Module dee3052 by mfaizal jel

All rights reserved. No part of this publication may be reproduced, copied, stored in any system retrieval and transmitted in any form or by any means â&#x20AC;&#x201C; electronic, mechanical, photocopying, recording or otherwise; without prior permission in writing from the Head of Department, Electrical Engineering Department, Sultan Haji Ahmad Shah Polytechnic (POLISAS), Semambu, 25350 Kuantan, Pahang Darul Makmur, Malaysia. First Printed

COURSE DEE3052 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. ramlypress@yahoo.com +609-5130020 (Head Office)

AUTHORS

Project director Mohd Yusof bin Zakaria

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

Publisher Electrical Engineering Department Sultan Haji Ahmad Shah Polytechnic 2015

Course DEE3052 Electronic Equipment Repair Wan Ghani Bin Wan Pi 1, Mohd Faizal Bin Mustapha2, Muhamad Adha Bin Shamdin@Shamsudin3 Electrical Engineering Department, Sultan Haji Ahmad Shah Polytechnic, Pahang 1,2,3 Email,wanghani@polisas.edu.my1: mfaizal@polisas.edu.my2 : adha.polisas.edu.my3

ABSTRACT This book is a necessity for Electronic Equipment Repair DEE3052 courses. It was written and produced specifically for third semester student of Electrical and Electronic Engineering at Sultan Haji Ahmad Shah Polytechnic. It can also be used by all students in Electrical Engineering Department of Polytechnic Malaysia. Among the topics contained in the book are Hand tools and Soldering Technique, Testing Equipment, Diagnosis Technique, Power Supply, Amplifier and Electronic Domestic Appliances. This book has been prepared in accordance with chapter by DEE3052 Course syllabus. This book can help students to be better understands the repair of electronic equipment.

KEYWORD: Course DEE3052 ,Electronic Equipment Repair, Hand tools and 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 DEE3052 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 Sultan Haji Ahmad Shah Polytechnic, Semambu 25350 Kuantan Pahang Darul Makmur

CONTENTS Topic

Page

HAND TOOLS AND SOLDERING TECHNIQUE Hand tools Soldering tools Desoldering tools Soldering technique

1 3 4 5

TESTING EQUIPMENT Multimeter Digital multimeter Analogue Oscilloscope Digital Oscilloscope Audio signal generator Transistor tester Color TV test pattern generator

9 12 13 13 14 15 16

DIAGNOSIS TECHNIQUES Symptom finding Printed circuit tracing Signal injection Voltage measurement technique Resistance measurement technique Fault finding by physical technique Identify the terminal of component

17 18 19 21 22 24 24

POWER SUPPLY Linear dc power supply Protector Transformer Rectifier Filter Regulator Switching mode power supply

27 28 28 28 30 31 35

AUDIO AMPLIFIER Introduction Audio amplifier block diagram Audio pre-amplifier Tone control Driver amplifier Power amplifier

37 38 38 39 39 39

Topic

Page

Faults at amplifier Single ended amplifier Push pull amplifier Complementary symmetry Coupling transformer circuit Direct biasing Diode biasing Distortion Public address system PA systems troubleshooting

41 41 42 43 44 44 45 46 48 50

DOMESTIC ELECTRONIC APPLIANCES AM radio FM radio Advantages of FM radio Disadvantages of FM radio TV systems CRT TV systems Beam scanning and raster development Composite video signal The different type of TV displays Backlight LED TV block diagram Television user controls Television troubleshooting

52 54 55 56 56 57 60 61 62 63 63 64

ELECTRONIC LABORATORY EQUIPMENTS Audio signal generator Wien bridge oscillator circuit RF signal generator Types of RF signal generator RF signal generator functions

67 68 69 70 72

Printed 2015

First Printed

COURSE DEE3052 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. ramlypress@yahoo.com +609-5130020 (Head Office)

AUTHORS

Project director Mohd Yusof bin Zakaria

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

Publisher Electrical Engineering Department Sultan Haji Ahmad Shah Polytechnic 2015

ABSTRACT This book is a necessity for Electronic Equipment Repair DEE3061 courses. It was written and produced specifically for third semester student of Electrical and Electronic Engineering at Sultan Haji Ahmad Shah Polytechnic. It can also be used by all students in Electrical Engineering Department of Polytechnic Malaysia. Among the topics contained in the book are Hand tools and Soldering Technique, Testing Equipment, Diagnosis Technique, Power Supply, Amplifier and Electronic Domestic Appliances. This book has been prepared in accordance with chapter by DEE3052 Course syllabus. This book can help students to be better understands the repair of electronic equipment.

