Download Notes

TROUBLESHOOTING
 An Electronic circuit is a collection of components connected together to perform a
particular function and every component has a role to play in the overall operation of
the circuit.
 If any component fails, then the operation will be drastically changed.
 As an example consider the simple relay circuit shown below:
3
D1
DIODE_VIRTUAL
AC S1
C1
R2
5.6kΩ
R1
47kΩ
1
6
Q1
2
Q2
4
2N2102
V1
24 V
2N2102
5
FIG 1.
For normal operation there are 4 paths for the current to flow: 24V Battery to Ground
 24V Battery biases Q1, so at Point 4, you should measure no more than 0.7V
 Q1 goes into saturation (conducts), so you should measure almost 0.1V (ground) at Point
1, which is applied to the base of Q2, and therefore Q2 should be off.
 Point 2 Voltage should be 24V and the relay is off.
EQUATION:
24V=IB1*R1+0.7 ------- (1)
24V=IC1*R2+VCE1 ------(2)
24V=IRL1*RL1+VCE2----(3)
 If R1 becomes open circuited, IB1=0, VB1=0,VCE1=0.7V (B-E of Q2,since Q1 is off.
Current is flowing from 24 V power supply ,through R2 into the B-E of Q2. Q2 will
conduct and the relay will be permanently energized.
Normal Voltage
1
2
0.7V
0.1V
3
+2V
With R1 open
1
2
0
0.7V
3
0.15V
 Same symptoms will appear if B-E of Q1 becomes short.
 We need to check the resistance of R1 and Q1 both to determine the exact cause. This
is called root cause analysis.
 Thus it is clear that skillful troubleshooting requires both theoretical knowledge and
the understanding of circuit operation as well as practical experience.
 The technologist must understand the purpose of each component in a circuit and full
circuit operation.
COMPONENT AND COMMAN FAULTS
 A Component fails if one of its specified values exceeds its limits e.g. 5.6Kohm +_ 5%
resistor value changes to 6K or 5K, it has failed or a capacitor’s specified leakage
current e.g 10uA shows a leakage of 150 micro AMP, then it has failed.
KINDS OF FAULTS
There are two kinds of failure
(1) Slow failure - affects circuit operation over time.
- Faults are intermittent
- Difficult to troubleshoot.
(2) Catastrophic failure
- A sudden and complete failure.
We shall first study the catastrophic type of failure. For e.g. a resistor goes very high in value or
becomes open, a diode shorts between anode and cathode or a transistor B-E junction shorts.
Such failures result in complete loss of performance and drastic changes in DC voltage values.
As a general rule certain types of components fails in a particular way
(a) When resistors, especially the film type, fail, they often go open circuit, since a small
break in the resistance spiral is much more likely than a complete short circuit across the
whole resistor.
(b) Electrolytic capacitor is more likely to become short-circuited.
(c) Modern day electronic components are very very reliable. Notice that we are talking
about why they fail, not how often they fail.
(d) Table below indicates the more probable types of failure for various types of electronic
components.
Component
Common types of faults
Resistor
Variable Resistance
Capacitor
Inductor
High in value or open circuit
Open circuit
Open or short circuit
Transformer open circuit or shorted
turns, short circuit coil to frame (iron
core type)
Open or short circuit at any junction
Diodes, Transistor, FETS, SCR’s
Why Components fails?
 Manufacturing Defects
 Overloads
 Ageing, Continuous stress
Stresses are of two kinds
(a) The operating stress: It is due to design conditions. The life of the component can be
prolonged by operating it well within its specified rated maximum value of current,
voltage and power. This is called derating.
(b)Environmental stress:
 Caused by surrounding conditions such as High temperature, high humidity,
mechanical shock, vibration, high and low pressure, corrosive chemicals, dust in
the air.
 All these conditions cause the components to fail.e.g. If the component is subjected
to continual cycles of heating and cooling, eventually the material it is made of
becomes brittle and a slight mechanical shock will break it open.
 Another cause of component failure is high voltage or high voltage spikes caused
by switched inductive load, being transmitted along power lines. These spikes can
easily lead to breakdown of junction in semiconductor device.
Measuring instruments and Testing Methods






