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Do-It-Yourself
SERVICING
So you would like to service electronic gear for yourself and
your friends. Provided you have some basic test equipment
such as a multimeter and oscilloscope, there is a surprising
range of electronic equipment than can be repaired,
provided you have the time and persistence to do it. This
article gives some useful hints and tips.
B y D AV I D R E I D
E
LECTRONICS IS A BLACK art to
the uninitiated. They marvel at all
those colourful little components and
wonder how on earth we “experts”
can navigate that secret world armed
only with a schematic diagram – which
to them is somewhat akin to a magician’s grimoire written in a long-dead
language. That is why, upon seeing
that you own a soldering iron and
multimeter, your next-door neighbour
may disappear briefly, only to return
with his TV, DVD player, lawnmower
or quite possibly, his wife’s pacemaker.
“I don’t suppose you’d mind having
a look . . .”
You either decline and explain that
it’s not really your field and that it
should be seen to by an expert or you
spend a couple of hours poking around
inside, after which you say “look, it’s
not really my field and it should be
seen to by an expert”.
In reality though, many fault-finding
techniques are really just variations
on common sense. You might be surprised at just how many repairs you
can successfully complete.
Play it safe
Electrolytic capacitors degrade with age and heat. Inspect them carefully for
signs of leakage and bulges in the case (especially the top).
42 Silicon Chip
Get a portable Residual Current Device circuit breaker and “don’t leave
home without it”. Never assume that
you don’t need one or assume that it
will magically protect you from every
eventuality. It won’t.
Never dismantle any unit with the
240VAC mains still connected. It is too
easy to inadvertently touch exposed
circuitry with the metal lid or drop a
screw inside.
Similarly, never power up a dismantled unit unless you are absolutely
certain of the voltage distribution inside. Some manufacturers physically
isolate mains and low-voltage circuitry
siliconchip.com.au
but it is common to find a single PC
board with everything mounted on it,
including the mains transformer. Tape
exposed mains connections and never
make assumptions.
Treat every power supply as if every
component were live, as there are
many supplies where this is the case.
If you do not find a decent-sized mains
transformer in the unit, be especially
suspicious, as you will probably be
dealing with a switchmode power
supply. In the case of colour television sets, they frequently have a live
chassis. This means that it is not safe
to touch any part of the electronics,
including anything that looks like an
earth, while the set is operating.
If you feel uncertain as to the nature
of a power supply, limit yourself to
passive tests and measurements with
the power off. Watch very carefully for
high-voltage electrolytic capacitors.
These are common in switchmode
power supplies and 300V of DC can
easily blow the tip off your favourite
screwdriver!
Finally, don’t work alone but ask
for children to be removed from your
vicinity, as they can be a dangerous
distraction.
Video heads are very brittle, and can be snapped by a cotton bud. Use a clean,
flat piece of card moistened with alcohol.
Initial observations
Many faults are intermittent or
become intermittent as soon as a
technician enters the room, so keep
your screwdriver holstered while you
determine what the symptoms are.
Check every function on the unit, as
many faults have multiple symptoms
which collectively point directly at
the faulty circuit.
Consider a DVD player, for example
In spite of the owner’s claim that it
“isn’t working”, a thorough check may
reveal that the disc tray opens and
closes, that the display is illuminated
and that it seems to be spinning up
and playing. As it turns out, most of
it works and the actual fault is that
there is no audio.
When you dismantle the unit, you
may find that the fault is gone but
because you have done some preliminary work, you will have an idea
where to look and it will later come
as no great surprise if you discover a
muting fault or a hairline fracture of
the PC board next to the line output
sockets. You repair the crack, hand
the unit back and all that remains is to
congratulate yourself on how terribly
clever you are.
siliconchip.com.au
Don’t be tempted into thinking that you can adjust the fault out of a unit by
playing with trimpots. It’s highly unlikely and can render many appliances
inoperable.
Sometimes, this initial analysis
can save you the grief of attempting
a dead-end repair. You have a friend
who wants his beloved tape deck repaired. The fault is similar: no sound.
However, you noticed that the meters
are showing a signal on playback, so
you very cleverly grab some headphones. Yes, there is audio. However,
the speed is all over the place, one of
the meter lamps is gone and the heads
have a groove in which you could
park a bicycle. Can you get the parts
required? How badly does he want it
fixed? Is he willing to pay what it’s
likely to cost?
If you are serious about saving
time, see if you can get the brand and
model number of the unit and hit the
web. More and more schematics are
finding their way online and there
are some very specific fault FAQs on
various products. Even if you can’t
find something on the exact model,
February 2006 43
Switchmode power supplies are potentially lethal. Some of the parts operate
at 240V AC mains potential and that can include heatsinks. Never work on a
switchmode supply with power applied unless you are an expert and know
exactly what you are doing.
something similar will often yield
useful clues.
