1. INTRODUCTION TO MAINTENANCE AND REPAIR
Repair: Is to restore something damaged, faulty or worn out of a good condition such as electrical
equipment, machine, vehicle, appliance, house, road and telecommunication equipment.
Maintenance: This involves functional checks, servicing, repair or replacing of necessary devices,
equipment, machinery, electrical, building infrastructure, and supporting utilities in industrial and
residential installation.
Types of Maintenance
a. Preventive Maintenance: Is a routine for periodically inspecting with the goal of noticing
small problems and fixing them before major ones develop. In a nutshell, nothing breaks
down.
The main goal behind preventive maintenance is for the equipment to make it from one
planned service to the next planned service without any failures caused by fatigue, neglect, or
normal wear.
Maintenance activities include partial or complete overhauls at specified periods, oil changes,
lubrication, minor adjustments, etc.
The main objectives of Preventive Maintenance are as follows:
i.
Enhance capital equipment productive life.
ii.
Reduce critical equipment breakdown.
iii.
Minimize production loss due to equipment failures.
b. Corrective maintenance: Is a type of maintenance used for equipment after equipment break
down or malfunction is often most expensive – not only can worn equipment damage other
parts and cause multiple damage, but consequential repair and replacement costs and loss of
revenues due to down time during overhaul can be significant.
c. Predictive Maintenance: Most recently, advances in sensing and computing technology have
given rise to predictive maintenance. This maintenance strategy uses sensors to monitor key
parameters within a machine or system, and uses this data in conjuction with analysed
historical trends to continuously evaluate the system health and predict a breakdown before it
happens.
This strategy allows maintenance to be performed more efficiently, since more up to date data
is obtained about how close the product is to failure.
Essential Electronics Lab. Equipment
Setting up an electronics laboratory requires just a few essential pieces of equipment and tools. While
specialty pieces of equipment may be essential for your application, the essential pieces of equipment
are the same for nearly any electronics lab.
i.
ii.
iii.
iv.
v.
vi.
vii.
viii.
ix.
x.
Multimeter
LCR Meter (Inductance, Capacitance and Resistance)
Oscilloscope
Soldering Iron
Precision Mechanical Tools eg cutter, wire strippers, needle nose pliers etc
Network Analyzers Rental (RF Test Equipment)
Optics (Magnifying lenses)
Power supply
Function generator
Signal generator
xi.
xii.
xiii.
xiv.
xv.
Spectrum Analyzer
Signal Analyzer
Pattern Generator
Protocol Analyzer
Transistor Tester
2. RESISTORS AND CAPACITORS COLOUR CODE
Resistor Colour Code
Resistor Colour Coding uses coloured bands to easily identify a resistors resistive value and its
percentage tolerance.
There are many different types of Resistor available which can be used in both electrical and
electronic circuits to control the flow of current or to produce a voltage drop in many different ways.
But in order to do this the actual resistor needs to have some form of resistive or resistance value.
Resistors are available in a range of different resistance values from fractions of an Ohm to millions
of Ohms.
Resistors are manufactured in what are called preferred values with their resistance value printed onto
their body in coloured ink.
The resistance value, tolerance, and wattage rating are generally printed onto the body of the resistor
as numbers, when the resistors body is big enough to read the print.
The Resistor Colour Code Table
Colour
Digit
Black
0
Brown
1
Red
2
Orange
3
Yellow
4
Green
5
Blue
6
Violet
7
Grey
8
White
9
Gold
Silver
None
Multiplier
1
10
100
1,000
10,000
100,000
1,000,000
10,000,000
100,000,000
1,000,000,000
0.1
0.01
Tolerance
±1%
±2%
±0.5%
±0.25%
±0.1%
±0.05%
±5%
±10%
±20%
For example, a resistor has the following coloured markings:
Yellow Violet Red = 472 = 47*10^2 = 4700 Ohms or 4k7 Ohm
Capacitor Colour Codes
Generally, the actual values of Capacitance, Voltage or Tolerance are marked onto the body of the
capacitors in the form of alphanumeric characters.
