PC Power Supply Troubleshooting

Technical Manual
In this module, your will learn how computer power supplies function and how to repair them when they dont. Youll also conduct a couple experiments to exercise your troubleshooting skills. While measuring voltages, youll collect data and compare your results with what are considered benchmark values. Safety is of key concern. This module is one of few that will expose you to lethal voltages if you dont follow directions carefully. You must adhere to all safety rules and ask for your instructor to inspect your setup when directed to do so in the exercise. In this module, your goal is to learn how computer power supplies operate, recognize when theyre not operating correctly, and repair them when theyre malfunctioning.

POWER SUPPLY FUNDAMENTALS

All computers require reliable electrical power to operate. However, PCs do not operate directly off of the electrical power found at a wall outlet. The alternating current (AC) available in homes, schools, and offices must be converted to direct current (DC) and then stepped down to a voltage that computers can safely use. If electricity is cut off or the power supply fails the PC will not operate. There is also a chance of component failures due to sudden power outages, brownouts, or other types of intermittent power supply problems. As already mentioned, the purpose of the power supply is to convert AC line power to the DC voltages computers use to operate the various circuits in a PC. Line power voltage and frequency vary in different countries. For instance the USA, Canada, and Japan use 115Volt / 60Hertz power where as Germany, Italy, and England use 220 Volt / 50 Hertz power. Therefore, the PC power supply has to be able to switch between different line voltages for companies that market their products worldwide. This is not necessarily true for other appliances you might find around your home, school, or office. Devices like vacuum cleaners, electric can openers, or even computer printers have power supplies that are hard wired for a specific operating voltage and frequency. Because of the expense involved with designing a universal power supply, theyre manufactured to meet the voltage requirements found in the country that the appliance will be sold in. Since the PC is so pervasive in the worlds economy, manufactures have designed most computers to accept power from these different power sources either automatically or with a simply flip of a switch. Making one power supply to fit all simply makes good business sense for personal computers. Once the power is converted from the AC line voltage to the low level DC voltage outputs, theres little danger of electrical shock from outside the power supply. The line voltage from the wall outlet and many connection points inside the power supply are dangerously high and can cause injury or death. For this reason, only certified technicians are permitted to open and service computer power supplies. Ultimately, the power supply is considered to be a field replaceable module (FRM). This point cannot be over stated It is essential that you understand that there are dangerously high voltages inside the PC power supply even when it is disconnected. Capacitors contained in the power supply can hold a charge for months after the power supply has been unplugged. Unless you are qualified to work inside of the unit, do not open the power supply for any reason.
PC Power Supply Troubleshooting Revised 8/19/2002 Frank C. Pendzich 28 April, 2002

Figure 1. AC Power Connections and Controls

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Power Ratings
PC power supplies are rated by their power output or Watts. The very first IBM compatible computer, the IBM XT, had a power supply rating of 65 watts. As personal computers started getting more powerful(i.e. more memory and more processor capabilities) XTs and ATs required power supplies that could pump out at least 130 watts. Todays PC power supplies have to operate an incredible amount of electronic circuitry and now range from 200 to 400 watts of electrical power. The formula for electrical power (Measured in Watts) is:

Power = Current Voltage

or

P = I E

Algebraic manipulation of the formula can reveal the maximum amount of current a PC actually uses (or I measured in Amps).

Current = Power

Voltage

or

I =P

In the United States a fully loaded state-of-the-art PC could use nearly 3.5 Amps of current.

3.48 Amps = 400Watts

115Volts

However, in Germany the same computer would use about half of that.

1.81Amps = 400Watts

220Volts

This power formula is very useful to determine how many computer systems you can place on a circuit for a classroom or office. In the United States you could safely install four computers on a 16 Amp circuit. In Germany, you could install eight of the same computer systems before you run the risk of blowing a circuit breaker. USA Germany

4.60 Systems = 16 Amps 8.84 Systems = 16 Amps

3.48 Amps 1.81Amps

Regarding power supplies Its important that the PC power supply has a high enough power rating to accommodate the components that are used in the system. Hardware upgrades and new components usually increase the need for power. Therefore, the power supply rating is an important consideration when deciding to upgrade or change the hardware in a PC. Its also important to know the amount of current a PC draws so it wont overload the wiring in a school, home, or office.

Keeping Things Cool

Overheating is the primary cause to electronic equipment failure and heat is always generated when electrical power is being consumed. Common sense dictates that getting rid of heat is not only a good idea, but essential to prolonging the life of a personal computer. The PC power supply is cooled by a fan thats mounted in the power supply housing. The fan moves air over the electronic components inside the power supply and carries away heat. As a power supply fan becomes older, the bearings wear causing them to become noisy. Quieter replacement fans are available but should only be replaced by a technician qualified to work inside the power supply. Several manufacturers produce an auxiliary temperature-regulated fan that can be installed into the PC case as an upgrade instead. Preventive maintenance, such as removing dust and other debris from the fan can improve airflow and therefore reduce heat inside the power supply case. The technician can remove any PC Power Supply Troubleshooting Page 2 of 7
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dust buildup with a vacuum cleaner or some compressed air. AC power plugs into the power supply using a standard International Electrical Connector (IEC) type 320 plug/receptacle on the rear panel of the power supply case. Figure 1 - Shows the AC power connector, main power switch, and voltage selector switch wiring.

Power Supply Outputs

The PC power supply voltage output schematic in Figure 2 shows the distribution of DC voltage to the motherboard and drives. As shown, the power supplys output voltages are +5, -5, +3.3, +12, and -12 VDC. All of these output voltages run to the motherboard. The +5 and +12 VDC outputs also power each of the drives using either dedicated or daisy-chained power cable.

