When a system starts but crashes/reboots intermittently

1. The system randomly crashes or reboots for no apparent reason:

• Check for viruses. Some viruses (especially memory-resident viruses) can cause the PC to crash or reboot unexpectedly. If you haven't run your virus checker yet, do so now.
• Check the power supply cables. Verify that the power supply cables are attached properly and securely to the motherboard.
• Check the powersupply voltage. As I have said earlier use a voltmeter to verify that each output from the power supply is correct. If any output is low (especially the +5-volt output), replace the power supply.
• Check the CPU. with all power off, check to see that the CPU is cool, that the heat-sink/fan assembly is fitted on correctly, and that the CPU itself is inserted properly and completely into its socket. If the CPU overheats, it will stall, taking the entire system with it. 
• Check the CPU socket. If the Cpu is seated in a ZIF (sero insertion force) socket, make sure that the socket's tension lever is closed and locked into place.
• Check all SIMMs. With all power off, make sure that all SIMMs are seated properly in their holders and locked into place. you may try removing each SIMM, clean the contacts, and reinstalling the SIMMs.
• Check the expansion boards. Make sure that all expansion boards are seated properly. Any boards that are not secured properly, or that are inserted unevenly, can short bus signals and cause spurious reboots. If you have recently installed now expansion hardware, make sure that there are no hardware conflicts between interrupts. DMA channels, or I/Q addresses. 
• Check the motherboard for shorts. Inspect the motherboard at every metal standoff and make sure that no metal traces are being shorted against a standoff or screw. You may want to free the motherboard and see if the crashes or reboots go away. If so, use nonconductive spacers (such as a small piece of manila folder to insulate the motherboard from each metal standoff. If the system continues to crash or reboot (and all voltages from the power supply are correct). replace the motherboard.   

Start up Problem

If there is no power light, but you do hear the cooling fan running:

• Check the ac voltage. Use a voltmeter and conform that there is adequate ac voltage at the wall outlet. Unusually low ac voltages (such as during "brownout" conditions) can cause the power supply to malfunction.
• Check the power supply cables. Verify that the power supply cables are attached properly and secure to the motherboard. A typical connection scheme is shown in figure above.
• Check the power supply voltages. Use a voltmeter to verify that each output from the power supply is correct. If any output is very low or absent (especially the +5-volt output), replace the power supply.
• Check the "Power Good" signal. Use a voltmeter and verify that the "Power Good" signal is +5 volts. If this signal is below 1.0 volt, It may inhibit the CPU from running by forcing a continous Rest condition. Since the "Power Good" signal is generated by the power supply, replace the power supply. 

Evaluating Startup Problems

There are many problems that can plague the PC, but perhaps the most troubling problems occur during startup-when computer fails to start at all or does not start completely. Startup problems make it most impossible to use diagnostics or other utilities that we depend on to help isolate problems. With the advent of windows 95, there are even more difficulties that can develop. This part of the chapter offers you a series of possible explanations for full and partial system failures.

When a system doesn't start at all

1. If there is no power light, and you cannot hear any cooling fan:

• Check the ac voltage. Use a voltmeter and confirm that there is adequate ac voltage at the wall outlet.
• Check the ac cord. The ac cord may be loose or disconnected.
• Check the power supply fuse(s). The main fuse may have opened. Replace any failed fuse.  

1           1   Orange     Pwr Good
             2   Red          +5
             3   Yellow       +12
             4   Blue          -12
             5   Black        Gnd
 6          6   Black        Gnd

                                                                   Motherboard power connector
1           1   Black        Gnd
             2   Black        Gnd
             3   White        -5
             4   Red          +5
             5   Red          +5
6           6   Red          +5

NOTE: If you replace a main fuse and the fuse continues to fail, you may have a serious fault in the power supply. Try replacing the power supply.

Guard and shielding

Modern PCs and peripherals often employ an assortment of metal and plastic shields or guards within the device. Shields and guards serve a variety of purposes, but all should be replaced when service is complete.

