PCWorkz

 

MOTHERBOARD

---

   For the great majority of personal computers in the marketplace today, the motherboard (also called the main board) represents the single, most complex and expensive subassembly in the system. The motherboard contains the core processing elements (i.e., the CPU, main chipset, cache, and RAM), manages the system resources, such as IRQ lines and DMA channels, and supports the system’s expansion bus architectures (i.e., ISA, AGP, VL, and PCI). More than any other part of the PC, a motherboard defines the capabilities and over-all performance of a system .
   As a PC ages, however, the motherboard limits the system’s upgradeability. True, you can add RAM and upgrade a CPU. Although these tactics can prolong the working life of older systems, they have a limited impact on the overall performance of a motherboard (especially older i286 and early i386 motherboards). As PC technology surges ahead and the price of advanced motherboards continues to drop, replacing an outdated motherboard outright is becoming an ever-more cost-effective upgrade option. If however replacing your motherboard is not a option then we will show you solutions to common problems older or even newer boards may have.
   While all possible problems can not be foreseen, we will illustrate the most common problems and solutions most user have experienced. Also be aware that often times when we experience problems with the motherboard, more times then not the problem stems from a cable has become disconnected or the board itself has grounded out, i.e. It has come in contact with the frame of the tower. So if you experience problems with your board check these two options first before delving into more in depth solutions.

START WITH THE BASICS

    Because motherboard troubleshooting represents a significant expense, be sure to start any motherboard repair by inspecting the following points in the PC. Remember to turn all power off before performing these inspections:

My motherboard is installed, but the system won’t boot?
   This is a classic sign of installation problems. Start with the basics. Check all of the cables and connectors—especially the power connectors. Also be sure that no metal standoffs or brackets are shorting the motherboard from underneath. Next, check for any wiring or cables that might be installed backward. Although this will rarely keep a PC from booting, it is possible. Be sure that pin 1 on each cable aligns with pin 1 of each connector. Finally, double-check the socket-mounted ICs, such as the CPU, BIOS ROM(s), and SIMMs. They should all be aligned properly, inserted evenly and completely. If you locate an incorrectly installed IC, it might be damaged. Remove it from the motherboard, check it for bent or broken pins, reinsert it correctly, and try the motherboard again. If the IC is damaged, it should be replaced.

The power light is on, but the system hangs during initialization?
Video might be active, but there might be no text in the display. The Power On Self-Test (POST) has detected a fault, and is unable to continue with the initialization process. BIOS makers mark the completion of each POST step by writing single-byte hexadecimal completion codes to port 80h. Turn off and unplug the PC, then insert a POST board to read the completion codes. Reboot the computer and find the last code to be written before the initialization stops—that is the likely point of failure. You can determine the meaning of that POST code by finding the BIOS maker (usually displayed in the initial moments of power-up), then locating the corresponding error message below. Without a POST board available, it will be extremely difficult to identify the problem.

The power light is on, but you hear two or more beeps There is no video?
   Check the video board first. Video problems can easily halt the initialization process. Turn off and unplug the PC, then be sure that your video board is inserted completely into its expansion slot. Consider the beep code itself—a catastrophic fault has been detected in the Power On Self-Test (POST) before the video system could be initialized. BIOS makers use different numbers and patterns of beeps to indicate failures. You can determine the exact failure by finding the BIOS maker (usually marked on the motherboard BIOS IC), then finding the error message below. In the vast majority of cases, the fault will be traced to the CPU, RAM, motherboard circuitry, video controller, or drive controller.

My motherboard starts, but it will not boot from the hard drive or recognize the correct amount of RAM in the system?
   You might see an error message, such as “CMOS error: press to run Setup.” This error generally indicates that the motherboard is working, but the system CMOS contains incorrect information. Either you forgot to enter the new CMOS variables, you forgot to save the settings when you updated them, or the backup battery is not installed and CMOS contents were lost after the system was powered down. Check the backup battery first. If the battery is a coin cell, see that it is inserted properly and completely into its holder. If the battery is a “pack-type,” check to see that it is plugged into the proper motherboard connector in the right polarity. If the battery is installed correctly, try a new one. Run the Setup program and check each drive and memory setting. If you entered a drive parameter or RAM amount improperly, correct the settings and save CMOS again. Reboot the PC. If new CMOS settings are lost after the PC is powered down, the backup battery has failed—try a new battery.