Figure 2. Power Supply Output Diagram

The Concept of Ground

Notice that the AC line voltage input, as well as every device in the diagram are connected to ground (Gnd). Ground is an electrical concept sometimes difficult to grasp. The concept of ground inside the computer has an additional feature of being a common reference point for the power supply. This is important to understand. Its a voltage reference point thats also considered to be 0 volts. Ground (common) is usually the chassis of an electrical component such as the power supply or disk drive and serves as a return path for both power circuits and signal sources. If you look around your house, school, or office youll notice that just about every appliance with a metal case has a three-prong plug attached to it. Two of the prongs are Line (Hot) and Neutral. This is where the appliance gets its power. The third prong is a safety feature called the ground and appliances that feature this kind of electrical plug are referred to as grounded appliances. Fortunately, computers are grounded appliances. The idea behind grounding is to protect people from electric shock. Its the metal casing thats connected directly to the ground prong on an AC power plug and its the casing thats common to all devices in a computer. From a safety perspective Let's say that a wire carrying line (hot) voltage comes loose inside an ungrounded computer and touches the metal case. If the loose wire is hot then the metal case is now hot and anyone who touches it will get electrocuted. With the case grounded, the electricity from the hot wire flows straight to ground and blows the circuit breaker in the rooms fuse box. The appliance won't work but it won't kill anyone either. Confused? Lets look at the everyday automobile as an example. Cars are typically made out of metal and we all know that metal conducts electricity. Cars operate using a battery and the battery of an automobile has two terminals, positive (+) and negative (-). An electrical PC Power Supply Troubleshooting Page 3 of 7
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device on a car like a lamp or electric motor only works when electrons flow through it and of course we know that electrons travel from negative to positive. Now If we run a wire from the negative terminal of the battery one connector of the lamp and then run a wire from the positive terminal of the battery to the other electrical connector of the lampThe lamp will glow. Thats because electrons are streaming from the negative terminal of the battery, through the lamp, and then to the positive terminal of the battery. Heres the part that explains the concept of ground Since the car is made of metal we can connect the negative terminal of the battery directly to the metal chassis of the car and use the chassis as an electrical conductor or wire. To illuminate our cars tail lamps all we have to do is attach one of the lamps electrical connectors to the metal on the rear fender and then run only one wire to the remain connector on the lamp from a switch. Since the metal on the fender is bolted to the chassis, one of the lamps electrical connectors will in fact be connected to the negative terminal of the battery though the chassis. The point here is that the metal on the car is grounded to the negative terminal of the battery and is common to every device in a car that uses voltage from the cars electrical system. The same is true for a computer. The case is grounded to the power supply and is essentially 0 volts in relation to the other power supply outputs. When a technician measures +5 VDC at the output of the power supply theyre actually measuring between Ground (0 Volts) and +5 Volts. Since voltmeters measure the difference in voltage between to points, the voltage displayed on the meter face will be +5 VDC.

How PC Power Supplies Work

Below is a simplified diagram of a switching power supply (also called a switch-mode power supply) for a PC. The power supply is connected to either 115/230 VAC through the plug P1.

Figure 3. Switching Power Supply Diagram. NOTE: Only some computer power supplies feature an extra AC power connector. Closing the main power switch connects AC power to this auxiliary connector. Located on the rear panel of the power supply, the computer monitor can be plugged into this auxiliary receptacle. Its important to notice that this fuse protects only the power supply and not anything connected to the auxiliary PC Power Supply Troubleshooting Page 4 of 7
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AC receptacle. When the main power switch (S1) is closed, AC power is connected to the auxiliary AC power socket (P2) and to the Front-End Supply through the fuse (Fl). The Front-End Supply provides DC power to a Switching Regulator Control Circuit once its turned on by the PS ON signal from the motherboard. Its also responsible for selecting the right operating voltage using the Voltage Selector Switch (S2). As current begins to flow into the switching regulator, a high frequency DC pulse train is produced and is sent to the Switching Transformer (T1). Rectifier diodes (D1D8) at the output of the switching transformer convert the AC output of the transformer to the +5 VDC, +3.3 VDC, +12 VDC, and -12 VDC voltages required for PC operation. The rectified DC output voltages are filtered by inductive-capacitive filters (LC1-LC4). L is the symbol used to represent an inductor (coil) and C represents a capacitor. The filtered output of the +12 VDC rectifier is also supplied to cooling fan motor M at the rear of the power supply. The -5 VDC power supply output is actually the regulated branch from the -12 VDC output after its passed through a separate voltage regulator circuit. The power supply output voltages are regulated by the Switching Regulator Control Circuit. This circuit monitors and controls the level of the +5 VDC output. The +5 VDC output is returned to the switching regulator as feedback voltage. Any variation in the +5 VDC output causes the switching regulator to vary the input to the switching transformer to insure a constant level of +5 VDC. This in turn affects the +12 VDC, -12 VDC, -5 VDC, and +3.3 VDC voltage levels since they are also created by the switching transformer output. The power supply output voltages are monitored by an over-voltage protection circuit in the switching regulator. If either the +5 VDC output voltage exceeds +10 VDC or the +12 VDC output voltage exceeds +24 VDC, the over-voltage protection circuit kicks in and shuts down the output of the switching regulator. Pin 14 of the ATX connector, PS-ON, is used to turn on/off the power supply from the motherboard. The power supply also produces a POWER GOOD signal that is used to protect the system anytime the DC voltages go out of tolerance. The POWER GOOD signal is at logic HIGH when the power supply output voltages are within tolerance and goes to logic LOW when the power supply voltages are out of tolerance. During system startup, the POWER GOOD signal is delayed from going High for approximately 100 ms to allow the power supply outputs to stabilize. The logic Low POWER GOOD signal is used by the motherboard to prevent the system from operating before the +5 VDC has stabilized and to provide a warning before power is lost. As long as both the AC source voltage and the +5 VDC are within tolerance, the POWER GOOD signal will be at logic High, allowing the system to operate. If the AC source voltage falls out of tolerance, the POWER GOOD signal will rapidly switch to the logic Low state before the power supply outputs are out of tolerance. This permits the system to respond before the output voltage of the power supply falls below the specified thresholds.

Power Supply DC Connectors

NOTE: There are two types of motherboard power connectors in use today. The AT dual connectors (P8/P-9), and the ATX (20-pin block). Although much of the information contained in this lesson can be applied to the older style AT dual connector power supply, the exercises will concentrate on the power supply that features the ATX 20-pin power connector. Its also important to note that some Pentium IV motherboards require both an ATX 20 pin block and the older AT dual connector to operate. ATX 20 Pin Block AT Dual Connector (1) Large DC Connector Medium DC Connector

Figure 4. PC Power Supply Connectors

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The power supply voltages are distributed to the PC components through a set of cables with different types of output connectors. The 20-pin ATX power connector and its associated cable furnish power to the motherboard. Additionally, most power supplies have several large, medium, and small connectors for disk drives and other components. One type of cable with a large 4-pin DC power connector is used to furnish power for 5-1/4 floppy disk drives, hard disk drives, and CDROM drives. The second type of cable with a medium 4pin DC connector is used to feed 3-1/2 floppy disk drives. Figure 4 shows different types of DC power connectors.

Motherboard

Figure 5. ATX 20 Pin Power Connector The ATX power connector plugs into a single polarized connector on the motherboard. Polarization prevents the ATX power connector from being plugged in backwards and prevents the connector from being misaligned. The orientation diagram and wiring data is shown in Figure 5. Determining pin numbers can be tough on these types of connectors. However, if you look very closely youll see that the pin numbers are stamped on the connector on the wire end. This is also true for the other power supply connectors.