1. Replace EMI shields. PCs operate at very high frequencies, and the signals they generate can sometimes be transmitted to nearby recivers such as radios and televisions. Ideally, the PCs design should prevent such EMI (electromagnetic interference), but it may also be necessary to add metal shields to attenuate excessive interference. Whenever you remove metal housings or shields from a PC, be sure to replace them before returning the system to service. 
2. Replace x-ray shields. Monitors use extremely high voltages at the CRT, which in turn can liberate x-rays through the CRT glass. The lead contained in CRT glass is usually a sufficient shield, but larger CRT funnel. When you remove x-ray shields from a monitor, be sure to replace them before running the monitor or returning it to the service. 
3. Replace all guards and other mechanical assemblies. Printers typically employ a large assortment of guards and covers (both plastic and metal) to protect delicate mechanical assemblies from dust and accidental contact. You can usually operate a printer for short periods without guards in place, but you should always make in a point to replace any protective assemblies before returning the device to service.   

Electricity Control

PCs and their peripherals use raw alternating current as a power source. While the myriad of plugs, outlets, and line cords used today are generally regarded as quite sage for end users, technicians must often work in close proximity to exposed circuitry. In reality, the odds of electrocution are quite slim, but electricity can injure or kill when handled carelessly.

• Keep the PC unplugged when working inside. As a rule, unplug the PC (don't just turn it off) during upgrades or repairs.
• Use only one hand for "hot" measurements. If you make measurements or probe around inside a powered system (especially inside the power supply), keep one hand behind your back. If you should contact a live wire, there is no pathway through your heart.
• Use properly rated test probe's. If you attemp to measure high through commercial test probes, you can be electrocuted right through the probe's insulation. Make sure the prob you're using is rated for the expected voltage levels.

Rules

• Use wrist straps. Grounding wrist straps are the first line of defence against ESD. They attach to your wirst and connect to a grounded surface or outlet through a wire. When properly connected, a wrist strap "bleeds off" any charge on your body and clothing, make it safer to handle delicate electronics.
• Use antistatic containers. You have probably noticed that all delicate electronics comes packaged in blue or pink bag. These act as "Faraday cages" which dissipate charges before thay can reach the device contained inside. Always keep devices inside antistatic containers until you are ready to actually install them, then place any removed device into an antistatic container immediately.
• Use and antistatic mat. A mat works like a wirst strap by connecting to ground and bleeding off any accumilated charges. You can place boards, ICs or SIMMs sagely on a properly connected anti static mat without having to place them in container. Antistatic mats are very popular on PC repair workbenches where sensitive items are regularly installed and removed.
• Use antistatic chemicals. Monitor screens, most synthetic surfaces, and virtually all plastic enclosures are major sources of ESD. when properly and regularly applied, antistatic chemicals can go a long way towards preventing ESD damage from accidental or casual contact with sensitive electronics.
• Manage temperature and humidity. Static builds up easily in cool, dry environments. work in a warm area with adequate relative humidity (RH). Use a humidifier if necessary to maintain adequate RH.

Troubleshooting Guidelines

Static Control
Modern PCs depend on extermely complex integrated circuits, and those ICs are very sensitive to ESD (electrostatic discharge). Unfortunately, static electricity means as passing a comb through our hair or putting on a sweater. When ESD is allowed to discharge through an IC, the IC is destroyed. They are no outware signs of ESD damage-no smoke, no fire, and rarely and shock or other physical sensation. Still, the damage is quite real. ESD is controlled by combination of grounding, protective materials, and environmental management.

Disclaimer & Cautions

It is IMPORTANT that you read and understand the following information. Please read it carefully!

PERSONAL RISK & LIMITS OF LIABILITY

The repair of personal computer and their peripherals involves some amount of personal risk. Use extreme caution when working with ac and high-voltage power sources. Every resonable effort has been made to identify and reduce areas of personal risk. You are instructed to read this carefully before attemping the procedures disussed. If you are uncomfortable followin the procedures do not attempt them - refer your service to qualified service personnel.

PREPARING FOR SERVICES

Troubleshooting is a lot like taking a test-there is a problem that needs to be solved, and you are "graded" on the speed and success with which you solve that problem. As with any test, a certain amount of preparation is required, and you often have to make some basic assumptions about the problem in order to solve it. Unfortunately, many less-experienced technicians are quick to jump at a solution without performing essential checks and observations. Too often, this results in lost time and wasted materials. We present you with a set of checklists and guidelines that can help to speed your diagnosis of a problem and keep your troubleshooting on track.