Friday, February 26, 2010

ELECTRONIC TROUBLESHOOTING AND REPAIRS

A REPORT OF THE
STUDENT’S INDUSTRIAL WORK EXPERIENCE SCHEME
(SIWES 400)


DONE AT
LASER CONCEPTS
43, ALHAJI TOKAN STREET, ALAKA ESTATE, WESTERN AVENUE, SURULERE, LAGOS.


WRITTEN BY
OKERE HENRY OKECHUKWU
20051472533
ELECTRICAL/ELECTRONICS ENGINEERING


SUBMITTED TO
THE SIWES COORDINATOR,
DEPARTMENT OF ELECTRICAL/ELECTRONICS ENGINEERING,
SCHOOL OF ENGINEERING & ENGINEERING TECHNOLOGY,
FEDERAL UNIVERSITY OF TECHNOLOGY, PMB 1526, OWERRI, IMO STATE.

IN PARTIAL FULFILMENT OF THE REQUIREMENT FOR THE AWARD OF A BACHELOR’S DEGREE (B.ENG) IN ELECTRICAL/ELECTRONICS ENGINEERING.



NOVEMBER 2009


DEDICATION
I dedicate this report to the Almighty God for sparing my life while the training lasted, to my parent for their financial support and to every student who quest for meaningful knowledge.

































ACKNOWLEDGEMENT

Taking a retrospective look at the challenges encountered prior to securing a placement and in the course of my training, I sincerely express profoundest gratitude to the following personalities:

To my parent, Mr. & Mrs. Innocent Okere, for their immense support and care. May they reap bountiful reward of their labour.

To my supervisor, Mr. Chukwuemeka Ogbue, for his dogged and relentless efforts in ensuring that this report becomes a reality.

My appreciation also goes to my colleagues Emmanuel, Seun, Fisayo, Reuben and to all friends innumerable to mention.
A special thanks accords to my typist Miss Karoline Anyawu, for her job well done.

Finally, to the Almighty God for His continual guidance, protection and good health while my training lasted, I remain most grateful.











ABSTRACT

In Nigeria, a comprehensive study revealed that within the last decade of the 20th century, only a handful graduates go the extra mile to acquire substantive knowledge in Information and Communication Technology (ICT).

In recent times, irrespective of the course studied at school it is recorded that thousands of school leavers sought after knowledge in ICT as part of ways to increase chances of being employed. It is in this regard that I present this write-up in unambiguous terms.

This report has been painstakingly written to expose students who desire to learn as well as improve their abilities in troubleshooting and repairs of electronic gadgets. Technical terms used have been thoroughly explained to aid understanding. The general structure of this report is as follows:

Chapter one presents the introductory part of the report.

Chapter two introduces the concept of electronics as a study of the flow of electrons in certain materials. It also covers the physical and symbolic structure of electronic component and how to determine when a component is faulty.

Chapter three titled “Soldering Practice” explains the concept of soldering. An explanation on high resistance joint (dry joint), how to cure dry joint and various de-soldering techniques are also contained herein.

Fault diagnoses and troubleshooting techniques have been clearly emphasized in chapter four of this write-up.

Chapter five covers frequently occurring faults in electronics and their remedies. This chapter teaches from start to finish how to repair faulty electronic devices.

Finally, chapter six recaptures the salient points encountered in the course of reading. It also presents in a concluding tone the relevance of the training and recommendations.
























TABLE OF CONTENT

TITLE PAGE……………………………………………………………
DEDICATION………………………………………………………….
ACKNOWLEDGEMENT………………………………………………
ABSTRACT…………………………………………………………….
TABLE OF CONTENT…………………………………………………

CHAPTER ONE………………………………………………………..
1.0 INTRODUCTION……………………………………………….
1.0.1 WORKFLOW DIAGRAM………………………………………
1.1 RELEVANCE OF TRAINING………………………………….