Power Cable: 5 Floppy, Hard Disk, and CDROM Drives

Figure 6 shows a diagram of the large 4-pin DC power connector and wiring used on each power cable for the following drives: 51/4 floppy disk, hard disk, and CD-ROM. The connector is keyed with two chamfered corners to prevent it from being inserted backwards. A Y connector is sometimes used with this power cable to feed dual floppy disk drives. It plugs into the cable between the power connector and the disk drive receptacles when used.

Connector Pin-Out Pin 1 2 3 4 Color Red Black Black Yellow Voltage +5 VDC Ground Ground +12 VDC

Figure 6. Power Cable for 5 Floppy, Hard Disk and CD-ROM Drives.

Power Cable: 3 Floppy Disk Drive

Figure 7 shows a diagram of the medium 4-pin DC power connector and wiring used on the 3 floppy disk drive power cable. The connector is polarized by its shape to prevent it from being inserted backwards. A special adapter cable is sometimes used to supply power to 3 floppy disk drives. It has a large 4-pin DC power receptacle on one end and a medium 4-pin DC power cable connector on the other end. This allows a transition from a large to a medium 4-pin DC power connector, which is required for mating with the connector on the 3 floppy disk drive.
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Connector Pin-Out Pin 1 2 3 4 Color Red Black Black Yellow Voltage +5 VDC Ground Ground +12 VDC

Figure 7. Power Cable for 3 Floppy Disk Drive.

Power Requirements
The type and mix of components determine the power requirements of a specific PC configuration. Each component in the system draws current at the voltage ratings used by the component. Some, such as disk drives draw different amounts of current depending on their operations (reading, writing, or idling). Table 1 lists some typical current requirements for common components found in a PC. Notice that the most current is drawn by the +5 VDC circuit for the digital electronics. The +12 VDC devices also draw a substantial amount of current. Only a small amount of current is drawn by -5 VDC and -12 VDC circuits. The serial ports use the 12 VDC power source. Its important to note that the actual current that any device uses varies widely from component to component. However, the individual components of older systems usually consume more power than the latest and greatest the computer industry has to offer. The total power consumption is greater in newer systems simply because these systems have more components and are more powerful.

+3.3 VDC -5 VDC -12 VDC Power Supply Fan Motherboard (1 MB RAM) 3.0 0.05 0.05 Video Adapter Card 0.5 I/O Card 0.25 0.1 0.1 Sound Card 0.5 Floppy Disk Drive 0.5 0.8 Hard Disk Drive 1.2 1.3* CD-ROM Drive 1.0 * This is the current used during normal operation. At spin-up the current draw can reach 4.0 amps. Table 1. Typical current requirements of PC components. Energy Star, or Green machines are computers and peripherals that feature reduced power consumption ratings. These components require less current than their early counterparts. Although theres a big advantage with reduced electricity consumption, but the complexity of the circuitry has caused some problems as well. When theres a substantial reduction in load, poor regulation gives the appearance of the power supply being faulty. This happens with the low-load conditions that are typical of Energy Star compliant machines. This occurs when theres not enough current for the power supply regulators to perform their function. As a computer repair technician you need to be aware of the effects of under-loading when troubleshooting PC power supplies.

Component

Current in Amps
+5 VDC +12 VDC 0.25

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PC Power Supply Troubleshooting

Exercise 1 - SERVICING INFORMATION
Exercise Objective:
When you have completed this exercise, you will be familiar with how the power supply converts 110/220 VAC to the DC voltages required to operate your computer. You will also learn how to determine if the power supply in a PC is functioning correctly.

Name: Period: Date:

Discussion of Fundamentals:
Most electronic equipment are powered by linear power supplies that operate at line frequencies between 50 and 60 hertz (Hz). Because of the size of the power supplys line transformer, these units are typically pretty heavy. The more power thats required of a linear power supply, the heavier and bulkier it has to be. Besides the line transformer, this type of power supply also requires rectifier diodes to produce the DC power and large capacitors to filter out the unwanted AC component. The filtered DC voltage is then processed through linear voltage regulators to maintain the desired output voltage. Personal computers do not use linear power supplies. Instead, the PC uses a switch mode or switching power supply. It differs from conventional heavy transformer-based power supplies because it has a switching regulator that operates at a frequency hundreds of times greater than the line frequency of 50 or 60 Hz. The PC switching (switch mode) power supply is more complex and more expensive than conventional power supplies. It does, however, offer significant advantages over conventional power supplies: The switch mode regulator consumes less power than a linear regulator. It is much lighter and more compact because of the smaller transformer and filter components used for high frequency operation.

During this exercise youll be directed to have your instructor check over your work before continuing. Its very important to have your instructor inspect your setup to make sure that youre safe and that you wont damage the computer or test equipment.

Research Resources:
Company How Stuff Works PC Power and Cooling Computer Power Supply Web Site http://www.howstuffworks.com/power-supply.htm http://www.pcpowercooling.com/home.htm http://www.amtrade.com/pc_power/switching_power_supplies.htm Description How Computer Power Supplies Work Build Your Own PC Specs and Sales

Required Materials:
Computer Trainer Technician Tool Kit Digital Logic Probe Digital Multimeter Jumper Wires Page 1 of 6

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Procedure:
In this exercise youll complete four power supply tests: 1. Testing Fan Operation 2. The POWER GOOD signal The logic probe is used to test The POWER GOOD Signal. In this exercise, the key-lock connector on the motherboard supplies power to the probe. The connection of the probes lead clips must be done with extreme caution. 3. DC voltage Tests You will measure DC voltages and record the readings in the table below. The measurements will be taken with the power supply under the full load of the motherboard and disk drives. When you have finished, you will compare the measured voltages to the nominal voltages listed in the exercise. The power supply is considered operational if the voltages at the motherboard are within the minimum and maximum limits. However, this measurement does not guarantee that proper voltages are being furnished to the disk drives. You must measure power at the disk drive inputs. NOTE: You can DESTROY the motherboard if you do not correctly perform the procedure below. DO NOT start this procedure before asking your instructor for permission. Your instructor may decide to perform the procedure for the class.

4. Power Cable DC Voltages To verify power at the disk drive inputs, you should measure the voltages at the large and medium 4pin DC connectors that feed those drives. Appropriate voltages for the power cable connectors for the hard disk and CD-ROM drives are listed in the exercise. The same is true for the cable connector voltages for the 3 floppy disk drive.

Fan Operation
O 1. O 2. O 3. Check the power supply fan for obstructions. Press in the power switch and observe if the fan is running. Turn OFF the power using the power switch.