There is no power light, and you cannot hear any cooling fan?
   Chances are that there is insufficient power to the computer. Use a voltmeter and confirm that there is adequate ac voltage at the wall outlet. Check the ac cord next—it might be loose or disconnected. See that the power switch is turned on and connected properly. Check the power-supply fuse(s). The main fuse might have opened. Replace any failed fuse.

I cannot use one or more drives in the PC?
   Either the drive(s) have been entered improperly in the CMOS setup routine or the drive(s) are cabled improperly. Start by checking the CMOS setup. Be sure that proper references for the floppy drives and hard drives are in the system. Fill in any missing information and reboot the computer. If the CMOS settings are all correct, check the data cables and power cables at each drive—loose power or signal cables can easily disable a drive.

The system boots and runs, but it locks up unpredictably?
   Be sure that the CPU and all system RAM is installed correctly and securely. Try reseating the RAM. Check the system CPU for excessive heat. An overheated CPU can lock up without warning. If the CPU is fitted with a heatsink, be sure that it is securely attached and that you have used ample amounts of thermal compound to aid heat transfer. If the CPU runs hot and there is no heatsink, try adding one.
   Another factor to consider is the possibility of controller conflicts. For example, if a video port is on the new motherboard, but you also have a video board installed in an expansion slot, you will have to set jumpers to disable the motherboard’s video port. The same thing is true for drive-controller conflicts, as well as serial or parallel-port conflicts. Take another close look at the expansion boards in your system and be sure that the board functions do not conflict with the functions provided on the motherboard.

There is no power light, but you hear the cooling fan running?
   This usually means that some level of ac power is reaching the system. Use a voltmeter and confirm 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. Verify that the power-supply cables are attached properly and securely to the motherboard. Use a voltmeter to verify that each output from the power supply is correct. If this signal is below 1.0 V, it might inhibit the CPU from running by forcing a Reset condition. Because the Power good signal is generated by the power supply, try replacing the power supply.

The system is active, but the video is erratic or absent?
   Check to see no video conflict is between the motherboard and a video expansion board. If a video port is on the motherboard, it probably should be disabled when an expansion video board is used in the system. Also check that the monitor is turned on and that the video cable is securely connected to the video port. If problems persist, check that the video board is installed correctly and that if extra memory must be excluded from the upper memory area, the proper command-line switches are included with EMM386 in the CONFIG.SYS file.

The power light is on, but there is no apparent system activity?
   Check the power-supply voltages. Use a voltmeter to verify that each output from the power supply is correct. Table 4-2 lists the proper voltage for each wire color. If any output is very low or absent (especially the 5-V output), replace the power supply. Use a voltmeter and verify that the Power good (or PwrOK) signal is +5 V. If this signal is below 1.0 V, it might inhibit the CPU from running by forcing a continuous Reset condition. Because the Power good signal is generated by the power supply, try replacing the power supply.
    Check to see that the CPU is cool, that the heatsink/fan assembly is fitted on correctly, and that the CPU itself is inserted properly and completely into its socket. Check the CPU socket—if the CPU is seated in a Zero Insertion Force (ZIF) socket, be sure that the socket’s tension lever is closed and locked into place. If there is a separate math coprocessor on the motherboard (i286 and i386 systems), be sure that the MCP is inserted properly and completely into its socket. Next, check the expansion boards and be 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 prevent the PC from starting. Check the motherboard for shorts. Inspect the motherboard at every metal standoff and see that no metal traces are being shorted against a standoff or screw. You might want to free the motherboard and see if the system starts. If it does, use nonconductive spacers (such as a small piece of manila folder) to insulate the motherboard from each metal standoff. If the system still fails to start (and all voltages from the power supply are correct), replace the motherboard.