Look, listen & sniff
Many faults can be found by simple
observation, so have a careful look
around before you start exploring with
your meter. You might find a plug that
isn’t seated properly in its socket or a
solder splash bridging two tracks on a
PC board. Either way, the fault could
rectify itself while you are attempting
to fix it.
In many cases, components actually look faulty; transistors that have
cracked open, swollen or ruptured
electrolytic capacitors or obviously
overheated resistors. In the case of the
latter, do not touch the resistor until
you have had a chance to measure it.
They have a nasty tendency to disintegrate at the slightest touch, leaving
you with no idea of the original value.
And you you’d be surprised how many
technicians will take the back off a TV,
take one sniff and mutter “caps”.
Poor connections
Poor connections are responsible
for a huge proportion of faults. These
come in a multitude of forms: dry
solder joints, plugs and sockets (dirty
or badly crimped, including edge
connectors and IDC plugs), switches,
44 Silicon Chip
board fractures and dirty pots.
This brings us to the question of
contact cleaning. There are dozens
of contact sprays around and they all
excel in their specific applications.
However, they can cause trouble when
used inappropriately. The only type of
spray that can be used with confidence
in most circuits is one designed for
cleaning television tuner contacts or
a plain isopropanol spray. Don’t fill
your VCR with something designed
for auto electrical systems.
There are two cases where contact
cleaners of any type should be used
with caution: slide potentiometers
and conductive rubber switches. In the
case of the pots, contact cleaners can
wash the lubricants out along with the
dirt, causing them to stick and bind.
Try to drop a tiny amount directly onto
the carbon tracks. This often cures
them. The rubber switches can be hard
to recognise, but they are so common
that it is better not to clean any switch
unless you are reasonably sure that it
has metallic contacts.
I have had three memorable experiences involving sprays. Once, I
was using a common furniture polish
to restore an electronic organ. As I
wiped the rag across the faces of the
tab switches, the red lettering, which
was embossed into the faces of the
switches, disappeared! On another oc-
casion, also with an electronic organ,
a customer decided to try spraying
the key contacts himself. However, he
also liberally sprayed the plastic keys,
chilling them to the point where they
became brittle and snapped under
tension!
Finally, a totally predictable near
disaster was caused by yours truly.
My wife had been complaining of
some intermittent fault in the washing
machine. I wasn’t in the mood for any
serious fault-finding, so I removed the
top cover and soaked the beast with
my favourite contact cleaner. I leaned
up close to observe the results of my
handiwork and hit the ‘go’ button. The
‘explosion’ that ensued was worthy of
Francis Ford Coppola. I was unharmed
and considered myself fortunate to
have learned my lesson in an outdoor
laundry, rather than in a customer’s
living room!
The ubiquitous dry joint
Ubiquitous: u-bi’kwi-tus. (adj).
Existing or being everywhere; omnipresent.
Although common, dry joints (DJs)
can be hard to locate as many perfectly
sound joints look dry and often a crook
joint looks fine. Fortunately, some
components form DJs more frequently
than others so you can usually save
time by looking at the most likely
culprits first: ie, any components that
are physically large, conduct high
currents or get hot. These three factors
usually go hand in hand.
The expansion and contraction associated with heating and cooling can
fatigue joints, as can the vibrations
from improperly secured large components. Consequently, the areas that are
worth inspecting are power supplies,
amplifier output transistors and drivers, servo motor drive transistors, TV
line output transistors and so forth.
Technically speaking, the best way
to rework a DJ is to completely remove
the old solder, clean the joint and then
resolder it. In the real world, most
of the time you will get away with
reheating the old joint and adding a
little fresh solder. If the joint doesn’t
look right when you are through,
you can always go back and do it the
hard way.
While we’re on the subject of soldering, one of the greatest causes
of damaged PC boards is the act of
removing an IC from a double-sided
board. Unless you have a good vacuum
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Sometimes it’s easier to remove a part by cutting its pins rather than desoldering
it and it’s usually easier on the PC board tracks as well.
One the part is out, you can desolder
and remove the individual pins.
desoldering station, the safest way is
to carefully cut the component’s pins
off one by one and then desolder them
individually.
Surface-mounted components are
a different matter entirely. I’ve seen
boards destroyed by salesmen who
were demonstrating hot-air SMD
rework stations. Many components
are glued to the board, so it can be a
challenge to remove even a 3-legged
device.
