PC Hardware in a Nutshell, 3nd Edition

26.6 Troubleshooting Power Supplies

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

Parity check errors

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.

Sporadic or regular boot failures

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.

Spontaneous reboots or system lockups during routine operations, not attributable to running a particular program

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.

Lockups after installing a new processor, memory, disk drive, or expansion card

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 significantly—e.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.

Failure to function with a Wake-on-LAN (WOL) motherboard

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.

Slow disk performance

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 subtle—occasional memory errors, lockups, slow disk performance, and so on—and 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:

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 warranty—good units often have three- to five-year warranties—call 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.