EE302: Electronic Equipment Repair

HAK CIPTA Department of Electrical Engineering Department, Polytechnic Sultan Haji Ahmad Shah (POLISAS) Printed 2014

All rights reserved. No part of this publication may be reproduced, copied, stored in any retrieval system or transmitted in any form or by any means – electronic, mechanical, photocopying, recording or otherwise; without prior permission in writing from the Head of Department, Electrical Engineering Department, POLISAS, Semambu, 25350 Kuantan, Pahang Darul Makmur Malaysia.

First Printed

COURSE EE302 ELECTRONIC EQUIPMENT REPAIR No ISBN: 978-967-11550-7-3

Printed by: Ramly Press Sdn.Bhd 3116, Taman Sekilau Maju, Bukit Sekilau 25200 Kuantan, Pahang.

SIDANG REDAKSI

Pengarah Projek Mohd Yusof bin Zakaria

Editor Mohd Faizal Bin Mustapha Wan Ghani Bin Wan Pi Muhamad Adha Bin Shamsudin.

Penerbit Jabatan kejuruteraan Elektrik Politeknik Sultan Haji Ahmad Shah 2014

Course EE302 Electronic Equipment Repair Mohd Faizal Bin Mustapha1 , Wan Ghani Bin Wan Pi 2, Muhamad Adha Bin Shamsudin3 Jabatan kejuruteraan Elektrik, Politeknik Sultan Haji Ahmad Shah Pahang1,2,3 Email: mfaizal@polisas.edu.my1,wanghani@polisas.edu.my2,adha.polisas.edu.my3

ABSTRAK Buku ini merupakan kajian terkini yang menyeluruh terhadap keperluan Kursus EE302 Electronic Equipment Repair. Ia ditulis dan diterbitkan khas untuk pelajar semester 3 bidang Kejuruteraan Elektrik dan Elektronik, Elektronik(Komunikasi) dan Elektronik (Komputer) di Politeknik Sultan Haji Ahmad Shah. Ia juga boleh diguna pakai oleh seluruh Politeknik Malaysia amnya. Antara topik-topik yang terkandung dalam buku ini adalah Handtools, Soldering Technique, Testing Equipment, Diagnosis Tecnique, Power Supply, Amplifier dan Domestic Electronic Appliances. Buku ini telah disusun mengikut bab seperti dalam silibus Course EE302 supaya senang pensyarah dan pelajar untuk membuat rujukan dan carian bahan. Manakala pendekatan yang digunakan adalah prinsip operasi litar yang mudah difahami, contoh dan penggunaan secara menyeluruh termasuk langkahlangkah baikpulih. Terdapat juga masalah yang diberikan bagi memantapkan lagi kefahaman pelajar.

Kata Kunci : Course EE302,Electronic Equipment Repair, Hand tools, Soldering Technique, Testing Equipment, Diagnosis Technique, Power Supply, Amplifier , Domestic Electronic Appliances.

Acknowledgement

On behalf of Polytechnic Sultan Haji Ahmad Shah, I’m hereby acknowledge lecture effort as writer of Notes and Reference for COURSE EE302 ELECTRONIC EQUIPMENT REPAIR This Notes and Reference can be used in teaching and learning purposes.

MEJ. (K) DATO’ HAJI ABU BAKAR BIN AHMAD, DIMP., SMP.,AAP., AMN. Director Polytechnic Sultan Haji Ahmad Shah, Semambu 25350 Kuantan Pahang Darul Makmur

CONTENT SECTION A

B

SUB-SECTION / TITLE LIST OF FIGURES

PAGE ii - v

LIST OF TABLE

vi

ABSTRACT

vii

ACKNOWLEDGMENT

viii

CHAPTER 1

HANDTOOLS

1–6

CHAPTER 2

SOLDERING TECHNIQUE

CHAPTER 3

TESTING EQUIPMENT

15 – 28

CHAPTER 4

DIAGNOSIS TECHNIQUE

29 – 45

CHAPTER 5

POWER SUPPLY

46 – 68

CHAPTER 6

AMPLIFIER

69 – 87

CHAPTER 7

DOMESTIC ELECTRONIC APPLIANCES

88 – 108

7 –14

i

LIST OF FIGURES

Figure 1.1

Long-nose pliers

1

Figure 1.2

Side Cutter

2

Figure 1.3

Tweezer

2

Figure 1.4

Wire stripper

3

Figure 1.5

Types of Screwdriver

4

Figure 1.6 (a)

Soldering Iron

4

Figure 1.6 (b)

Soldering Gun

4

Figure 1.7

Desoldering pump

5

Figure 1.8

Complete set of soldering tools

5

Figure 1.9

Others type of tools use in soldering

6

Figure 2.1

The Soldering process

11

Figure 2.2

Soldering Machine for Industrial

12

Figure 2.3

De-Solder Process: Solder Sucker

12

Figure 2.4

Removing soldering iron

13

Figure 2.5

Clear the component lead

13

Figure 2.6

Solder Wick

13

Figure 2.7

Solder wick desoldered connection

14

Figure 2.8

Clearly remove pad

14

Figure 3.1

Basic Construction of Cathode Ray Tube

16

Figure 3.2

Main Block Osiloskop

17

Figure 3.3

Measuring DC/AC Voltage

19

Figure 3.4

Current Measurement ( milliampere)

20

Figure 3.5

Measuring Resistance

20

Figure 3.6

Using MOV to perform testing on Capacitor

21

Figure 3.7

Capacitor Testing using ‘quick and dirty’ Method

21

Figure 3.8

Capacitor Testing Graph

22

Figure 3.9

Diode Testing

23

Figure 3.10

Transistor Testing Using an Ohmmeter.

24

Figure 3.11

Audio Generator Test Set

25

Figure 3.12

RF Signal generator

26

Figure 3.13 (a)

'PM5544' Philips Pattern

27

ii

LIST OF FIGURES

Figure 3.13 (a)

Samples of test pattern generator

28

Figure 4.1

Types of equipment such as Monitor, TV and

30

Radio Figure 4.2

Vertical Deflection Circuit

Figure 4.3

Vertical Deflection Circuit (continuation)

33

Figure 4.4

Feedback Amplifier on a PCB

34

Figure 4.5

Feedback Amplifier Circuit

35

Figure 4.6

Audio Amplifier in TV Block

35

Figure 4.7

Basic Audio Amplifier Block Diagram

36

Figure 4.8

Speaker Testing

36

Figure 4.9

Push-Pull Amplifier Circuit

37

Figure 4.10

Pra-Amplifier Circuit

37

Figure 4.11

Fault Finding Chart

38

Figure 4.12

Amplifier circuit

39

Figure 4.13

Horizontal Output Circuit

41

Figure 5.1

Power supply block unit

47

Figure 5.2

DC Power Supply.

48

Figure 5.3

Protector circuit

49

Figure 5.4

Alternating current wave

50

Figure 5.5 (a)

Transformer construction & symbol

51

Figure 5.5 (b)

Block diagram Supplies

52

Figure 5.6 (a)

Input and output wave

53

Figure 5.6 (b)

Full-wave rectifier circuits

53

Figure 5.6 (c)

Bridge rectifier circuits

54

Figure 5.7 (a)

Filter circuits and output wave

54

Figure 5.7 (b)

LC filter circuits

55

Figure 5.7 (c)

T filter and π filter

55

Figure 5.8 (a)

Single diode regulator circuits

56

Figure 5.8 (b)

Simple series transistor

57

Figure 5.8 (c)

Series transistor output stabilizer

33

58

iii

LIST OF FIGURES

Figure 5.8 (d)

Series transistor current limiter

59

Figure 5.8 (e)

Integrated package uA723

60

Figure 5.8 (f)

Internal circuit uA723

61

Figure 5.8 (g)

Voltage regulator circuit uA723 applications

61

Figure 5.9 (a)

Simple block diagram of a switching-mode

63

power supplies Figure 5.9 (b)

Simple schematic diagram

63

Figure 5.10

Power supply block diagram of a personal computer

64

Figure 5.11

Protective circuit

64

Figure 5.12

Rectifier circuit

65

Figure 5.13 (a)

Temporary supply circuit

65

Figure 5.13 (b)

Temporary supply circuit

66

Figure 5.14

Block diagram switching power supply unit

68

Figure 5.15

Block diagram of circuit switched regulator

68

Figure 6.1

Class A amplifier

69

Figure 6.2

Class B amplifier

70

Figure 6.3

Class C amplifier

70

Figure 6.4

Audio Amplifier Block diagram

71

Figure 6.5

Audio Pre-Amplifier

72

Figure 6.6

Tone control pre-amp

72

Figure 6.7

Single – ended amplifier

74

Figure 6.8

Push-Pull Amplifier +ve signal

74

Figure 6.9

Push-pull Amplifier –ve signal

75

Figure 6.10

Input Coupling Transformer Circuit

76

Figure 6.11

Direct biasing circuit

77

Figure 6.12

Diodes biasing circuit

78

Figure 6.13

Amplitude distortion wave

79

Figure 6.14

Type of phase distortion wave

80

Figure 6.15

Cross over distortion wave

81

Figure 6.16

Inter modulation distortion wave

82

iv

LIST OF FIGURES

Figure 6.17

PA system block diagram

83

Figure 6.18

PA system schematic diagram

85

Figure 7.1

Block Diagram of the AM Transistorized Radio

88

Figure 7.2

The block diagram of FM

91

Figure 7.3

The unused frequencies

95

Figure 7.4

The process of color tv

96

Figure 7.5

The composite transmitted signal

97

Figure 7.6

A block diagram of the composite color TV

98

modulating signal Figure 7.7

A block diagram of a color receiver

99

Figure 7.8

A sample of it on the back porch of the

100

horizontal blanking pulse Figure 7.9

A single hole in the shadow mask

101

Figure 7.10

A special color convergence yoke place around

102

the tube yoke Figure 7.11

Snowy, weak television reception

104

Figure 7.12

Horizontal tearing

106

Figure 7.13

Other vertical system problems

107

v

LIST OF TABLE

Table 4.1.1

The example of symptom

32

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CHAPTER 1 : HANDTOOLS

1.0 TYPE OF HANDTOOLS

1.1 Needle-nose pliers

Also known as long-nose pliers, pinch-nose pliers, or snipe-nose pliers are both cutting and gripping pliers used by electricians and other tradesmen to bend, reposition and cut wire.

Figure 1.1 : Long-nose pliers

Their namesake long gripping nose provides excellent control and reach for fine work in small or crowded electrical enclosures, while cutting edges nearer the pliers' joint provide "one-tool" convenience. Given their long shape, they are useful for reaching into cavities where cables (or other materials) have become stuck or unreachable to fingers or other means.

1.2 Side Cutter

Side cutters are used to remove plastic components from spurs or trimming tabs and vents from metal figures. They are also used for cutting single wires. Since they are used in the electrical industry the handle of the side cutter is made by an insulated material.

