Keyboard Troubleshooting and Repair

Keyboard errors are usually caused by two simple problems:

• Defective cables/bad connections

• Stuck keys

Other more difficult, intermittent problems can arise, but they are much less common.

Defective Cables or Bad Connections

Defective cables or bad connections between the keyboard and the system are easy to spot if the failure is not intermittent. If the keyboard stops working altogether or every keystroke results in an error or incorrect character, the cable or connection is likely the culprit. On a portable system, the cable is internal to the system, and it's usually permanently attached to the keyboard end and plugged into the motherboard at the other end. The cable itself is called a flex-film cable , which consists of a very thin dielectric ( insulating ) film with conductors bonded on, looking more like a flexible circuit board than a conventional cable. The fact that it is permanently attached at the keyboard end means that if the cable goes bad, you'll probably have to replace the entire keyboard assembly. Fortunately, most flex-film cable problems are not the cable itself so much as the connection between the cable and the system. In some cases, the contacts can become dirty or tarnished, or the thin film end can be partially or completely pulled out of the mating connector.

Troubleshooting a defective internal cable is simple:

1. Disconnect the film cable from the motherboard.

2. Clean the contacts using contact cleaner and a foam swab.

3. Reinsert the cable into the connector.

Many of the flex-film connectors have tiny latches that must be pulled out to release the grip on the cable, allowing it to be either removed or inserted. Once the cable is inserted, the latches are pressed inward, locking the flex-film end into place.

If after disconnecting, cleaning, and reconnecting the cable you find that the problem remains, it most likely lies with the motherboard or some other component in the system.

Stuck Keys

Many times you first discover a problem with a keyboard because the system has an error during the POST. Many systems use error codes in a 3xx numeric format to distinguish the keyboard. If you have any such errors during the POST, write them down. Some BIOS versions do not use cryptic numeric error codes; they simply state something such as the following:

 Keyboard stuck key failure 

This message is normally displayed by a system with a Phoenix BIOS if a key is stuck. Unfortunately, the message does not identify which key it is!

If your system displays a 3xx (keyboard) error preceded by a two-digit hexadecimal number, the number is the scan code of a failing or stuck keyswitch. Look up the scan code in the tables provided in the "Technical Reference" section on the CD-ROM to determine which keyswitch is the culprit. By removing the keycap of the offending key and cleaning the switch, you often can solve the problem.

If your problems are with an external non-USB keyboard, and replacing the keyboard doesn't seem to solve the problem, you can try testing the external PS/2-type mouse/keyboard connector. For a simple test of the external mouse/keyboard connector, you can check voltages on some of the pins. Using Figure 13.10 as a guide, measure the voltages on various pins of the mouse/keyboard connector. To prevent possible damage to the system or keyboard, turn off the power before disconnecting the keyboard. Then, unplug the keyboard and turn the power back on. Make measurements between the ground pin and the other pins according to Table 13.7. If the voltages are within these specifications, the motherboard keyboard circuitry is probably okay.

If your measurements do not match these voltages, the motherboard might be defective. Otherwise , the external keyboard cable or keyboard might be defective. If you suspect that the cable is the problem, the easiest thing to do is replace the keyboard cable with a known good one. If the system still does not work normally, you might have to replace the entire keyboard or the motherboard.

See the CD-ROM included with this book for a listing of the standard POST and diagnostic keyboard error codes used by some systems.

Cleaning a Keyboard

One of the best ways to keep a keyboard in top condition is periodic cleaning. As preventive maintenance, you should vacuum the keyboard weekly, or at least monthly. When vacuuming, you should use a soft brush attachment; this will help dislodge the dust. Also note that even most built-in keyboards have keycaps that can come off easily; if you're not careful, you'll have to dig them out of the vacuum cleaner.

You also can use canned compressed air to blow the dust and dirt out instead of using a vacuum. Before you dust a keyboard with the compressed air, turn the keyboard upside down so that the particles of dirt and dust collected inside can fall out.

On virtually all keyboards, each keycap is removable, which can be handy if a key sticks or acts erratically. For example, a common problem is a key that does not work every time you press it. This problem usually results from dirt collecting under the key. An excellent tool for removing keycaps on almost any keyboard is the U-shaped chip puller included in many computer toolkits. Although the process is slightly different with a built-in laptop keyboard, this tool is still useful. Simply slip the hooked ends of the tool under the keycap (with the ends of the tool pressing down the adjacent keys instead of fitting between them), squeeze the ends together to grip the underside of the desired keycap, and lift up. After removing the cap, spray some compressed air into the space under the cap to dislodge the dirt. Then replace the cap and check the action of the key.

When you remove the keycap on some keyboards, you are actually detaching the entire key from the keyswitch. Be careful during the removal or reassembly of the keyboard; otherwise, you might break the switch.

Spills can be a problem, too. If you spill a soft drink or cup of coffee into a keyboard, you do not necessarily have a disaster. Many keyboards that use membrane switches are spill resistant. However, you should immediately (or as soon as possible) remove the keyboard from the system and flush it out with distilled water. Depending on how much liquid was spilled, you may need to rinse or clean out other parts of the system as well. Most circuits can be washed, but mechanical devices like disk drives require more careful handling and cleaning, and they should not be immersed in water.

If the spilled liquid has dried , try soaking the keyboard in some water for a while. When you are sure the keyboard is clean, pour another gallon or so of distilled water over it and through the keyswitches to wash away any residual dirt. After the unit dries completely, you can reinstall it into the system, where it should be perfectly functional. You might be surprised to know that drenching your keyboard with water does not harm the components . Just make sure you use distilled water, which is free from residue or mineral content. (Bottled water is not distilled; the distinct taste of many bottled waters comes from the trace minerals they contain!) Also, make sure the keyboard is fully dry before you try to use it; otherwise, some of the components might short out.

Tip

If spills or excessive dust or dirt are expected because of the environment or conditions in which the system is used, you should purchase a thin membrane skin that molds over the top of the keyboard, protecting it from liquids, dust, and other contaminants . These skins are manufactured by several companies and are generally thin enough so that they don't interfere too much with the typing or action of the keys.