(1) Meters:
A good meter (and a scope) is all you need for basic troubleshooting.
You are basically checking for predicting voltages, currents, distorted outputs, incorrect
waveform, overheating, open or short circuit.
The multimeter should have at least 20Kohm/V or Dc ranges, otherwise loading effect will
give you incorrect reading.
Infact higher the meter resistances on volt scale (and lower on amp scale) better the meter.
Also when measuring voltages across a high resistance you must consider loading effect.
Figures below show the examples. Thus use the highest possible range when measuring
across high resistance.
2
R2
100kΩ
1
V1
20 V
+
R3
200kΩ
0.000
-
V
U1
AC 20k W
3
3
V1
20 V
R1
100kΩ
1
R2
200kΩ
R3
200kΩ
2
 The meter will read 10V instead of 13.33 V, because R2//R3=100K and this 100K
becomes in series with R1 (100K)

Now a days digital multimeters are available, which measures voltages, current or
resistance on 3 or more inline digital displays.
(3) Scopes:
 Most versatile equipment for troubleshooting.
 Measure DC, AC voltage, current, time, phase angle, frequency etc.
 Must be properly calibrated for accuracy.
 Typical Zi of scope is 1M to 10Mohm with a capacitance of 20PF in parallel.
 Sometimes probes are used to attenuate the signal.
Component testing;
 When equipment is being serviced and checks indicate that a certain component is
suspect, it is then necessary to confirm the defect.
 Often simply replacing the component is sufficient check, but it is always good practice to
test the faulty component to verify the type of fault.
 It is important to collect data on component failure and properly document fault report.
 A fault may be caused by defects in component manufacture, due to ageing,
environmental stresses, a design error, circuit conditions, poor production methods etc.
Checking for open circuit:
 Use the multimeter to check the open and short.
 Turn off the power and lift one end of the component before making measurement
(unsolder)
 An Alternative method of checking for a open circuit resistor is to bridge the suspect
component with the known good one, and then recheck the circuit conditions.
Checking Capacitor:
 A leaking capacitor can also be checked using ohmmeter, again by disconnecting one
end of the capacitor from the circuit.
 A good electrolytic capacitor should indicate a low resistance initially as a capacitor
charges and rapidly approaches infinity.
 If the capacitor does not indicate the very high resistance, it is leaky.
 An open circuit capacitor can be confirmed by placing another good capacitor of the
same value in parallel and checking the circuit operation or
 By removing the capacitor from the circuit and testing up a simple setup as shown
below:
C1
1
C2
2
1uF
1uF
V1
3
120 Vrms
60 Hz
0°
4
100kΩ
R1
R2
1kΩ
50%
Key=A
V2
5
120 Vrms
60 Hz
0°
Testing Diodes:
 First determine the polarity of the internal battery.
 Take a known good diode.
 If black lead connected to anode and red lead to cathode indicates low resistance
(<1Kohm) and reverse shows >100Kohm, then black lead is positive.
 Once the lead polarity is established, a good diode should display low one-way
<1Kohm and high >100K, the other way it is good diode.
Testing transistors:
 Determine device leads if not known.
 Measure the forward and reverse resistance between pairs of leads until you find two leads
that show high >100K in both directions.
 These must be collector and emitter and the third must be the base provided the transistor is
good.
 Now measure the resistance between the base and the one of the leads .It should be low in
one end and high in the other. If low occurs with positive lead on the base, it is NPN. If it
occurs with negative lead on the base it must be PNP
 The above check also indicates that the transistor is good.
 If it shows high in both directions it is open, if it shows low in both direction it is short.
Q1
Q2
2N2102
2N1132A
NPN
PNP
When testing Components
(a) Check the power supplies near the actual component and for ICs directly
on the appropriate pins.
(b) Don’t use large test probes, because they can easily cause shorts.
(c) Avoid the use of excessive heat when unsoldering the components.
(d) Do not unsolder with the unit switched ON
(e) Never remove a plug in a device without first switching off the power
supply. Components can be damaged easily by the excessive current
surges.
NOTE ON TROUBLESHOOTING AND ANALYSIS

Troubleshooting begins with the knowledge of the circuit under test.

