Hardware Troubleshooting Techniques

Hardware Troubleshooting Tools

The level of troubleshooting most often performed on PC hardware is exchanging Field Replaceable Units (FRUs). Due to the relative low cost of computer components, it is normally not practical to troubleshoot failed components to the IC level. The cost of using a technician to diagnose the problem further, and repair it, can quickly exceed the cost of the new replacement unit.

However, a few hardware diagnostic tools can be very helpful in isolating defective hardware components. These tools include

• Software diagnostic disk

• Multimeter

• Cable tester

• POST card

Software Diagnostic Packages

Several commercially available disk-based diagnostic routines can check the system by running predetermined tests on different areas of its hardware. The diagnostic package evaluates the response from each test and attempts to produce a status report for all of the system's major components. Like the computer's self-tests, these packages produce visual and beep-coded error messages. Figure 3.1 depicts the Main menu of a typical self-booting software diagnostic package.

Figure 3.1 A typical software diagnostic main menu.

This menu is the gateway to information about the system's makeup and configuration, as well as the entryway to the program's Advanced Diagnostic Test functions. You can find utilities for performing low-level formats on older hard drive types and for managing small computer system interface (SCSI) devices through this menu. In addition, options to print or show test results are available here, as is the exit point from the program.

The most common software-troubleshooting packages test the system's memory, microprocessor, keyboard, display monitor, and the disk drive's speed. If at least the system's CPU, disk drive, and clock circuits are working, you might be able to use one of these special software-troubleshooting packages to help localize system failures. They can prove especially helpful when trying to track down non-heat-related intermittent problems.

If a diagnostic program indicates that multiple items should be replaced, replace the units one at a time until the unit starts up. Then replace any units removed prior to the one that caused the system to start. This process ensures that there are not multiple bad parts. If you have replaced all the parts, and the unit still does not function properly, the diagnostic software is suspect.

Using a Multimeter in a PC

A number of test instruments can help you isolate computer hardware problems. One of the most basic pieces of electronic troubleshooting equipment is the multimeter. These test instruments are available in both analog and digital readout form and can be used to directly measure electrical values of voltage (V), current in milliamperes (mA) or amperes (A), and resistance in ohms. Therefore, these devices are referred to as VOMs (volt-ohm-milliammeters) for analog types, or DMMs (digital multimeters) for digital types.

Figure 3.2 depicts a digital multimeter. With a little finesse, you can use this device to check diodes, transistors, capacitors, motor windings, relays, and coils. This particular DMM contains facilities built in to the meter to test transistors and diodes. These facilities are in addition to its standard functions of current, voltage, and resistance measurement; however, in computer repair work, only the voltage and resistance functions are used extensively.

Figure 3.2 A digital multimeter.

The first step in using the multimeter to perform tests is to select the proper function. For the most part, you never need to use the current function of the multimeter when working with computer systems; however, the voltage and resistance functions can be very valuable tools.

In computer troubleshooting, most of the tests are DC voltage readings. These measurements usually involve checking the DC side of the power-supply unit. You can make these readings between ground and one of the expansion-slot pins, or at the system board power-supply connector. It is also common to check the voltage level across a system board capacitor to verify that the system is receiving power. The voltage across most of the capacitors on the system board is 5V (DC). The DC voltages that can normally be expected in a PC-compatible system are +12V, +5V, –5V, and –12V. The actual values for these readings might vary by 5% in either direction.

The DC voltage function is used to take measurements in live DC circuits. It should be connected in parallel with the device being checked. This could mean connecting the reference lead (black lead) to a ground point and the measuring lead (red lead) to a test point to take a measurement, as illustrated in Figure 3.3.

Figure 3.3 DC voltage check.

As an approximate value is detected, you can decrease the range setting to achieve a more accurate reading. Most meters allow for overvoltage protection; however, it is still a good safety practice to decrease the range of the meter after you have achieved an initial value.

The second most popular test is the resistance, or continuity test.