CHAPTER TWO……………………………………………………….
2.0 COMPONENTS IDENTIFICATION AND CODING………….
2.1 PHYSICAL AND SYMBOLIC IDENTIFICATION……………
2.1.1 Resistor Colour Coding………………………………………….
2.1.2 Capacitor Identification………………………………………….
2.1.3 Identification of Diodes………………………………………….
2.1.4 Identification of Transistors……………………………………..
2.1.5 Identification of Integrated Circuits……………………………..
2.2 PRECAUTIONS REQUIRED WHEN HANDLING AND
TESTING OR INSTALLING COMPONENTS………………..
2.3 HOW TO DETERMINE WHEN A COMPONENT IS
FAULTY…………………………………………………………
2.3.1 Failure modes of components……………………………………

CHAPTER THREE……………………………………………………..
3.0 SOLDERING PRACTICE……………………………………….
3.1 DRY JOINT………………………………………………………
3.1.1 Cure of dry joint………………………………………………….
3.2 MAKING A GOOD SOLDERED JOINT……………………….
3.3 DESOLDERING TECHNIQUES………………………………..

CHAPTER FOUR……………………………………………………....
4.0 TROUBLESHOOTING FAULTY ELECTRONICS…………….
4.0.1 Troubleshooting Techniques……………………………………..

CHAPTER FIVE………………………………………………………...
5.0 FREQUENTLY OCCURING FAULTS IN ELECTRONICS
AND REMEDIES……………………………………………….
5.0.1 Possible faults in CRT Televisions and Monitors and Remedies.
5.0.1.1 Power Faults……………………………………………………
5.0.1.2 Horizontal Fault…………………………………………………
5.0.1.3 Sound Fault……………………………………………………..
5.0.1.4 Picture/Display Fault……………………………………………
5.0.1.5 Tuner/Receptor Fault……………………………………………
5.0.1.6 Possible faults in DVD/CD/Home Theatres and Remedies……..
5.0.1.7 General Troubleshooting………………………………………..
5.0.1.8 Power, Lens and Picture Board Faults………………………….
5.0.3 Possible faults in UPS, AVR, and Adaptors and Remedies…….

CHAPTER SIX……………………………………………………………
SUMMARY AND RECOMMENDATIONS…………………………….
CHAPTER ONE

1.0 INTRODUCTION

Owing to the declining standard of education in the Nation’s tertiary institutions, the Student’s Industrial Work Experience Scheme (SIWES) was initiated to curb the impending danger. This initiative was brought about by the joint effort of the National Universities Commission (NUC) and the Industrial Training Fund (ITF) in collaboration with the Federal Government and it is duly funded by the latter.

The aim of this initiative was to bridge the gap between theoretical and practical learning and to integrate both in order to yield a befitting output.
Job specifications for all degree programmes were drawn out to make this initiative a worthy cause.
In line with this initiative, universities all over the country mandated a 6-month Industrial Training programme for her 4th year students in which on completion a work report would be presented.

Laser Concepts, located at 43, Alhaji Tokan Street, Alaka Estate, Western Avenue, Surulere, Lagos, is a private owned ICT consulting firm which has been in operation since 1992 and has its specialty in the following areas:
Repair & Maintenance of Computer Accessories and Electronic gadgets,
System Analysis, Design, Implementation and Support;
Training (Human Capital Development) and other related field.
Their major products are the services rendered and their income is derived from the charges placed on services rendered.

Being a private owned establishment, the manager deals directly with clients and allocate tasks to his few workers including the IT students. Below is a workflow diagram of the firm.

1.0.1 Workflow Diagram













The content of this write-up is a product of judicious time spent at Laser Concepts. This write-up is not only a teacher but also a guide for persons interested in the field of electronics. The report is divided into six chapters with explicit contents to enhance maximum understanding. I therefore deem it fit to recommend this report to all and sundry willing to acquire knowledge in repairs and troubleshooting of electronic gadgets.