First step is to determine, calculate and sketch the expected DC/AC voltage and waveform at the output
of each functional block under normal operating conditions.

Second step is to actually observe and measure (using multimeter and scope), the actual output at these
test points and compare with the expected values.

If the circuit is operating properly, the measured and observed values will match the expected values.

If they don’t, the next task is to isolate the defective block, the defective component or fault condition.

Standard technique is to inject a valid signal and divide the circuit into half and measure/observe the
output at halfway points.

If the fault is in the first half of the circuit, you will observe an invalid output; if you measure a valid
output, the fault is in the later half of the circuit.

Repeat the procedure, dividing the circuit into half until you locate the faulty block.

Next step is to locate the faulty component or faulty condition.

Repeat the process measuring and observing the voltage/waveform at various points in the block and
comparing with the expected output until you locate the defective component or fault condition.

Always look for obvious visual problems first. Power connections, ground connections, disconnected
wires, solder open on the PC board, a small solder block bridging two tracks on the PC board etc.

Sometimes an effective and quick technique is to swap the defective board with a known good working
board.

Look for open or shorted components, resistors, capacitors, incorrectly connected diodes, transistors, OPamps, incorrect power supply connections.

You will apply these techniques in the lab. You will be given a pre built board with fault introduced and
you will troubleshoot and isolate the faults.

