Troubleshooting the power supply basically means isolating the supply as the cause of problems within a system and, if necessary, replacing it. Many symptoms lead to suspect that the power supply in a system is failing. This can sometimes be difficult for an inexperienced technician to see because at times little connection seems to exist between the symptom and the cause—the power supply.

For example, in many cases a parity check error message can indicate a problem with the power supply. This might seem strange because the parity check message specifically refers to memory that has failed. The connection is that the power supply powers the memory, and memory with inadequate power fails.

It takes some experience to know when this type of failure is power related and not caused by the memory. One clue is the repeatability of the problem. If the parity check message (or other problem) appears frequently and identifies the same memory location each time, I would suspect that defective memory is the problem.

However, if the problem seems random, or if the memory location the error message cites as having failed seems random, I would suspect improper power as the culprit. The following is a list of PC problems that often are related to the power supply:

• Any power-on or system startup failures or lockups

• Spontaneous rebooting or intermittent lockups during normal operation

• Intermittent parity check or other memory-type errors

• Hard disk and fan simultaneously failing to spin (no +12V)

• Overheating due to fan failure

• Small brownouts that cause the system to reset

• Electric shocks felt on the system case or connectors

• Slight static discharges that disrupt system operation

• Erratic recognition of bus-powered USB peripherals

In fact, just about any intermittent system problem can be caused by the power supply. I always suspect the supply when flaky system operation is a symptom. Of course, the following fairly obvious symptoms point right to the power supply as a possible cause:

• System that is completely dead (no fan, no cursor)

• Smoke

• Blown circuit breakers

If you suspect a power supply problem, some of the simple measurements and the more sophisticated tests outlined in this section can help you determine whether the power supply is at fault. Because these measurements might not detect some intermittent failures, you might have to use a spare power supply for a long-term evaluation.

If the symptoms and problems disappear when a known good spare unit is installed, you have found the source of your problem. Following is a simple flowchart to help you zero in on common power supply–related problems:

1. Check the AC power input. Make sure the cord is firmly seated in the wall socket and in the power supply socket. Try a different cord.

2. Check the DC power connections. Make sure the motherboard and disk drive power connectors are firmly seated and making good contact. Check for loose screws.

3. Check the DC power output. Use a digital multimeter to check for proper voltages. If it's below spec, replace the power supply.

4. Check the installed peripherals. Remove all boards and drives and retest the system. If it works, add items back in one at a time until the system fails again. The last item added before the failure returns is likely defective.

Many types of symptoms can indicate problems with the power supply. Because the power supply literally powers everything else in the system, everything from disk drive problems to memory problems to motherboard problems can often be traced back to the power supply as the root cause.

Overloaded Power Supplies

A weak or inadequate power supply can put a damper on your ideas for system expansion. Some systems are designed with beefy power supplies, as if to anticipate a great deal of system add-ons and expansion components. Most desktop or tower systems are built in this manner.

Some systems have inadequate power supplies from the start, however, and can't adequately service the power-hungry options you might want to add. The wattage rating can sometimes be very misleading. Not all 300-watt supplies are created the same.

People familiar with high-end audio systems know that some watts are better than others. This is true for power supplies, too. Cheap power supplies might in fact put out the rated power, but what about noise and distortion? Some of the supplies are under-engineered to just barely meet their specifications, whereas others might greatly exceed their specifications.

Many of the cheaper supplies provide noisy or unstable power, which can cause numerous problems with the system. Another problem with under-engineered power supplies is that they can run hot and force the system to do so as well. The repeated heating and cooling of solid-state components eventually causes a computer system to fail, and engineering principles dictate that the hotter a PC's temperature, the shorter its life.

Many people recommend replacing the original supply in a system with a heavier-duty model, which solves the problem. Because power supplies come in common form factors, finding a heavy-duty replacement for most systems is easy, as is the installation process.