diagnosing and fixing motherboards faults

7/29/2019 Diagnosing and fixing Motherboards faults

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When a PC fails or exhibits in-termittent faults, the first re-sort is often to send for a

specialist engineer. Yet there is a lotthat PC support staff with the rightle vel o f co m p et en ce a n d u n d er -standing can do to filter out easily-fixed problems, leaving only those thatabsolutely require the services ofa p ro-fessional repair-shop. Failure of a sock-eted part is one thing, but replacing asurface mounted component normallydemands specialist equipment.

Reco rd- Keepi ng

When you re diagnosing faults, it isimportan t to keep a record of wh at youhave done. If nothing else, this willhelp a specialist later on if you are n otable to fix the system you rself. In add i-tion, it is vital to know w hat was h ap-pening when the problem occurred,particularly with interm ittent faults.

As long as the PC is still in its case,static safety shouldnt be a problem,but ideally you should ground your-

self with a static strap attached to thewrist and earthed to the workbench(which itself should be earthed). Fail-ure to take the n ecessary p recautionscan cause intermittent faults and/ ordamage components on the mother-board.

Starting Work

Does the PC do anything at all or isit dead? Ifyou h ave a dead PC, the firstthing to check is obviously the powersupp ly.Is the fan running? Ifit is, is thepower connection to the motherboard

OK?Most PSUs connect to the m other-board with two connectors, with theblack cables meeting in the midd le. Its

a good idea to keep a spare powersupply handy, as its not unusual forthem to burn out completely or for thefan to fail. Removing the case from apower supply normally involves re-moving four small screws to exposethe circuit board. Theres nothing onthe board that you can fix, but itsworth looking at the board in order tocheck for obvious signs of d amagesuch as a blown fuse or some gungeleaking from the large cylindrical ca-pacitors.

Never replace a blown fuse on apower supply. If the fuse blows, it hasprobably blown for a good reason(such as one of the PSU componentsburning out). Replacing the fuse isdangerous, and can also damage com-ponents on the PCs motherboard. Fita new power supply instead - theydon t cost more than a roun d US$30.

Assuming the power supply checksout, try a d ifferent monitor. Maybe thePC is working fine, but the monitor isbroken. If this turns out to be the case,you should not attempt to fix the moni-

tor yourself. Taking the case off amonitor exposes you to voltages thatcan kill, and its best left to a specialistwho knows which areas are safe totouch. Remember that, because of thehigh capacitance of a monitor, thoselethal voltages can exist even if themonitor has been turned off for manyhours.

The next step is to remove all theexpansion cards an d disconnect thehard and floppy drives to see if theyare holding the power rails down.(With EISA systems, make a note ofwhich boards were in which slots toavoid the need to rebuild the systemconfiguration later.) If the systemstarts to boot without the drives at-tached, re-attach them one by one untilit no longer boots, to see which is theproblem. Known-good cables shouldalso be tried w ith the dr ives, wh ile thereset and other switches should be d is-connected, in case they are causingproblem s. Leave the speaker con-nected - those beeps might be youronly clue as to w here the fault lies.

M odern PC motherboards are very modular in their design, and so easily accessible.

A nyone wit h a basic u nderstanding of how they work should be able to diagnose and

repair basic motherboard-related fault s on a PC.

By Bryan Betts

W hen youre diagnosing faults, it isimportant to keep a record of what you

have done. If n othing else, this will helpa specialist later on if you are not able

to fix the system yourself. In addition,it is vital to know what was happening

when the problem occurre.d

D iagnosing And Fixing

Motherboard Faults

Problem Solving:Hardware


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Phy si cal Checks

A visual check of the motherboardshould look for discoloured compo-nents and other obvious signs of over-heating. Socketed components shouldbe carefully pressed down to m akesure they are securely seated - this isparticularly important if the systemhas failed after being moved. Ensuretoo that they do not have bent legs:these can cause interm ittent failures if

they make contact to begin with bu t goopen circuit as the system warms up.After this, the processor and mem-

ory can be swapped for known-goodequivalents, ensuring that the proces-sor clock speed jumpers are correctlyset and that the memory is the correctspeed and compatible with the board.Wrongly setting the BIOS jum pers canin some cases wipe a Flash EPROM, sothe BIOS should be verified by swap-ping it with a know n-good chip.

If the BIOS is OK, the system will

normally beep when it completes itspower-on self test (POST). Most sys-tems also have a range of other recog-nisable beep codes to report certainfaults. A list of these should be sup-plied in the m otherboard or systemmanual and is essential for fault find-ing.

A h igh-imped ance oscilloscope canbe used to check that the clock is work-ing, by probing onto the underside of

the crystal or the capacitors around it.It should typically be producing a fre-quen cy in th e region of 14 MHz w hichis then mu ltiplied u p by oth er circuitryto the PCs bus frequency.

If the board fails to beep at all, andthe processor and BIOS have been re-placed and verified, it may m ean thatthe processor cannot access the BIOS.In this case, a major fault such as abroken PCB track or a loose SMT com-ponent should be suspected, and theboard will need sp ecialist analysis and

repair.

Subsy st em Faults

If the POST beep is present and thedisk access light is active but nothingappears on the screen, a video fault ispossible. (Incidentally, if the floppydisk light ever remains permanentlyactive after you ve p ut the m achineback together, this is a sure sign thatyouve p lugged in the floppy driveconnector the wrong way rou nd.)

If the screen is indeed blank, theon-board video should be disabled and

a known-good video card installed in-stead. (Again, keep a known-goodvideo card in your spares box. Theydont cost more than $50, and it saveshaving to take someone elses machineapart to borrow their video card.)

The video subsystem comes in twoparts: the controller generates the digi-tal image, and th e DAC (digital to ana-logue converter) turns this into theanalogue signal u nderstood by themonitor.

If there are synch signals presentb u t n o o u t p u t , t h e D A C m a y h a v efailed. If there is no synch signal a con-

troller p roblem is more likely. A blownDAC will get hot, but testing for this

The ToolkitAlongside screwdrivers and the like, the PC engineers toolkit should

includ e the following:

q A good software test suite such as PC-Check, Checkit or QA+.q Serial and parallel loopback connectors.q Multimeter.q Known-good components, including processors, SIMMs and batteries,

plu s video, IDE and serial/ par allel cards.q Anti-static wrist strap .

An oscilloscope is useful but probably overkill - the sort of problems itcan locate will probably requ ire specialist equ ipment to fix, anyw ay.

If the floppy disk light ever remainspermanently active after youve put themachine back together, this is a sure sign

that youve plugged in the floppy driveconnector the wrong way roun d.

Checklist

q Start with the power supply. Ensure that its working, and that itssupp lying pow er to the motherboard.

q Next, try a different monitor.q Remove all expan sion cards. If the machine boots, replace the cards one

by one until it doesnt.q Check motherboard for signs of blow components.q Try swapping the CPU with a known -good one.q If the video controller is built in, disable it and try another vid eo card.q Buy or borrow a POST card.q Check the CPU fan.q Check the RAM chips by replacing them with known-good ones.q Disable external cache.q Remember to keep a record of everything you do.q Take an ti-static precautions.



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needs caution as failed chips can beh otenoug h to bur n flesh. In any case, somemodern video chipsets combine theDAC and controller, making fault-finding harder.

A DPMS power-managed m onitorcould be useful as it will usually indi-cate, either w ith an on-screen m essageor by v arying the colour or blink p at-tern of the power LED, whether it isreceiving synch signals or n ot.

Key bo ard FuseA common problem isthe keyboard

fuse, which often blows if a keyboardis plugged in or removed when the PCis running. The keyboard fuse is usu-ally a subminiature surface-mountedcomponent that is not immediatelyrecognisable as such. Look for some-thing labelled F1 or F2an d located nearthe keyboard connector (on the moth-erboard, n ot in th e PC). The best wayto test it is with a multimeter or with aspare keyboard - if the keyboard lightsflash as the system pow ers on, the fuse

is OK.As always, dont replace the fuse

unless you have a p retty good idea ofwhy it blew in the first place. Thewhole idea of the fuse blowing is toprotect the other componen ts. Replac-ing it will expose the rest of the moth-

erboard to th e same fault again, unlessyou fix it first (eg, by replacing thekeyboard or remembering not to p lugit in while the PC is running).

Send A Post card

A useful diagnostic tool is a POSTcard, though good versions are hard tocome by and you should be wary ofpaying too much for one that you havenot h ad a chan ce to evaluate. PCs sendtheir POST results out on a particular

I/ O port - m ost use port 80, whileCompaqs may use port 84 and IBMsuse 300. The POST card, which plugsinto the ISA bu s, decodes these signalsand displays them either on a set ofLEDs or a seven-segment nu meric dis-play. A list of POST codes is needed,

and can be used to identify the resultsbeing produ ced by the self-test rou-tine.

If the POST card show s FF, an oscil-loscope can be used to see if the proc-essor is accessing th e BIOS, looking forclock signals, the address and datastrobes on the CPU, and for resets.

No Clock

Real-time clock (RTC) failure can befixed fairly simply, by replacing the

battery or, ifit is a combined clock/ bat-tery unit, by replacing the whole mod-ule. However, if this would requiresoldering, it should not be attempted:motherboards contain heavy groundplanes that require a lot of heat to beapplied very quickly to avoid damage.If this heat is applied too slowly, thelonger exposure to i t can burn theboard.

It is also important to replace thebattery with the correct type.Some PCsuse non-rechargeable Lithium cellsand others rechargeable N iCads, andifthe former is substituted for the latter

an explosion could result.Some clock modules allow you to

simply connect a standard householdbattery or two (eg, as used in a Walk-man) to override the exhausted built-inbattery. How ever, you n ormally needto set a jum per before doing this. Ify ouare in any doubt, dont do it.

Int ermi t t ent Fault s

Intermittent faults are the hardest todetect. If the system runs for a whilebut then halts, check the CPU fan. Ifthis has failed the CPU will overheat

until its thermal cut-out shuts it down.Although this cut-out protects the chip

Faulty joints or components can alsomake the system hang as it warms up and

causes them to expand and go open-circuit, but testing for this requires a

temperature chamber and is a specialist job.

Motherboard Faults



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to some extent, continued attempts touse the system in this state may causethermal damage to the processor chip.

Unfortunately, CPU fans seem toburn out w ith alarming regularity. Besure to keep one or two in your sparesbox, along with some of that specialheat-cond ucting glue to fix them to th eCPU chip itself.

Faulty joints or components canalso make the system han g as it warm sup and causes them to expand and go

open-circuit, bu t testing for this re-quires a temperature chamber and is aspecialist job. It is possible to diagnosesome faults of this kind by bend ing theboard but this is highly inadvisable asit can cause more problems by break-ing comp onents, joints an d tracks. Fora long life the board should be prop-erly supported and kept well cooled.

A motherboard that boots and thenhangs may have cache problems, inwhich case the level 1 and 2 cacheshould be d isabled in th e BIOS. Cacheproblems show mor e often when boot-ing from the floppy drive. If the prob-

lem goes away, the caches can th en bere-enabled one at a time to see wherethe fau lt lies. Alternatively, if the sys-tem w ill not boot from flopp y, this canbe a sign of comp onent failure or a d ry

joint on th e m otherb oard .Ifthe cache isOK,the main memory

needs to be tested with diagn osticsoft-ware, as a memory parity error willcause the system to hang , usu ally witha w arning message. You also n eed tocheck that the memory fitted matchesthe specifications laid down by theboard or system manufacturer and iscompatible. Adding a wait state in theCMOS set-up m ay alleviate a memorytiming problem. Check also whetherthe motherboard supp orts or requires

parity memory, and whether this isindeed fitted.

Once the base system is working,the expansion cards can be put backone at a time. How ever, before doingthis the test and diagnosis softwareshould be run to determine the free

I/ O space and resources on the PC.These programs willnot find problemssuch as timing errors but can provebase system functionality. Loopbackconnectors for the serial and parallelports are another essential diagnostictool needed here. If built-in ports orcontrollers are faulty it may be possibleto disable them and fit a card instead.

Once the base system is working, theexpansion cards can be put back oneat a time. However, before doing this

the test and diagnosis software shouldbe run to determine the free I/O space

and resources on the PC.

PCSA

The AuthorBryan Betts (br ya [email protected] k) is afreelance wr iter on IT issues. Ourthanks to Piyuskumar Shah at Re-sponse Computer Maintenance

(+44 181 965 322) for their helpwith this article.


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