However, when the value of the capacitance is of a decimal value problems arise with the marking of
the ‘Decimal Point’ as it could easily not be noticed resulting in a misreading of the actual capacitance
value. Instead letters such as p (pico) or n (nano) are used in place of the decimal point to identify its
position and the weight of the number.
For example, a capacitor can be labelled as n47=0.47nF, 4n7=4.7nF, 47n=47nF etc.
Capacitor Colour Code Table
Band
Digit
Digit B
Colour
A
Black
Brown
Red
Orange
Yellow
Green
Blue
Violet
Grey
White
Gold
Silver
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
Multiplier
D
Tolerance
(T>10pf)
Tolerance
(T<10pf)
*1
*10
*100
*1000
*10,000
*100,000
*1,000,000
±20%
1
2
3
4
5
±2.0%
0.1
0.25
*0.01
*0.1
*0.1
*0.01
+80, -20
10
5
10
Capacitor Voltage Colour Code Table
Band Colour
Type J
Type k
Black
4
100
Brown
6
200
Red
10
300
Orange
15
400
Yellow
20
500
Green
25
600
Blue
35
700
Violet
50
800
Grey
900
White
3
1000
Gold
2000
Silver
Capacitor Voltage Reference
Type J - Dipped Tantalum Capacitors
Type k – Mica Capacitors
Type L – Polyester/Polystyrene Capacitors
Type M – Electrolytic 4 Band Capacitors
Type N – Electrolytic 3 Band Capacitors
Capacitor Letter Codes
Picofarad (pF)
Nanofarad (nF
Microfarad (uF)
0.5
Temperature
Coefficient
(TC)
-33*10^-6
-75*10^-6
-150*10^-6
-220 “
-330 “
-470 “
-750 “
1.0
Voltage Rating (V)
Type L
Type M
10
100
1.6
250
4
40
400
6.3
16
630
25
2.5
Type N
10
35
6
15
20
25
3
3. Testing Methods
Observation Test Method
a. Visual: Testing method can be carried out by looking for a defective or burnt component in a
circuit.
b. Touch: Is to determine or feeling the temperature of the component in a circuit, if the
temperature is too high i.e is too hot, a check on the component or stage must be carried out.
c. Smell: We also use our smelling organ to perceive an odour, that is, the nose to detect if a
component is burnt or burning.
d. Hearing: Listen to the performance of a set as it is very essential when testing. The hearing
organ is used to detect a sound or noise coming out from a set, whether it is normal or
abnormal.
Types of Electrical fault
•
Open circuit: Is an electrical circuit in which the continuity is broken so that current does not
flow. Also it is a discontinuous circuit through which no current can flow. Voltage is at
maximum while current is at minimum.
•
Short circuit: Is an electrical circuit that allows a current to travel along an unintended path
with no or very low electrical impedance. This results in an excessive current flowing through
the circuit. The opposite of a short circuit is an open circuit which is an infinite resistance
between two nodes. Voltage is at minimum while current is at maximum.
Methods of testing component
In circuit: Reading of electronics components on board. The reading of transistors, diode and resistors
are taken without removing it from the board or panel.
Out of circuit: Is a procedure or a way of removing components on board before taking its readings to
determine the status of the component whether it is good or bad.
Selecting Equivalents for Diodes
i.
ii.
iii.
iv.
v.
vi.
The variety of diode (silicon or germanium point contact or junction) must be the same.
The maximum rated current of the replacement diode must be at least the same as the old one
as preferably somewhat greater.
The peak reverse voltage rating of the replacement must be the same or grater.
The voltage across the diode at its normal operating current must be the same or less.
Replacements of zener diodes must have the same voltage and power ratings.
Replacements for LEDs must have the same colour, forward voltage and current ratings.