Using a Logic Probe

In this procedure, the logic probes lead clips are connected to ground and +5 VDC. Before starting, read all the steps completely so that you will be sure of what you are doing. O 1. O 2. O 3. O 4. Turn the computer OFF and remove its case. Locate the ATX 20 pin block power connector on the motherboard. Locate a +5 VDC power source in the computer for the logic probe. Set the logic probe function switches to TTL and PULSE. Figure 1. Connecting Power to the Logic Probe.

NOTE: There are two good methods for connecting the logic probe leads to +5 VDC. You can use either the Medium DC Power Connector or Main Board Front Panel Header method.

A. Medium DC Power Connector This is probably the best method to power the logic probe. You can use the same connector that provides power to the 3 floppy disk drive to power the logic probe. O O O O Find a free medium DS power connector or temporarily disconnect the 3 disk drive. Carefully insert a short wire into pin 2 or 3 (black) of the medium DV connector plug. Connect the black (-) lead of the logic probe to the short wire. Insert a short wire to pin 4 (red) of the connector. PC Power Supply Troubleshooting Page 2 of 6
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5. 6. 7. 8.

O 9. Connect the red (+) lead of the logic probe to the short wire inserted in pin 4 of the connector. O 10. Have your instructor check your work before you proceed. NOTE: Its very important that the bare ends of the short wires or metal ends of the logic probe leads dont touch each other or any computer case metal of circuitry. This will short out the +5 VDC power supply and destroy it. O 11. With the logic probe power leads connected - Turn the computer ON. O 12. Test the logic probe by touching the probe tip to the +5 VDC and ground clips on the connector as shown in inset drawing of Figure 1. The logic probe should read HIGH when probing the +5 VDC and LOW when probing the ground connectors. O 13. Skip Method B and continue with the POWER GOOD signal test. B. Main Board Front Panel Header This connection is a little risky due to the close proximity of the header connectors. Its also more difficult to find the right pins to connect to. Since various motherboards have different header configurations, theres really no clear-cut way of using the header to tap power for the logic probe. If youre not careful, youll short out the +5 VDC power supply and destroy it or the motherboard. Refer to Figure 2 as a guide for connecting the logic probe power leads. O 4. Carefully examine the front panel header on the motherboard and locate a pin that will supply Ground or (-) for the logic probe. A good place to start is the negative Speaker pin. It may be necessary to remove the speaker header connector to get access to the pin. Which is ground? Thats tough to know for sure sometimes its screen printed on to the top of the motherboard. It could also be the pin the black speaker was connected to Maybe not. If all else fails, you could simply clip the lead to the metal computer case. Connect the black (-) lead clip of the logic probe to the negative (-) pin of the speaker header. Carefully examine the front panel header and locate a pin that will supply +5 VDC. A good place to start here is the + Keyboard Lock pin. It may be necessary to remove a header connector to get access to the pin. Which is the (+) pin? There are two pins to the Keyboard Lock header... Pick One! If the (+) is not printed on the motherboard pick one. If the logic probe doesnt work try the other. Connect the red (+) lead clip of the logic probe to the positive (+) pin of the Keyboard Lock header pin. Have your instructor check your work before you proceed.

O 5. O 6.

O 7. O 8.

NOTE: Because the computer is OFF, you do not have to worry about accidentally shorting out the motherboard when connecting the probes lead clips. However, when you turn the computer ON, if the lead clips are touching anything but the specified pins, the motherboard will short and youll destroy it or the power supply.

Figure 2. Power Connections for Logic Probe.

Figure 3. Testing The Power Good Signal.

NOTE: Make sure that the probes lead clips are secure and only touching the specified pins. Remember, if the lead clips touch anything else on the motherboard, the motherboard will
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be shorted.

Power Good Signal

O 1. O 2. O 3. O 4. O 5. O 6. Turn the computer OFF. Probe the POWER GOOD signal at pin 8 (usually the gray wire) of the ATX motherboard power connector with the logic probe, as shown in Figure 3. Turn on the computer while observing the LED indicators on the logic probe. The LO indicator should momentarily go on and off. The HI indicator should then come on steady. This sequence of events demonstrates that a delay occurred in the POWER GOOD signal during system power up. Turn off the computer while observing the LED indicators on the logic probe. The HI indicator should turn off, followed by the PULSE and LO indicators momentarily going on and off. This sequence of events demonstrates that the POWER GOOD signal switches the system to a low voltage before the system power shuts down. This is the normal sequence for the POWER GOOD signal. Disconnect the logic probe from the motherboard.

O 7.

DC Voltages (Refer to Figure 4.)

O 1. O 2. O 3. O 4. O 5. O 6. O 7. O 8. O 9. O 10. O 11. O 12. O 13. Turn the computer ON and allow the system to boot up. Set the multimeter to a range that will measure 20 VDC. Attach the negative probe of the multimeter to metal case of the PC. This will act as the ground or negative connection. Have your instructor check your work before you proceed. Measure the DC voltage at pin 1 of ATX power connector. Record the voltage in Table 2. Measure the DC voltage at pin 2 of ATX power connector, Record the voltage in Table 2. Measure the DC voltage at pin 4 of ATX power connector. Record the voltage in Table 2. Measure the DC voltage at pin 6 of ATX power connector. Record the voltage in Table 2. Measure the DC voltage at pin 8 of ATX power connector. Record the voltage in Table 2. Measure the DC voltage at pin 9 of ATX power connector. Record the voltage in Table 2. Measure the DC voltage at pin 10 of ATX power connector. Record the voltage in Table 2. Measure the DC voltage at pin 11 of ATX power connector. Record the voltage in Table 2. Measure the DC voltage at pin 12 of ATX

power connector. Record the voltage in Table 2. Figure 4. Measuring Power Supply Voltages. O 14. Measure the DC voltage at pin 14 of ATX power connector. Record the voltage in Table 2.

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O 15. Measure the DC voltage at pin 18 of ATX power connector. Record the voltage in Table 2. O 16. Measure the DC voltage at pin 19 of ATX power connector. Record the voltage in Table 2. O 17. Measure the DC voltage at pin 20 of ATX power connector. Record the voltage in Table 2. O 18. Remove the negative probe from computer case. O 19. Shut OFF the power of the PC trainer. O 20. Compare the voltage readings you listed in Table 2 with the permissible values shown in Table 3. If the voltage readings are within acceptable limits, it is reasonable to assume that the power supply is operating properly. If the voltages are not within acceptable limits, you should take a second set of readings. If you get the same results a second time, then the power supply is probably at fault. NOTE: If you cannot obtain an acceptable set of voltage readings, on a second attempt, you should inform your instructor.