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Figure 1.2 : Side Cutter

1.3 Tweezers

Are tools used for picking up objects too small to be easily handled with the human hands. They are probably derived from tongs, pincers, or scissors-like pliers used to grab or hold hot objects since the dawn of recorded history. In a scientific or medical context they are normally referred to as forceps.

Figure 1.3 : Tweezer

Tweezers can be used for tasks whenever small objects have to be manipulated, including for example small, particularly surface-mount, electronic parts, and small mechanical parts for models and precision mechanisms.

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1.4 Wire Stripper

A wire stripper is a small, hand-held device used to strip the electrical insulation from electric wires.

Figure 1.4 : Wire stripper A simple manual wire stripper is a pair of opposing blades much like scissors or wire cutters. The addition of a center notch makes it easier to cut the insulation without cutting the wire. This type of wire stripper is used by rotating it around the insulation while applying pressure in order to make a cut around the insulation. Since the insulation is not bonded to the wire, it then pulls easily off the end. This is the most versatile type of wire stripper.

1.5 Screw drivers

A screwdriver is typically identified by its tip, which is shaped to fit, or mate with, a screw the head of which has a particular contour, or surface shape. A screwdriver is, thus, a mechanism to apply torque to a screw. Proper use of a screwdriver requires that the screwdriver's tip engages with the head of a screw having the same size and type designation as the screwdriver itself. Screwdriver tips are available in a large variety of shapes and sizes.

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Figure 1.5 : Types of Screwdriver

1.6 Soldering Iron

A soldering iron is a hand tool used in soldering. It supplies heat to melt the solder so that it can flow into the joint between two workpieces. A soldering iron is composed of a heated metal tip and an insulated handle. Heating is often achieved electrically, by passing an electric current (supplied through an electrical cord or battery cables) through a resistive heating element.

(a)

(b)

Figure 1.6: (a) Soldering Iron and (b) Soldering Gun

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1.7 Desoldering/Sucker

In electronics, desoldering is the removal of solder and components from a circuit board for troubleshooting, repair, replacement, and salvage. Specialized tools, materials, and techniques have been devised to aid in the desoldering process.

Figure 1.7: Desoldering pump

Figure 1.8 : Complete set of soldering tools

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1.8 Others type of tools use in soldering :

Figure 1.9 : Others type of tools use in soldering

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CHAPTER 2 : SOLDERING TECHNIQUE

2.0 Introduction

Soldering is a process by which you can join metal items together by applying heat along with special metallic alloys (solder) and allowing them to cool. This results in a metal bond between the metals that is strong and has good electrical conductivity under harsh mechanical environments.

2.1 Three requirements for process soldering: 

Low melting point metal (solder wire)



Heat source (soldering iron)



Flux (to clean the surfaces and prevent if from oxidizing)

2.1.1 Tin-Lead solders 60% Tin, 40% Lead - solid at 361° F, liquid at 374° F, 63% Tin, 37% Lead - eutectic point is 361° F . No “pasty” range so joint movement less a problem as the solder nearly instantaneously goes from liquid to solid.

Note: When there is a temperature gap between the solid and liquid state, the solder is “plastic” or “pasty” over that range as it cools

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2.1.2 Silver-bearing Solder 62% Tin, 36% Lead, 2 % Silver - solid at 354 ° F, liquid at 372 F. Often used for surface mount components whose contacts contain trace amounts of silver.The use of lead in solder is now increasingly prohibited in many countries. Various "lead free" alloys are becoming popular and typically contains 97% tin, 2.5% silver and 0.5% copper.These require higher soldering temperatures and do not "wet" as well as Lead-Tin alloys, as a result it requires more skill to produce a good quality solder joint.

2.1.3 Constant Wattage Iron is continuously “ON” and eventually reaches

equilibrium

temperature 20 to 30 watt iron sufficient for circuit board assembly.

2.1.4 Constant Temperature Tip incorporates a thermostatic element to maintain desired tip temperature Solder melts at around 190 C, and the bit reaches a temperature of over 250 C. This temperature is plenty hot enough to cause a nasty burn. The soldering iron must be placed into the specially designed stand, when not in use .

2.2 Temperature Controlled Solder Station Feedback control maintains tip at desired temperature. Adjustable, often with analog or digital temperature display. Many have grounded tip to help prevent ESD damage.

A list of things you don't want to do: 

Don't overheat the part or pad - Might damage your part or the

pads/traces 

Don't underheat the part or pad - Creates poor bonds, cold

joints

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

Don't melt the solder on cold parts - Creates poor bonds, cold

joints 

Don't use too much solder - Creates a mess, prevents

inspection of good bonds and could 'bridge' or connect onto other parts 

Don't use too little solder - Creates poor bonds

2.3 Action before soldering 

Plug in and turn on iron



Ensure iron is hot enough



Melt a little solder and visually inspect how fast it melted



Clean your tip on wet sponge to remove any oxidation, excess

molten solder or debris on your iron 

Tin (apply a little solder) your iron. This is CRITICAL for

good work 

Prepare your part for soldering



Place your part in the PCB



\

2.3.1 Inspect Your Bond What to look for: 

Bond should be a cone and shiny



Profile of bond should be concave, not convex



Bond surrounds part completely…. not 99%



Bond does not cover other connections – Smooth, Bright, Clean,

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Completely surrounded, Concave fillet and shiny .

2.3.2 Soldering Bond Inspection 

One connection is a cold joint. Note the molten metal is only

on the pin, not the pad 

Two other bonds are not around the entire part



Top left is a good bond

2.3.3 The Secret to a Good Soldered Joint 

Cleanliness



Temperature



Time



Adequate solder coverage

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The figure 2.0 is shown how the action must be taken and the basic soldering techniques.

Figure 2.1 : The Soldering process

2.4 De-soldering Techniques If you solder electronic components, you will find a need from time to time to desolder a joint to remove or re-position a wire or component on a circuit board. There are at least two good ways to de-solder simple components: 

Solder Sucker (Desoldering Pump): Is a device which sucks

the molten lead 

Copper Wick: Thin braided copper wires form a cloth like

wire. Some have flux on them, other are just copper. Solder will get wicked up the braid, away from the PCB connection

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2.4 Industrial method: Industry often uses an iron with a suction compressor built in.

There is a small hole in the iron, where the molten solder is sucked.

This reduces time and amount of heat required to accomplish the task.

Figure 2.2 : Soldering Machine for Industrial

2.4.1 The basic step for de-soldering technique

1

Apply heat to the connection to be desoldered. When the solder melts, trigger the solder sucker is shown in figure 2.2

Figure 2.3: De-Solder Process: Solder Sucker

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Repeat de-soldering as needed until all solder is removed.

Remove soldering iron & solder sucker from area is shown in figure 2.3 2 Figure 2.4 : Removing soldering iron

Next

step

is

remove

component lead clearly as shown in figure 2.4

3 Figure 2.5 : Clear the component lead

Solder wick is finely braided copper that is used to wick away excess solder from a de-soldered connection as shown in figure 2.5 4 Figure 2.6 : Solder Wick

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4

Apply the solder wick and soldering iron to the desoldered connection. The solder wick will draw the excess solder off of the PCB pad as shown in figure 2.6

Figure 2.7 : Solder wick desoldered connection

De-soldered PCB pad as shown in figure 2.7

5 Figure 2.8 : Clearly remove pad

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CHAPTER 3 : TESTING EQUIPMENT

3.0 INTRODUCTION In this unit, it’s explain the

equipments use for electricity and

electronic instruments testing such as oscilloscope, multimeter, transistor tester, audio generator and others.

3.1 OSCILLOSCOPE AND STORAGE OSCILLOSCOPE Oscilloscope is an electronic measuring device which is able to display signal that varies with time ( voltage versus time) on the screen. This is very useful for a technician to know the stated signal parameters value.

3.1.1 Basic Principles 

It comprise of main component, Cathode Ray Tube (CRT) which forms from several parts such as electron gun, fluorescent screen and 2 pairs of Harizontal's (H) and Vertical (V) deflection plate.



When voltage is applied to horizontal plate, electron will move

horizontally and when voltage is given to vertical plate, electron will move vertically . Belows is shown the construction of CRT's.

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Deflection Planes

Electron Gun

Vertical

Horizontal

V

H

Electron Beam

Spot

Fluorescent Screen

Figure 3.1: Basic Construction of Cathode Ray Tube

Usually the H plate is connected to a saw tooth signal, this causes the electron to move from left to right along the x- axis . The V- plate which is connected to a test signal will cause the electron to move according to this plate. 

The continuous voltage fed to H and V plate will produce a curve or sharp signal on the screen which is equal to the test signal.In fact the signal shown on the screen are very fast and thus invisible.

Therefore it’s needed longer time to stabilize the signal. Thus the oscilloscope has to repeatedly display the same signal. This is the reason why to have a stable displayed signal, it must be given a suitable periodical signal. To expediate this process, a special circuit known as trigger circuit is applied to ensure that the signal shown on the screen cause by the electron movements is same as the original signal.

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3.1.2 Function of Oscilloscope Parts

Below shown a main block of an oscilloscope.

Y - Deflection X - Deflection Y - Amplifier

X - Amplifier

X - Deflection Y - Deflection

Voltage/DIV

Trigger Circuit Comparator

SAWTOOTH OSCILLATOR

Trigger Level Frequency Time / DIV

Input

Figure 3.2: Main Block Osiloskop 

VOLTAGE/DIV Knob: To change the amplification scale for Channel Y amplifier and also to change the Y-axis scale on the screen.



TIME/DIV Knob: To change the periodic scale of the saw tooth signal that determine the time for the electron to move from leftright on the screen and to change the X-axis scale on the screen.



TRIGGER LEVEL Knob: To change the reference point of the test signal in order to determine the scale of saw tooth signal.

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3.1.3 Types of Oscilloscope i.

Storage Oscilloscope

It has memory that can keep the signal that was measured earlier.It can display test signal without any scale signal.

ii.

Multichannel Oscilloscope.

Able to display 2 or more measured signal at a time. It uses multiplexer principle. Every signal can be displayed one after another by the electron gun but in a shorter time. Thus it has sufficient time to displayed the visible signal.

iii.

Digital Oscilloscope.

By applying digital concept, the test signal will be change to into digital format and display ( as computer system) at the screen. It is able to perform extra measurement with higher accuracy and easy to use.

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3.2 MULTIMETER (VOM- Volt, Ohm dan Miliampere)

MOV can perform various measurement such as voltage DC/AC, current measurement, resistance, Capacitance, diods and transistors.

3.2.1

The measurement of MOV

i.

Measuring DC/AC Voltage

When measuring DC/AC, the correct and higher scale than the measured value must be selected to prevent MOV from being damage.Voltmeter must be connected in parallel to the load being measured.

Figure 3.3 :Measuring DC/AC Voltage

ii.

Measuring Miliampere (current)

Select the right DC mA scale from the MOV. The value must be higher than the measured current. Milliampere meter must be connected in series with the load as shown below:-

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Disconnect the

circuit

Figure 3.4 :Current Measurement ( milliampere)

iii.