KEYWORD: Course DEE3052 ,Electronic Equipment Repair, Hand tools and Soldering Technique, Testing Equipment, Diagnosis Technique, Power Supply, Amplifier, Domestic Electronic Appliances.

Acknowledgement

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

CONTENTS Topic

Page

HAND TOOLS AND SOLDERING TECHNIQUE Hand tools Soldering tools Desoldering tools Soldering technique

1 3 4 5

TESTING EQUIPMENT Multimeter Digital multimeter Analogue Oscilloscope Digital Oscilloscope Audio signal generator Transistor tester Color TV test pattern generator

9 12 13 13 14 15 16

17 18 19 21 22 24 24

POWER SUPPLY Linear dc power supply Protector Transformer Rectifier Filter Regulator Switching mode power supply

27 28 28 28 30 31 35

AUDIO AMPLIFIER Introduction Audio amplifier block diagram Audio pre-amplifier Tone control Driver amplifier Power amplifier

37 38 38 39 39 39

Topic

Page

41 41 42 43 44 44 45 46 48 50

52 54 55 56 56 57 60 61 62 63 63 64

ELECTRONIC LABORATORY EQUIPMENTS Audio signal generator Wien bridge oscillator circuit RF signal generator Types of RF signal generator RF signal generator functions

67 68 69 70 72

DEE3052 ELECTRONIC EQUIPMENT REPAIR

CHAPTER 1 : HAND TOOLS & SOLDERING TECHNIC A. Hand tools i.

Long-nose Plier

Long-nose plier is used for cutting and gripping, to bend, re-position and cut wire. 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.

Figure 1.1: Long nose plier ii. Plier Plier is used for bending and compressing a wide range of materials and fitted with wire cutter blades either built into the jaws or on the handles just below the pivot.

Figure 1.2: Plier iii. Side Cutter Side cutter is used for cutting wire and cut off the source terminal.

Figure 1.3: Side cutter iv. Manual Wire Stripper 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

DEE3052 ELECTRONIC EQUIPMENT REPAIR

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.

Figure 1.4: Manual wire stripper

v. Tweezers Tweezers is used for grip small objects such as wires and screws

Figure 1.5: Tweezers vi. Screwdriver The screwdriver is used for tighten and loosen the screw. Turning the screw clockwise or counter-clockwise

Figure 1.6: Screwdrivers

DEE3052 ELECTRONIC EQUIPMENT REPAIR

B. Soldering Technic Soldering Tools i.

Soldering iron

A soldering iron/gun is a hand tool most commonly used in soldering. It supplies heat to melt the solder so that it can flow into the joint between two work pieces. For electronics work, a low-power iron, a power rating between 15 and 35 watts, is used. Higher ratings are available, but do not run at higher temperature; instead there is more heat available for making soldered connections to things with large thermal capacity, for example, a metal chassis. Some irons are temperature-controlled, running at a fixed temperature in the same way as a soldering station, with higher power available for joints with large heat capacity. Simple irons run at an uncontrolled temperature determined by thermal equilibrium; when heating something large their temperature drops a little, possibly too much to melt solder.

Figure 1.7: Soldering iron ii. Soldering gun Soldering gun is an approximately pistol-shaped, electrically powered tool for soldering metals using tin-based solder to achieve a strong mechanical bond with good electrical contact. The tool has a trigger-style switch so it can be easily operated with one hand. Soldering guns are used where more heat is needed than from the lower-power soldering irons. The temperature of the soldering tip is regulated manually by holding the button until the solder melts, and then releasing it. When the solder is about to start solidifying, the button is pressed again, and so on.