Rules for Transistor substitution
When transistors are substituted for one another, the following rules should be obeyed:
i.
ii.
iii.
The substitute transistor must be of the same variety (silicon, NPN, switching as opposed to
amplifying etc.
The substitute transistor should have shown the same hfe value.
The substitute transistor should have the same ratings of maximum voltage and current.
Service Methods
Circuit Tracing: Is a means or procedure of identifying a defective component in a circuit or on board.
Spliting Method: Is a means of troubleshooting by isolating a stage one after the other, in order to
identify the faulty stage. A signal is fed in order to troubleshoot the circuit, stage by stage until the
faulty area or stage and the component which is faulty is being identified.
4. Diagnosing faults in a system
a.
Troubleshooting of a super heterodyne radio receiver
Symptom
Causes or Remedy
1, No power supply/system dead
Power stage: check and replace transformer,
capacitor, diode, cable lead and fuses.
2, Humming
Power/Audio Stage: check capacitor
3, No sound/output
Audio/Sound stage: check speaker, detector,
IF, volume control and sound IC/transistor
4, No reception
RF stage: Antenna, coil, ceramic and mica
capacitor.
Scanner: Scanners are the input peripheral device which is used to transfer images into digital form.
These images can be documents, pictures, graphics, photograph etc. The selection of scanner depends
on the volume and type of material to be scanned and can selected as drum scanner, flatbed scanner,
sheeted scanner and small hand held scanners etc. The small hand held scanners are widely used with
microcomputer. Scanners vary greatly in several basic ways i.e scan speed, bit resolution and signal to
noise ratio (SNR). There are tradeoffs to each of these parameters.
Scanner
Resolution: There is a direct correlation between the bit resolution and the AFE(analog front end).
Higher bit resolutions (more digitized bits per pixel) give smoother greys, more natural colour and
more ‘photorealistic’ images. The higher bit resolutions also help to give more dynamic range, the
difference between the black level and the white level. Dynamic range is particularly important for
seeing details in dark or shadowed regions or when an image includes both shadow and sunlit areas.
Scan Speed: There is direct tradeoff between scan speed and resolution and signal to noise ratio.
Slower scanners will tend to be higher resolution and have a higher signal to noise ratio, but these
specifications can be improved at higher scan speeds with a faster AFE and processor. Thus the more
precise scanner motor should be desired.
Signal to Noise Ratio: The signal to noise ratio or SNR is dependent upon the entire signal chain,
from the image sensors to the AFE. This is important for document scanners if OCR (Optical
Character Recognition) or other image processing will be performed. It is important in photo quality
scanners to eliminate graininess in images. A higher resolution necessitates a better SNR to maintain
quality.
Block diagram of a scanner
Depending on the volume and type of the material to be scanned we can use drum scanner, flat bed
scanner, sheeted scanner or even small hand held scanner. The small hand held scanners are
frequently used with microcomputer. Most of the manufacturer responded to user reluctance to use
scanners smaller in size and paper to be scanned at cheaper price. Most of these new devices sit
between keyboard and monitor and can be interfaced with fax machine. There purpose is to convert
scanned image into numeric digits before storing it into the computer.
Troubleshooting of a scanner
FAX MACHINE
A fax machine is designed to send and receive documents. So it contains a transmitter as well as
receiver section. The transmitter section is similar to computer scanner with a charged coupled device.
It scans the image line by line. It looks at each line separately, scans the lack areas and the white
areas, and transmits electric pulse on the phone line to represent it in black and white. The information
is transmitted to other side fax through phone line. The receiver side fax machine receives information
in the form of electrical pulses. If the receiving fax hears black, it draws a tiny black dot on the page,
if it hears white, it moves along slightly, leaving a white space. It takes about a minute to transmit a
single page of writing. In this way, fax machine works to transmit black and white lines from sending
end to receiving end.
Block diagram of a fax machine
Block Diagram of a Microwave oven