1.1 Relevance of Training
Indeed the idea of SIWES is in no doubt a milestone in the development of academic activities on the Nation’s tertiary institutions. The benefits derivable by employers, universities and students alike are immense and will go a long way to move the country forward technologically. Below are the relevance of the training:
(1) It provides an avenue for students involved to acquire industrial skills and experience in their chosen course of study.
(2) Affords students the opportunity to apply their theoretical knowledge in real work situation thereby bridging the gap between university work and actual practice.
(3) Prepare students for the work situation they are likely to meet after graduation.
(4) It makes the transition from the university to the world of work easier, and thus enhances student’s contact for later job opportunity.
(5) Expose students to work methods and techniques in handling equipments and machineries that may not be available in the universities.





















CHAPTER TWO

2.0 COMPONENTS IDENTIFICATION AND CODING

A good understanding of components and their limitations is an essential part of project building techniques. Components are identified at two levels- Physically and Symbolically.

In circuit diagrams, electronic components are represented as symbols. Ability to recognize these symbols and relate it to its corresponding physical component comes in handy when troubleshooting is being carried on an equipment.

Component code number
A component code number fully identifies that component physically. For example there are many brands of transistors and each individual transistor is identified by its code number.

Various components resemble one another, the main differentiating feature being the code number. A component must be known if it is polarized so as to avoid fixing it the wrong way up in the circuit, with disastrous consequences.

2.1 Physical and symbolic identification
The table below gives the physical appearance and symbolic representation of some components. It also shows whether it is polarized or not.












































2.1.1 Resistor colour coding

Resistance value of most resistors is determined by the colour band painted on the body. Colour coding is used to represent value and tolerance of a resistor. Four colour bands are mostly used. The first three bands give the value of the resistor while the last gives the tolerance. The table below gives the various colours and their respective codes.

COLOUR 1ST DIGIT 2ND DIGIT MULTIPLIER TOLERANCE
Black 0 0 1 --
Brown 1 1 10 1%
Red 2 2 102 2%
Orange 3 3 103 --
Yellow 4 4 104 --
Green 5 5 105 --
Blue 6 6 106 --
Violet 7 7 107 --
Gray 8 8 108 --
White 9 9 109 --
Gold -- -- -- 5%
Silver -- -- -- 10%
Colorless -- -- -- 20%

Resistors are of two types viz fixed resistors, which includes carbon resistors and wire wound; and variable resistors, also called potentiometer. Parameters considered when selecting a resistor are:
1. Power rating in watts- this relates to the maximum current that can pass through the resistor without overheating it. Carbon resistors are rated 1/8W, 1/4W, 1/2W, 1W, or 2W. wire wound have higher wattages up to 25W or more. The physical size of a resistor indicates its power rating.
2. Maximum voltage applied across the resistor
3. Working temperature range

2.1.2 Capacitor Identification
Capacitors are used to store electric charges, to separate AC from DC by coupling DC and to regulate the current in an AC. Some types of capacitors are polarized and must be used in DC circuits only. It must also be correctly oriented when placed in circuits. Non-polarized capacitors are used in both AC and DC.

Capacitors derive their names from the dielectric used in its construction. eg paper, mica, ceramic and electrolytic capacitors use paper, mica, ceramic and electrolytic materials respectively as dielectrics. Capacitors may be fixed or variable.
Parameters considered in choosing a capacitor are:
1 Voltage rating: This is the highest DC voltage the capacitor can withstand without physically breaking down. e.g 6.3v, 10v, 16v, 25v, 35v, 50v, 100v, 250v, 400v.
2. Polarization.

2.1.3 Identification of Diodes
Diodes are polarized and allow current flow in one direction only, from cathode to anode. Diodes are named according to their functions
*Rectifier Diodes- used for rectification of voltages from AC to DC
*Zener Diodes- for voltage stabilization.
*Varactor Diodes- employed in tuning of radios and TV when they act as variable capacitors
*Light Emitting Diodes-used as illumination indicator light.
Factors that determine the suitability of diodes are:
Peak Inverse Voltage (PIV): This is the maximum reverse voltage given by the manufacturer, beyond which the diode suffers a breakdown. Typical PIV rating are 50v, 100v, 200v, 400v etc.
Forward current rating: This is the maximum current the diode can pass without thermal runaway.