On RF board, look for grand loops, presence of noise spikes etc.
Lab Notes
Laboratory Staff
Prepared by the ARRL
(e-mail: [email protected])
ELECTRONIC
TROUBLESHOOTING
By Ed Hare, KA1CV,
ARRL Laboratory Supervisor
Q: Someone just gave me a
collection of used amateur
equipment, but some of it
doesn’t seem to be working
right. I’ve built a few kits, and
understand electronics fairly
well, but don’t know much
about troubleshooting.
Should I try to fix this stuff
myself?
A: In ham jargon, a collection of
used equipment is known as
“junque.” The pseudo-French
spelling is employed to remind
us that there is treasure in some
of the older equipment we
discover from time to time.
You don’t need to be an
engineer to fix electronic
equipment! In fact, some of the
engineers I’ve known were not
good troubleshooters. Nearly
anyone who is familiar with
basic electronic theory can learn
troubleshooting techniques and
fix many types of electronic
failures. To troubleshoot, you
simply follow logical, step-bystep procedures to arrive at a
solution.
You do need to be able
to read a schematic diagram
and understand basic circuit
functions. More importantly,
you need to understand basic
safety rules! Some of the
voltages found in electronic
equipment are lethal. Refer to
the important information in
the 1995 ARRL Handbook
safety and troubleshooting
chapters.
Q: What sort of test equipment
will I find useful?
A: Let’s start with the basics. If
you do a lot of troubleshooting,
you’ll need a good multimeter
for voltage, resistance and current
measurements. Modern multimeters can
sometimes include such frills as capacitance
and inductance meters, and even transistor
testers. An oscilloscope can be useful for
looking at RF, audio or digital waveforms. A
power supply is handy when you need to
power the equipment under test, or as a
troubleshooting substitute for a defective
power supply. If you’re troubleshooting old
tube-type rigs, a tube tester will help
diagnose defective vacuum tubes.
Many electronic problems are
sensitive to heat⎯ a unit may work well
when it is first turned on, then fail as it
warms up. A heat lamp and cold spray
may help isolate these thermal problems.
First, use the heat lamp to warm the
circuitry quickly (don’t overdo it!). When
the failure occurs, use the cold spray to
cool down components one at a time.
When the circuit suddenly starts working
again, you’ve found the bad part.
Q: I own most of the test equipment you
describe, so I guess I’m ready. Where
should I begin?
A: Start by reading through the owner’s
manual. Make sure you understand the
equipment and how it is supposed to work.
Does the owner’s manual contain a
schematic? Even better, can you get a
service manual for the unit? There are a
number of sources for manuals. The most
logical is the original equipment
manufacturer. The June 1992 issue of QST
featured a “Lab Notes” column that
described a number of companies that sell
reproductions of older equipment manuals.
Try the easy things first (if they
are real easy, try them twice!) Check
the obvious: it would be a shame to
spend hours of troubleshooting only to
discover a bad fuse.
Q: Are there any general guidelines to
follow?
A: Perhaps the most important rule is to
simplify the problem! If you’re
troubleshooting a complex system,
perhaps an entire amateur station, it may
be difficult to determine why no RF is
coming out if you have a number of units
hooked together in complex ways. In this
1
case, start by testing only the
transceiver, preferably into a
dummy load. If it tests okay,
start adding things back one
Most failures are
catastrophic. It is rare that a
circuit will half work. An
amplifier stage is usually dead
or working, a digital circuit
works, or is stuck in one state.
Many problems have multiple
causes, and the problem won’t
be fixed until you find them all.
Q: Okay, enough preliminary
chatter. When do I get to take
something apart?
A: Right now! Assuming
that you’ve obtained a
schematic, have run
through the preliminary
diagnosis, and are certain
that the problem isn’t
something obvious, it’s
time to take the unit apart.
Take notes while
you’re unscrewing chassis
plates and disconnecting
cables and wires. You’ll
need to remember how to
put it all back together. If
you have to order a part
that takes a month to
arrive, you may forget
which cables or screws
went where. If you own a
camcorder, use it to make
a video record of your
work.
Q: Well, the back is off and I
have my voltmeter at the ready.
What’s next?
A: Put the voltmeter down!
Before you begin making
measurements, spend about 15
minutes looking the unit over
from stem to stern. Think of it
this way: You’re preparing to
spend a few hours diagnosing a
complex unit. About half of the
problems I’ve seen over the
at a time. The problem will diagnose itself
pretty quickly. This principle can be
applied at many points in the
troubleshooting process.
years have exhibited a visible symptom⎯ a
broken wire, a burned resistor, a loose
connector or a cold solder joint. It is much
more efficient to find the problem visually, if
you can. In 15 minutes, you’ll be able to look
at every component, wire and connector.
Q: I found a broken wire and repaired it, but
the unit still doesn’t work. What else can I
do?
A: This demonstrates an earlier
point⎯ many problems involve multiple
causes. The next thing you need to do is to
isolate the problem to a single circuit.
You’re not an experienced troubleshooter,
so I recommend one of the systematic
approaches to troubleshooting⎯ signal
tracing or signal injection.
Let me give you an example. We’ll
use the block diagram of an AM broadcast
receiver shown in Figure 1. Before we can
trace a signal, we need to create one. Tune
the receiver to 1 MHz and set up a signal
generator to provide a 50-µV signal,
amplitude modulated with a 1-kHz tone.
