Troubleshoot Power Supplies

Troubleshoot Power Supplies
In the last topic, you identified and corrected problems with storage devices. Another personal computer component that is required for the system to operate is the power supply. In this topic, you will troubleshoot power supplies.
Underpowered systems, especially older systems with relatively small power supplies, can experience lockups, random reboots, and other quirky behavior. As an A+ technician, you will be required to test and troubleshoot power supplies.
Common Power Problems
Power problems can result in data loss, erratic behavior, system crashes, and hardware damage, and the more severe the problem, the more severe the consequences.

Power Problem
Line noise
Power sag
Power undervoltage or brownout

Possible Causes
EMI interference.
RFI interference. Lightning.
Defective power supply.
Many electrical systems starting up at once. Switching loads at the electric company utility. Electric company equipment failure. Inadequate power source.
This symptom can last from several minutes to several days and can be caused by any of the fol- lowing:

Power Problem
Overvoltage
Power failure

Possible Causes
Suddenly reduced loads.
Equipment with heavy power consumption is turned off.
Power company switches loads between equipment.
Lightning strikes.
Electrical power lines down. Overload of electrical power needs.

Common Power Supply Issues
Power supply damage from overheating, lightning strikes, or short circuits can produce a num- ber of symptoms.

Symptom
Fan will not work.

Possible Cause and Solution
The fan and openings around the power supply bring in air to cool sys- tem components, but they also allow dirt and dust to gather around the power supply. This can cause the fan bearings to wear and the fan to turn more slowly. You can use compressed air to remove this debris from the system. If the fan becomes damaged due to dust, replace the power sup- ply or have qualified personnel replace the fan.

Symptom
Computer won’t start or reboots after startup.

Possible Cause and Solution

• ●  If the computer doesn’t start at all, make sure that there is power to the outlet. You can do so by plugging in a lamp or other device that you know works. If that doesn’t turn on, you know that you have a bad outlet and not necessarily a bad power supply.
  
• ●  Check that the connections from the power supply to the system board are secure, especially on ATX systems. Make sure the master switch to the power supply, at the rear of the system, is on, before pressing the computer’s power button. Also on ATX systems, check the voltage of the power being supplied.
  
• ●  A loose power supply rail landing on exposed metal can short-circuit the power supply. The power supply can detect this problem and dis- able itself. If you fix the short (by putting the power cable onto the drive correctly), the power supply should start working again. Unused rails should be either covered (some bring rubber end caps) or tie- wrapped to a safe location (not too tight to avoid damaging the wire). Also check for loose screws or foreign metallic objects that can cause shorts.
  
• ●  Check power supply rail voltages with a digital multimeter, to verify that the necessary voltages are being provided to the board. This will not measure voltage under load, but will allow you to determine whether the rails are working properly. Most motherboards also pro- vide a voltage reading within the BIOS. If the system boots, access this BIOS option to obtain readings as detected by the motherboard.
  Other components, especially drives, can also sometimes make a lot of noise. Make sure this isn’t where the noise is coming from.
  A whine or squeal from the power supply area is usually from the fan. A damaged fan with worn bearings will cause a grinding whine that wors- ens with time. Sometimes, when the bearings begin to fail, the fan blade assembly will shift, rubbing against the fan grill or the case, and produce a high-pitched noise. Also possible, after cleaning with compressed air, a wire inside the power supply unit was shifted by the forced air and is now touching the fan, causing the very loud grinding noise, possibly stopping the fan altogether. With the power supply off, you can attempt to carefully shift the wire away from the fan by using a plastic tool (metal not recommended so as to avoid damaging any components).
  If the noise is not from the fan, but from another power supply compo- nent, replace the power supply or take it out and send it for service.
  

Noise coming from power supply.

How to Troubleshoot Power Supplies
Procedure Reference: Test a Power Supply With a Multimeter
To use a multimeter to test a power supply for proper output voltage:

1. Prepare the computer.
   
   1. Shut down the computer.
      
   2. Disconnect all cables externally connected to the chassis.
      
   3. Remove the computer enclosure (cover).
      
   4. Locate a spare Molex connector, and remove it from the bundle if necessary so that you can easily access it without having to reach inside the case to work with the connector.
      
   5. Reconnect the power cable.
      
   6. Power on your computer.
      
      1. Measure the 5 volt output from the power supply.
         
         1. Set your multimeter to measure DC voltage with a scale that will permit readings in
            the +5 volt range.
            
         2. Insert the multimeter’s black probe into the black (GND) lead of the power connector.
            
         3. Insert the multimeter’s red probe into the red (+5) lead of the power connector.
            
         4. Examine the voltage measured by the multimeter. It should be approximately +5 volts.
            
      2. Measure the 12 volt output from the power supply.
         
         1. If necessary, set your multimeter to measure DC voltage with a scale that will permit
            readings in the +12 volt range.
            
         2. Insert the multimeter’s black probe into the black (GND) lead of the power connector.
            
         3. Insert the multimeter’s red probe into the yellow (+12) lead of the power connector.
            
         4. Examine the voltage measured by the multimeter. It should be approximately +12 volts.
            
            4. Restore the PC to service.
            

            1. Shut down the computer.
               
            2. Disconnect the power cable.
               
            3. Rebundle wires and return the Molex connector to the location from which you got it.
               
            4. Replace the computer enclosure (cover).
               
            5. Connect all cables externally connected to the chassis.
               
            6. Power on your computer.
               

            Power Supply Wire Color Conventions
            System components cannot use the 120-volt power coming directly from the electrical outlet. The power supply steps the voltages down to 3.3-, 5-, and 12-volt connections for system components. Wires are color-coded as to their voltages. The following table shows the wire color for each voltage connection.
            Color or Component Voltage
            Yellow wire +12 Blue wire -12 Red wire +5 White wire -5 Motor +/-12 Circuitry +/-5
            Power Supply Testers
            Although you can use multimeters to test power supplies, there are also specialized tools called power supply testers that are simpler to use for this purpose, and that perform more compre- hensive tests. For instance, you can test the various power connectors (Berg, Molex, AT, and ATX). You can also use them to test the power supply under load.
            Procedure Reference: Troubleshoot Power Problems
            To troubleshoot power problems:
            1. If the fan does not appear to be working and the system doesn’t come on or abruptly shuts itself down, address the problem as soon as possible. Leaving the problem alone would allow heat to build up to dangerous levels, causing serious damage to the system, and possibly fire.
            

            1. Unplug the system and remove the system cover.
               
            2. Using compressed air, blow out any dust around the fan spindle.
               
            3. Verify that there is no obvious reason that the fan is not spinning.
               
            4. If these suggestions do not fix the problem, replace the power supply. Remember, do not open a power supply as there is a high danger of electrocution.