2.1.4 Identification of Transistors
Transistors are identified by coded numbers. Some of which are BC109, BC181, 2N3055 etc. two types of transistors are Bipolar junction (BJT) and Field Effect Transistors (FET). Parameters considered in selecting a transistor are:
- Application whether for low power audio, high frequency.
- The common emitter amplification factor (hfe)
- Maximum power it can dissipate
- Maximum collector current
- Maximum supply voltage.

2.1.5 Identification of Integrated Circuits(IC)
An IC is a complete circuit containing transistors, diodes, resistors and capacitors all contained in a single package (chip) and are available in many sizes with varying connecting pins. Maximum supply voltage, proper pin connections and function of device are parameters considered in selecting an IC.

2.2 Precautions required when handling and testing or installing components
Components can be damaged by careless handling during assembling and testing or when the device is being serviced. Some major causes of damages are:
Lead bending: this can generate excessive stress on components and cause it to crack.
Overheating and thermal shock: If the maximum temperature of the component is exceeded during soldering, it may be damaged.
Mechanical shock: dropping of component can cause damage by high impact shock.

2.3 How to determine when a component is faulty
Every component is supposed to work according to specification. Outside this, the component has failed.

2.3.1 Failure modes of components

Resistor Failure
Resistors can go open circuit
It can also go high in value
Capacitor Failure
Open circuit thus blocking useful voltage
Short circuit
It can also go leaky, passing more dc current than the specification allows
Inductor Failure
Open circuit due to break in the winding
Potentiometer Fault
Open circuit if wiper is not making proper contact with track
It may be noisy when dirt gets between wiper and track
Diode Fault
Open circuit at PN junction
Short circuit at PN junction
Transformer Fault
Open circuit in primary or secondary winding
Short circuit in primary-secondary winding
Short circuit between the core and primary/secondary winding.
Transistor Faults
Short circuit at base-emitter junction
Short circuit at base-collector junction
Open circuit at base-emitter junction
Open circuit at base-collector junction
Short circuit between collector and emitter
FET Fault
Short circuit at gate-source junction
Short circuit at gate-drain junction
Open circuit at gate-source junction
Open circuit at gate-drain junction
Short circuit at gate-drain junction for MOSFETs only.
IC Faults
Two methods are used in determining faulty ICs
Substitution method: Components around the IC are checked and the IC is replaced as a final resort when the surrounding components are good
Comparison method: Voltage and resistance measurements are made at the pins of a healthy IC and the suspected IC in similar circuit.

CHAPTER THREE

3.0 SOLDERING PRACTICE

When components have been placed on Vero board according to the circuit diagram, the next step is to solder the lead out wires on the copper sides.

To make a good soldered joint the soldering iron tip is applied to the lead out wire and the copper foil. The solder is allowed to melt and flow smoothly covering the junction with a blob. The soldering iron is withdrawn and then solder joint allowed to cool without shaking.

To prevent short circuit between adjacent copper strips, the tip of the soldering iron is run between the strips to eliminate solder hairs. A sharp pointed object is also used to scrap the area between strips.
Finally, resistance measurement should also be taken between strips. This will show any short circuit.

Soldering Iron and Bits


Dry Joint.
Another bane of poor soldering is dry joint or high resistance joint. Dry joint is a joint which may appear perfectly sound on the surface yet has substantial electrical resistance.

Joint with a relatively high resistance will have a voltage developed across it due to current flowing through it.
In DC circuits, this will result in voltage drop, loss of power and overheating due to I2R power loss.
In AC circuit, a signal voltage will develop across the joint causing instability like oscillations. This effect may show up as fizzles, crackles and similar disturbances in audio, radio and TVs.

3.1.1 Cure of Dry Joint
The cure to dry joint is to heat all the joints again until the solder melts. More solder may be applied where there is not enough of it.

