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 your 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 crating 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.
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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.
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.
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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.
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).
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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.
Thank you,
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