Failure to turn off the power when making resistance checks can cause serious damage to the meter and can pose a potential risk to the technician. Resistance checks require that you electrically isolate the component being tested from the system. For most circuit components, this means desoldering at least one end from the board.

The resistance check is very useful in isolating some types of problems in the system. One of the main uses of the resistance function is to test fuses. You must disconnect at least one end of the fuse from the system. You should set the meter on the 1k ohm resistance setting. If the fuse is good, the meter should read near 0 ohms. If it is bad, the meter reads infinite.

The resistance function also is useful in checking for cables and connectors. By removing the cable from the system and connecting a meter lead to each end, you can check the cable's continuity conductor by conductor to verify its integrity.

You also use the resistance function to test the system's speaker. To check the speaker, simply disconnect the speaker from the system and connect a meter lead to each end. If the speaker is good, the meter should read near 8 ohms (although a smaller speaker might be 4 ohms). If the speaker is defective, the resistance reading should be 0 for shorts or infinite for opens.

Only a couple of situations involve using the AC voltage function for checking microcomputer systems. The primary use of this function is to check the commercial power being applied to the power-supply unit. As with any measurement, it is important to select the correct measurement range; however, the lethal voltage levels associated with the power supply call for additional caution when making such measurements.

The second application for the AC voltage function is to measure ripple voltage from the DC output side of the power-supply unit. This particular operation is very rarely performed in field-service situations.

Cable Testers

The most frequent hardware-related cause of network problems involves bad cabling and connectors. Several specialized, handheld devices designed for testing the various types of data communication cabling are available. These devices range from inexpensive continuity testers, to moderately priced data cabling testers, to somewhat expensive time domain reflectometers (TDR).

Although inexpensive continuity testers can be used to check for broken cables, data cabling testers are designed to perform a number of different types of tests on twisted-pair and coaxial cables. These wiring testers normally consist of two units—a master test unit and a separate load unit, as illustrated in Figure 3.4.

The master unit is attached to one end of the cable and the load unit is attached to the other. The master unit sends patterns of test signals through the cable and reads them back from the load unit. Many of these testers feature both RJ-45 and BNC connectors for testing different types of cabling. When testing twisted-pair cabling, these devices can normally detect such problems as broken wires, crossed-over wiring, shorted connections, and improperly paired connections.

Figure 3.4 Cable tester.

TDRs are sophisticated testers that can be used to pinpoint the distance to a break in a cable. These devices send signals along the cable and wait for them to be reflected. The time between sending the signal and receiving it back is converted into a distance measurement. The TDR function is normally packaged along with the other cable testing functions just described. TDRs used to test fiber-optic cables are known as optical time domain reflectometers (OTDRs).

POST Cards

A POST card is a diagnostic device that plugs into the system's expansion slot and tests the operation of the system as it boots up. These cards can be as simple as interrupt and direct memory access (DMA) channel monitors, or as complex as full-fledged ROM BIOS diagnostic packages that carry out extensive tests on the system.

POST cards are normally used when the system appears to be dead, or when the system cannot read from a floppy or hard drive. The firmware tests on the card replace the normal BIOS functions and send the system into a set of tests. The value of the card lies in the fact that the tests can be carried out without the system resorting to software diagnostics located on the hard disk or in a floppy drive.

The POST routines located in most BIOS chips report two types of errors—fatal and nonfatal. If the POST encounters a fatal error, it stops the system. The error code posted on the indicator corresponds to the defective operation.

If the POST card encounters a nonfatal error, however, it notes the error and continues through the initialization routine to activate as many additional system resources as possible. When these types of errors are encountered, the POST card must be observed carefully because the error code on its indicator must be coordinated with the timing of the error message or beep code produced by the BIOS routines.

Simple POST cards come with a set of light-emitting diodes (LEDs) on them that produce coded error signals when a problem is encountered. Other cards produce beep codes and seven-segment LED readouts of the error code. Figure 3.5depicts a typical XT/AT-compatible POST card.

Figure 3.5 A typical POST card.