PC Repair and Maintenance: In-depth Look at Power Supply

• By Scott Mueller
• Feb 28, 2003
• Sample Chapter is provided courtesy of Que

Power Supply Troubleshooting

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 me 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 cause the system to reset.

• Electric shocks felt on the system case or connectors.

• Slight static discharges disrupt system operation.

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

4. Check installed peripherals. Remove all boards and drives and retest the system. If it works, add back in items 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 cannot 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 goes 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.

Inadequate Cooling

Some of the available replacement power supplies have higher-capacity cooling fans than the originals, which can greatly prolong system life and minimize overheating problems—especially for the newer, hotter-running processors. If system noise is a problem, models with special fans can run more quietly than the standard models. These power supplies often use larger-diameter fans that spin more slowly, so they run more quietly but move the same amount of air as the smaller fans. PC Power and Cooling specializes in heavy-duty and quiet supplies; Astec has several heavy-duty models as well.

Ventilation in a system is also important. You must ensure adequate airflow to cool the hotter items in the system. Many processors today use passive heatsinks that require a steady stream of air to cool the chip. If the processor heatsink has its own fan, this is not much of a concern. If you have free expansion slots, you should space out the boards in your system to permit airflow between them. Place the hottest running boards nearest the fan or the ventilation holes in the system. Make sure that adequate airflow exists around the hard disk drive, especially for those that spin at high rates of speed. Some hard disks can generate quite a bit of heat during operation. If the hard disks overheat, data can be lost.

Always be sure to run your computer with the case cover on, especially if you have a loaded system. Removing the cover can actually cause a system to overheat. With the cover off, the power supply fan no longer draws air through the system. Instead, the fan ends up cooling the supply only, and the rest of the system must be cooled by simple convection. Although most systems do not immediately overheat for this reason, several of my own systems, especially those that are fully expanded, have overheated within 15–30 minutes when run with the case cover off.

In addition, be sure that any empty slot positions have the filler brackets installed. If you leave these brackets off after removing a card, the resultant hole in the case disrupts the internal airflow and can cause higher internal temperatures.

If you experience intermittent problems that you suspect are related to overheating, a higher-capacity replacement power supply is usually the best cure. Specially designed supplies with additional cooling fan capacity also can help. At least one company sells a device called a fan card, but I am not convinced these are a good idea. Unless the fan is positioned to draw air to or from the area outside the case, all it does is blow hot air around inside the system and provide a spot cooling effect for anything it is blowing on. In fact, adding fans in this manner contributes to the overall heat inside the system because the fan consumes power and generates heat.

CPU-mounted fans are an exception because they are designed only for spot cooling of the CPU. Many newer processors run so much hotter than the other components in the system that a conventional, finned aluminum heatsink can't do the job. In this case, a small fan placed directly over the processor provides a spot cooling effect that keeps the processor temperatures down. One drawback to these active processor cooling fans is that the processor overheats instantly and can be damaged if the fans fail. Whenever possible, try to use the biggest passive (finned aluminum) heatsink you can find and purchase a CPU fan from a reputable vendor.

Using Digital Multimeters

One simple test you can perform on a power supply is to check the output voltages. This shows whether a power supply is operating correctly and whether the output voltages are within the correct tolerance range. Note that you must measure all voltages with the power supply connected to a proper load, which usually means testing while the power supply is still installed in the system and connected to the motherboard and peripheral devices.

Selecting a Meter

You need a simple digital multimeter (DMM) or digital volt-ohm meter (DVOM) to perform voltage and resistance checks on electronic circuits (see Figure 3.17). You should use only a DMM instead of the older needle-type multimeters because the older meters work by injecting 9v into the circuit when measuring resistance, which damages most computer circuits.

Figure 3.17 A typical DMM.

A DMM uses a much smaller voltage (usually 1.5v) when making resistance measurements, which is safe for electronic equipment. You can get a good DMM with many features from several sources. I prefer the small, pocket-size meters for computer work because they are easy to carry around.

Some features to look for in a good DMM are as follows:

• Pocket size. This is self-explanatory, but small meters are available that have many, if not all, of the features of larger ones. The elaborate features found on some of the larger meters are not really necessary for computer work.