ATX Power Connector Pin Number

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Voltage Measured
Ground Ground Ground

Ground Ground Ground Ground

Table 2. ATX Power Supply Exercise Measurements. Motherboard ATX Power Connector Pin Number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Wire Color Orange Orange Black Red Black Red Black Gray Purple Yellow Orange Blue Black Green Black Black Black White Red Red

Power Cables
Normally, technicians check every DC power cable as part of their troubleshooting procedure. Since we know the power cables in this computer trainer are good, youll just check one large and one medium 4-pin DC connector as part of the troubleshooting exercise. Most computers have a few power supply connectors free for the installation of additional drives or other components. You dont have to disconnect a power cable from a drive to take a measurement. Simply use one of the free connectors. However, Its still important to make sure the computer is OFF before disconnecting any power cables. O 1. O 2. O 3. Verify that the power for your PC Trainer is OFF. Locate an unused large power connector and an unused medium power connector. Turn on the computer.

Nominal Voltage 5% +3.3 VDC +3.3 VDC Ground +5 VDC Ground +5 VDC Ground POWER GOOD +5 VDC +I2VDC +3.3 VDC -I2VDC Ground POWER ON Ground Ground Ground -5 VDC +5 VDC +5 VDC

Table 3. Nominal ATX Power Supply Voltages.

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NOTE : Make sure that the probe makes a good connection with the metal part of the pin to get an accurate voltage measurement. You may need to insert a piece of wire into the end of the medium connectors in order to make a measurement. O 4. O 5. O 6. O 7. O 8. Set the multimeter to a range that will measure 20 VDC. Have your instructor check your work before you proceed. Use the multimeter to verify that +5 VDC and +12 VDC are at the large 4-pin DC power cable connector in the system. Compare your findings with Table 4. Use the multimeter to verify that +5 VDC and +12 VDC are at the medium 4-pin power cable connector in the system. Compare your findings with Table 5. Turn OFF the power on the PC trainer. Wire Color Yellow Black Black Red Nominal 5% +I2VDC Ground Ground +5VDC

Connector Pin Number 1 2 3 4

Table 4. Large 4-pin DC power cable voltages. Connector Pin Number 1 2 3 4

Wire Color Yellow Black Black Red

Nominal 5% +I2VDC Ground Ground +5VDC

Table 5. Medium 4-pin DC power cable voltages.

CONCLUSIONS
The PC switching or switch mode power supply is different from conventional linear power supplies. It has a switch mode regulator that consumes less power than a linear regulator and is much lighter because of the smaller high frequency transformer and filter components. Most modern PC power supplies range from 200 to 400 watts, and furnish the DC voltages that are required for the PC to function properly. Three different types of power connectors are used to connect output power to the motherboard and the disk drive. All voltages can be measured at the ATX power connector on the motherboard. For a complete operational check of the power supply, however, the output voltages must also be tested at the both the large and medium 4-pin DC power connectors. The POWER GOOD signal from the power supply is used as a reference voltage by the system to ensure that power is acceptable. Out of tolerance power supply voltages will cause this signal to shut down the system or prevent it from starting up during the initial boot sequence. The power supply requires a load for proper operation. The best load is the system components. However, a test resistor may also be used for testing purposes. Do not operate a PC power supply without some kind of load being used. It will damage the power supply.

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PC Power Supply Troubleshooting

Exercise 2 Troubleshooting and Repair
Exercise Objective
Upon completion of this exercise, you will be familiar with various symptoms regarding a faulty power supply. Your goal is troubleshoot and repair the PC power supply.

Name: Period: Date:

Discussion

Power supply malfunctions can look like system failures or like intermittent glitches that cause erratic problems. Either way, therere systematic troubleshooting procedures that can be followed to determine if the power supply is the cause of the problem. Troubleshooting starts with a simple test of the senses. Visual inspection and even a few well-placed sniffs can help you zero in on the problem. Other troubleshooting methods require tools and test equipment but are still relatively easy to perform. As emphasized in the Basic Troubleshooting lesson, you should always perform the easiest tests first. This often saves time and allows you to perform repairs without any disassembly or complicated testing.

Research Resources:
Company How Stuff Works PC Power and Cooling Computer Power Supply Web Site http://www.howstuffworks.com/power-supply.htm http://www.pcpowercooling.com/home.htm http://www.amtrade.com/pc_power/switching_power_supplies.htm Description How Computer Power Supplies Work Build Your Own PC Specs and Sales

Required Materials:
Defective Power Supply Module Trainer Technician Tool Kit Digital Logic Probe Digital Multimeter Mini-Gator Test Leads AC Receptacle Tester Test Load Resistor Assembly Jumper Wire

Symptoms
Typically, the symptoms of a faulty power supply are: Completely Dead. Theres no sign of life when the power switch is turned on. The power indicator LED doesnt light on the PCs front panel. Comes On and then Dies. This is similar to a dead PC. The computer may power up, but after a short period of time turns off. Crashes After a Period of Operation. The system comes on and operates normally for a while. The PC fails after it operates normally for a period of time. Locks Up. The system comes on and operates normally for a while. The computer then seems to freeze in the middle of an operation. There is no response to keyboard inputs. PC Power Supply Troubleshooting Page 1 of 11
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Reboots Itself. The system comes on and operates normally then, without warning reboots itself.

Completely Dead
A completely dead PC is a catastrophic failure that is relatively easy to diagnose. Before doing any work in the unit, you should perform a visual inspection that includes: A check of the power source to ensure that it is operating. A check of the power cords to ensure that they are properly connected. A check of the voltage selector switch on the rear of the power supply. It should be set to match the local line voltage (115 VAC in the United States or 230 VAC in Germany). Check to determine the local voltage that is supplied in other countries. Check the ATX power connector PS-ON (pin 14) to ensure it is active.

Comes On and then Dies

You should approach a PC with this symptom in the same way you approach a dead PC. One additional consideration, however, is the fact that the POWER GOOD signal may be shutting down the system because of an overload condition. Overload means that there is too much current being drawn from the power supply, and the power supply cannot meet the demand. This is usually caused by adding new system components that increase demand on a power supply with too small a rating for the systems needs. It may, however, be caused by a faulty component. If the problem is caused by a peripheral device or expansion card, you may be able to isolate an overload condition by temporarily removing an I/O device or unplugging a floppy disk drive to alleviate some of the load on the power supply.

Crashes After a Period of Operation

One of many causes of failure is overheating after a period of operation. If the cooling fan fails completely, it will be conspicuous by its silence. If the speed of the fan is too low, the temperature can rise enough to cause system shutdown. Therefore, you should check for overheating anytime a PC shuts down after a period of operation. If the operating temperature is normal, perform all the dead system checks. Vibration, for example, can cause a power cord to work loose and disconnect power from the unit.

