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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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..
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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).
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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.
If you've reached this
point without getting the system to power up, you probably have a defective
power supply or motherboard. Try replacing the power supply first since they're
cheaper than motherboards. Repairing power supplies requires a good knowledge
of electronics as there are usually "no user serviceable parts." Even
when power supplies are unplugged, they can give nasty zap from stored power in
the electrolytic capacitors. If the power supply or motherboard is new, they
may be incompatible with one another due to poor adherence to ATX standards or
support for different generations of the ATX standard.
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