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Microcomputer care and repair: part II.

Last month, we discussed preventive maintenance of microcomputers, safety practices, and ways to keep hardware and software inventory. This concluding article deals with troubleshooting microcomputer problems.

Troubleshooting begins with the site selection for the computer system. It should be near electric outlets in a temperature- and humidity-controlled room, removed from heavy traffic, out of the sun, and protected from dust and dirt.

Allow enough room for cables, so that they will not be stretched or crushed. Leave adequate room around air vents. If at all possible, try to preserve the electrical circult exclusively for computers only. Large motors, as in refrigerators or centrifuges, can cause voltage to drop, and an overloaded circuit can fuses or circuit breakers to open, resulting in no voltage at all. Either outcome can cause the computer to turn off.

Use a good power strip (extension cord) with surge protectionone that will not be rendered useless by the first power surge. Buy one that costs at least $40, to insure high quality. The power strip allows you to apply power to all units simultaneously. Most power strips have an on/off switch that is lighted when it is on.

If you are using a power strip, all LEDs and indicator lights on attached components should be lighted after a normal power-up. Some monitors and printers have LEDs or switches that will light up to show they have power.

While the computer is working normally, become familiar with the sights and sounds of the power-up. Knowing what is supposed to happen and the order in which it happens will make the problem solving easier in the event of a failure.

Listen for the sound of the fan that keeps the heat down in the computer. Also listen for beeps, and look for the blinking dash in the upper left-hand corner of the monitor screen. The computer will be going through its built-in power-on self test (POST). This test will check random access memory, monitor, keyboard, optional RAM, serial and parallel ports, diskette drives, fixed drive, and hard drive.

Beeps, cursors, and numerical error codes are different ways POST indicates problems. Watch closely for the POST error codes on the monitor screen-they appear for only a short time.

For example, a continuous beep points to a problem with the power supply; a repeating short beep, a system board problem; and one long and two short beeps, a display circuit problem. Examples of numerical error codes will follow as we discuss problems with computer components.

Microcomputer troubleshooting is detective work requiring you to observe and analyze the problem before jumping in. Restart the computer and record any error codes. Then run the diagnostics diskette to determine the next step. The diagnostics diskette can alert you to simple problems. Fix the obvious first. Check the power source, and make sure that plugs are properly seated and located. Take the time to perform a good visual inspection.

Among the insulated repair tools you should have are small screwdrivers, nut drivers, an extractor (chip puller), and a magnifying glass to check for breaks in traces (printed circuits) or for chips with pins bent under and not making contact.

Never use force, and do not go into an energized circuit.

Take notes for future reference and for a repair person if it becomes necessary to call one in. List the methods you used, what was successful, and how you would fix the problem next time. In this manner, amass your own troubleshooting file.

IBM's "Guide to Operations" offers only a partial list of effor codes. The rest of the codes are presumably intended for service representatives, but some of the references below will help a user decipher them. The IBM guide does summarize the power-on self test, and it provides error examples, a printer problem chart, and an excellent explanation of the diagnostic diskette that accompanies the guide.

The diagnostic diskette's routines are for devices attached to the computer. The user is asked to indicate which devices are attached. Then the computer checks each and displays error codes when it finds something wrong. Run the diagnostic routine a few times while the computer is functioning.

Now let's start troubleshooting common problems.

* No computer power. Listen for the fan; if it works, there is power from the wall. Similarly, see if the LED is lit on the disk drive (indicating POST is testing the drive). Check the power, brightness, and contrast knobs on the monitor.

The monitor's power could be switched off or the contrast turned down, making it only seem as though the computer does not have power.

Are all cords properly located and seated? Is the wall outlet energized? You can find out by plugging a lamp into the outlet.

Make sure the fuses are good, both for the wall circuit and the computer. Compaq, an IBMcompatible microcomputer, has a thermal cutoff when heat builds up if the unit has been on too long or the air vent is obstructed. Give the computer 15 minutes to cool off, then try again.

* If there's still no power. You will have to strip the computer down to its basic parts by removing unessential controller cards or boards, such as those for memory, printers, and modems. Take these precautions before going into the computer: Make sure all power is turned off; remove plugs from sockets; and ground yourself by touching the bright metal cabinet of the power supply.

Next, write down the cards' original system board memory switch numbers. Then remove the cards, reset the switches to this minimal configuration, plug in the power cords, and try to boot (start up) the computer. There are two possibilities:

1. If the computer boots, you know the problem doesn't lie with the basic pans. Now insert each card back into the system, one at a time. Changethe card's switch setting and run the diagnostics after each addition. Continue replacing cards and verifying that the computer is functional. The first card to cause a failure is probably the culprit. To guard against the unlikely event of two failures, do not reinstall all of the other cards at once after correcting the first error. Go on adding single cards and running the diagnostics.

2. If the computer does not boot in its stripped-down state, send it out for repair.

* Computer problems after you have powered up. If you get no monitor response, check whether the monitor has power, is connected, and is tumed on. Twist the contrast and brightness controls through their entire range to see if there's screen activity. Should the problem persist, remove all unessential cards and follow the steps outlined in the "no power" and "still no power" sections.