Locks Up
While this is normally a software problem, it can sometimes be caused by static or voltage spikes from the AC supply source. If the problem occurs regularly, you should have the source supply tested because the power supply can be damaged from this condition.

PC Reboots Itself
This is also normally a software problem. It can however, be caused by a low source voltage that triggers a POWER GOOD signal reset. If the problem occurs regularly, you should have the source supply tested, because the power supply can be damaged from this condition.

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Figure 1. Power Supply Troubleshooting Diagram.

Troubleshooting Procedure
Figure 1 is a troubleshooting diagram that shows a logical approach for locating faults in a malfunctioning power supply. It combines visual inspections and observations with the use of test equipment to conduct a logical evaluation of both the AC and DC circuits of the power supply system. In following the logical flow of the troubleshooting diagram, you should make the following four major checks of the power supply, using the procedures described in Exercise 1 Servicing Information: POWER AVAILABLE? FAN RUNNING? VOLTAGES OK? POWER CABLES OK? POWER GOOD OK?

These checks are shown in the troubleshooting diagram in Figure 1. If the results of all fours checks are good, then you can assume that the power supply system is operating properly and the trouble is elsewhere in the PC. If a negative result is realized during any of the four checks, then you must assume that the power supply or a component in the power supply subsystem is not functioning properly. You must then perform the checks shown to further isolate the cause of the problem. The logical troubleshooting paths fall into two categories: AC circuits DC circuits The AC circuit checks follow a logic path that verifies proper functioning of the AC source, AC input wiring, AC power switch, and AC power cord. The DC circuit checks expand on the DC voltage tests of PC Power Supply Troubleshooting Page 3 of 11
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Exercise 1 by checking the DC output under a load and the operation of the +12 VDC fan. The paragraph headings of the individual procedures are set in bold type and correspond with the titles in the boxes of the troubleshooting diagram. This will allow you to conveniently track the result of each procedure against the troubleshooting flow logic.

AC CIRCUIT CHECKS
As with all computer troubleshooting, the first thing you should do when trying to diagnose power supply problems is to make a visual check of the major components. Checking out the power supply fan, electrical outlet, power switch, and power cord can often reveal the problem without the need for more sophisticated testing. Perform these checks as described below.

Power Supply Fan

Check to see that the power supply fan is running when the computer is turned on. You should also make sure that there are not obstructions in the fan housing that may be blocking airflow. If the fan is running, then you know that the power supply is at least producing enough voltage to operate the fan. Certainly, this is not a comprehensive test of the cooling fan let alone the power supply. Its just an indicator that the power supply is receiving AC power. If the fan isnt working, you need to make sure the computer is actually getting power. Use an electrical outlet tester, AC outlet polarity tester, or even a desk lamp to test for the presence of AC power.

Video Monitor
A simple means of checking for AC power in the system is to turn on the video monitor thats plugged into the same circuit and see if the power indicator comes on. If it does, AC power is present. If not, the next step is to check the AC power switch on the power supply.

Power Supply AC Switch and Auxiliary Outlet

There are many manufactures of computer power supplies. The configuration of an AC power switch differs from power supply to power supply. These instructions detail the procedure for testing the AC power switch for those power supplies that feature an auxiliary power outlet and main power switch. Skip the next test if your computer power supply doesnt include an auxiliary AC power outlet. NOTE: Continuity is a test of a component, circuit, or wire to see if the circuit can conduct electricity well. Multimeters can be used to test for continuity in two ways. The best way is to use the built-in Continuity Function. Its usually marked by a diode symbol on the function switch. This setting features an audible beep when theres continuity so you dont have to watch the meter face while your testing a circuit. You can also use the resistance function () to measure circuit resistance. The reading for a good continuity check would be virtually zero on the multimeter.

Testing the Auxiliary Power Outlet

O 1. O 2. O 3. O 4. O 5. O 6. O 7. O 8. Turn OFF the computer and unplug the AC power cord from the wall outlet and the power supply. Unplug the monitors AC power cord from the power supply if applicable. Setup your multimeter. Set the multimeter function switch to Continuity or Resistance (). Be absolutely sure that the AC power cord is unplugged from the computer and the power source. Have your instructor check your work before you proceed. Figure 2. Power Supply Auxiliary AC Wiring. Refer to Figure 2. Connect the multimeter across the Line pins of AC input connector and auxiliary outlet. Page 4 of 11

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O 9. O 10. O 11. O 12. O 13.

Turn the power switch to the ON position while observing the multimeter. Return the power switch to the OFF position. Was there continuity across the Line connections with the power switch on? If yes, the wiring of the auxiliary outlet is good so go on to the AC Power Switch Test. If no, proceed with the next step. On the auxiliary outlet only, move the multimeter lead to Neutral pin. Turn the power switch to the ON position while observing the multimeter. Return the power switch to the OFF position. Was there continuity across the Line and Neutral connections with the power switch on? If yes, the wiring of the auxiliary outlet is good so go to the AC Power Switch Test. If no, have your instructor check your work.

AC Main Power Switch Test

If theres no continuity between the AC input connector and the auxiliary AC outlet when the main power switch is turned on, then the malfunction may be either the wiring to these three components or the main power switch itself. Examine the schematic in Figure 8. Switching Power Supply Diagram in the Power Supply Troubleshooting lesson at the beginning of this unit. SI is the main power switch for the power supply. If there is an open circuit in any of the wiring connected to S1, there can be no continuity between the AC input connector and the auxiliary AC outlet. You can isolate the fault by checking the power switch and the wiring as described below. NOTE: Use the Defective Power Supply Module Trainer for this section of instruction. If you dont have this module you may use the power supply from a computer. Ask your instruction for assistance. O 1. O 2. O O O O 3. 4. 5. 6. Turn OFF the computer and unplug the AC power cord from the wall outlet and the power supply. Unplug the monitors AC power cord from the power supply if applicable. Remove the computers case. Disconnect and remove the power supply. Remove the power supply case. Have your instructor check your work before you proceed. Locate the electrical connections for the main power switch. Notice that there are two black and two white wires (There may be other colors used but the following procedure is essentially the same) attached to the power switch. Figure 3. Power Switch Continuity Test. Sketch of the terminals and wires of the main power switch. Indicate the terminal numbers in the drawing. Be sure to include the position of the terminals and the color of the wires. Record your data in the table below. Wiring Sketch Terminal Number 1 2 3 4 Wire Color

O 7. O 8.

O 9.

Table 1. AC Power Switch Wiring.