3.2 Making a good soldered joint.
1. Tin the soldering bit by applying resin cored solder to it coating it with solder.
2. Apply the tinned tip immediately to the joint, holding it in contact while the temperature of the joint metal rise to melt the solder.
3. The application of the heat should not be prolonged to avoid ruining the component and the circuit conductors
When soldering delicate components like transistors, diodes and ICs it is very important to make sure that heat conduction to the components is kept minimal. This is achieved by:
i. Employing a heat shunt on the component lead out wire to conduct heat away from the component.
ii. Using a soldering iron with hotter bit so that the solder melts quickly, thus minimizing the time of contact between the bit and the component.
iii. Pre-tinning the joint metal to accelerate soldering process.

3.3 De-soldering Techniques
Occasions arise in the course of troubleshooting a circuit for the removal of a defective component so that known good replacement can be tried. One has to be careful because removal of a component can affect other adjacent components and the copper foil.

In these instances, a more scientific approach is desired and appropriate de-soldering tool is employed.
The de-soldering gun, shown below is a hand operated device which work by suction.
First, the gun is loaded by depressing the plunger. The nozzle, made of tough PTFE (heat resistant plastic) is then placed in a position over the joint in which the solder is melted by the use of a soldering iron; the plunger is released by activating a lever conveniently situated on the body.
This result in a strong suction which extracts the molten solder from the junction drawing it up into the gun where it solidifies and is pushed out in small pellets when the plunger is depressed again. The de-soldering gun is operated by one hand and it is ideal for printed circuits




A De-soldering Gun













CHAPTER 4

4.0 TROUBLESHOOTING FAULTY ELECTRONICS/PROJECTS

Many projects hardly ever work first time they are switched on. Some of them malfunction while a few work for sometime and then pack up
When a project malfunctions or does not work, a number of issues comes to mind. The first impulse is to tackle the fault finding in a haphazard way and end up changing many good components.

When faced with a silent non-functional project, one does not at once reach for meter or soldering iron. Rather, we should first apply observation test using the sense of sight, hearing, smelling and feeling.

First, the plug is checked for proper contact, and then the following observations are carried out:
i. The equipment is observed for flying wires, unconnected leads and damaged PCB tracks
ii. The components are checked for overheating by touching them. This is applied especially to transistors, resistors and transformers.
iii. The smell of burning transformers or resistors is noted
iv. Distorted sounds emanating from amplifiers or oscillators are noted.
If the fault is not discovered then the circuit layout diagram is crossed checked with the circuit diagram and then checked with the PCB or vero board wiring.

4.0.1 Troubleshooting Techniques
When observation test failed to reveal the fault, then systematic troubleshooting techniques are employed. These are:
(a) Symptom-Function Techniques
This involves knowing the working of the gadget built. Thorough knowledge of the working parts of the gadgets is essential. The more you know about the equipment, the easier it is to troubleshoot.
(b) Signal Injection and Tracing Techniques.
This is most useful in radio and audio circuits. A signal generator is used to provide the signal. An audio amplifier is used to trace the signal until it disappears after a faulty component is identified. When the faulty components have been located, it is de-soldered and resistance check is used to confirm whether it is good or faulty.
© Voltage and Resistance measurement:
Owing to problems involved in removing components for substitution tests and possibly placing them back, components are removed for testing as a last resort. Basic testing equipment is a multimeter. A digital meter gives a more accurate reading than the analog type and we can check the working of a circuit by checking the voltage across resistive elements.

Since transistors and ICs are delicate and not easily removed, components around these semiconductors are thoroughly checked for breakdown before they are placed.









CHAPTER FIVE

5.0 FREQUENTLY OCCURING FAULTS IN ELECTRONICS AND REMEDIES

Most, if not all electronic devices develop fault(s) after being put to use foir sometime. The rate of breakdown depends on the degree of usage in which the device is subjected.
Every electronic device comes with a manual, containing information on how the device should be used in other to ensure safe working.
The device also has some specifications written on its body. The specification contains information on the input/output voltage, the frequency and the wattage of the device. Deviation from these specifications may reduce the life span of the device consequently damaging it.
This chapter contains remedies to frequently occurring faults in common electronic devices.

