Download Principle of Troubleshooting

Principle of
Troubleshooting
Topic
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Problem solving analysis
Circuit faults
Testing basic components
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Troubleshooting methods
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Voltage measurement
Amperage measurement
Resistance measurement
Substitution
Bridging
Heat
Freezing
Signal tracing & injection
Component testers & test lamps
Resoldering
Adjusting
Bypassing
Problem solving analysis

Servicing a device has 3 phases:
1) Situational analysis
2) Problem solving
3) Decision making
1) Situational analysis
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Asking questions and observations as follows:
 Discuss
the defect with the owner or operator
 Compare the problem with others from past
experiences
 Perhaps there is no problem at all, and it is an
operating error
 Identify the existing state of operation with the desired
state
 Make an overall observation of the situation, noting
symptoms and relevant changes.
2) Problem solving
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Get organized:
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Describe the problem
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Compare the problem situation with known operating conditions prior to the
breakdown
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Describe all known differences such as the symptoms, noises, and smells
noticed when defect occurred
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Compare the ‘what is’ with the ‘what is not’. Which components are okay and
which are not, and to what degree are the components defective?
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Analyze differences through testing by paying close attention to obscure and
indirect relationships. For example, slight tolerance changes in components or
physical color changes can signal causation.
3) Decision making
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Examining various solutions or repair
alternatives and selecting the best option
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Consider all the advantages and disadvantages
for each alternative along with contingency
planning
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Contingency planning: future changes in the
overall system such as expected life, operating
conditions and model changes.
Circuit Faults
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Basic causes of electrical or electronic breakdowns:
1. Heat
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Increases the resistance of circuits
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Increases the current
Causes the materials to expand, dry out, crack, blister, and wear
down much more quickly
2. Moisture
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Causes circuits to draw more current
Causes expansion, warping, quicker wear, and abnormal current
flow (short circuits).
3. Dirt and contaminants (fumes, vapors, abrasives, soot, grease,
and oils)
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Causes devices to ‘clog’ or ‘gum’ up and operate abnormally until
they finally break down
Circuit Faults (cont..)
4. Abnormal or excessive movement
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Vibrations and physical abuse can lead to breakdowns
5. Poor installation
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Unqualified installer, careless and rushing installation
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Failure to tighten a bolt
Failure to properly solder a wire connection
6. Manufacturing defect
7. Animals and rodents
Circuit Faults (cont..)
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4 most common causes of circuit faults:
1. Short circuit
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Draw more current because resistance decreases and
results in voltage decreases.
Typical signs of short circuits are blown fuse, increased
heat, low voltage, high amperage, smoke
2. Open circuit
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Infinite resistance and zero current
Typical signs are infinite resistance, zero amperage, and
completely inoperable device
Circuit Faults (cont..)
3. Ground
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Wires with poor insulation, pinched wires or misplaced
components.
Typical signs are abnormal amperage, voltage and
resistance reading, shocks, abnormal circuit performance,
tripped ground-fault interrupters, and periodic blown fuses or
circuit breakers
4. Mechanical fault
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Too much friction, wear, abuse, or vibration
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Broken belts, worn bearings, loose bolts, damaged chasis etc
Typical signs are noisy operation, abnormal operation, visual
clues, and circuit failure
Troubleshooting methods
1.
Voltage measurements
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Amperage measurements
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Use voltmeter or oscilloscope, in parallel with the circuit
A zero voltage may identify an open circuit, a low voltage may
indicate a short circuit.
Use ammeter or a ‘clamp-on’ ammeter in series with the
circuit.
Indicates and locates common circuit faults such as short
circuit, open circuit and grounds.
Resistance measurements
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Use ohmmeter. Always shut off the power before measure
resistance.
Locates short circuit, open circuit and grounds.
Troubleshooting methods
4.
Substitution
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5.
Replacing a suspected faulty component with a
known good component.
Bridging
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‘jumps’ or placing a known good component across
the suspected faulty component in a circuit.
Limited to open-circuited components. Bridging a
short-circuited components may have no effect or
may cause damage to the new component
Troubleshooting methods
6.
Heat
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7.
Freezing
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8.
Usually by using a hot blower
Applied to a suspected ‘intermittent’ component. Thermally
intermittent component breaks down under heat.
Use of cold air from a fan or a chemical coolant.
Cools the suspected thermally intermittent component, thus
temporarily restore the component to normal operation.
Signal tracing & injection
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Most often used in servicing radio receivers.
A signal is injected into various points of a process (stage) in
order to locate the specific inoperable stage.
Troubleshooting methods
9.
Component testers, test lamps
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10.
Re-soldering, adjusting and aligning.
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11.
Instruments used to test the quality of a component
Megohmmeters, capacitor checkers, test lamps, transistor or
diode testers, cathode-ray-tube (CRT) checkers, IC testers
etc.
Poor electric solder connection (cold solder joint)
Bypassing
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To locate a suspected problem.
Requires unplugging one of several circuits
Testing basic components
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Most common defects of resistors are physical –
cracking and charring. Excessive current and
heat tend to increase the resistance.
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Ohmmeter – measure continuity or resistance in
resistors or other components.