NOTE: Sketching and noting the placement of wires on terminals is a tried and true method of
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troubleshooting and repair when wire are removed from terminals. If this were actual repair work Who knows when you might get back to this job? Dont rely on your memory. Sketch the terminals and make a table of the placement of the wires. The best place for this information is on your work order. O 10. Setup your multimeter to measure Continuity or Resistance (). O 11. Be absolutely sure that the AC power cord is unplugged from the computer and the power source. O 12. Have your instructor check your work before you proceed. O 13. Refer to Figure 3. O 14. Do not remove the wires from the switch terminals. O 15. Connect the multimeter leads across the switch terminals that have the same color wires. O 16. Turn the power switch to the ON position while observing the multimeter. Return the power switch to the OFF position. O 17. Was there continuity across these switch terminals with the power switch on? If yes, these switch contacts are good. If no, have your instructor check your work. O 18. Connect the multimeter across the switch terminals with the other color wires. O 19. Turn the power switch to the ON position while observing the multimeter. Return the power switch to the OFF position. O 20. Was there continuity across these switch terminals with the power switch on? If yes, these switch contacts are good. If no, have your instructor check your work. O 21. Reassembly the Power Supply and return the Defective Power Supply Module Trainer to the instructor. If the power switch had continuity between both sets of contacts then the switch is good otherwise the switch is defective and needs to be replaced. If you still suspect a problem, then the fault is probably in the wiring to and from the switch. You can isolate the specific fault by performing continuity checks of the wiring. A faulty switch or the wiring should be repaired as described later in this exercise.

Power Cord Continuity Test

The following procedure is used to test the AC power cord. O 1. Check to make sure the computer is OFF and then disconnect the power cord from the AC outlet and the rear of the power supply. Dangerously high voltages of 115 VAC or 230 VAC are present in the power supply unit unless the power plug is disconnected from the AC source.

Warning:

O 2. O 3. O 4.

Set the multimeter to measure resistance or continuity. Refer to Figure 4. Check the continuity between the ground, line, and neutral pins on the power cord.

There should be continuity between the ground, line, and neutral pins on the AC power cord. If not, repair or replace the cord.

AC Power
The AC power source should only be repaired by a qualified electrician. However, you can verify whether its working or not by performing the following procedure: O 1. O 2. O 3.

Figure 4. Power Cord Continuity Test

Plug a working AC receptacle tester, lamp, or other electrical device into an electrical outlet. If the device works, the power source is probably good. If the device does not work, report the problem to your instructor. Page 6 of 11

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O 4.

Disconnect the device after you finish the test.

That completes the AC circuit checks. If no faults can be found with the AC source, AC wiring, power switch and power cord, then you can assume that the cause of the trouble is in the power supply itself.

DC Circuit Checks
The procedures that follow describe the checks that you should make if unsatisfactory results were obtained during the tests of the DC voltages and power cables that were performed in Exercise 1. Also, if the fan is not running, yet the video monitor has power to it, the + 12 VDC output is suspect. The following steps may be more conclusive in determining if the DC power is at fault than those in Exercise 1.

Dummy Load Test Resistor

What sometimes appears to be a dead power supply may be a problem with the motherboard or other related components. The best way to determine if the power supply is actually at fault is to test it while its completely disconnected from the main board and drives. Switch mode power supplies cannot operate correctly without a load. In fact, you can damage the power supply trying to operate it without a load. The power supply must have a minimum load on the +5 VDC output in order for the output voltage regulator circuit to work properly. Therefore, a dummy load resistance is needed to simulate the load of a motherboard. You will connect a 10 Ohm 10 Watt test resistor between the +5 VDC line and ground to provide a 0.5-amp load. Refer to Figure 5. while following these directions. O 1. Youll need these parts to build a dummy load test resistor. a. 10 Ohm, 10 Watt Ceramic Resistor b. 18-20 Gauge Solid Hook-up Wire, 2 Inch c. Solder d. Electrical Tape Strip the insulation from the solid hook-up wire. Solder the solid hook-up wire to one of the resistors leads. Bend the lead against the resistor and secure in place with the electrical tape. Have your instructor check your work before you proceed. Figure 5. Dummy Load Test Resistor

O 2. O 3. O 4. O 5.

Figure 6. Load Testing Power Supply

CAUTION:

The dummy load test resistor will get hot. Be careful to not touch the resistor until it cools. Be sure to position the resistor away from any wires or parts that may be damaged by heat. Also, the bare leads to the resistor cannot touch any metal on computer case or power supply. This will short out the +5 VDC power supply and damage it.

Power Supply DC Load Test

If the voltages in Exercise 1 are incorrect, a final check should be made of the power supply, using a
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dummy load resistance. This will isolate the fault to either the power supply or one of the other components in the PC. Refer to Figure 6 and 7 while following these directions very carefully. For this test you will need a multimeter and the dummy load test resistor that was supplied with your kit. All measurements will be made with the test resistor connected between the +5 VDC supply and ground (GND) pins. All voltage measurements will be referenced to ground (GND).

Testing +5 Volts DC
O 1. O 2. O 3. Be sure the power switch is OFF. Unplug the computer from the AC outlet. Disconnect the power supply cables from the motherboard, hard disk drive, CD-ROM drive, and floppy disk drive. O 4. Adjust the voltmeter to read 20 VDC. O 5. Find the dummy load test resistor. O 6. Locate the 20 pin ATX power supply connector. Place the shorting wire between pins 13 & 14. O 7. Position the test resistor as shown in Figure 6. O 8. Insert one test resistor lead into ATX power connector pin 3 (black wire). O 9. Insert the other resistor lead into ATX power connector pin 20 (red wire). O 10. Connect the red (+) voltmeter lead to the resistor lead that is connected to the pin 20 (red wire). O 11. Connect the black (-) voltmeter lead to the resistor lead that is connected to the pin 3 (black wire). Warning: For safety, make sure that neither the resistor nor any bare wires are touching the case, power supply, or each other.

O 12. Plug the computer into an AC power. O 13. Turn ON the computer. The power supply fan should turn on. O 14. Record the voltage O 15. Turn OFF the computer using the switch on the back of power supply O 16. Unplug the AC power cord from the outlet. O 17. The voltage reading should be between the minimum and maximum values listed in Exercise 1 Table 3 for +5 VDC.

Testing -5 VDC
O 1. O 2. O 3. O O O O 4. 5. 6. 7. Move the red (+) voltmeter lead to the ATX power connector pin 18 (white wire) Figure 6. Plug the computer into AC power. Press the power switch to turn ON the computer. Record the voltage reading: Figure 7. Voltage Test Meter Set-Up Turn the computer OFF. Unplug the computer from the AC receptacle. The voltage reading should be between the minimum and maximum values listed in Table 3 in Exercise 1.