Measuring Resistance

Select the right scale. Calibrate the meter to 0 ohm... Connect the things to be measured as shown below:-

Calibrate here Figure 3.5 : Measuring Resistance

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iv.

Capacitor Testing

a) There are some MOV that can perform Capacitor testing.

It can be shown below but the Capacitor has to be fully discharged before testing is done.

Figure 3.6 : Using MOV to perform testing on Capacitor

b) Another testing method is called ‘quick and dirty’. Power must be disconnected from the circuit before carrying any testing on the Capacitor which is soldered on PCB.

Figure 3.7: Capacitor Testing using ‘quick and dirty’ Method

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One of the terminals (leg) must be desoldered from the board and connect the meter as shown in Fig 3.4.1.

For a good

Capacitor, MOV will show charging and discharging activities and the end result can be seen in the graph at Fig 3.4.2

If the Capacitor is found to be at fault, the result as stated below:1. Open circuit : R = Infinity, Constant.

2. Short circuit : R = Zero, Constant.

Resistance

Figure 3.8: Capacitor Testing Graph

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v.

Diode Testing

Diode Testing can be done if it has the added feature such as ‘diode check’ or ‘high-ohm’ scales. With this feature, it can forward bias the diode. The figure below shown hoe diode can be tested using a MOV.

Figure 3.9: Diode Testing a) Forward-bias b) Reverse-bias

i.

Set the MOV to ‘diode test’ and off the power. For forward-bias testing, the result as stated below:Normal –typed Diode : 200 to 700 ohm

ii.

Small-signal diode

: 200 ohm

Rectifier diode

: 600 ohm

For a good diode, the reverse-bias value R = Infinity.

For malfunctions diodes, it could yield both results as stated below:Open Circuit : R = Infinite Short Circuit : R = Zero

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vi.

Transistor Testing

Ohmmeter can be used to test whether a transistor is functioning or not. A transistor is formed by connecting 2 PN junctions almost similar to diodes. It has a lower forwardbiased resistance and the reversed-biased resistance is very high. For a normal NPN transistor, the measurement obtained with an ohmmeter is as shown below:-

Figure 3.10 :Transistor Testing Using an Ohmmeter.

If both forward and reversed biased reading is high, the transistor is having an open-circuit fault but vice-versa if the reading is low, then it

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has a short-circuit fault. If both readings are same, the transistor is faulty. The normal forward-biased rating is around 300 to 700 ohms and the reversed biased is around 10 to 60Kohms

3.3 AUDIO SIGNAL GENERATORS Signal generators that produce waveforms in the audio spectrum are used by technicians to aid in repairing audio equipment. In conjunction with a sound level meter, audio-signal generators are used in setting up recording studios and serious home theater environments. The goal of a studio is to have a "flat" frequency response across the audio spectrum. Ideally, the volume of all frequencies that the ear can hear should be unaffected by the room or the amplifier and speaker system, neither boosting nor cutting the volume of any frequency band. This ensures that the mixing engineer is hearing the true sound, and one that is not colored by the room's acoustics or amplification system. Adjustments can be made using electronic equalizers or by making physical changes to the room (such as adding drapes, rugs or acoustic foam blocks) to help give the listening environment a flat response.

Figure 3.11: Audio Generator Test Set

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Audio signal generators usually produce several standard waveforms: sine, pulse, triangle, sawtooth, white noise and pink noise. Each waveform is made up of different harmonics (overtones). A sine wave, for example, is a pure tone, with only the fundamental frequency (no harmonic overtones). A square wave is made up of the fundamental plus odd harmonics, with each successive harmonic reduced in volume. White noise is composed of all audio frequencies in equal amounts (which sounds similar to when you have your TV tuned to a channel where all you see is "snow"). Pink noise contains equal energy per octave. Analog electronic music synthesizers have oscillator modules (which are a type of signal generator) that create these waveforms, and when controlled by a piano-like keyboard and other circuitry, they are used to create music.

3.4 RF SIGNAL GENERATORS RF signal generators produce waveforms in the radio frequency band.

These are used by engineers who design radio equipment and service technicians who repair them. RF generators, along with oscilloscopes and other test equipment, are valuable tools for troubleshooting radio receivers and transmitters.

Figure 3.12: RF Signal generator

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3.5 TEST PATTERN GENERATORS A test signal generator generates test patterns, and other useful test signals, for troubleshooting and analyzing television systems. These devices are generally intended for off-line use (test patterns are seldom broadcast, unless a station is not operating properly or is off the air at the time), as they output complete television signals.

Examples of signals output by such a device include: 

Color bars, one of several test signals used to verify the

proper reproduction of a TV system's color gamut, and/or that a television signal or plant is compliant with the appropriate analog transmission standards. 

Flat fields, a signal consisting of nothing but a specific color

(typically white, black, a shade of gray, or one of the primary colors (red, green, and blue) at maximum saturation). A red field is especially important in PAL applications, as it is the "red difference" portion of the chroma signal whose phase alternates every line; the red field should appear as a solid block of color, with no visible "bands" going across the screen.

Figure 3.13 (a) : 'PM5544' Philips Pattern

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

Multibursts, sweeps, and pulse signals, used to test the

frequency response of a television system 

Ramp signals and staircase signals, used to check the

voltage linearity of a television system 

Convergence patterns, used to check the alignment and

linearity of a receiver 

The bowtie signal, used to check the relative (inter-channel)

timing of a component video signal.

Figure 3.13 (b) : Samples of test pattern generator

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CHAPTER 4 : DIAGNOSIS TECHNIQUE

4.0 INTRODUCTION

Any component parts that are interconnected together to make a complete system has its own hierarchy. It is very essential for a computer technician or assistant engineer to know the systems hierarchy before works on the system as well as any electronics equipment.

Before troubleshooting is carried out, a few question must be answer:i.

What type of equipments to be repaired

ii.

What is the fault symptom of the equipment

iii.

Do the equipment provides any schematic diagram

iv.

What is the suitable techniques to be used.

To answer the first question, we must recognize the equipments and how it’s work. Fo example, function generator is used to generate sine or square wave. The function of a radio is to receive signal, process the signal to produce audio signal.

The television is to receive signal, process and produce video and audio. Then we can determine the fault of the equipments.

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Figure 4.1 : Types of equipment such as Monitor, TV and Radio

The second question involves the symptom such as output product of the equipment. For example, no oscillation at the function generator output, no sound from the radio speaker, no picture from TV screen.

From here, then we can determine which part of the circuit malfunction without wasting time. We can only visualize the circuit and it’s function if we know which

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part is malfunction, but not the exact part of the circuit that is at fault. Thus we need a schematic diagram in order to pin point the exact fault location.

4.1 ANALYZING AND FAULT FINDING TECHNIQUES To find fault for an electronic equipment, we must rely on it’s usage. Before finding fault, we should understand the basic function of the equipment. From then, we can analyze what is the fault or symptom. Every techniques used must be relevant to each other to prevent time-wasting.

The techniques can be line up as follows:i.

Symptom Finding

ii.

Signals Tracing and Injections

iii.

Voltage Measurement

iv.

Resistance Measurement or Continuity

v.

Physical Fault Detection

4.1.1

Symptom

Every equipments normally display some symptom to give indication of the faults.

The troubleshooter must be well versed in the system and functions of the equipments in order to determine the fault. But for the users, they can only recognized a fault such as by the sound output of a radio either no sound, upnornal sound etc. Below is the examples of fault and functions for some equipments:-

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1

Equipments

Symptom & Functions

Colour TV

Coloured

Pictures

&

Sound 2

Radio

Stereo Sound

3

Oscilloscope

Display

4

Audio Generator

Audio Signal Output

Table 4.1 : The example of symptom

4.1.2 Tracing And Signals Injection Techniques

By referring to the fault symptom, the faulty area can be traced. In the absence of schematic, circuit tracing is used to determine the connection and components placing. This can hasten the tracing process and fault detection.

4.1.2.1 Printed Circuit Tracing

Tracing through the printed circuit is essential to determine the location of the components and techniques to be used.

The most popular systematic fault location methods are:-

i.

Determine the input and output

ii.

Determine the power source and earth

iii.

Determine the numbering of the components

Tracing can be done either from the input to output or vice-versa. Below is an example of the printed circuit.

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Figure 4.2: Vertical Deflection Circuit

Figure 4.3: Vertical Deflection Circuit (continuation)

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We can find the wire that is connected to the deflection coil at the TV yoke as the output reference, then trace inward until it met the amplifier circuit.

For a simple circuit such as audio amplifier, pra amplifier, tone circuit etc, tracing the whole circuit is needed. Fig 4.3 shows the feedback amplifier circuit:-

Figure 4.4: Feedback Amplifier on a PCB

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Figure 4.5 : Feedback Amplifier Circuit

The components numbering on the printed circuit board indicate the beginning of the circuit, for example C1 and R1 is the first component in the circuit. The 1st digit of the number represent the block grouping of the components.

4.2.2.2 Signal Injection

Figure 4.6: Audio Amplifier in TV Block

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After isolating the fault area such as audio amplifier, signal injection and tracing is applied to this area. Function generator is one of the equipments that is used to inject signal to the isolated block. Oscilloscope is then used to trace the output signals of the circuits. Follow this step before we begin the fault finding process:i. Identify the amplifier blocks ii. Identify the supply voltage to prevent from over-current. iii. Adjust a suitable signal of 1KHz or 400Hz from the function generator

Pra Amp

Tone Circu it

Driver/ Pra Amp 2

Power Amp

Figure 4.7 :Basic Audio Amplifier Block Diagram

4.3

Trobleshoot the speaker

i. Check the speaker to determine whether it is functioning. It can be done by ohmmeter or signal generator

Figure 4.8: Speaker Testing

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ii. If the speaker is functioning, then signal can be injected at the input of power Amplifier

Figure 4.9: Push-Pull Amplifier Circuit

iii. If normal, place the audio generator probe at the driver input i.e before or after C3. Observe the output iv. If normal, check the tone circuit. The output will be small or the tone cannot be adjusted if this circuit is faulty.

Figure 4.10: Pra-Amplifier Circuit

v.

If the circuit is normal, inject signal at the pra-Amp input. Level the function generator to obtained an undistorted output signal.

The above process must be done several times to confirm the output in order to determine the faulty parts.

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The above procedured can be summarized in Figure 4.3.3 below:-

Start

No sound

Speaker O.K

Not normal

Power Amplifier Output

Test Voltages

O.K

Not

normal

Driver

Test resistance

O.K No output

Tone Circuit

Repair O.K

Not normal

Pra Amplifier

Norma l

Figure 4.11: Fault Finding Chart

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4.4

Voltage Measurement Technique

Once the fault area is located, voltage measuring technique is used. Supply voltage to this area must be determined . For example, a supply of +12V is available at the junction of R1 and R3. If lower or none, it meants the supply is not normal.

If the supply voltage is normal, voltage must be be measured around the active component i.e transistor Q1.