If you are desperate to have a go
anyway, find a wrecked board and
get some practice. You need an ultrafine tip, fine gauge solder and a very
narrow probe that will allow you to
lift individual legs clear of the board.
Use a magnifier to make certain that
all the legs are clear and then attempt
to rotate the component on the board,
rather than pulling it off. The glue is
very strong but usually quite brittle.
Pulling on the part might lift tracks
but twisting it will often make it come
away with very little force.
All of the above presupposes that
you have found a dry joint to resolder.
If you cannot locate it visually, a little
freeze spray often helps.
Gentle tapping of components or
the PC board with a (non-conducting)
screwdriver often helps to localise the
fault to a particular area. If all else
fails, you can always try tracing out
the fault.
solder mask from the tracks and polish
the copper at the same time. Tin the
tracks, using an absolute minimum of
solder and tack down the finest tinned
copper wire you can get your hands
on, such as 3A fuse wire.
spot. If gently flexing the PC board
seems to have an effect on the fault,
then you may well be looking for a
fracture. In this case, try removing
the board completely and looking at
it with a bright light behind it. Many
boards are translucent to some degree
and frequently, when backlit, the
extent of the fracture will show up
clearly. Again, large components are
more likely to have fractures beneath
them.
Look closely around any mounting
holes and any components which protrude into the “outside world”, such
as volume controls, input and output
sockets, etc.
The repair of fractures is an art in
itself and a shoddy job can introduce
more problems than it cures. Modern
equipment often has very fine tracks,
narrowly spaced. The trick is to get the
tracks really clean on both sides of the
fracture. An ink eraser will remove the
Component failure
Even without a schematic, it is possible to pin-point many component
failures, once you know where to
look. Those same components which
I mentioned as being prone to DJs are
also the most likely to fail. The most
reliable electronic appliances are those
with little mechanical complexity and
low power consumption. Hifi tuners
and equalisers are generally more reliable than amplifiers, VCRs and DVD
players.
To begin with, don’t get too theoretical with the problem. Check the
components which seem to have some
bearing on the problem, starting with
Fractured PCBs are
remarkably common.
Backlighting helps
determine the extent of
the crack.
Board fractures
Board fractures can be difficult to
siliconchip.com.au
February 2006 45
Warning labels are there for a purpose. Reading them is
good for the soul, among other things.
easily-tested components like output
transistors and rectifier diodes. Desolder at least two legs of any transistor
before measuring it. Any low value
resistor is worth measuring. Often
these are used as fuses in low voltage rails.
Electrolytic and tantalum capacitors
are also worth a look. These are often
used to provide localised filtering of
power supplies and can present real
problems when they go short circuit. A
short circuit on a 5V rail can be a nightmare in a digital circuit, but always
look at the bypass caps before you start
removing LSI chips. Most capacitors
have 38 fewer legs to desolder!
You can often determine what
voltages should be present in a circuit without needing a schematic if
Use a quality photographic blower-brush to clean the lens
of a CD or DVD player.
you have a degree of familiarity with
the components involved. 3-terminal
regulators are wonderful devices - they
have their voltages printed on them.
Integrated circuit families usually
work off predictable rails, so a quick
peek at pin 14 of most TTL ICs will
often save time. Look for other old
standards, such as LM741s, 555 timers,
etc. Even if you have no idea what an
IC does, it should usually have some
sort of rail on one of the pins!
Because much consumer equipment
tends to be under-engineered, you
often find that filter electros in power
supplies are working very close to their
rated voltages, so that 4700mF 63V cap
in the power supply is not likely to be
filtering the 5V rail.
Transistors usually fail either open
or short-circuit, so don’t worry about
beta, hfe and all that other stuff unless you want to make a career of one
repair. Diodes frequently go leaky and
this is easily checked with a multimeter but make sure you are measuring
leakage and not the resistance between
your fingers.
Resistors tend to go open-circuit and
mostly these will be either very low or
fairly high in value; less than 1kW or
more than 100kW.
This may sound like a very simple
approach and may lead you to think
that there is not much science involved
in electronic repair. However, the really hard repairs can present such a
challenge that a busy technician won’t
have the time to worry about style on
the easy ones.
Dead shorts
PC boards with surface-mount components are easily damaged during rework
due to the small pad sizes. Note that most components are physically glued to
the board as well as being soldered.
46 Silicon Chip
A short circuit on a power supply
rail can be one of the most difficult
faults to find. Imagine a shorted bypass
capacitor on a 5V rail on a good-sized
logic board. In this case, practically
every single component will be connected directly to the rail and any one
of them could be the culprit. The real
trick is not be overwhelmed by the
magnitude of the task.