Testing +12 Volts DC

O O O O O O O 1. 2. 3. 4. 5. 6. 7. Move the red (+) voltmeter lead to the ATX power connector pin 10 (yellow wire). Plug the computer into AC power. Turn ON the computer. Record the voltage reading: Turn OFF the computer. Unplug the computer from AC power. The voltage reading should be between the minimum and maximum values listed in Exercise 1 Table 3 for +12 VDC. Page 8 of 11

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Testing -12 Volts DC

O O O O O O O Move the red (+) voltmeter lead to the ATX power connector pin 12 (blue wire). Plug the computer into AC power. Turn ON the computer. Record the voltage reading: Turn OFF the computer. Unplug the computer from AC power. The voltage reading should be between the minimum and maximum values listed in Exercise 1 Table 3 for 12 VDC. O 8. Disconnect both meter leads and remove the test resistor and shorting wire. O 9. Unplug the computer from AC power. O 10. Reconnect the power cables to the motherboard, hard disk drive, CD-ROM drive, and floppy disk drive. If any of these tests did not pass, then the problem is in the power supply. If any of these tests just slightly fells, the problem may be in the power supply or a component. If the power supply checks out okay, then the problem could be with one of the components that is connected to it. 1. 2. 3. 4. 5. 6. 7.

REPAIR
The procedures in this section of the exercise cover repair procedures for the power supply, AC power switch, and AC wiring. The procedures are typical of those that might be required for any of the many IBM compatible PCs that you will encounter in the field.

Power Supply
Defective power supplies arent normally repaired in the field. Theyre usually removed and replaced because its faster and the generally more reliable. Replacing a power supply varies from computer to computer, but its pretty simple task. With a little flexibility, the procedure below can be applied to most every computer system that needs the power supply removed and replaced. REMOVAL O 1. Turn the computer OFF and disconnect the power cord from the AC outlet and computer. O 2. Disconnect any power cable from the auxiliary AC outlet at the rear of the power supply. O 3. Remove the cover from the computer. O 4. Sketch the connections of the DC cabling from the power supply to the components before disconnecting any cables or wiring. NOTE: Good technicians sketch cable and wiring connections to make reassembly easier. Carefully label wire colors and cable positions before disconnecting cables and wiring. O 5. O 6. O 7. O 8. O 9. Disconnect the DC power cables from the motherboard, hard disk drive, CD-ROM drive, and floppy disk drive as applicable. Disconnect any remote cables to a front panel power switch if applicable. Remove any cable ties or clamps that will interfere with the removal of the power supply when the unit is removed. Remove the power supply mounting screws at the rear of the case and set them aside. Normally, there are four screws. Remove the power supply from the computer case.

INSTALLATION O 1. Insert the new power supply in the system case and secure it in place with the mounting screws. O 2. Connect the ATX power connector to the motherboard. O 3. Connect the power cables for the hard disk drive, CD-ROM drive, and floppy disk drive according to your sketch. O 4. Connect the remote front panel power switch to the power supply if applicable. O 5. Set the power supply voltage selector switch to the appropriate voltage for your area. O 6. Have your instructor check your work before you proceed. O 7. Connect the computer to the AC power outlet with the power cord. O 8. Turn the computer ON and check for proper boot-up and video monitor operation. PC Power Supply Troubleshooting Page 9 of 11
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O 9.

If the computer doesnt operate have your instructor check your work. There may be additional troubleshooting required. O 10. Turn the computer OFF and disconnect it from the AC power outlet. O 11. Replace the computer cover. O 12. Reconnect the power cords to the computer. The system is now ready for use.

AC Power Switch
The AC power switch cannot be repaired. The switch must be removed and replaced and it must be replaced with exactly the same switch model. There are many styles of switches used with todays PC power supplies. Some are push-button switches while other might be rocker or toggle types. Regardless of the style, they are all rated by the amount of voltage and current they can handle. Make sure the replacement part has the same power rating. You should note that most main power switches are located inside the power supply unit.

Replacing The AC Power Switch

Perform the following steps to remove and replace the AC Power Switch. The AC Power Cord should NOT be connected to an AC outlet or the computer during this procedure. Please note that some main power supply switches feature push-on connectors rather than solder to make connections. This type of connection makes switch removal and replacement and snap. Simply pull the connector off the terminal with a pair of needle nose pliers. Switches that are soldered in place require that the wires be desoldered from the switch before the switch can be removed. Power Switch and AC Connections O 1. Follow the instructions in the AC Main Power Switch Test to get access to the switch. O 2. Unplug the power cord from the AC outlet and the computer. O 3. Refer to Figure 3 and Table 1 to record the information about the switch youre about to replace. O 4. Desolder or disconnect all four (4) wires from the power switch. O 5. Remove the screws the secure the main power switch to the power supply case. O 6. Separate the power switch from the mounting bracket (if any). O 7. Install the new power switch at the same location on the mounting bracket. Make sure the terminals are in the same orientation as the old switch. O 8. Mount the power switch to the bracket (if any) using the screws you removed earlier. O 9. Install the main power switch and mounting bracket into to the power supply case. O 10. Secure the main power switch to the powers supply case with the screws you removed earlier. O 11. Check the alignment of the switch to make sure the mechanism move freely. Misalignment between the power switch and button is a common problem and will cause the switch not to work. O 12. Make sure the AC power switch is OFF. O 13. Solder or Connect the four (4) AC power supply wires using the information you collected in Table 1. Refer to the figure to determine the correct wiring of the switch. O 14. Verify that each wire is connected correctly and that the wire insulation meets the connection point to the terminal. O 15. Have your instructor check your work before you proceed.. O 16. Replace the power supply cover and reinstall the power supply unit in the computer. O 17. Test the operation of the power supply. If the power supply switch works, youve successfully completed this portion of the exercise. Otherwise, you made a mistake. Retrace these steps to determine what youre done wrong. Have your instructor check your work before you proceed.

Wiring
Wiring repairs should be made with comparable wiring of the same gauge and color whenever possible. Be sure to replace any cable ties that you have to cut to replace defective wires.

Conclusions
Power supply failures are indicated by a number of different symptoms. You must be able to recognize the various symptoms to effectively isolate troubles and make repairs. Simple visual inspection and checks can often be used to isolate a power related problem without the use of tools or test equipment. Develop good work habits that include a visual check for a functioning power source, a check that the power cord is connected and plugged into the power source, and the computer PC Power Supply Troubleshooting Page 10 of 11
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power switch is turned on when power troubles are reported. Basic testing of the power supply subsystem must check both the AC and DC power circuits to ensure that the system is functioning properly. The DC output voltages of the power supply must be tested with a proper load to get accurate results. Using a test resistor as a load can help determine whether a trouble exists in the power supply or some other component of the PC. Replacement is the general method used to correct a faulty PC power supply. Do not attempt to work on the power supply unless you are a qualified technician who has been trained for the job.

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