Figure 4.12: Amplifier circuit

To analyze the measured voltage, we have to know the normal voltage value for the circuit. For example:Collector Voltage (Vc) = 5.5V Base Voltage ( VB)

= 2.5V

Emitter Voltage (VE) = 1.7V Then we can analyze the faulty area and determine the fault.

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Example 1 If Vc = 0.1V, which component faulty and what is the fault?

Answer : R3 open-circuit ( no supply to the collector) Example 2 If Vc = 11.8V, what is the fault?

Answer : i.

R1 open-circuit or

ii.

CE Q1 open or

iii.

R4 open

Example 3 If VB

0.1V

=

Answer : i.

BE Q1 shorted or

ii.

R1 open

i.

C2 shorted or

ii.

E Q1 open

Example 4 If VE

= 0V

Answer :

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4.5

Resistance Measurement Technique

This method is used for components with known resistance such as resistor, diodes, transistors etc.

4.5.1

Resistor

Resistor can be measured in the circuit with the absence of other lower resistance, inductance and transformer. An open circuit resistor give an infinity or higher resistance.in the circuit,

4.4.2

Diode

The same applied to measuring diode which including checking on it’s biasing.If both forward and reverse biased yields the same results as when it was measured outside the circuit, then the diode is in a good condition.

4.4.3

Transistor

Transistor testing must be done outside the circuit. We must identify transistor type whether it is NPN or PNP. If readings are normal, then the transistor is in good condition. For power transistor, the input signal is fed through the transformer. In Fig 2.13, the resistance between base and emitter is low. Thus by measuring the resistance between base and collector, fault can be determine.

Figure 4.13: Horizontal Output Circuit

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4.6

Fault Finding by Physical Technique

It involves a few senses such as visual observation, hearing, smell and taste/touch.

i. Visual Observation By observation, we can located the faulty area such as burning trace, loose connection, broken etc.

ii. Hearing and Smelling Sometimes a burnt or spark cannot be seen but can be heard or smelt. For example a high voltage transformer normally give sparks if its insulation cracked or hiss sound, or else yield a burning smell if over heated.

iii. Taste This method only applied to fault such as broken components, dry joints, dislodge and etc.

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Aktivity Activity 1:

Try your comprehension in this activity 1.

List THREE fault finding techniques 2.

State TWO differences between voltage measurement technique and measuring reasistance technique.

Answer: 1.

The three techniques in fault finding is signal injection & tracing, voltage measurement and resistance measurement.

2.

Voltage Measurement is only measuring the potencial differences between components terminal while measuring resistance only measuring resistance.

Voltage measurement can be used for all components but resistance measurement only used for components of higher values .

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Aktivity 2:

To test your comprehension in this unit, answer this self assessment questions.

Output Input

Referring to the circuit, given the normal reading is ;Vc = 5.5V, VB = 2.5V, dan VE = 1.7V. Determine the faulty components according to the voltages belows:i. Bacaan Vc = 11.8V ii.

Bacaan VB = 0.1V

iii.

Bacaan VB = VE

iv.

Bacaan VE = 0V

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Answer i.

R1 open or CE Q1 open or R4 open

ii.

R1 open

iii.

BE Q1 shorted

iv.

C2 shorted

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CHAPTER 5 : POWER SUPPLY

5.0 Introduction

Ideal power supply is the battery, because the voltage stability produces. But only used to power small and portable. To supply durable and convenient, direct current power supply unit (at), the supply of alternating current (ac) is the sensible choice. The main function is to supply DC power supply current and voltage at a certain, with a small ripple level and stability of the regulator. Some current DC power supply has a current-limiting and transient voltage detector that protects the load exceeds the maximum value.

In this case, we will discuss two ways DC power supply is obtained from the ac supply:

i. Linear dc power supply ii. Switching mode dc power supply

5. 1 Damage to Power Supply Unit

Damage early simple power supply input is in effect, because it consists of plugs, cables, switches, fuses and power factor capacitor. Damage which may be found here is a loose plug, touch disturbed (Intermittent), loose-no touch switch (open) and the fuse is broken. If the fuse is found broken (open), damage may occur before the circuit is shorted. For example, a transformer, rectifier, filter or regulator.

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Basic linear DC power supply consists of the following parts: i. protector ii. power transformer iii. Rectifier iv. filter v. regulator

See Figure 5.1 sequence of the power supply circuit block connections.

Figure 5.1 : Power supply block unit

In Figure 2 below show the basic blocks of a dc power supply. Almost every piece of electronic equipment that makes use 120V ac as a source of power will have a built-in dc power supply, as show in Figure 2(b). Standalone dc power supplies, such as the one show in figure 2(c), are also available for use in laboratory experimentation.

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Figure 5.2: DC Power Supply. (a) Block Diagram (b) Built in Subsystem. (c) Stand-Alone Unit

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Let us now refer to the block diagram of the dc power supply in Figure 1 and describe the function of each block.

a. Protector

Figure 5.3 : Protector circuit

Figure 5.2 above shows the standard protection circuits used in switched-mode power supplies. For protective linear power supply usually consists of switches, fuses and capacitors only. However in this section, we discuss the function and operation of the entire circuit. Switch functions as a circuit breaker manually. Input ac supply can be controlled by the user so that the power supply does not continue to work. Fuse serves as overload protector which decide circuit when the output current exceeds a certain limit ampere Loader works in two situations, namely as a transient voltage protection (transient) and stabilizer power factor. It also absorbs noise brought by the main supply.

* V is veristor or varactor, in order to limit the peak input voltage where the main, sometimes that supply is transformed into higher than normal. If this happens, verister will limit the peak to its original value, eg 240 V. Refer to Figure 5.3.

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Figure 5.4 : Alternating current wave

* Choke - voltage transients (transient) usually occur quickly. This means that the voltage has a high frequency of high frequency through the inductor if it will act as a high resistance in turn prevents the tide into the next section.

Example: XL = 2Ď€fL

f

XL

Similarly, C1 functions as a transient voltage protection, when the high frequency to pass through, impedance become the next low tide to overtake the earth.

Example: XC = 1/2Ď€fc f

XC

* C2 - such as C1, it works correct the power factor and reduce noise. * Bleeder resistor - usually a low resistance value and have a watt (power) higher purpose is as real as the load is shorted load. It can also limit the voltage and worked as a secondary fuse (secondary fuse).

ii. Transformer Damage occurs either open or short in the primary. Most of the transformer (Figure 4) at low power supply, has a fine wire winding and are relatively resistant inner primary .If the current excessive use, heat up and melt the transformer winding insulation. Primary wire size can determine the maximum current transformer. If high current winding can be cut off.

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Figure 5.5(a): Transformer construction & symbol

The basic function of a power transformer is:

a Reducing or increasing the voltage alternating current This function is available in many low power supply as in radio receiver, oscilloscope waveform generator and so on.

Transformer reducing main supply voltage from 240Vac to 6V ac , 9Vac or 12Vac. Sometimes there is also a greater reduction in the supply of the power of television as a black / white, from 240V ac to 120 or 180V ac. But rarely do we see the power supply transformer is used to increase the voltage unless the cause of the supply transformer high in television.

b. Input and output circuit isolator (isolation transformer) Isolating transformer is required when we want to touch the test point (test point) tool to probe the earth, measuring instruments such as osiloskop.Ini very important because the probe earth measuring instrument usually merge with the main supply earth. If the probe touched to casing tool to be measured, the possibility of a 'Neutral' or 'Live' joined with the chassis. This seems we head off beam 'Neutral' or 'Live' to earth mains. Referring to Figure 6, indicates transformer circuit used in power supplies, circuit 51

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equipment (load) will be isolated from the mains.

Figure 5.5(b): Block diagram Supplies

i.

iii. Rectifier Consisting of a power diode features a limited durability. For example, a reverse bias voltage of the diode power is there 50V.Jika voltage transients (shock) is larger than the maximum voltage, it will be broken or shorted. Current factors are also taken into account. Suppose the light depends on the bias current tolerances future. If the current load is used exceeds the maximum level of current the diode, it can damage the diode.

The main task of the rectifier is to convert ac voltage to dc voltage .Circuit design to determine the function and efficiency of the work.

Common circuit design is divided into three, namely:

a. single diode Referring to Figure 5.6(a), a single diode will produce a half-wave output and relatively low average voltage (0.318 Vp), which also has a ripple higher than full wave. Rectifier is mainly used for ac voltage with a higher frequency as the output of the transformer high voltage (Fly back transformer - FBT) in circuit television or output mode-switching supply transformer as it is easier and cheaper.

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Figure 5.6(a) : Input and output wave

b. Twin diodes (center tap) For twin diodes can also produce a full wave. But the design of the circuit as shown in Figure 5.6(b) must be combined with the center tap transformer.

Figure 5.6(b) : Full-wave rectifier circuits

c. Diode bridge

Bridge rectifier is very popular, in order to produce a full wave output. In addition to easy installation, it also saves space.

Sometimes there are also cubic form as in Figure5.6 (c). It does not require a transformer center tap.

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Figure 5.6(c ) : Bridge rectifier circuits

iv.Filter Usually components of a filter is capacitor. If capacitor is shorted the fuse will be open .Capacitor can affect the output voltage. If ripple interference in the output load current is relatively high need such as audio power amplifier, it can make the output voltage of the oscillating or ' motoboting ' or 'Humming'. Apart from short and open, filters regularly also occur 'leak'.

This will cause the current unstable.

Referring to Figure 5.6 (c), the filter works as a lubricant that has rectify dc voltage to produce a more pure product. It will also raise the level of dc voltage to the peak voltage value .The ripple voltage also be small. Simplest filter is a single capacitor filter.

Figure 5.7 (a) : Filter circuits and output wave

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1 - level dc voltage without the filter 2 - the voltage level at the filter

To get less noise disturbance as shown in Figure 5.7 (a), the filter L is designed. It contains the inductor and capacitor. An example might be 'suppressor' in power supply radio / cassette car.

Figure 5.7 (b): LC filter circuits

Another filter that is often used to avoid interference and noise is T or π filters as shown in Figure 5.7(b). Both have the same function, just only design that distinguishes it.

T filter

π

filter Figure 5.7 (c) : T filter and π filter

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v. Regulator The shorted regulator, there was no output or disconnect fuse protector. Regulator circuit, there is a transistor that controls the output current series. If the excessive burden it can make short or open .

Damage transistor in the regulator control, such as 723 can degrade or break off the output voltage.

Basic task of the regulator is to set the voltage level or stabilize the voltage level at which level. In many DC power supply, regulator in place before the output voltage. Any design in order to limit the current and the variable output voltage. There are several types regulator which is often used in linear DC power supply:

i.

Single diode regulator ( zener diode )

Figure 5.8 (a) : Single diode regulator circuits

With features such zener diodes, the output voltage at the same level, i.e + 12V, even if the input voltage varies. Simple circuit in Figure 5.8 (a) is usually a low supply current.

If the regulator is used for higher currents, the transistor must be used to avoid damage. Zener diode regulator using transistor is called transistor regulator.

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ii.

