PC Hardware in a Nutshell, 3rd Edition [Electronic resources] نسخه متنی

26.6 Troubleshooting Power Supplies

Suspect
a power supply problem if you experience any of the following
symptoms, particularly in combination:

Such errors may be caused by defective or poorly seated memory or by
overheating, but insufficient or poorly regulated +3.3VDC or +5VDC
(depending on memory type) from a failing or inadequate power supply
is a likely cause.

Obviously, such errors may instead be caused by hard disk, cable, or
disk controller problems, but inadequate or poorly regulated +12VDC
(less commonly, +5VDC) is also a common cause of this problem.

Numerous other factors can cause this problem, but one common cause
is insufficient or poorly regulated +3.3VDC and/or +5VDC being
provided to the memory and/or processor.

Driver issues and resource conflicts aside, this problem commonly
occurs when new components overload a marginal power supply. This is
particularly likely to occur if you make dramatic changes to the
system, such as replacing a slow CPU with a fast, high-current CPU;
if you expand memory significantlye.g., from 128 MB to 512 MB;
if you add a high-current expansion card such as a fast AGP video
card or internal modem; or if you add a high-current drive such as a
high-performance SCSI hard disk or a CD burner to the system. Note
that the power supplies provided with commercial systems,
particularly inexpensive ones, often have very little reserve.

The motherboard and power supply may both be operating properly but
be incompatible. Many early ATX power supplies (and some current
models) provide 100 mA or less of +5V_SB.
Although that output met the ATX 2.01 requirements, WOL motherboards
require +5V_SB of at least 720 mA.

Although this may seem an odd symptom to be related to power supply
problems, inadequate voltage and current can cause disk retries on
both reads and writes. The error correction circuitry built into hard
disks and controllers means that this problem often (usually) goes
undiagnosed. People often say to us something like,
"I replaced the power supply as you suggested, and
now my hard disk seems a lot faster. Is that
possible?" Yes, it is.

A very common source of problems is using a noncompliant ATX-like power supply. We say "ATX-like" because many power supplies that fit ATX cases are not ATX-compliant. Motherboards vary in their tolerance for voltages that are slightly out of spec, and a marginal power supply that works fine with one motherboard may not work with another, even of the same model.

Troubleshooting power supplies is difficult for several
reasons:

Other than an outright
power supply failure, problems caused by an inadequate or failing
power supply are likely to be subtleoccasional memory errors,
lockups, slow disk performance, and so onand easily
attributable to something other than the power supply.

It is difficult to
"bench test" a suspect power supply
because PCs use switching power supplies rather than linear power
supplies. Unlike linear power supplies, switching power supplies by
design do not operate unless minimum loads exist on specific voltage
rails. The minimum load required varies from model to model, but in
practical terms you must connect at least a motherboard, processor,
and hard drive to most power supplies before they function at all.

Systems draw varying amounts of
current at different voltages during routine operations. For example,
a drive spinning up draws down +12VDC, which in turn may cause
another voltage rail to fluctuate, causing problems that are not
clearly linked to the +12VDC load. Even processors use varying
amounts of current, depending on what they happen to be doing at the
moment. This constant variation in draw and the interdependency of
currents on different voltage rails make troubleshooting very
difficult.

Working inside a power
supply is dangerous because high voltages are present, but testing
only external connectors makes it difficult to troubleshoot
effectively. Despite this,

we do not recommend removing the
cover from a power supply for any purpose . If you do so
and electrocute yourself, don't blame us.

With the high cost of labor, it is
usually more cost-effective in a business environment simply to swap
out a suspect power supply for a new or known-good unit, particularly
if the suspect unit is old and/or was inexpensive to start with.
Paying for an hour or two of technician time makes little sense when
the alternative is installing a new $50 power supply.

If you're working on your own system, however,
and if you have a DMM, you can do a few quick tests that may isolate
the problem to the power supply. These steps involve testing voltages
on specific wires of the main power supply connector while it is
connected to the motherboard. Some connectors have built-in probe
contact points that provide easy access to each signal. If yours does
not, slide the probe down inside the body of the connector until it
contacts the crimp-on connector to which the wire is secured.
Ideally, use a DMM that permits logging maximum and minimum voltages
over a period of time while you use the system. If
you're using an inexpensive DMM,
you'll have to settle for instantaneous readings,
but those often suffice.

To test your power supply,
have a list of pinouts and signals for your power supply type (AT or
ATX) handy, and take the following steps:

With the black probe touching the power
supply case, touch the red probe in turn to each Ground/Common
(black) wire on the main power supply connector, on any subsidiary
connectors, and on the Peripheral Connectors and Floppy Drive
Connectors. The DMM should show 0.00V. Significant voltage present on
any ground wire indicates a serious problem in the power supply.

If the system is completely dead
when it is plugged in and turned on, the power supply may not be
asserting Power Good. Even if the system runs, check the Power Good
voltage because voltage variations on Power Good commonly cause
subtle system problems. With the black probe of your DMM touching the
power supply case or other grounding point, touch the red probe to
the Power Good line. Power Good is nominally +5VDC. The DMM should
indicate between +4.0V and +6.0V. Most motherboards trigger at from
+2.0V to +2.5V, so a reading below +4.0V may allow the motherboard to
boot, but indicates a possible power supply problem. If the DMM
indicates less than +3.0V or more than +6.0V, replace the power
supply. If no voltage is present, the power supply is not asserting
Power Good, and is likely defective.

Test each voltage rail against ground to verify that they are
within specifications. If a particular voltage appears on multiple
pins, test that voltage at each pin.

For each Peripheral Connector and Floppy Drive Connector, test
each of the two voltages present against ground. That is, touch the
red probe to +12V (yellow wire), and then touch the black probe to
the adjacent ground pin (black wire). The DMM should read +12V within
tolerance. Then touch the red probe to +5V (red wire) and the black
probe to the adjacent ground pin. The DMM should read +5V within
tolerance. Finally, touch each probe to one of the ground pins. The
DMM should read 0.00V or something very near it.

If any of these tests fails, a
defective or overloaded power supply is the most likely cause. In
that event, replacing the power supply is usually the best choice. We
have never attempted to repair a power supply ourselves, and do not
recommend doing so. If the power supply is under warrantygood
units often have three- to five-year warrantiescall the vendor
for an RMA number and ask if they are willing to cross-ship a
replacement unit. If the power supply is not under warranty but is an
expensive (high-wattage or redundant) and relatively new unit,
contact the vendor about having it repaired. Some vendors quote a
fixed price, while others charge time and materials. Be wary of the
latter.