One problem that may come up is signaled on the screen by either a Cassette Basic display (if you have an early-model IBM microcomputer with a cassette jack on the back panel) or a dash (on models without a cassette jack). With either symptom, you will not be allowed access to any programs or data. Check to see that you have the correct DOS diskette and that it is inserted in the proper drive; you may need to try another copy of DOS. Reboot and watch for error codes and listen for the beeps. The disk drive may have failed.

If you fail to get the DOS prompt, do a cold reboot by tum ing off the power, waiting about 25 seconds, and turning on the power again. Watch for error codes and listen for beeps. (A warm reboot, done by pressing the ctrl-alt-del keys together while the computer is still on, takes less time, but it won't retest memory.)

* Problems after DOS prompt. If the computer won't go any further, determine whether the error is in hardware or software. Try another diskette with the same program. Does it work? If yes, the problem could be a diskette error.

If the replacement diskette does not work, try it in a different computer. Does it work? If yes, then the original disk drive has failed. Run the diagnostics, and adjust the speed, or have a service person make the alignment.

* Cables. Here are some IBM error codes and the problems they signify: 301, keyboard malfunction, keyboard cable disconnected; 601, diskette or disk drive interface malfunction (drive adapter, cable, drive A); 611-613, bad drive data cable or disk drive adapter card; 14xx (xx is any digit between 00 and 99), printer interface malfunction; 15xx, system unit or communications adapter cable malfunction; 18xx, malfunctioning expansion unit or expansion unit cable malfunction; and 20xx-21xx, system unit or communications adapter cable malfunction.

Cables can be stressed, crushed, or crinkled, and thus cause failures. Always handle a cable by the connector or plug.

Occasionally if you move a malfunctioning, permanently attached cable around, or put a little stress on it, you can make it function on an intermittent basis, but

replacement is the only sure cure.

* Printer and/or printer cables. IBM printer error codes include: 199,432,901, indicating a printer adapter card or printer malfunc

tion; and 7xx, 9xx, system unit I/O (input/output) malfunction.

If the indicator light is on, there is power. Run a printer self test (see printer manual for instructions) to rule out printer failure. If the self test will not run, the printer's probably the problem. Replace it if possibble for confirmation.

On the other hand, if the self test does run, check the cables by bringing up a program and trying the printscreen key. If printscreen works, the cables are all right; otherwise, replace them.

*Monitors. Limit any monitor repairs to cleaning the outside, and substitution of monitor, cables, or card. Do not go into the monitor. A black and white monitor can have a few thousand volts and a color monitor as much as 25,000 volts.

If there's no display, remember the obvious: Check to see that the power switch is on, that all cables are properly seated, and that the brightness and contrast controls are turned up.

If that does not cure the problem, do a cold boot and watch and listen for the error codes. IBM system error codes for monitor problems include 401 for a monochrome adapter card malfunction and 501 for a color/graphics adapter card malfunction.

In continuing to troubleshoot, start with the easiest substitution, turning off the power whenever you connect or disconnect any peripheral. Substitute a similar monitor, first with the original cable, then with another cable. If that doesn't work, substitute a working controller card. Next, check the system board memory switches.

Send the monitor back if it is under warranty; otherwise, ask an experienced repair person to determine whether it should be fixed or replaced.

* Keyboard. IBM error code 301 indicates a keyboard malfunction or a disconnected keyboard cable; xx301 means a keyboard circuitry malfunction (xx, called the hex value, represents the scan code of the malfunctioning key). A diagnostic diskette will confirm that a keyboard is faulty. Since keyboards are inexpensive, replacement is often the best solution.

*Disk drives. An audio clueone short beep-in conjunction with a Cassette Basic display and failure of the diskette to start in the disk drive means a problem with the diskette or disk drive.

IBM system error codes include 601 for a diskette or disk drive interface malfunction (drive adapter, cable, drive A); 606 for a drive assembly or drive adapter malfunction; 613 for a faulty drive data cable or disk drive adapter card; and 621 -626 for a drive assembly problem.

You must determine if the diskette, disk drive, or disk drive cable is at fault. A bad diskette can make it appear the disk drive is faulty, so always eliminate diskette error.

Occasionally a diskette will not load because the program is too large for memory. An over-edited disk can cause slow performance due to excessive head movement and rotational delays involved in finding, reading, or writing a file. Instead of using the command DISKCOPY, use COPY *.* This will copy one file at a time from the diskette, into sequential sectors.

No LED activity or no motor sound indicates power source or power supply problems.

If you cannot open a file or if the diskette cannot be read by all drives, it indicates that the drive that wrote the file is misaligned or rotating at a speed outside tolerances. Disk drives are so inexpensive now that it is probably more cost-effective to purchase a new one than to attempt extensive repairs.

Diskettes written by a drive that is out of alignment can be read by that drive but not by another. If you discard the drive, you will have nothing with which to read the diskettes-their information will be lost. (Norton's Utilities, a commercially available program, can occasionally retrieve data from a bad diskette.) Therefore, before replacing the misaligned drive, use it to copy the diskettes to a good drive.

Errors can be avoided by running the diagnostics often enough to keep alignment and speed within tolerances.

I have tried to present some troubleshooting basics. As the references below suggest, much more can be written about microcomputer repair.
COPYRIGHT 1989 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1989 Gale, Cengage Learning. All rights reserved.

Article Details
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Author:Schneider, Nancy Dankes
Publication:Medical Laboratory Observer
Article Type:column
Date:Jun 1, 1989
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