5.0.1 Possible faults in CRT Televisions and Monitors/Remedies
A number of faults occur in CRT Televisions and Monitors but I have examined the most frequently occurring fault and their respective solutions.
5.0.1.1 Power Faults
Every electronic device has a power board from which voltage distributes to other parts of the device. The power section is prone to fault if the device is carelessly used some of such faults are stated in the table below.

Faults Diagnoses Remedies

TV failed to power 400V Power capacitor burnt. Replace capacitor and
Also affected choke resistor resistor with same
close by. kind and rating

TV failed to power Partial contact (high resistance Scrap off the area
Joint) around power region. Affected and apply
More solder.

TV failed to power Leakage of 400V capacitor, Replace the three
Power IC damaged and components with
Power transistor burnt. Same kind and
Rating.

TV failed to power Thermistor (high voltage Replace both with
Protector) burnt as well same kind and
as fuse. Rating.

LCD Monitor failed Internal connections to Reconnected
to power. Power module wrongly
placed.

TV Failed to power Power section burnt Substitute old power
Pack with a known
working one.


Note: To use a working power pack of one panel in place of another, remove the power capacitor, power transistor, Starting IC, Fuse and other essential power component from the non-functioning power pack, then connect the chopper of both panels using the diagram shown below.






Chopper connection









Note: the chopper works in the same principle as the transformer, but it stabilizes the output voltage whereas transformers do not. For this reason, modern electronic devices employ chopper to maintain the voltage even if the supply voltage is low.

The primary side of the chopper has four pins, the first and last two pins connected as shown in the diagram. The secondary side can have as many as 6-10 pins depending on its configurations. It thus supply different voltages on each secondary pins, which in turn passes the voltage to diodes, resistors and finally to capacitors.

To determine the exact voltage on each secondary pins of the chopper, simply locate the nearby capacitor connected o the pin via a diode or resistor. Find out the voltage of that capacitor, and then use the table below to determine the likely voltage of the pin.

Capacitor voltage Chopper pin voltage
10V 2-5V
16V 6-10V
25V 12-20V
35V 20-25V
50V 25-35V
63V 35-40V
100V 50V
160V 110V

5.0.1.2 Horizontal Fault

The TV panel is divided into horizontal and vertical section. The horizontal section consists of the line transformer (supplies voltage to the tube, the panel and the RGB board). The line transformer works handy with a large transistor (horizontal transistor). The horizontal section also includes the power pack and all its components including the chopper.

To determine if the line transformer is functioning.
Test: place the negative of test probe to earth and the positive to the collector of horizontal transistor. If the line transformer is good, 110V will be registered on the test meter.

Alternative test: Remove the cap from the tube, earth the cap to dissipate voltage stored on it. Turn on the TV, and then bring the cap very close to a conducting material. If the line transformer is good, a bluish light will be seen.
To purchase a new line transformer that will suit your TV panel, compare the continuity reading for both the old and new one. They must correspond.

5.0.1.3 Sound Faults

Sound apparatus or components are found at the vertical section of the TV panel. The possible faults are:

Faults Diagnoses Remedies

TV not able to control Function IC bad. It Clean up affected area with
or change between could result from rats fuel then replace function IC
stations and volume depositing urine on the with same kind and rating.
control faulty. panel.
NB: Function IC is
responsible for TV
settings such as
volume adjustment,
contrast, brightness,
channel selection etc.

TV’s sound not check the speakers to A small resistor was found
Functioning confirm if working by be blocking voltage from
probing its terminals, entering the IC, which we
check components replaced.
around sound IC.

TV’s sound not The speaker, sound IC, Re-solder components
Consistent and other components around the sound region
still OK. In case of dry joint.


Construction of an Alternative sound system using TDA 2001-3 IC to replace a burnt IC not available or scarce to find in the market.

Some sound equipment may be abandoned as a result of unavailability of a sound IC. In such a situation, an alternative sound apparatus can be constructed to suit that purpose using the diagram below.