First, disconnect every single item
which is not soldered in - plugs and
sockets, edge connectors, socketed ICs,
etc. Check the resistance to ground
after each removal. Next, desolder
the “easy” components; large electros,
regulators, transistors and so on.
If you have access to a schematic,
use logic (the intellectual type, not the
electronic kind!). If your short circuit
measures only a few ohms, then a transistor which has a 1kW emitter resistor
siliconchip.com.au
is highly unlikely to be the cause of the
fault. However, if the same transistor
had a 10mF capacitor across that 1kW
resistor, then it would be conceivable
for the capacitor to have developed an
internal short, which in turn, could
cause the transistor to go shorted.
Thirty seconds taken to establish that
a component could not possibly be
the cause of the fault can save a lot of
fruitless soldering.
There is another approach which is
a little unorthodox and if you decide
to try it, you should be aware of the
possible side effects. It involves attempting to ‘smoke it out’. By applying a power source to the rail (at no
more than the rail’s rated voltage and
preferably somewhat less) at a reasonably high current, it is often possible
to heat the component enough to
cause it to smoke or at least enough
to make it hot. However, you should
know that you could be forcing the
board tracks to carry considerably
more than their rated current. In most
cases, I am reluctant to use more than
500-odd milliamps. As long as you do
not exceed the rated voltage of that
particular rail, the other components
on that rail should be safe enough,
even if the short should actually burn
itself out. Of course, this assumes that
you have access to a current-limited
supply. If you cannot set the current
separately from the voltage, give this
technique a big miss. You will need to
use your own judgment on this one,
as it does involve potential risk to the
PC board.
Know your test gear
I am continually surprised to see
experienced technicians making fundamental mistakes in taking measurements. Because we mainly use high
impedance digital meters these days,
many of us tend to be blissfully ignorant of the bad old, “20kW/V” days and
usually, this is a blessing. However, a
10-megohm input impedance is still a
long way from being an open-circuit
and loading effects can happen. Similarly, few crystal oscillators function
properly, if at all, unless your oscilloscope probe is set to the X10 position and even then, they might not
run. Nevertheless, I saw a very senior
Technical Officer employed by a major
manufacturer who was unaware of the
loading effect.
A typical multimeter will have poor
frequency response on its AC volts
siliconchip.com.au
Do basic safety checks before and after every repair. Is there earth continuity?
Is the appliance lead damaged? Is the polarity correct? This mains lead has
obvious damage and must be replaced.
ranges. However, if you have access
to a meter with a good AC bandwidth,
you can use it to trace an audio signal
through an amplifier.
To fix or not to fix
It’s a sad fact that many appliances
today are not economically repairable.
DVD players are essentially throwaway items, VCRs aren’t far behind
and even TVs are cheaper than ever
before in real terms.
If you get a sense of personal
satisfaction from being able to get
something going again or if you are
fixing your own appliances, then it is
probably worthwhile spending a few
hours tinkering. However, be careful
not to get into a vicious circle in doing repairs for friends. The scenario
can go like this: you agree to look at
something. A week goes by and you
still haven’t got around to taking the
covers off. The guilt kicks in, so you
spend a while and get some idea of the
fault. You order a part and a week later
you fit it. And it still doesn’t work.
With every day that goes by, you
will feel more pressure to complete
the repair, although the chances of you
being able to fix it are decreasing at the
same time. The trick is to be able to cut
your losses and cut them early.
Final checks
There’s one important aspect to repairing an appliance which is easily
overlooked. You must be certain that
you have not overlooked a fault which
might render it unsafe or worse, introduced one. For a non-professional,
this is primarily a moral responsibility.
Although there is an Australian Standard (AS/NZS 3760:2001) which details
the tests required after repair of any
appliance, it is unrealistic to expect
full compliance from someone who
is performing a “homer” for a mate.
Nevertheless, basic tests can (and
should) be performed which go some
of the way towards keeping your
friends and family intact:
Inspect the power lead for any
damage.
• Check the wiring of the plugtop
for correct polarity and anchoring of
the cable.
• Check the strain-relief where the
cable enters the appliance.
• Check that external fuses are of the
correct rating.
• Ensure that the resistance of the
earth connection from the power plug
to any exposed metal surface is less
than 1W.
• Measure the resistance from the Active pin to Earth and from the Neutral
pin to Earth. Both should measure
open circuit.
Finally, note that these tests are not
a substitute for a proper high-current
earth-bond test, nor for a high-voltage
leakage test, but they will give you
an indication of gross conditions and
a warm and fuzzy feeling when you
hand the appliance back.
SC
February 2006 47
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