Transistor regulator

Transistor regulator design, which consists of two transistor regulator . Regulator transistor series and parallel . In this case, we only discuss the type of transistor regulator, transistor regulator series parallel only because very rarely in use, it is also less practical due to the waste stream. In the series regulator transistors, some circuit design can we use depending on their needs and functions below are some design of transistor regulator series: a. Simple series transistor

Figure 5.8(b): Simple series transistor

In Figure 5.8(b) transistor TR acts as a current controller, depending on the bias given by R. Dz, as a control voltage is +16 V bruised A and at point B is 12.7V (according to the zener voltage), we know the voltage difference between the base (b) and emitters (e) silicon transistor is 0.7V. From configuration NPN transistor circuit design, the voltage at b is greater than e. So the output is +12 V. This voltage remains stable even input variable. This is called the stabilizer input. If the load is supplied using a high current limit regulator, this will affect the stability of the output voltage to be less than the output voltage and stable reference. If the design of the regulator which can stabilize the output required.

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b. Series transistor output stabilizer

Figure 5.8 (c): Series transistor output stabilizer

This circuit design is based on a series of simple transistor circuit as the circuit Figure 5.8 (c). Here TR2 is used to control the output voltage. It is called the error amplifier (error amp). R2 and R3 as bias voltage resistor. Suppose that the output voltage is less than +12 V caused by overloads. Voltage in R3 fixed. But the voltage at R2 becomes less and less current through it also. This will make the current bias TR2 also be less. Next the voltage dropped impedes TR2 collector and producer of increase and making the product upgrade and become stable. This is called the stabilizer output.

b. Series transistor current limiter

Figure 5.8 (d) : series transistor current limiter

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Figure 5.8(d) shows the circuit diagram of a current-limiting transistor series. By adding TR3 and Rs, we eligible limit the maximum current that comes out of the regulator or power supply. It also serves as over current protection. Rs as the sensing current, the resistance value are low and high power. High current through Rs will drain the voltage.

This voltage is equal to the voltage b-e TR3. When VRS than BE, transistor TR3 to work. Ib current to TR1 and TR3 will be attracted to make TR1 stop working. High current can be block by transistor as current limiter. Current value if the maximum current is 2 A and the transistors used is silicon, so: Rs = VBE / Max = 0.7 / 2 = .035 â„Ś

c. Integrated circuit (IC) regulator

Regulator integrated circuits are available in various designs and types.

It is based on series transistor regulator. For more easily understand this type of regulator, we discuss the following: i. Regulator integrated circuit fixed ii. Regulator integrated circuit Op-Amp ii. Regulator integrated circuit 723 723 uA regulator integrated circuit

Based regulator transistor series and Op Amp, a more cohesive design that has been made of the voltage regulator 723 as shown in Figure 5.8(e). It is very popularly used in power supply circuits at low, as it has the characteristics of a variable, the protective circuit and can modify for high current.

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Figure 5.8 (e) : Integrated package uA723

From this integrated circuit internal circuits, we see all the blocks that are in it have a specific task. Zener diode as a voltage stabilizer feature precision reference has even temperature changes. This is in aid of constant current source (constant current source). Op Amp amplifier used as the reference voltage (Vref Amp). Error amplifier is made of Op Amp with high efficiency at which the change in output voltage can stabilize quickly. It also has a current-limiting circuit and 'Freq Compensation' as a noise reducer. As low current regulator transistor series are built in it. This arrangement for its current voltage not exceeding 400 mA. If you need more current, modifying the external series transistor should be added as shown in Figure 5.8(f).

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Figure 5.8(f) : Internal circuit uA723

Examples of application of the voltage regulator circuit 723 as shown in Figure5.8(g) below:

Example 1

Figure 5.8(g): Voltage regulator circuit uA723 applications

The output of this circuit is dependent on Vref and resistance R1 and R2.Output can be calculated if R1 and R2 in the specified value. For

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this design, the output voltage can be obtained between +7 V to +37 V. Vo = Vref (R2/(R1+R2)); where Vref R1 + R2 standard is 7.15 V ISC = V23 / RSC; where V23 is the site - the producer of the transistor

Current limiting (current limit) of 0.65 V

If we want to find the resistance of the RSC at a certain maximum current, we can use the formula in the maximum current, if limit is 2 A;

RSC = 0.65 / 2 = 0325 â„Ś

In most linear power supplies, a larger current is needed. For regulator integrated circuit, the maximum current is supplied only 400 mA. So impose external series transistor to drive a larger current.

5.2 Switching Mode Power Supply

This type of power supply used in many electronic devices and digital today .Power supply is used for computers, televisions, video players and etc because it has the advantages of: i. Can supply high voltage and current ii. Does not require input transformer iii. Have an oscillator that can decide 'switching' if there is a short in the load. iv. Rectifying and simple filters v. Stable output voltage vi. Having feedback correction signal of the power supply at the computer - working when no signal 'power good' from the computer.

However there are also disadvantages of this type of power supply: i. It is no separating the mains. ii. Sufficiently skilled and very careful when repairing the damage.

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Figure 5.9(a): Simple block diagram of a switching-mode power supplies

Figure 5.9(b) : Simple schematic diagram Circuit of Figure 5.9(a) above can produce high current and voltage smoothness can be obtained for high Freq (ie 20 kHz). To simplify the discussion we note switching mode power supply, for example a personal computer.

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Figure 5.10: Power supply block diagram of a personal computer

Figure 5.11 : Protective circuit S - the power switch F - Fuse (overload protector) V - Verister (protector / stabilizer main input voltage changing) C1 - capacitor power factor - voltage protection shock (transient) - Change the power factor - Absorb noise (noise) that can interfere with other electronic devices Choke - Freq swing protect stray from the main supply C2 - C1 functions the same as above Bleeder - serves as a fuse

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Figure 5.12 : Rectifier circuit The main rectifier circuit is shown in Figure 5.12consists of a diode bridge produce at maximum output voltage is: 1,414 x 240 V= 340 V dc

This means that the light used must have the characteristics of high power.

Each diode has a small value capacitor use to protect the shock voltage across the diode. Example 47nf 1kv.it also function as noise (noise). To get a more smooth dc voltage and stable, the filter capacitor charged. Capacitor values are usually large, i.e C1 = 2200 uf 400V, 400V C2 = 200uf.

5.2.1 Temporary supply (starter) Small at the beginning of the supply voltage, required to turn on the oscillator

Figure 5.13(a): Temporary supply circuit

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Figure 5.13(b): Temporary supply circuit Schematic diagram of the two above , while the 12V supply is generated. In figure 28 above transformers is used to reduce the voltage. R1 usually low value use to protect the transformer. Capacitor C in place so that the output voltage at serial key not continue to supply current to the transformer .

Similarly, in figure 29 above, the resistance R2 330 kâ„Ś in use as a limiter that produces an output voltage at +12 V.

5.3 Type of Computer Power Supply Unit

Stable power supply in a computer system is to be in serious note when using computer .Maybe most users do not bother, but are probably right cause data loss. Problem of power supply should be well in priority need.

Computer power supply unit serves as the receiver power supply 240/110 V ac from main supply source and convert it into some form of dc voltage and distribute them to the main parts of a computer. If a computer does not work at all, the main reason is because of the problems in power supply unit, whether it is from a mains supply or it can not function properly.

Dc voltage level of the power supply

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Load

Voltage

Use

+5V

4.8A

supply for TTL IC

Wire Color

Red

Microcontroller -5V

0.5A

IC TTL, Op –Amp, Speaker

+12V

1.5A

IC CMOS, Motor Drive,

White Yellow

Communication IC -12V

0.5A

Op-Amp, Motor Drive,

Blue

Black wire is for Ground Orange wire to the Power Good

5.4 Universal Power Supply Unit

This type of power supply is a linear / constant output. In any load change will not affect many outputs. Although load (should be supplied), but it cannot properly control the output or otherwise, that causes trouble and waste energy Di where this type of power supply efficiency is 50% of the input.

Among the functions of each block are: Transformer: Functional decline or increase the value of the supply voltage 240 V to a lower value, i.e +12V and - 12V. Rectifier: Work convert alternating voltage to form voltage direct current (dc).

In this section diodes used.

Filter: Functioning get a fixed dc voltage value and genuine. The main component is a capacitor.

Regulator: This section sets the value of the desired output voltage so that it really is at a constant value which is required even if there are changes to other parts of the input.

Voltage divider: Setting voltage products in need such as +12 V,-12V, +5 V and-5V.

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Figure 5.14: Block diagram switching power supply unit

Figure 5.15 : Block diagram of circuit switched regulator

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CHAPTER 6 : AMPLIFIER

6.0 Introduction

In electronics equipment, one of the vital parts is the amplifier. Generally, amplifier is used to amplified signals, voltage and current. It is vital in electronic circuit such as power amplifier, RF and IF amplifier, high voltage and so on. The classes of amplifier is determine by their functions.

i.

Class A Input

Output

Class A amplifier

Figure 6.1 : Class A amplifier Class A is an amplifier that produce a full output signal of ď‚ą 360ď‚°.

Example is audio pre-amp

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ii.

Class B Produce ½ cycle of the signal i.e  180.

Figure 6.2 : Class B amplifier Class B amplifier is usually comes with twin amplifier + ve and – ve to produce an output signal of 360. The advantages is that it can produce two times the gain of class A. Example is push-pull power amplifier and complementary symettery.

iii.

Class C Class C amplifier produce signals in pulse i.e the output is less than 180.

Example is IF and RF amplifier .

Class C

Output

Input

Figure 6.3 : Class C amplifier

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6.1 Faults at audio amplifier blocks

As being mentioned earlier, every audio equipment has certain stages to produce a better controlled and audio qualities.

Example : Selector switch Mic Input

Pre Amp

Tone Control

Pre

Driver /Pre Amp 2

Pow er Amp

Amp

Aux Pre Amp

Phono Figure 6.4 : Audio Amplifier Block diagram

Audio Pre - Amplifier Pre Amplifier is designed for the following function:i.

Amplifying small signal from the input

ii.

Input impedance Coupling

iii.

Limits Signal to Noise ratio

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Single ended amplifier is one example of the Class A Example:

Figure 6.5 : Audio Pre-Amplifier

6.2 Tone Control Pre-Amplifier In the tone control network, pre-amplifier is needed to stabilize or replace the signal losses at the circuit. It comprises of :i.

Treble

ii.

Bass

iii.

Volume

iv.

Balance

TREBLE

BASS

BALANCE

VOLUME

Figure 6.6 : Tone control pre-amp

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Tone Control Treble – is to control high frequency audio or high pitch sound. It’s circuit is based on high pass filter.

Bass – is to control low frequency audio that is designed by low pass filter to control the low pitch sound.

Volume – control the overall loudness of the audio i.e by reducing or increasing the signal amplitudes. It is designed for voltage or attenuator.

Driver / Pre-Amplifier 2 It is similar to the 1st pre-amplifier but is use to prevent attenuation for output signal from the tone control. It is also used to drive the output of the power amplifier.