(The ARRL Handbook has a testequipment chapter that will explain how a
signal generator functions.) Feed the
generator output to the antenna jack.
Then, use a signal tracer to follow
the test signal through the receiver,
starting at the input and working toward
the output. When you find the stage
where the signal disappears, you’re very
close to the problem. In Figure 1, if the
IF (intermediate-frequency amplifier)
stage was defective, you would find the
signal at point “A,” but not at point “B.”
Q: Wait a minute. What is a signal tracer?
A: A signal tracer gives an audible or
visible indication of the presence of a
signal. Several different types of test
equipment can function as signal tracers.
An oscilloscope can be used to visually
measure a number of different signal
types. A diode RF detector can be used
with an audio sound system to detect and
listen to a signal. The same
detector
can sometimes be used with a
voltmeter to follow a signal through a
system. Refer to the Handbook for more
information.
injection?
Q: What about signal
A: Signal injection is the
opposite of signal tracing. You
inject a signal at various
stages in the system, starting
at the
output and working your way
toward the input. You may
need to select different
frequencies for each stage. In
the block
diagram shown in Figure 1,
you would begin by injecting
an audio signal at the output of
the AF (audio frequency)
amplifier. You should hear
sound coming from the
receiver’s speaker. You would
then move the AF signal to the
input of
the AF amplifier. If the AF
amplifier is functioning, you
would still hear the sound. You
triggered if R2 was open. (This
is where general electronic
knowledge can pay off, by
suggesting what types of
component failures can cause
what symptoms.)
You may need to test
each of these components.
By using a modern
voltmeter that uses only a
few millivolts to measure
resistance, it should be easy
to measure the components
in-circuit. In more complex
circuits, especially those
using inductors, some
components may need to be
removed before testing.
Watch out for multiple
failures. If R4 is open, there is
a good possibility that the
failure was caused by a short
should then use a modulated signal
at the frequency of the IF amplifier. Inject
that signal at the output of the IF amplifier.
If you hear sound, move to the input.
If the IF amplifier is defective, you would
hear sound when you inject a signal at
point “B,” but the sound would disappear
when you inject a signal at point “A.”
Q: Well, I found a defective AF amplifier in
my receiver. What should I do now?
A:Start with voltage readings. If your
schematic includes voltage readings,
compare those in your circuit against those
in
the schematic. Any variance greater than
about 20% is cause for concern. Refer to
the sample circuit shown in Figure 2. If
the collector voltage is near 0 V, I would
suspect a shorted transistor, or open
resistor (R4). This symptom could also be
circuit in Q1. These types of problems are
common in electronic circuits.
Q: I discovered that the +12 V was
missing entirely. When I checked a few
other places in the radio, it was missing
there, too. Is my power supply bad?
A: Bingo! Actually, I goofed; I should
have told you to check the power supply
first. Power supplies often need to supply
and dissipate quite a bit of energy. This
makes power supply failures fairly
common.
Q: I got the radio working. Is it time to do an
alignment?
A: Beware the urge to align radio circuits!
Most equipment does not require such
adjustments on a regular basis. The most
common reason a circuit needs alignment is
because someone else botched it the first
time. If frequency-determining components
have been changed in the RF or IF
amplifiers, if the circuits are fairly old or have
been subject to severe environmental
conditions, or if someone
“tightened all the loose screws”
in those little cans (the IF
transformers), the unit needs to
be realigned.
Q: I think I’ll pass on the
alignment for now. The rig is
working so I guess it’s time to
put it all back together, right?
A: Not so fast! First, make a
final visual inspection. You
want to check the quality of
your work. Look at solder
joints, double check the
replaced parts, make sure
there are no pinched or
burned wires. Once again,
power it up and look for signs
of trouble. Operate the rig for
a while and let it get good and
warm. Once you’re satisfied
that it’s good to go, round up
your screws and bolts. Even
after the last screw is in place, check the
unit one more time.
Q: Thanks for the tips. I was able to fix
some of the equipment. Others looked
too far gone to warrant any serious
attention. But there was one H-T that I’d
love to fix, but it has layers of small parts
in hard-to-find places. I couldn’t make any
headway with that one. Is it time for a
professional?
A: When my H-T broke, I sent it
away for repairs. Some repairs are
best left to the pros. The factory
repair staff has experience with their
products and can usually fix them
efficiently.
(1) Freeze spray is distributed by GC
Thorsen, PO Box 1209,
Rockford, IL 61102; tel 800443-0852 (call for nearest
distributor). It is also sold by
most electronic-component
distributors.
Figure 1⎯ A simplified block diagram of an
AM broadcast receiver.
Figure 2⎯ A one-transistor audio-frequency
(AF) amplifier stage.
“Live” Troubleshooting
When you’re tracing signals through an active circuit, remember that dangerous voltages exist
inside most electronic equipment. Even solid-state gear often has 117 V ac at the input side of the
power supply. Some equipment uses circuitry that can put 117 V on the chassis. Simply turning off the
power is not enough; some components can store a charge for a surprisingly long time. If you’re not
certain that you’re qualified to work on live circuits, leave this to the professionals.
Hyder khoja
Page 13
5/6/2017