Com 226 comp trouble shooting II theory book
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| Keyboard Troubleshooting Although their appearance might seem daunting at first glance, keyboard systems are not terribly difficult to troubleshoot. This ease is primarily because of the keyboard’s modularity— if all else fails, it’s a simple matter to replace a keyboard outright. The keyboard’s great weakness, however, is its vulnerability to the elements. Spills, dust, and any other foreign matter that finds its way between the key caps can easily ruin a keyboard. The keyboard’s PC board is also a likely candidate to be damaged by impacts or other physical abuse. The following procedures address many of the most troublesome keyboard problems. Symptom 20-1. During initialization, an error message indicates that no keyboard is connected Check your keyboard cable and see that it is inserted properly and completely into the PC connector. Remember that you will have to reboot your system to clear this error message. Try another compatible keyboard. If anew keyboard assembly works properly, there is probably a wiring fault in the original keyboard. Given the very low price of new keyboards, it is usually most economical to simply replace a defective keyboard. If you’re working on a file or network server, see that the CMOS setup has enabled the keyboard. If a known-good keyboard fails to function, try the original keyboard on a known-good PC to verify that the keyboard itself is indeed operational. If so, your trouble now lies in the PC. Check the wiring between the PC keyboard connector and the motherboard. Check the connector pins to be sure that none of them have been bent or pushed in (resulting in a bad connection. You might also want to check the soldering connections where the keyboard connector attaches to the motherboard. Repeated removals and insertions of the keyboard might have fatigued the solder joints. Reheat any defective solder joints. If the keyboard connector is intact, it is likely that the keyboard controller IC (KBC) has failed. Try booting the PC with a POST board installed (as covered in Chapter 15). A KBC failure will usually be indicated by the system stopping on the appropriate POST code. You can attempt to replace the KBC or replace the motherboard outright. If a POST board indicates a fault other than a KBC such as the programmable interrupt controller, which manages the KBC’s interrupt, you can attempt to replace that component, or simply exchange the motherboard anyway. Symptom 20-2. During initialization, an error message indicates that the keyboard lock is on In many cases, the detection of a locked keyboard will halt system initialization. Be sure that the keyboard lock switch is set completely to the unlocked position. If the switch is unlocked, but the system detects it as locked. The switch might be defective. Turnoff and unplug the system, then use a multimeter to measure continuity across the lock switch (you might need to disconnect the lock switch cable from the motherboard. In one position, the switch should measure as an open circuit. In the opposing position, the switch should measure as a short circuit. If this is not the case, the lock switch is probably bad and should be replaced. If the switch measures properly, there is probably a logic fault on the motherboard (perhaps the keyboard controller. Your best course is to try another motherboard. SYSTEM DATA AND TROUBLESHOOTING Symptom 20-3. The keyboard is completely dead—no keys appear to function at all other computer operations are normal. In this symptom, it is assumed that your computer initializes and boots to its DOS prompt or other operating system as expected, but the keyboard does not respond when touched. Keyboard-status LEDs might not be working properly. Your first step in such a situation is to try a known-good keyboard in the system. Notice that you should reboot the system when a keyboard is replaced. If a known-good keyboard works, the fault is probably on the keyboard-interface IC. You can attempt to replace this IC if you wish, but it is often most economical to simply replace the keyboard outright. If another keyboard fails to correct the problem, use a multimeter and check the +V supply at the keyboard connector (refer to Fig. 20-5). If the +V signal is missing, the female connector might be broken. Check the connector’s soldering junctions on the motherboard. Reheat any connectors that appear fatigued or intermittent. Many motherboards also use a “pico-fuse” to protect the +V supply feeding the keyboard connector. If your +5 V is lost, locate and check the keyboard-connector fuse. If problems continue, replace the motherboard. Symptom 20-4. The keyboard is acting erratically One or more keys appear to work intermittently, or it is inoperative. The computer operates normally and most keys work just fine, but one or more keys do not respond when pressed. Extra force or repeated strike might be needed to operate the key. This type of problem can usually range from a minor nuisance to a major headache. Chances are that your key contacts are dirty. Sooner or later, dust and debris works into all key switches. Electrical contacts eventually become coated and fail to make contact reliably. This symptom is typical of older keyboards, or keyboards that have been in service for prolonged periods of time. In many cases, you need only vacuum the keyboard and clean the suspect contacts with a good-quality electronic contact cleaner. Begin by disconnecting the keyboard. Use a static-safe, fine-tipped vacuum to remove any accumulations of dust or debris that might have accumulated on the keyboard PC board. You might wish to vacuum your keyboard regularly as preventive maintenance. Once the keyboard is clean, gently remove the plastic key cap from the offending keys. The use of a keycap-removal tool is highly recommended, but you might also use a modified set of blunt-ended tweezers with their flat ends (just the tips) bent inward. Grasp the key cap and pull up evenly. You can expect the cap to slide off with little resistance. Do not rip the key cap off—you stand a good chance of marring the cap and causing permanent key switch damage. Use a can of good-quality electronics-grade contact cleaner and spray a little bit of cleaner into the switch assembly. When spraying, attach the long, narrow tube to the spray nozzle—this directs cleaner into the switch. Work the switch in and out to distribute the cleaner. Repeat this maintenance once or twice to clean the switch thoroughly. Allow residual cleaner to dry thoroughly before retesting the keyboard. Never use harsh cleaners or solvents. Industrial-strength chemicals can easily ruin plastic components and housings. Reapply power and retest the system. If the suspect keys) respond normally again, install the removed key caps and return the system to service. As a preventive measure, you might wish to go through the process of cleaning every key. Membrane keys must be cleaned somewhat differently from mechanical keys. It is necessary for you to remove the rubber or plastic boot to clean the PC board contacts. Depending on the design of your particular membrane switch, this might not bean easy task. If you are |