NB: - All earth should be connected to negative terminal
- Use negative terminal of gram to touch the entire holes one after the other. When a loud sound is heard, solder the negative of gram to that hole.
- For 15V supply, connect an ohmless resistor to positive of pin 5.

5.0.1.4 Picture/Display Faults

Fault Diagnoses Remedies

A white horizontal It is a vertical Replace vertical IC with same type
line appears on screen fault. The potentiometer around vertical
with no display on IC also adjusted so that the picture
screen. covers entire width of screen.

Picture on screen not A colour on Replace RGB board.
Clear. RGB board NB: each color on RGB receives a
Missing. Voltage of 160V.

TV picture displays White plastic on White plastic on board replaced.
after 20s or more but RGB board
begins with dots like condemned.
haze/hazy picture on
screen.

TV displays picture but 35V or 160V For panels with transistorized
with several horizontal capacitor vertical IC, search for a small
lines on screen. unnoticeably 160V capacitor around line
burnt around transformer region. For panels
vertical section. With normal vertical IC, search
within IC area for a 35V capacitor
which has direct connection to the
IC and replace it with same type.

Picture not displayed Conduct necessary Re-solder components on RGB
on TV screen. test within RGB board in case of dry joint.
board.

Picture on TV screen 400V power Replace power capacitor with
wavers. Capacitor bad. same kind and rating.
although not leaky.

Picture not displayed White plastic on Replace white plastic and resolder
On screen, only a hazy RGB board components on RGB board.
Blue impression is condemned.
Seen covering screen.


NB: Before troubleshooting a TV set with picture fault, check the line transformer to ensure it’s in good condition.

5.0.1.5 Tuner/Receptor Fault.
The tuner on a TV panel is in charge of receiving signals from the outdoor/indoor antenna so as to produce clear picture on screen. Its resistance is 75 ohms.


Tuner Conversion
From Single-band to 3-band.
Some TVs come with single-band tuner (6 pins), therefore not capable of receiving signals from channels in the VHF series (NTA 5 and NTA 10). In order to cover channels in both UHF and VHF series it is necessary to convert such single-band tuner to a three-band tuner. See connections below.















NB: TV- Tuning Voltage for fine tuning.
More space is purposely left between IF and MB top indicate pin 1 (IF pin).
Connect VH to MB of 3-band tuner
Both tuners must be earthed before connection is made.

AUDIO-VIDEO (AV) CONNECTION

This is necessary for a TV without AV apparatus.
Materials needed for this connection are: Tuner from TV; IF from video scrap; connecting wires; red and yellow ports.
NB: MB from tuner supplies 12VDC to IF which is the required voltage for connection.
Both tuner and IF must be earthed as shown below.







After connection has been made, the fine tune button on TV should be used to search for any free channel, which will automatically be allocated to video.

5.0.2 Possible Faults in DVD/CD/Home theatre and Remedies.
All DVD/CD/Home theatre players consist of three main panels namely: power board, picture/display board (compatible) and Lens apparatus.

5.0.2.1 General Troubleshooting
Before troubleshooting faulty DVD, check the power cable, check if supply enters the power board, check if supply is available at output of power board, if not, check the components on power board by observation and taking meter readings to determine if any is faulty.

If power board is OK, then proceed to lens region. Rotate the tub at the centre point of lens. If rotating freely, then its rotor is good. Move the lens mechanism backwards or forward, turn on the power if the mechanism returns to its previous position, then the rotor on the mechanism is good. We can replace the lens with a sure working one to confirm if lens is faulty.

If DVD can’t still play, then we proceed to the picture board. The latter is machine soldered. We can only check for dry joint. The board can also be washed with soap and water to enhance its efficiency.

The flex that connects the picture board to the lens should also be checked.
Possible faults on a DVD player are:

Fault Diagnoses Remedies

DVD player can only Lens faulty Replace lens.
read from VCD but NB: some DVD player are
not DVD and vice lens selective.
versa.

DVD player with Sound IC on picture Replace picture board.
sound/volume board bad.
problem.