You see no drive light activity The boot drive cannot be located?
    The most frequent cause of drive problems is power connections. Inspect the 4-pin power cable and see that it is attached properly and completely to the drive. Check the power-supply voltages next. Use a voltmeter and verify that the +5- and +12-V levels (especially +12 V) are correct at the 4-pin connector. If either voltage is low or absent, replace the power supply. Locate the wide ribbon cable that connects to the drive and be sure that it is attached correctly and completely at the drive and controller ends. Look for any scrapes or nicks along the cable that might cause problems. Start the CMOS setup. If you are working on an older system (early i386 and i286 systems), you will probably need to boot the PC from a setup disk.
   Check the CMOS setup next. Review the drive parameters entered in the CMOS setup, and be sure that the CMOS entries accurately reflect the actual boot drive installed on your system. If not, correct the error(s), save your changes, and reboot the system. Also, be sure that the drive-controller board is installed properly and completely in its expansion slot, and see that any jumpers are set correctly. Try booting the system from your boot floppy. If the system successfully boots to the A: prompt, your problem is limited to the hard-drive system. Now try switching to the C: drive. If the drive responds (and you can access its information), there might be a problem with the boot sector. Try a package, such as PC Tools or Norton Utilities, to try and “fix” the boot sector. If you can’t access the hard drive, try a diagnostic to check the drive controller and drive. Check for boot-sector viruses. A boot-sector virus can render the hard drive unbootable. If you haven’t checked for viruses yet, use your anti-virus work disk now, and focus on boot-sector problems. If you cannot determine the problem at this point, try replacing the drive with a known-good working drive. Remember that you will have to change the CMOS setup parameters to accommodate the new drive. If all else fails, try a new drive-controller board.

PINOUTS OF ATX AND BABY AT POWER CONNECTORS

ATX POWER CONNECTOR

COLOR	VOLTAGE	PIN
Orange	+3.3 Vdc	1
Orange	+3.3 Vdc	2
Black	GND	3
Red	+5 Vdc	4
Black	GND	5
Red	+5 Vdc	6
Black	GND	7
Gray	PwrOK	8
Purple	+5V standby	9
Yellow	+12 Vdc	10
Orange(22AWG) Brown (22AWG)	+3.3 Vdc 3.3 V sense	11
Blue	–12 Vdc	12
Black	GND	13
Green	PS-ON	14
Black	GND	15
Black	GND	16
Black	GND	17
White	-5 Vdc	18
Red	+5 Vdc	19
Red	+5 Vdc	20

BABY AT POWER CONNECTORS 

COLOR	VOLTAGE	PIN
Orange	PwrOK	1 (P8)
Red	+5 Vdc	2 (P8)
Yellow	+12 dc	3 (P8)
Blue	-12 Vdc	4 (P8)
Black	GND	5 (P8)
Black	GND	6 (P8)
Black	GND	1 (P9)
Black	GND	2 (P9)
White	-5 Vdc	3 (P9)
Red	+5 Vdc	4 (P9)
Red	+5 Vdc	5 (P9)
Red	+5 Vdc	6 (P9)

My motherboard reports a fail, but goes away when the PC’s outer cover is removed?
An intermittent connection is on the motherboard. When the housing is secured, the PC chassis warps just slightly—this might be enough to precipitate an intermittent contact. When the housing is removed, the chassis relaxes and hides the intermittent connection. Replace the outer cover and gently re-tighten each screw with the system running. Chances are that you will find one screw that triggers the problem. You can leave that screw out, but it is advisable to replace the motherboard as a long-term fix.