6.3 Power Amplifier

There are few types of power amplifier circuits i.e:i.

Single Ended Amplifier

ii.

Push – Pull amplifier

iii.

Complementary Symmettery

iv.

Intergrated circuit

Faults at the amplifiers Single-Ended Amplifier It is a class A amplifier and is coupled to an output transformer to match the speaker. The bias signal is fed into The driver stage whether it is transistor circuit or intergrated circuit will feed a bias signal to drive the transistor to work in Class A.

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Figure 6.7 : Single – ended amplifier

Push Pull Amplifier

Figure 6.8 :Push-Pull Amplifier +ve signal

This ciruit works in Class B for each of the output transistor. Started from driver circuit, signal is fed through T7, then coupled to the secondary winding that is tapped to isolate the +ve and –ve signal. Both bases of the Q4 and Q5 becomes the commons for R7,D3 and Rth.

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The +ve signal flows into secondary winding of T7-A. The current cannot flow into B windings because it is against the direction of A signal and the earh biasing through diode D3.

Figure 6.9 :Push-pull Amplifier –ve signal For –ve signal, B into base of Q5 transistor and A winding is cutoff to earth.

The same applies to T8 whereby the +ve and -ve signal flows through primary winding oa C and D respectively.

Probable Faults Transistor Q4 or Q5 – short/open , amplitude distortion Resistor R8 & R6 burnt - open- amplitude distortion T7 and T8 Biasing resistor

- short/open – no sound – cross-over distortion

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Complementary Symmetry Complementary symmetry is also another example of power amplifier. It operates in Class B. To produce a full cycle of output signal, the output transistors is connected in series using the same characteristic transistor but diferent types that compliment each other. i. Input Coupling Transformer Circuit

Figure 6.10 : Input Coupling Transformer Circuit T605 is used as input coupling. The secondary winding is connected to biasing resistor R616, R617, R618, R619 and R622. Without signal, the bias voltage is half the supply i.e 6V. During +ve signal, A winding is induced and yields voltage drops. This voltage causes the increase of voltage at BE TR25, current will flow reducing the CE voltage. The same applies during – ve signal, B winding will operates.

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ii. Direct Biasing

Figure 6.11 : Direct biasing circuit The biasing voltage for output transistor Q205 and Q206 depended on the collector voltage for Q204 transistor. While the biasing voltage for Q204 is taken from the supply through R231, Vbe and R227.

Without any signal, the bias voltage of Q204 will work at certain dc and this voltage is fed into the base of Q205 and Q206. The biasing voltage level will change when there is input signal. Thus the +ve and –ve signal is isolated by the driver transistor Q204.

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iii. Diodes biasing

Figure 6.12 : Diodes biasing circuit

Output transistor is connected in Darlington pairs that complement to each others. The operation is similar to Example 2 but depended on the driver transistor TR1 and the +ve and –ve signals is isolated using diode D1 and D2.

Probable Faults i. Output Transistor: Open/short – distorted output or not functioning ii. Biasing: Symmetry amplifier is biased by driver transistor. Without this transistor, the output transistor cannot function properly. No output due to none signal flow into the output amplifier or phase distortion at the output. iii. Resistor RE: Each transistor is designed with a series resistor at the emitter, other than used to limit an output, it is also acted as fuse.

Consideration must be taken on the rating of the resistor if the needs to change a rises, to prevent it from causing faults to the output transistors. The faulty resistor can cause amplitude distortion. iv.

Diode Biasing: For certain design, biasing diode is used to ensure the phase output is 180ď‚°. If faulty diode, should be replaced by the bias of the transistor driver.

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6. 4 Distortion There are few types of distortion caused by faulty amplifier. It is due to over-drive, filtering, modulation and noise

6.4.1 Amplitude Distortion

Figure 6.13 : Amplitude distortion wave The output signal became flatten at the upper or lower side or both side. This is caused by larger input signal or varying biasing or un linearity of the transistors.

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6.4.2 Phase Distortion

Figure 6.14 : Type of phase distortion wave The output phase distortion is compared to the input signal. Every amplifier has a certain phase differences such as 0 and 180. Other than these phases, the output is considered phased distortion.

It is difficult to analyze this type of distortion because the original audio signal usually comes with harmonics noise cause the phase to be distorted.

But we can refer to the results given by the square wave testing.

6.4.3 Frequency Distortion

Frequency distortion describes the condition where different frequencies within the amplifier’s bandwidth are amplified by different amounts. The gain over the bandwidth is no longer flat. Under these

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conditions the various frequency components of complex waves are amplified by different amounts and the wave becomes distorted. Frequency distortion can be caused by the frequency dependent effects of reactive components (capacitances and inductances) in the circuit.

6.4.5 Cross-over Distortion

Figure 6.15 : Cross over distortion wave This fault is commonly occur in push-pull or complementary amplifier. The output signal is less than 180 degrees. This is caused by wrong biasing or drifted characteristics of the active components.

6.4.6 Inter modulation Distortion This is caused by the un linearity of the various input frequencies than added to produce a new frequency. Inter modulation distortion was very low, the 1kHz difference component with the very demanding 19+20kHz test signal remaining at -90dB (0.003%), even just below the onset of visible clipping.

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Figure 6.16 : Inter modulation distortion wave

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6.5 Public Address System (PA system) A public address system or PA system is a combination of a set of audio equipment that allows broadcasts over a designated area. Often found in schools, shopping complex and office buildings, PA systems can be used for announcements or emergency information and provide a simple way to get information out quickly. PA systems can be basic or advanced, and can be adapted to fit a variety of need.

Basic PA systems are comprised of loudspeakers placed in convenient locations around the broadcasting area, an amplifier to increase the sound, and a mixer, which allows variation in sound levels. The user speaks into a microphone, and the sound is transmitted through connected cables to the area surrounding the speakers. Some systems also include microphones or intercoms near the speaker locations, allowing the listener to reply to the central location, although not to the entire system.

Figure 6.17 : PA system block diagram

6.5.1 Category PA Systems These systems can be categorized into two parts: i. ii.

High level of system Low level system

High Level System High level of public address systems is used for the area to be covered by the

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speakers as broad as in the hall, the mosque, the stadium and the auditorium.

The speaker is in a diagonal space so that no reflections occur and this avoid-tailed noise.

Low Level System Low level system used for small area and a small number of listeners. It's like classrooms, meeting and briefing rooms.

6.5.2 PA system basic components i. ii. iii. iv. v. vi.

Input Devices Mixer Equalizer Crossovers Amplifiers Speakers

i. Input Devices An input device is the interface between a sound source and the sound system. In most cases, it will consist of either a microphone or a direct input (DI) box. These devices convert the sound or the electrical signal from an instrument or voice into an electrical signal that is compatible with the components of a sound system. ii. Mixer

The mixing console (mixer) is the heart of the sound system. It is the device that controls how the input signals are routed to various signal processors and output devices (loudspeakers) connected to the system. They are available in analog and digital flavors, but they are all designed with the same architecture. Mixers can appear intimidating with their dense array of knobs, buttons, faders, lights and displays. Their operation is, however, very straightforward and logical if you keep the purpose of their various functions in perspective. iii. Equalizer (EQ) The equalizer (EQ) on the channel strip is used to modify the tone of each input signal. Equalization at this stage is most commonly used to adjust the desirable and undesirable qualities of each instrument or voice as well as

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those of the input device (microphone or DI box). iv. Crossovers Crossovers are specialized filters that split an input signal into a few different output signals based on the specific frequency range that a speaker’s drivers (tweeters, woofers, and subwoofers) are designed to handle. Once the signals are emitted from their respective drivers as sound waves, they recombine acoustically to form the original signal. v. Amplifiers Amplifiers convert the electrical signal used amongst the sound system components into a much more powerful one to drive the loudspeakers.

Figure 6.18 : PA system schematic diagram

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6.5. 3 PA systems Troubleshooting A public address system is vital for communicating with large groups. The combination of microphone, cabling, amplifiers, mixers and speakers are all necessary to raise the speaker's voice to where everyone can hear. Public address systems, however, may have problems such as no sound, interference (static and feedback) or variable sound (fading in and out).

Understanding how the public address system works is the first step to finding and correcting any problems. i.

Microphone

The microphone (or "mic") picks up natural sound and converts it to an electrical impulse. Mics also have screens needed to filter out wind and breathing. If there is trouble with the mic check the screen and make sure it is clean of any debris. Some mics are battery powered to increase the signal to the amplifier. Always make sure a battery-powered mic has fresh batteries before use. Also, if it is a wireless microphone make sure the receiver is plugged in and receiving the battery-powered signal. A row of lights on the receiver will indicate signal strength (more signal, more lights).

ii.

Cabling/wireless

Modern public address systems can use either cables to connect all the components, wireless transmitters and receivers (transceivers) or a combination of both. If the public address system has crackling static or cuts on and off, check the cables for broken connections. A moving wire can connect and disconnect and interrupt the signal. Also, make sure there are no obstructions interfering with wireless transceivers. Try and keep the area around the public address system free of other device--even something simple like an electric fan or other appliance. They generate electrical signals that interfere with transceiver signals.

iii.

Amplifier

The amplifier is the heart of the public address system. It takes sound from the mic, amplifies it and sends it to the speakers. If there is feedback (loud whistles whenever someone speaks into a mic) check the GAIN (sound

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coming in) and the volume (sound going out). Adjust the two until the feedback dissipates. Also, if the amplifier stops operating, be sure to check the fuse. Amps will have a fuse that's usually accessible on the back panel.

Pull the fuse and check to see if the wire is burned through. If it is, replace it with a fuse having the same capacity.

iv.

Speakers

The speakers are the source of the sound. If they are connected wirelessly, make sure there are fresh batteries. Of course, make sure all the cables are intact. If the speaker's sound drops to a barely audible whisper no matter the volume, one of the sound-producing cones might be damaged. If this is the case, replacement is the only repair.

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CHAPTER 7 : DOMESTIC ELECTRONIC APPLIANCES

7.0 Introduction The basic principle of block diagram AM Radio

Figure 7.1 :Block Diagram of the AM Transistorized Radio

Antenna / Aerial Accept signal wave from the transmitter and choose frequency is wanted.

Frequency value want to be selected is 540Khz until 1600Hz Rf Amplifier Strengthen wave received and selected by antenna Local Oscillator Produce frequency a value berbentuk gelombang signature. Value fekuensi produce is higher 455Khz than the value accepted frequency by antenna.

Mixer Mix frequency who come from amplifier radio frequency with frequency from alatayun local and produce frequency a value whose name Frekuensi Antara (IF). Process made one new value from two stated frequency call

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action SUPERHETRODYNE. IF Amplifier Strengthen frequency among those accepted from mixer. Amplifier type in amplifier IF is tertala where wabble / transformer IF tuned to frekueni 455Khz

Detector Detector level is separate frequency audio from frequency IF's modulation Audio Amplifier Strengthen frequency signal audio from detector. Exist reinforcement two tier namely driver amplifier and power amplifier.

