagnostics process, but it introduces all sorts of risks, not the least of which is absence of the case ground.

Normally, a short circuit will result in a burnt smell and a ruined motherboard, sometimes damaging any of the attached components (memory, CPU, adapters) as well. In many instances, you'll be able to figure out which component is ruined by the presence of burn marks or a strong odor of smoke coming from the component, though if it happens in a closed case, the smoky smell can stick to everything. If you can't locate a failed component by visual inspection, you need to have access to a test-bed system (an inexpensive but completely functioning PC for testing questionable parts). Don't test parts that may be fried in a good system, because some types of failures will cause damage to the next machine.