 Component
having continuity has a resistance near
zero
 Component having no continuity has infinite
resistance.
Testing basic components
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Continuity measurement: (fuse, wires, cables or
cords)
A
good fuse, wire, cable or cord will have continuity (0
ohm)
 A blown fuse or broken wire, cable or cord will have
no continuity (infinite resistance).
 Gently bend the wire at various points especially
along common faults such as near connections.
Testing basic components
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Continuity measurement: (switch)
A
single pole, single throw switch should have
continuity when closed and no continuity
when opened.
 Gently
rock the switching mechanism to
identify potential intermittent circuit faults.
Testing basic components
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Potentiometer
1
lead of the ohmmeter placed on the center terminal,
and the other lead on one of the end terminals.
 The potentiometer should vary the resistance
accordingly when turned.
 Another way to test a potentiometer is to turn the
potentiometer while it is in the circuit. If a scratchy,
rough sound is heard in the speaker, the
potentiometer needs cleaning or replacing.
Testing basic components
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Batteries
 Voltage
or amperage testing
 An excellent battery should exceed its rated
value. For example, a new dry cell rated at
1.5 V should measure 1.5 V to 1.6 V. Old and
weak battery will read less than 1.5 V
Testing basic components
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Speakers
 Rattles
and heavy vibration are often signs of
a defective speaker.
 Visually inspect for cracks, dirt, and other
faults.
 Substitution is the best testing technique.
Replace with a speaker that have the same
impedance and power ratings.
Testing basic components
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Speakers (cont..)
 Defective
due to a bad voice coil, connection:
gently pressing on the speaker cone may
operate the speaker.
 Place an ohmmeter across the voice coil
terminal
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Listen and watch for a small ‘pop’ sound and
movement of the speaker cone. A defective
speaker will show no movement or popping sound.
Testing basic components
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Speakers (cont..)
 Phasing
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2 or more speakers:
Place the ohmmeter on the voice coil terminal and
note whether the cone moves in or out. Reversing
the polarity will reverse the movement. Mark the
positive side of each cone when it is in the out
position. Then connect each speaker to the audio
amplifier while observing the correct polarity.
Testing basic components
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Capacitors
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Techniques to test capacitors:
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Resistance measurement (ohmmeter)
1)
2)
3)
4)
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Make sure the capacitor has discharged
Place the ohmmeter on a high scale such as 10,000 ohm
Place the leads of the ohmmeter across the leads of capacitor
The needle should deflect upward and then slowly drop back
down to near zero
 Failure to deflect : open capacitor
 Failure to drop down: short circuited capacitor
Capacitance measurement (capacitor checker)
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To check capacitance, leakage and opens.
Testing basic components
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Capacitors
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Techniques to test capacitors:
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Spark test
1.
2.
3.
Momentarily (1 sec) connect the capacitor across the
voltage source. Make sure the voltage does not exceed
the voltage rating specified on the capacitor.
Short its terminals together using a screwdriver or a
similar device with an isolating handle.
A good capacitor will show a spark
Testing basic components
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Capacitors
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Techniques to test capacitors:
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Bridging
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Suspected defective capacitor is bridged (jumped) with
a known good capacitor within +10% of the rated value.
Substitution
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Replaced with a capacitor of the same
rating/specifications
Inductor
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It can be tested using meter ohm analog,
meter ohm digital and RCL meter
Condition
Meter Ohm
Meter RCL
Good
10Ω, 50Ω, 100Ω
Reading
Broken
∞ (open loop)
No reading
Fuse
Use ohm meter for testing
 Good fuse: reading must be between 0 –
10 ohm
 Broken fuse: reading infiniti
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Diode
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Use meter ohm
Forward bias
Reverse bias
-Probe +ve is connected to
anode
-probe –ve is connected to
cathode
Good diode: reading is low
resistance
-Probe +ve is connected to
cathode
-probe –ve is connected to
anode
Good diode: reading is
infinity
Transistor
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transistor is two back-to-back diodes in
one package as shown.
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The letters C, B, E stand for COLLECTOR,
BASE, EMITTER which are the names of the
three leads which come out of a transistor.
Transistor (cont)
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Assuming you know if the transistor is NPN or
PNP, and assuming you know where B, C, and
E are, then just test the B-C junction and the B-E
junction as if they were standard diodes. if one
of those junctions is a "bad diode", then the
transistor is bad.
Also, check the resistance from C to E using a
higher Ohms scale.
If the transistor is good, you should get an opencircuit reading from collector to emitter.
Transistors (cont)
USING METER TO SEPARATE NPN FROM PNP
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If you have a transistor but you don't know if it is NPN or PNP, then
you can find out which it is using Ohm-meter.
Assuming C, B, and E are known on the transistor, do the following:
- Connect the positive lead of your Ohm-meter to the base.
- Touch the other lead of your meter to the collector. If you get a
reading, the transistor is NPN.
- To verify, move the lead from the collector to the emitter and you
should still get a reading.
If your meter reads open-circuit, then connect the negative lead to
the base and touch the positive lead to the collector. If you get a
reading, then the transistor is PNP. Verify by measuring from base
to emitter.
THANK YOU