Speaker Exchange frequency audio to information voice or sound.

Power Supply Exchange voltage au to voltage at that is required by device receive radio.

Type rectifier scales and rectifier full-wave much used at this stage Automatic Gain Control Control amplifier gain radio frequency automatically although accepted wave by changing antenna

7.1 Radio receiver of AM and FM

Radio frequency signals broadcast containing both frequency carrier and frequency audio. voltage audio stated will be modulate frequency amplitude stated carrier and stated modulation known as modulation amplitude or AM transmission and reception.

7.1.1 AM Receiver Principle

AM's signal stated join made one envelope to be broadcast. On the part receiver, accepted information signal still staying in case envelope but far weak(several mV).On the part RF's amplifier and volume control, stated

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signal to be amplified and further double by power amplifier and speaker.

Front end receiver comprise from the division RF's amplifier, mixer and oscillator local.User can tune up to frequency is wanted inside 540Khz1600Khz's circle On the part local oscillator mixer will generate frequency that more 455Khz from frequency received. Errand to get frequency product middle(IF)to the value 455Khz's frequency. Example: Frequency receiver signals =1Mhz Frequency LO Frequency different

= 1Mhz + 455Khz = 455Khz = LO frek-RF frek = 1.455Mhz – 1 Mhz =455Khz

This blending gospel known as the principle heterodyning.

Gospel heterodning signal conversion is the process accepted RF to signal RF to frequency that different which is known as frequency middle(IF). IF's signal produce from mixer in to pass through to circuit tertala and further strengthen by amplifier IF. To generate signal IF, accepted signal must be mix signally generate in stated receiver. Life signal stated receiver lived by circuit local oscillator. Circuit use to replace accepted signal to signal IF known as mixer.

FM Radio With AM, the frequency of the carrier is fixed and the modulating signal controls carrier amplitude. With FM, the amplitude of the carrier is kept constant and its frequency varied by the modulating signal. This variation in carrier frequency is called DEVIATION. The amount that the carrier deviates in frequency is proportional to the loudness of the Audio modulating signal. Deviation is expressed in kHz per Volt.How often the carrier deviates is determined by the frequency of the modulating audio.If you whistle it deviates more frequently than if you hum into the microphone.

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7.1.2 The FM receiver principle Since FM signals occupy a wide bandwidth there is no room for them on LW or MW. They use the FM band of 88-108 MHz where there is plenty of band space available. Advantages of FM are higher quality and low noise.

The diagram shows how the carrier varies in frequency as the modulating signal changes in amplitude

Figure 7.2 : The block diagram of FM

Most of these blocks are discussed individually, and in more detail, on other pages. See filters, mixers, frequency changers, am modulation and amplifiers. The f.m. band covers 88-108 MHz. There are signals from many radio transmitters in this band inducing signal voltages in the aerial. The rf amplifier selects and amplifies the desired station from the many.

It is adjustable so that the selection frequency can be altered. This is called TUNING. In cheaper receivers the tuning is fixed and the tuning filter is wide enough to pass all signals in the f.m. band. The selected frequency is

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applied to the mixer. The output of an oscillator is also applied to the mixer.

The mixer and oscillator form a FREQUENCY CHANGER circuit. The output from the mixer is the intermediate frequency IF The IF is a fixed frequency of 10.7 MHz. No matter what the frequency of the selected radio station is, the IF is always 10.7 MHz. The IF signal is fed into the IF amplifier. The advantage of the IF amplifier is that its frequency and bandwidth are fixed, no matter what the frequency of the incoming signal is.

This makes the design and operation of the amplifier much simpler.

The amplified IF signal is fed to the demodulator. This circuit recovers the audio signal and discards the r.f. carrier. Some of the audio is fed back to the oscillator as an AUTOMATIC FREQUENCY CONTROL voltage.

This ensures that the oscillator frequency is stable in spite of temperature changes. The audio signal voltage is increased in amplitude by a voltage amplifier. The power level is increased sufficiently to drive the loudspeaker by the power amplifier.

FM Radio block receiver

All new FM broadcast receivers are being built with provision for receiving stereo, or two-channel broadcasts. The left (L) and right (R) channel signals from the program material are combined to form two different signals, one of which is the left-plus-right signal and one of which is the left-minus-right signal The (L - R) signal is double-sideband suppressed carrier (DSBSC) modulated about a carrier frequency of 38 kHz, with the LSB in the 23- to 38-kHz slot and the USB in the 38- to 53-kHz slot. The (L + R) signal is placed directly in the 0- to 15-kHz slot, and a pilot carrier at 19 kHz is added to synchronize the demodulator at the receiver. The output from the FM detector is a composite audio signal containing the frequency-multiplexed (L + R) and (L - R) signals and the 19-kHz pilot tone. This composite signal is

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applied directly to the input of the decode matrix.

The composite audio signal is also applied to one input of a phaseerror detector circuit, which is part of a phase locked loop 38-kHz oscillator.

The output drives the 38-kHz voltage-controlled oscillator, whose output provides the synchronous carrier for the demodulator. The oscillator output is also frequency divided by 2 (in a counter circuit) and applied to the other input of the phase comparator to close the phase locked loop. The phaseerror signal is also passed to a Schmitt trigger circuit, which drives an indicator lamp on the panel that lights when the error signal goes to zero, indicating the presence of a synchronizing input signal (the 19-kHz pilot tone). The outputs from the 38-kHz oscillator and the filtered composite audio signals are applied to the balanced demodulator, whose output is the (L - R) channel.

The ( L + R) and (L - R) signals are passed through a matrix circuit that separates the L and R signals from each other. These are passed through de-emphasis networks and low-pass filters to remove unwanted highfrequency components and are then passed to the two channel audio amplifiers and speakers. On reception of a monaural signal, the pilot-tone indicator circuit goes off, indicating the absence of pilot tone, and closes the switch to disable the (L - R) input to the matrix. The (L + R) signal is passed through the matrix to both outputs. An ordinary monaural receiver tuned to a stereo signal would produce only the (L + R) signal, since all frequencies above 15 kHz are removed by filtering, and no demodulator circuitry is present. Thus the stereo signal is compatible with the monaural receivers.

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7.1.3 Principle of CD Player

CD players can employ a number of ways to improve performance, or reduce component count or price. Features such as oversampling, one-bit DACs, dual DACs, interpolation (error correction), anti-skip buffering, digital and optical outputs are, or were, likely to be found. Other features improve functionality, such as track programming, random play and repeat, or direct track access. Yet others are related to the CD player's intended target, such as anti-skip for car and portable CD players, pitch control and queuing for a DJ's CD player, remote and system integration for household players. Description of some features follows: 

Oversampling is a way to improve the performance of the low pass filter present at the output of most CD players. By using a higher sampling frequency, a multiple of the 44.1 kHz used by CD encoding, it can employ a filter with much lower requirements.



One-bit DACs were less expensive than other types of DACs, while providing similar performance.



Dual dacs were sometimes advertised as a feature because some of the early CD players used a single DAC, and switched it between channels. This required additional supporting circuits, possibly degrading sound quality.



Anti-skip or Antishock, is a way for the CD player to avoid interrupting the audio output when mechanical shock is experienced by the disc playback mechanism.

It consists of an additional data processor and a RAM chip installed on the player that reads the disc at double speed and stores various frames of audio data in a RAM memory buffer for later decoding. Some players may compress the audio data prior to buffering to use less capacity (and less expensive) RAM chips. Most players can store about 44 seconds of audio data on a 16 mbit RAM chip.

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7.2 The Principle of Color TV receiver

We have thus far been concerned mainly with black-and-white or monochrome television. While color TV presents a much greater degree of sophistication, the student who has mastered monochrome principles reasonably well can advance to the color set by adding a few more basic ideas.

Our system for color TV was instituted in 1953 and is termed compatible. That is, a color transmission can be reproduced in black-andwhite shades by a monochrome receiver, and a monochrome transmission is produced in black and white by a color receiver. To remain compatible, the same total 6-MHz bandwidth must be used, but more information(color) must be transmitted.This problem is overcome by a form of multiplexing as when FM stereo was added to FM broadcasting.It turns out that the video signal information is clustered at 15.75-kHz (the horizontal oscillator frequency) intervalsthroughout its 4-MHz bandwidth.Midway between these 15.75-kHz clusters (harmonics) of information are unused frequencies,as indicated in Figure 7.3.

Figure 7.3 : The unused frequencies

By generating the color information around just the right color subcarrier frequency (3.579545 MHz),it becomes centered in clusters exactly between

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the black-and-white signals.This is known as interleaving.

At the color TV transmitter,the scene to betelevised is actually scanned by three separate pickup sensors in the camera,each camera sensitive to just one of the three primary colors:red,blue,and green.Because various combinations of these three colors can be mixed to form any color to which the human eye issensitive,an electrical representation of a complete color scene is possible. The three color cameras scan the scene in unison, with the red, green and blue color content separated into three different signals. This process is accomplished within the color TV camera as shown in Figure7.4.

Figure 7.4: The process of color tv

The lens focuses the scene onto a beam splitter that feeds three separate light filters.The red filter passes only the red portion of the scene,resulting in the R (red) electrical signal.A blue and a green filter accomplish the same process to generate the B and G signals.At the receiver,these three separate signals are made to illuminate properly groups of red,green,abd blue phosphor dots (called triads),and the original scene is reproduced in color.

After generation these three separate color signals are fed into the transmitter signal processing circuits (matrix) and create the Y,or luminance,

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signal and the chroma, or color, signals I and Q.The Y signal contains just the right proportion of red, blue, and green so hat it creates a normal blackand-white picture. This proportion is: Y = 0.3R + 0.95G + 0.11B It modulates the video carrier just as does the signal from a single black-andwhite camera with a 4-MHz bandwidth. The chroma signals, I and Q,are used to phase-modulate the 3.58-MHz color subcarrier,which then interleaves their color information in the gaps left by luminance Y signal’s sidebands. The proportions for I and Q are: I = 0.6R + 0.28G + 0.32B Q = 0.21R – 0.52G + 0.31B This modulation by the I and Q signals is accomplished in a balanced modulator. thus suppressing the 3.58-MHz subcarrier because it would cause interference at the receiver. The composite transmitted signal is shown in Figure7.5.

Figure 7.5 : The composite transmitted signal

At the receiver, a monochrome set simply detects the Y signal and thus presents a normal black-and-white rendition of a color picture. The chrome signals (I and Q) cannot be detected in a monochrome set because their 3.58-MHz subcarrier was suppressed and is not present in the received

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signal. Thus, a color set must have a means to generate and reinject the3.58MHz subcarrier to enable detection of the I and Q signals. Notice in Figure 12 that the Q signal is a full DSB signal with sidebands extending + 500kHz around the color subcarrier,which is 3.58MHz above the overall carrier frequency. The I signal has a lower sideband 1.5MHz below the color subcarrier.It is a vestigial sideband signal,however,because the upper sideband is attenuated after 500kHz. A block diagram showing the generation of the composite color TV modulating signal is shown in Figure7.6.

Figure 7.6: A block diagram of the composite color TV modulating signal

It is called the NTSC (National Television Systems Committee) signal and was approved by the FCC in 1953.Noticed that the I and Q signals are summed with the Y signal to modulate the TV carrier frequency. The chrominance signals (I and Q) modulate the 3.58MHz subcarrier in separate balanced modulators.These subcarriers are 90 degree out of phase (in quadrature).The two double-sideband signals created (I and Q) can be separately recovered at the receiver because of this quadrature modulation process.

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7.2.1 Color receiver block diagram A block diagram of a color receiver, from the video detector onward, is shown in Figure7.7.

Figure 7.7: A block diagram of a color receiver After video amplification, the Y signal is immediately available. It is given a delay of about 0.000001s,as shown, so that it will arrive at the CRT at the same time as the I and Q signals. This is necessary because the I and Q signals undergo considerably more processing, which takes about 0.000001s.The chrome signals are amplified and then sent into a 2-to 4.2MHz band pass amplifier and then to the I and Q detectors. These detectors also have inputs from the 3.58-MHz crystal oscillator so that the difference signal in the I detector is the 0-to 1.5-MHz I signal, and in the Q detector, it is the 0-to 0.5-MHz Q signal. Notice in Figure 14 that the 3.58MHz signal for Q detector is given a 90ยบ phase shift, which is how the Q signal was generated at the transmitter. This phase shift makes them separable the receiver.

Once the I and Q signals are detected and passed through their

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respective low pass filter, they are given a phase inversion that allows for both + and – chroma signals. This I necessary because Green = -0.64Q – 0.28I + Y Blue = 1.73Q – 1.11I + Y Red = 0.62Q + 0.9I + Y The I, Q, and Y signals are summed in the three-color adder circuits, with the resistor value providing the proper proportion of each signals. The output of each color adder is then applied to the appropriate CRT grid to control beam intensity notice the rheostat in each adder circuit. It allows for the intensity of each color signal to be varied in proportion to the other colors.

The color sub carrier crystal oscillator is not precise enough by itself to allow proper chrome signal detection. This is surprising because crystal oscillator are extremely stable and accurate. An accuracy of 1 part of 10^12 is necessary to obtain the correct chroma signal. Recall that color transmissions eliminate this carrier from the video signal but do include a sample of it on the back porch of the horizontal blanking pulse, as shown in Figure 7.8.

Figure 7.8: A sample of it on the back porch of the horizontal blanking pulse The color burst amp shown in Figure 7.7 is receptive to that portion of the overall video signal. Its frequency is compared with the 3.58-MHz crystals in the phase detector, and if they are not precisely equal, the phase detector applies a dc level to vary the reactance of the reactance modulator. It, in turn, causes the crystal’s frequency to “pull” in the proper direction to bring

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it back into precise synchronization with the color burst frequency.

Notice that the phase detector in Figure 14 also has an output that is applied to the color killer. The name is descriptive because a monochrome broadcast has no color burst, and thus the phase detector has a large dc output that the color killer circuit uses to “kill� the 2- to 4.2-MHz band pass amplifier. The purpose is to prevent any signals out of the chrome circuits during a monochrome broadcast. A defective color killer result in colored noise, called confetti, on the screen of a color receiver during a black-andwhite transmission. The confetti looks like snow but with larger spots, in color. The color killer also kills the color if a weak RF signal is received.

7.3 The color CRT and convergence Color receiver CRTs are a marvel of engineering precision. As previously mentioned, they are made up of triads of red, blue, and green phosphor dots.

The trick is to get the proper electron beam to stick its respective colored phosphor dot. This is accomplished by passing the three beams through a single hole in the shadow mask, as shown in Figure7.9.

Figure 7.9: A single hole in the shadow mask

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The shadow mask prevents the “red” beam from spilling over onto an adjacent blue or green phosphor dot, which would certainly destroy the color rendition. A typical color CRT has over has over 200 000 holes in the shadow mask and triads of phosphor dots. To make the three beams converge correctly on their color dot of phosphor throughout the face of the tube requires special modification to the horizontal and vertical deflection systems.

Static convergence refers to proper beam convergence at the center of the CRT’s face. This adjustment is made by dc level changes in the horizontal and vertical amplifiers. Convergence away from the center becomes more of the problem and is referred to as dynamic convergence. It is necessary because the tube face away from the center is not a perfectly spherical shape (it is more nearly flat), and thus the beams tend to converge in front of the shadow mask away from the tube center. Special dynamic convergence voltages are derived from the horizontal and vertical amplifier signals and are applied to a special color convergence yoke place around the tube yoke, as shown in Figure 7.10.

Figure 7.10 : A special color convergence yoke place around the tube yoke The dynamic convergence of a set involves the shown magnet adjustment and several adjustment (usually 12) on the convergence board that have interaction effects. The process is quite involved and time consuming.

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7.4 The TV Troubleshooting

7.4.1 Weak Picture and Sound

If both the sound and the picture are weak and distorted, a possible cause is a defective signal input -- antenna system, computer cable connection, or television cable hookup. For example, the antenna system may have a bad antenna, loose connection, bad cabling, or an improperly oriented antenna. Check for the proper signal, using a signal level meter or substituting another TV. Should the antenna system prove good, the problem is likely to be in the tuner section. On old mechanical tuners, selection of channels is accomplished by changing frequencies of the oscillator by rotating a different coil into position. You should check it for proper alignment and clean all the contacts. Modern tuners are all solid-state and electronically selected. There is usually nothing that you can do in the field to repair these tuners.

7.4.2 Good Picture, Weak Sound

If the picture is good but the sound is weak and distorted, chances are the audio IF amplifier, FM detector, AF amplifier, or speaker is causing the problem. The FM detector is the most likely cause. Check the FM detector for proper voltage and signal first. If the test voltage and signal match the manufacturer's specification, then the problem is the AF amplifier or speaker. An easy way to check the audio stage is to increase the volume and note the noise. If there is noise as the volume increases, then the AF amplifier output and the speaker are working; therefore, the problem must be in the signal or the previous stage. If not, then the problem is in the FM detector or audio IF amplifier.

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7.4.3 Weak Picture with Normal Sound

If the picture is weak but accompanied by normal sound and a bright screen (raster), the probable stages that can affect the picture are the antenna system, RF amplifier, converter, local oscillator, IF amplifier, video detector, AGC system, or, most likely, the video amplifier. The sound could be perceived as normal after leaving the video detector if the audio section has a number of amplification stages which would amplify a weak signal. Once again, a decrease in voltage from the low-voltage power supply can be the source of the problem. Another possible cause is the video detector.

7.4.5 No Picture with Normal Sound

If the TV receiver does not produce a picture but has a raster, or illuminated picture tube, then the defect exists in the stages before the sound pickoff. However, it is possible that the video amplifier circuit is at fault. If there is no snow on the screen, the trouble is most likely in the video detector or the IF amplifier stage. However, if the picture is snowy, the RF amplifier in the tuner or antenna / cable system is probably defective. See Figure below for an illustration of a snowy screen.

Figure 7.11 :Snowy, weak television reception

To determine whether the tuner or the antenna / cable system is at fault,

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simply substitute a good TV in its place. If the snow disappears, the tuner is the problem. If the snow remains, the problem is in the antenna or transmission line. Often a defective RF amplifier causes the picture to be snowy. Also, many older tuners contain silver contacts which have a tendency to tarnish and become dirty.

7.4.6 Sound Normal but No Raster

If the receiver lacks a raster, then the fault could exist in the highvoltage power section. There could be a problem in the horizontal deflection stage, such as the flyback transformer or damper. Check the high voltage with a high-voltage DC probe to determine if there is high voltage at the anode of the CRT. Be careful when you check this voltage, because dangerous arcs can be drawn. If the voltages match the manufacturer's specifications, then the picture tube may be at fault. However, if there is a lack of DC voltage, check to see if there is AC voltage from the flyback transformer. Newer circuitry may have only DC voltages at this stage.

Compare the positive and negative DC voltages in the circuit with the values indicated in the service manual schematic to isolate the problem. A blue arc indicates AC voltage; DC voltages produce a white arc. If an AC arc can be drawn at the flyback transformer; the high-voltage rectifier is defective.

Lack of an ac arc could indicate a faulty flyback transformer or horizontal circuit.

7.4.7 Sound Normal, Picture Out Of Sync

If the sound is normal but there are heavy slanted streaks across the screen, then the problem is that the horizontal deflection is out of synchronization. Figure 7.12 illustrates the horizontal synchronization problem, which is commonly called horizontal tearing. Check the horizontal deflection control to verify that it is set properly. If it is, then the fault exists in the horizontal oscillator. If it stops running, there will be no pulse to drive

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the horizontal output or deflection coil in the yoke.

Figure 7.12 :Horizontal tearing

7.4.8 Sound Normal but Picture Tearing, Reduced Width

If your picture has tearing or heavy slanted streaks across the screen as well as vertical roll, first check for proper setting of the horizontal and vertical controls. If these are properly set, then probable causes are defects in the sync separator or sync amplifier stages. There is a possibility that defects can exist in both horizontal and vertical deflection systems. Figure 7.12 illustrates a typical vertical deflection system. It consists of a vertical oscillator, a vertical driver, and a vertical output stage which is tied to the yoke.

7.4.9 Sound Normal but Picture Rolls and Folds, with Reduced Height

Other vertical system problems are shown in Figure 7.13. If the picture rolls vertically, a probable cause is a defect in the vertical oscillator.

If the oscillator stopped oscillating, there would be no vertical deflection; and all that would be seen on the screen would be a bright horizontal line. If you experience reduced height in the picture, the problem is a weak vertical output. Referring to Figure 5.19 again, several likely causes could be a shift in the oscillator or output bias, low dc supply voltage, or a shorted or open

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component. It probably is not a shorted oscillator or output transistor if there is a partial picture. However, if there is only a single horizontal line, then a shorted oscillator or output transistor may be suspected. In this case, the likely cause is a defective component, such as C306, C308, or R310. For example, if C306 is shorted, then the sawtooth-waveform process is interrupted and the bias of Q302 is shifted, which reduces the amplification and oscillation.

7.4.10 Normal Picture but Poor Sound

If the picture is normal but the sound is missing, check the IF amplifier, FM detector, or AF amplifier stages. There could also be a defective speaker coil. A weak sound suggests improperly adjusted finetuning control or a shift in the local oscillator due to changing component values. Since multiple stages are involved, use the half-splitting troubleshooting technique to quickly identify the defective stage. Check for a defective transistor, IC, or module in the sound section. A change in a component value or defective cable in an audio stage can affect the gain of an amplifier. Sound distortion can be caused by a defective interstage coupling capacitor.

Figure 7.13 : Other vertical system problems

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Dead Set Like the radio, if a TV set is dead, check the power supply. Possible causes include a blown fuse, tripped circuit breaker, open line cord, shorted or open cable, or a defective component -- on/off switch, transformer, diode or rectifier, thermistor, or power supply IC module.

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