Keyboard Troubleshooting and Repair

Keyboard errors are usually caused by two simple problems. Other more difficult, intermittent problems can arise, but they are much less common. The most frequent problems are as follows:

• Defective cables

• Stuck keys

Defective cables 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 is likely the culprit. Troubleshooting is simple, especially if you have a spare cable on hand. Simply replace the suspected cable with one from a known, working keyboard to verify whether the problem still exists. If it does, the problem must be elsewhere.

If you remove the cable from the keyboard, you can test it for continuity with a digital multimeter (DMM). DMMs that have an audible continuity tester built in make this procedure much easier to perform. To test each wire of the cable, insert the DMM's red pin into the keyboard connector and touch the DMM's black pin to the corresponding wire that attaches to the keyboard's circuit board. Wiggle the ends of the cable as you check each wire to ensure no intermittent connections exist. If you discover a problem with the continuity in one of the wires, replace the cable or the entire keyboard, whichever is cheaper. Because replacement keyboards are so inexpensive, it's almost always cheaper to replace the entire unit than to get a new cable, unless the keyboard is a deluxe model.

For more information about using digital multimeters for testing hardware, see Chapter 22, "PC Diagnostics, Testing, and Maintenance."

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 encounter 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 usually 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 disc 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.

For a simple test of the motherboard keyboard connector, you can check voltages on some of the pins. Using Figure 16.9, which was shown earlier in the chapter, as a guide, measure the voltages on various pins of the 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 16.2, shown earlier in the chapter. 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 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.

In many newer systems, the motherboard's keyboard and mouse connectors are protected by a fuse that can be replaced. Look for any type of fuse on the motherboard in the vicinity of the keyboard or mouse connectors. Other systems might have a socketed keyboard controller chip (8042-type). In that case, you might be able to repair the motherboard keyboard circuit by replacing this chip. Because these chips have ROM code in them, you should get the replacement from the motherboard or BIOS manufacturer. If the motherboard uses a soldered keyboard controller chip or a chipset that integrates the keyboard controller with other I/O chips, you'll need to replace the motherboard.

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

Keyboard Disassembly

Although disassembling a keyboard is possible, most likely you won't need or want to do that given the reasonable prices of keyboards. If you do want to disassemble your keyboard, see "Keyboard Disassembly" in the Technical Reference section of the disc accompanying this book.

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 many keyboards have keycaps that can come off easily. Be careful when vacuuming; otherwise, you'll have to dig them out of the vacuum cleaner. I recommend using a small, handheld vacuum cleaner made for cleaning computers and sewing machines; these have enough suction to get the job done with little risk of removing your keytops.

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 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 tool kits. Simply slip the hooked ends of the tool under the keycap, squeeze the ends together to grip the underside of the keycap, and lift up. IBM sells a tool designed specifically for removing keycaps from its keyboards, but the chip puller works even better. 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.

Caution

When you remove the keycaps, be careful not to remove the spacebar on the original 83-key PC and the 84-key AT-type keyboards. This bar is difficult to reinstall. The newer 101-key units use a different wire support that can be removed and replaced much more easily.

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'll break the switch. The classic IBM/Lexmark-type keyboards (now made by Unicomp) use a removable keycap that leaves the actual key in place, enabling you to clean under the keycap without the risk of breaking the switches. If your keyboard doesn't have removable keycaps, consider using cleaning wands with soft foam tips to clean beneath the keytops.

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) disconnect the keyboard and flush it out with distilled water. Partially disassemble the keyboard and use the water to wash the components. (See "Keyboard Disassembly" in the Technical Reference section of the disc accompanying this book for disassembly instructions.) If the spilled liquid has dried, soak the keyboard in some of the 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, 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 PC is used, several companies make thin membrane skins that mold over the top of the keyboard, protecting it from liquids, dust, and other contaminants. These skins are generally thin enough so that they don't interfere too much with the typing or action of the keys.