The POST (software diagnostic) reports a CPU fault?
This is a fatal error, and chances are that system initialization has halted. CPU problems are generally reported when one or more CPU registers do not respond as expected, or has trouble switching to the protected mode. In either case, the CPU is probably at fault. Fortunately, the CPU is socket mounted and should be very straightforward to replace. Be sure to remove all power to the PC and make careful use of static controls when replacing a CPU. Mark the questionable CPU with indelible ink before replacing it.
Zero-Insertion Force (ZIF) sockets are easiest because the IC will be released simply by lifting the metal lever at the socket’s side. Slide out the original CPU and insert a new one. Secure the metal lever and try the PC again. However, many CPUs are mounted in Pin-Grid Array (PGA) sockets, and a specialized PGA-removal tool is strongly suggested for proper removal. You should also be able to use a small, regular screwdriver to gently pry up each of the four sides of the CPU, but be very careful to avoid cracking the IC, the socket, or the motherboard—never use excessive force. If the IC is free, install the new CPU with close attention to pin alignment, then gently press the new CPU into place.
A word about heatsink/fans. Most i486 (and later) CPUs are equipped with a metal heatsink (or heatsink/fan) assembly. It is vital to the proper operation of your system that the heatsink be reinstalled correctly. Otherwise, the new CPU will eventually overheat and lock up or fail. Be sure to use good-quality thermal compound to ensure proper heat transfer to the heatsink (remember that a sound mechanical connection does not guarantee a good thermal connection).

The POST (or software diagnostic) reports a problem with the floating-point unit?
Math coprocessor (also called the Floating-Point Unit, FPU) problems are generally reported when one or more MPC registers do not respond as expected. Fortunately, MCP faults are not always fatal. It is often possible to remove the MCP or disable the MCP availability through the CMOS setup. Of course, programs that depend on the MCP will no longer run, but at least the system can be used until a new one is installed. On older systems that use separate MCPs, the device is socket mounted, and should be very straightforward to replace. Be sure to remove all power to the PC, and make careful use of static controls when replacing an MPC. Mark the questionable MPC with indelible ink before replacing it. If the MCP is integrated into the CPU (i386DX, i486DX, Pentium, and later CPUs) are a bit more expensive because you’ll need to replace the entire CPU, but the replacement process is no more difficult (remember to remount any heatsink/fan assembly properly).

The POST (or software diagnostic) reports a BIOS ROM checksum error?
The integrity of your system BIOS ROM is verified after the CPU is tested. This is necessary to ensure that no unwanted instructions or data could easily crash the system during POST or normal operation. A checksum is performed on the ROM contents, and that value is compared with the value stored in the ROM itself. If the two values are equal, the ROM is considered good and initialization continues. Otherwise, the BIOS is considered defective and should be replaced. Chapter 6 provides an index of major BIOS manufacturers.
Traditionally, BIOS ROM is implemented as one or two ICs that are plugged into DIP sockets. They can be removed easily with the blade of a regular screwdriver, as long as you pry the IC up slowly and gently (be sure to pry the IC evenly from both ends). When installing new DIP ICs, you might have to straighten their pins against the surface of a table or use a DIP pin-straightening tool. Ultimately, the IC pins will fit nicely into each receptacle in the DIP socket. You can then ease the IC evenly into the socket. Alignment is crucial to ensure that all pins are inserted. If not, one or more pins might be bent under the IC and ruin the new ROM. Also, be sure to insert the new IC(s) in the proper orientation. If they are accidentally installed backward, they might be damaged.
Newer BIOS ICs use flash EEPROM technology, which allows the device to be erased and reprogrammed in the field without having to replace the entire BIOS ROM IC. When a flash BIOS fails its checksum test, it also has probably failed. Because flash BIOS devices are often fabricated as PLCC ICs, it is a bit easier to replace them, but you will need a PLCC-removal tool to take the original IC out of its socket—there simply is not enough room for a screwdriver.

A keyboard error is reported on my system, but a new keyboard has no effect?
The keyboard fuse on the motherboard might have failed. Many motherboard designs incorporate a small fuse (called a pico-fuse) in the +5-Vdc line that drives the keyboard. If this fuse fails, the keyboard will be dead. Use your multimeter and measure the +5-Vdc line at the keyboard connector. If this reads 0 Vdc, locate the keyboard fuse on the motherboard and replace it (you might have to trace the line back to the fuse, which looks almost exactly like a resistor).