DVD not displayed Dry joint at DVD-TV Re-solder to overcome
On TV screen. Interface high resistance joint.

DVD not able to Eject switch bad Replace switch.
eject.

DVD failed to 400V power capacitor, Replace respective
power. Fuse and 2.2 ohm choke components with same kind
resistor burnt. and rating.

DVD failed to Entire power board Replace power board.
power. condemned. See notes below.


NB: before buying a new power board for DVD, compare the output voltages of the old board to the output voltage of the new board. Ensure that the voltages on both boards correspond. For instance









To fix in new board into DVD, we have to rearrange flex for old board to suit voltages on new board when compared. Board 1 has voltages (12v, -5v, 5v, G, -12v) which corresponds with pin 1, 2, 3, 4, 5 on flex. To configure board 2 to work with DVD, do the following connections.

Remove wires 2, 3, and 4 on flex since they do not correspond with voltages on old and new board. Interchange appropriately i.e. let wire 2 on old be 4 on new; let 3 on old be 2 on new and let 4 on old be 3 on new.

5.0.3 Possible Faults in UPS, AVR, Adaptors and Remedies.

Most frequently occurring faults in the above equipments are power faults which are usually caused by burnt capacitor, fuse diodes and resistors.
Simply carry out tests and voltage tracing to determine which component is faulty, and then replace appropriately with same type and rating.

To generate 12VDC when given a transformer with centre tap
This setup can be used to charge up battery faster and more economical.
Materials required are transformer, bridge rectifier or 4 diodes, capacitors and 6v or 12v battery.
NB: the capacitor used should have a voltage double that of the transformer output voltage. See diagram below.












CHAPTER SIX
SUMMARY AND RECOMMENDATION

Indeed SIWES has proven to be a worthy cause by way of the level or extent of exposure it gives to students who actively participate in it.

To recapture the salient points in the foregoing chapters, electronics is a study of the flow of current in certain materials and for this to be permissible, a conductor must be present.

Electronic devices consist of multifarious components which can be identified physically and symbolically. Each component has their respective code number and a good knowledge of their properties is inevitable when deciding their respective function.

Resistors resist the complete flow of current in a circuit.
Capacitors stores voltages in them and commonly found in DC equipments.
For rectification purposes, diodes are very good resorts while transistors find usefulness in signal amplification.
Integrated circuit is a combination of the aforementioned components in a single package called chips.

Electronic components have their respective failure modes and it is imperative to consult the manufacturer’s data book when in doubt.

Having placed components on board according to a specified circuit diagram, the next step is to solder. A good soldered joint is a function of the proficiency of the solderer, the soldering temperature and the kind of soldering iron used.

One bane of poor soldering is dry joint. This joint appears perfectly sound on the surface, yet has substantial electrical resistance. Dry joint results to loss of power and overheating in DC circuits.
In AC circuits, it generates signal voltage causing instability like oscillations across the joint.

Some component may be found defective in the course of troubleshooting. A most suitable approach is to use a de-soldering gun(a hand operated device which works by suction) with the help of soldering iron to remove such component.

A rudimentary approach in handling any faulty device is to troubleshoot it. Some techniques devised for effective troubleshooting are
The symptom-function techniques
The signal injection and tracing techniques and
The voltage and resistance measurement.

Troubleshooting uncovers he problem zone in a faulty device thus proferring solution to them. Before using a first-hand equipment, it is wise to go through the manual provided. A number of fault arise I a devise as a result of mishandling or deviation from that which is specified in the manual

Likely faults in CRT monitors and televisions are power failure resulting from burnt power capacitor, thermistors, power pack or dry joint around power region; horizontal or line transformer mishap; sound/volume adjustment fault; picture/display fault and tuner/receptor fault.

Other equipment like DVD/CD players, UPS, AVR, Adaptors etc are prone to faulty lens and picture board, power failure and high resistance joints.

To draw the curtain, I deem it necessary to sing the praises of all stake holders of SIWES for making this training possible and to also bring to their notice the difficulties students encounter in their search for IT placements.