• Overload protection. This means that if you plug the meter into a voltage or current beyond the meter's capability to measure, the meter protects itself from damage. Cheaper meters lack this protection and can easily be damaged by reading current or voltage values that are too high.

• Autoranging. This means that the meter automatically selects the proper voltage or resistance range when making measurements. This is preferable to the manual range selection; however, really good meters offer both autoranging capability and a manual range override.

• Detachable probe leads. The leads easily can be damaged, and sometimes a variety of differently shaped probes are required for different tests. Cheaper meters have the leads permanently attached, which means you cannot easily replace them. Look for a meter with detachable leads that plug into the meter.

• Audible continuity test. Although you can use the ohm scale for testing continuity (0ohms indicates continuity), a continuity test function causes the meter to produce a beep noise when continuity exists between the meter test leads. By using the sound, you quickly can test cable assemblies and other items for continuity. After you use this feature, you will never want to use the ohms display for this purpose again.

• Automatic power off. These meters run on batteries, and the batteries can easily be worn down if the meter is accidentally left on. Good meters have an automatic shutoff that turns off the unit when it senses no readings for a predetermined period of time.

• Automatic display hold. This feature enables you to hold the last stable reading on the display even after the reading is taken. This is especially useful if you are trying to work in a difficult-to-reach area single-handedly.

• Minimum and maximum trap. This feature enables the meter to trap the lowest and highest readings in memory and hold them for later display, which is especially useful if you have readings that are fluctuating too quickly to see on the display.

Although you can get a basic pocket DMM for as little as $20, one with all these features is priced in the $100–$200 range. Radio Shack carries some nice inexpensive units, and you can purchase the high-end models from electronics supply houses, such as Newark or Digi-Key.

Measuring Voltage

To measure voltages on a system that is operating, you must use a technique called back probing on the connectors (see Figure 3.18). You cannot disconnect any of the connectors while the system is running, so you must measure with everything connected. Nearly all the connectors you need to probe have openings in the back where the wires enter the connector. The meter probes are narrow enough to fit into the connector alongside the wire and make contact with the metal terminal inside. The technique is called back probing because you are probing the connector from the back. You must use this back-probing technique to perform virtually all the following measurements.

Figure 3.18 Back probing the power supply connectors.

To test a power supply for proper output, check the voltage at the Power_Good pin (P8-1 on AT, Baby-AT, and LPX supplies; pin 8 on the ATX-type connector) for +3v to +6v of power. If the measurement is not within this range, the system never sees the Power_Good signal and therefore does not start or run properly. In most cases, the power supply is bad and must be replaced.

Continue by measuring the voltage ranges of the pins on the motherboard and drive power connectors. If you are measuring voltages for testing purposes, any reading within 10% of the specified voltage is considered acceptable, although most manufacturers of high-quality power supplies specify a tighter 5% tolerance. For ATX power supplies, the specification requires that voltages must be within 5% of the rating, except for the 3.3v current, which must be within 4%. The following table shows the voltage ranges within these tolerances.

The Power_Good signal has tolerances that are different from the other voltages, although it is nominally +5v in most systems. The trigger point for Power_Good is about +2.4v, but most systems require the signal voltage to be within the tolerances listed here:

Replace the power supply if the voltages you measure are out of these ranges. Again, it is worth noting that any and all power supply tests and measurements must be made with the power supply properly loaded, which usually means it must be installed in a system and the system must be running.

Specialized Test Equipment

You can use several types of specialized test gear to test power supplies more effectively. Because the power supply is one of the most failure-prone items in PCs today, you should have these specialized items if you service many PC systems.

Digital Infrared Thermometer

One of the greatest additions to my toolbox is a digital infrared thermometer. They also are called noncontact thermometers because they measure by sensing infrared energy without having to touch the item they are reading. This enables me to make instant spot checks of the temperature of a chip, a board, or the system chassis. They are available from companies such as Raytek (http://www.raytek.com) for under $100. To use these handheld items, you point at an object and then pull the trigger. Within seconds, the display shows a temperature readout. These devices are invaluable in checking to ensure your system is adequately cooled.