The POST (or software diagnostic) reports a CMOS or RTC fault?
With either error, the same device is usually at fault. The CMOS RAM and RTC are generally fabricated onto the sane device. RTC problems indicate that the real-time clock portion of the IC has failed or is not being updated. CMOS RAM failure can be caused by a dead backup battery or by the failure of the IC itself. When dealing with a CMOS or setup problem, try the following protocol. First, try a new backup battery and reload the CMOS setup variables. If a new battery does not resolve the problem, the CMOS/RTC IC should be replaced. Often, the CMOS/RTC IC is surface mounted, and will have to be replaced (or the motherboard will have to be replaced). However, the growing trend is toward making the IC socket mounted and including the battery into a single replaceable module (such as the Dallas Semiconductor-type devices). Modules are typically replaceable DIP devices.

The POST (or software diagnostic) reports a fault in the first 64KB of RAM?
The first RAM page is important because it holds the BIOS Data Area (BDA) and interrupt vectors—the system will not work without it. When a RAM error is indicated, your only real recourse is to replace the motherboard RAM. On older motherboards, if the diagnostic indicates which bit has failed and you can correlate the bit to a specific IC, you can sometimes replace the defective IC (typically, surface mounted). Otherwise, you will need to locate and replace all of the motherboard RAM, or replace the motherboard entirely. Newer motherboards utilize SIMMs or DIMMs for all system memory, so it should be relatively simple to cycle through each SIMM or DIMM with a known-good unit and isolate the defective memory.

My “jumperless motherboard” receives incorrect CPU Soft Menu settings and now refuses to boot?
This is usually caused by accidental settings during system configuration. Fortunately, this type of problem can be corrected by removing power from the motherboard— try turning off the system and unplugging it for several minutes. When you restore power to the system, the CPU soft menu will automatically reset the CPU frequency for the lowest setting and allow the motherboard to boot. You can then go back into the CPU soft menu and correct any speed-setting errors. If this were a jumpered motherboard, you would need to find the CPU speed jumper and set it correctly.

After installing Windows 95, my system can no longer find the CD-ROM drive on the secondary IDE channel?
You might also find that the IDE drives are running in MS-DOS “compatibility mode.” This problem occurs often with motherboards using the Intel 430HX chipset—Windows 95 is not recognizing the Intel 82371SB drive controller on the motherboard, which causes BIOS to disable the secondary IDE channel—devices on the secondary channel are not being detected after the system is rebooted. In most cases, you can upgrade the BIOS to correct this problem or move the IDE devices to a separate IDE controller. You might also be able to find an update to the MSHDC.INF file, which will force Windows 95 to recognize the 82371SB controller.

My Pentium motherboard is unusually picky about which SIMMs it will accept?
This occurs even though the SIMMs are all within the proper type and rating. Consider several possible problems. First, Intel chipsets are very discriminating when it comes to memory speed, so be sure that the memory speed is well within the required range (usually 70 ns or faster). Second, try changing the wait states in the CMOS setup to a lower speed (e.g., 4-4-4-4). If your system works under this low speed, then increase the speed (e.g., 3-3-3-3, 3-2-2-2, 3-1-1-1, etc.) and keep trying until the best number has reached. Finally, the memory itself might be of questionable quality— try good-quality memory, bought from a reputable vendor. Be sure that the vendor offers a liberal return policy so that you can return questionable memory easily.

Your customer forgets their password The PC password is stored in the CMOS RAM, which is located in either the motherboard chipset or the real-time clock chip. If it is stored in the chipset, the CMOS memory is backed up by a coin-shaped lithium battery (or other battery). If it is stored in the RTC chip, it has an internal battery to back up the CMOS RAM. For the external battery, follow these steps: First, make a complete backup of the CMOS settings. Turn off the system, and remove the battery for at least two hours. This should clear the CMOS setting and erase the password. For the RTC battery, follow these steps: Determine which RTC chip you have—the five different kinds of real-time clock CMOS chips are: