When a PC fails or exhibits intermittent
faults, the first resort
is often to send for a
specialist engineer. Yet there is a lot
that PC support staff with the right
level of competence and understanding
can do to filter out easilyfixed
problems, leaving only those that
absolutely require the services of a professional
repair-shop. Failure of a socketed
part is one thing, but replacing a
surface mounted component normally
demands specialist equipment.
Record-Keeping
When you’re diagnosing faults, it is
important to keep a record ofwhat you
have done. If nothing else, this will
help a 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 occurred,
particularly with intermittent faults.
As long as the PC is still in its case,
static safety shouldn’t be a problem,
but ideally you should ground yourself
with a static strap attached to the
wrist and earthed to the workbench
(which itself should be earthed). Failure
to take the necessary precautions
can cause intermittent faults and/or
damage components on the motherboard.
Starting Work
Does the PC do anything at all or is
it dead? If you have a dead PC, the first
thing to check is obviously the power
supply. Is the fan running? If it is, is the
power connection to the motherboard
OK?Most PSUs connect to themotherboard
with two connectors, with the
black cables meeting in the middle. It’s
a good idea to keep a spare power
supply handy, as it’s not unusual for
them to burn out completely or for the
fan to fail. Removing the case from a
power supply normally involves removing
four small screws to expose
the circuit board. There’s nothing on
the board that you can fix, but it’s
worth looking at the board in order to
check for obvious signs of damage
such as a blown fuse or some gunge
leaking from the large cylindrical capacitors.
Never replace a blown fuse on a
power supply. If the fuse blows, it has
probably blown for a good reason
(such as one of the PSU components
burning out). Replacing the fuse is
dangerous, and can also damage components
on the PC’s motherboard. Fit
a new power supply instead - they
don’t cost more than around US$30.
Assuming the power supply checks
out, try a different monitor. Maybe the
PC is working fine, but the monitor is
broken. If this turns out to be the case,
you should not attempt to fix themonitor
yourself. Taking the case off a
monitor exposes you to voltages that
can kill, and it’s best left to a specialist
who knows which areas are safe to
touch. Remember that, because of the
high capacitance of a monitor, those
lethal voltages can exist even if the
monitor has been turned off for many
hours.
The next step is to remove all the
expansion cards and disconnect the
hard and floppy drives to see if they
are holding the power rails down.
(With EISA systems, make a note of
which boards were in which slots to
avoid the need to rebuild the system
configuration later.) If the system
starts to boot without the drives attached,
re-attach them one by one until
it no longer boots, to see which is the
problem. Known-good cables should
also be tried with the drives, while the
reset and other switches should be disconnected,
in case they are causing
problems. Leave the speaker connected
- those beeps might be your
only clue as to where the fault lies.
Update 108 (October 1997) Page 3 File: P1001.1
Modern PC motherboards are very modular in their design, and so easily accessible.
Anyone with a basic understanding of how they work should be able to diagnose and
repair basic motherboard-related faults on a PC.
By Bryan Betts
“When you’re diagnosing faults, it is
important to keep a record of what you
have done. If nothing else, this will help
a 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
Diagnosing And Fixing
Motherboard Faults
PC Support Advisor
Problem Solving:Hardware
Physical Checks
A visual check of the motherboard
should look for discoloured components
and other obvious signs of overheating.
Socketed components should
be carefully pressed down to make
sure they are securely seated - this is
particularly important if the system
has failed after being moved. Ensure
too that they do not have bent legs:
these can cause intermittent failures if
they make contact to beginwith but go
open circuit as the system warms up.
After this, the processor and memory
can be swapped for known-good
equivalents, ensuring that the processor
clock speed jumpers are correctly
set and that the memory is the correct
speed and compatible with the board.
Wrongly setting the BIOS jumpers can
in some caseswipe a Flash EPROM, so
the BIOS should be verified by swapping
it with a known-good chip.
If the BIOS is OK, the system will
normally beep when it completes its
power-on self test (POST). Most systems
also have a range of other recognisable
beep codes to report certain
faults. A list of these should be supplied
in the motherboard or system
manual and is essential for fault finding.
A high-impedance oscilloscope can
be used to check that the clock isworking,
by probing onto the underside of
the crystal or the capacitors around it.
It should typically be producing a frequency
in the region of 14MHz which
is then multiplied up by other circuitry
to the PC’s bus frequency.
If the board fails to beep at all, and
the processor and BIOS have been replaced
and verified, it may mean that
the processor cannot access the BIOS.
In this case, a major fault such as a
broken PCB track or a loose SMT component
should be suspected, and the
boardwill need specialist analysis and
repair.
Subsystem Faults
If the POST beep is present and the
disk access light is active but nothing
appears on the screen, a video fault is
possible. (Incidentally, if the floppy
disk light ever remains permanently
active after you’ve put the machine
back together, this is a sure sign that
you’ve plugged in the floppy drive
connector the wrong way round.)
If the screen is indeed blank, the
on-boardvideo shouldbe disabled and
a known-good video card installed instead.
(Again, keep a known-good
video card in your spares box. They
don’t cost more than $50, and it saves
having to take someone else’smachine
apart to borrow their video card.)
The video subsystem comes in two
parts: the controller generates the digital
image, and the DAC (digital to analogue
converter) turns this into the
analogue signal understood by the
monitor.
If there are synch signals present
but no output, the DAC may have
failed. If there is no synch signal a controller
problem ismore likely.Ablown
DAC will get hot, but testing for this
The Toolkit
Alongside screwdrivers and the like, the PC engineer’s toolkit should
include the following:
l A good software test suite such as PC-Check, Checkit or QA+.
l Serial and parallel loopback connectors.
l Multimeter.
l Known-good components, including processors, SIMMs and batteries,
plus video, IDE and serial/parallel cards.
l Anti-static wrist strap.
An oscilloscope is useful but probably overkill - the sort of problems it
can locate will probably require specialist equipment to fix, anyway.
File: P1001.2 Update 108 (October 1997) Page 4
“If the floppy disk light ever remains
permanently active after you’ve put the
machine back together, this is a sure sign
that you’ve plugged in the floppy drive
connector the wrong way round.”
Checklist
l Start with the power supply. Ensure that it’s working, and that it’s
supplying power to the motherboard.
l Next, try a different monitor.
l Remove all expansion cards. If the machine boots, replace the cards one
by one until it doesn’t.
l Check motherboard for signs of blow components.
l Try swapping the CPU with a known-good one.
l If the video controller is built in, disable it and try another video card.
l Buy or borrow a POST card.
l Check the CPU fan.
l Check the RAM chips by replacing them with known-good ones.
l Disable external cache.
l Remember to keep a record of everything you do.
l Take anti-static precautions.
PC Support Advisor
Problem Solving:Hardware
needs caution as failed chips can be hot
enough to burn flesh. In any case, some
modern video chipsets combine the
DAC and controller, making faultfinding
harder.
A DPMS power-managed monitor
could be useful as it will usually indicate,
either with an on-screen message
or by varying the colour or blink pattern
of the power LED, whether it is
receiving synch signals or not.
Keyboard Fuse
Acommonproblemis the keyboard
fuse, which often blows if a keyboard
is plugged in or removedwhen the PC
is running. The keyboard fuse is usually
a subminiature surface-mounted
component that is not immediately
recognisable as such. Look for something
labelled F1 or F2 and located near
the keyboard connector (on the motherboard,
not in the PC). The best way
to test it is with a multimeter or with a
spare keyboard - if the keyboard lights
flash as the systempowers on, the fuse
is OK.
As always, don’t replace the fuse
unless you have a pretty good idea of
why it blew in the first place. The
whole idea of the fuse blowing is to
protect the other components. Replacing
it will expose the rest of the motherboard
to the same fault again, unless
you fix it first (eg, by replacing the
keyboard or remembering not to plug
it in while the PC is running).
Send A Postcard
A useful diagnostic tool is a POST
card, though good versions are hard to
come by and you should be wary of
paying too much for one that you have
not had a chance to evaluate. PCs send
their POST results out on a particular
I/O port - most use port 80, while
Compaqs may use port 84 and IBMs
use 300. The POST card, which plugs
into the ISA bus, decodes these signals
and displays them either on a set of
LEDs or a seven-segment numeric display.
A list of POST codes is needed,
and can be used to identify the results
being produced by the self-test routine.
If the POST card shows FF, an oscilloscope
can be used to see if the processor
is accessing the BIOS, looking for
clock signals, the address and data
strobes on the CPU, and for resets.
No Clock
Real-time clock (RTC) failure can be
fixed fairly simply, by replacing the
battery or, if it is a combined clock/battery
unit, by replacing the whole module.
However, if this would require
soldering, it should not be attempted:
motherboards contain heavy ground
planes that require a lot of heat to be
applied very quickly to avoid damage.
If this heat is applied too slowly, the
longer exposure to it can burn the
board.
It is also important to replace the
battery with the correct type.SomePCs
use non-rechargeable Lithium cells
and others rechargeable NiCads, and
if the former is substituted for the latter
an explosion could result.
Some clock modules allow you to
simply connect a standard household
battery or two (eg, as used in a Walkman)
to override the exhausted built-in
battery. However, you normally need
to set a jumper before doing this. If you
are in any doubt, don’t do it.
Intermittent Faults
Intermittent faults are the hardest to
detect. If the system runs for a while
but then halts, check the CPU fan. If
this has failed the CPU will overheat
until its thermal cut-out shuts it down.
Although this cut-out protects the chip
Update 108 (October 1997) Page 5 File: P1001.3
“Faulty joints or components can also
make the system hang as it warms up and
causes them to expand and go opencircuit,
but testing for this requires a
temperature chamber and is a specialist job.”
PC Support Advisor
Motherboard Faults
Problem Solving:Hardware
to some extent, continued attempts to
use the system in this state may cause
thermal damage to the processor chip.
Unfortunately, CPU fans seem to
burn out with alarming regularity. Be
sure to keep one or two in your spares
box, along with some of that special
heat-conducting glue to fix themto the
CPU chip itself.
Faulty joints or components can
also make 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. It is possible to diagnose
some faults of this kind by bending the
board but this is highly inadvisable as
it can cause more problems by breaking
components, joints and tracks. For
a long life the board should be properly
supported and kept well cooled.
A motherboard that boots and then
hangs may have cache problems, in
which case the level 1 and 2 cache
should be disabled in the BIOS. Cache
problems show more oftenwhen booting
from the floppy drive. If the problem
goes away, the caches can then be
re-enabled one at a time to see where
the fault lies. Alternatively, if the system
will not boot from floppy, this can
be a sign of component failure or a dry
joint on the motherboard.
If the cache isOK, themainmemory
needs to be testedwith diagnostic software,
as a memory parity error will
cause the system to hang, usually with
a warning message. You also need to
check that the memory fitted matches
the specifications laid down by the
board or system manufacturer and is
compatible. Adding a wait state in the
CMOS set-up may alleviate a memory
timing problem. Check also whether
the motherboard supports or requires
parity memory, and whether this is
indeed fitted.
Once the base system is working,
the expansion cards can be put back
one at a time. However, before doing
this the test and diagnosis software
should be run to determine the free
I/O space and resources on the PC.
These programswill not find problems
such as timing errors but can prove
base system functionality. Loopback
connectors for the serial and parallel
ports are another essential diagnostic
tool needed here. If built-in ports or
controllers are faulty itmay be possible
to disable them and fit a card instead.
“Once the base system is working, the
expansion cards can be put back one
at a time. However, before doing this
the test and diagnosis software should
be run to determine the free I/O space
and resources on the PC.”