Figure 1.8: Soldering gun 3

DEE3052 ELECTRONIC EQUIPMENT REPAIR

Desoldering Tools i.

Solder sucker

Suction pumps are used to suck away molten solder, leaving previously joined terminals disconnected. They are primarily used to release through-hole connections from a PCB. The disordering head must be designed so that the extracted solder does not solidify so as to obstruct it, or enter the pump, and can be removed and discarded easily. It is not possible to remove a multi-pin part by melting solder on the pins sequentially, as one joint will solidify as the next is melted; pumps and solder wick are among methods to remove solder from all joints, leaving the part free to be removed.

Figure 1.9: Solder sucker ii. Solder wick Desoldering wick or solder wick, is finely braided 18 to 42 AWG copper wire coated with rosin flux, usually supplied on a roll. The end of a length of braid is placed over the soldered connections of a component being removed. The connections are heated with a soldering iron until the solder melts and is wicked into the braid by capillary action. The braid is removed while the solder is still molten, its used section cut off and discarded when cool. Short lengths of cut braid will prevent heat being carried away by the braid instead of heating the joint.

Figure 1.10: Solder wick

DEE3052 ELECTRONIC EQUIPMENT REPAIR

Soldering Technique 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 Three requirements:  Low melting point metal (solder wire)  Heat source (soldering iron)  Flux (to clean the surfaces and prevent if from oxidizing) a. Soldering Process    

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

Figure 1.11: Soldering process

DEE3052 ELECTRONIC EQUIPMENT REPAIR

Inspect Your Bond    

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, Completely surrounded, Concave

Good Soldered Joint    

Cleanliness Temperature Time Adequate solder coverage

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 under heat the part or pad - Creates poor bonds, cold joints 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.

b. The Ratio of Pb/lead in soldering Tin/lead solders, also called soft solders, are commercially available with tin concentrations between 5% and 70% by weight. The greater the tin concentration, the greater the solder’s tensile and shear strengths. Alloys commonly used for electrical soldering are 60/40 Tin/lead (Sn/Pb) which melts at 183 °C (361 °F) and 63/37 Sn/Pb used principally in electrical/electronic work. The 63/37 is a eutectic alloy, which:  has the lowest melting point (183 °C or 361 °F) of all the tin/lead alloys; and  the melting point is truly a point — not a range. c. Type of lead i.

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.

DEE3052 ELECTRONIC EQUIPMENT REPAIR

ii. Silver-bearing Solder ď&#x201A;ˇ 62% Tin, 36% Lead, 2 % Silver - solid at 354 Â° 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 contain 97% tin, 2.5% silver and 0.5% copper. These require higher soldering temperatures and do not "wet" as well as Lead-Tin alloy, as a result it requires more skill to produce a good quality solder joint. d. Soldering and desoldering station

Figure 1.12: Soldering & Desoldering station You can easily make a soldering station that can be used for any type of work. Though you need to make sure you follow the correct safety procedures. Soldering uses high temperatures and there is potential for splattering and other accidents. You also have concerns due to power and other issues to consider. Soldering & Desoldering Station Standard Operating Procedure (SOP) : Use Heatproof Materials Soldering uses high heat as what you are doing is to create joints or joining different metals together. Basically you are melting a metal onto another wire or metal to create a connection. In order to do this, the tip of the iron will reach temperatures of about 400 degrees Celsius. So this can produce very serious burns if it should touch your skin. You always want to put the soldering iron back on its stand as you can burn the table or workstation if you place it down on another surface, even if it is for a moment. Your safest procedure is to solder on a mat that is fire resistant. 7

DEE3052 ELECTRONIC EQUIPMENT REPAIR

Work in Ventilated Area The fumes produced during soldering can be very irritating and it may not be enough to simply turn your head away from the fumes. Make sure you set up the soldering station in a well ventilated area and you may even need to set up a fan to get rid of the fumes. You will need to set up the ventilation and air circulation to you preference.

Go for Regular Maintenance You want to make sure that the power cable does not wear out or become damaged. You also want to make sure that the iron is properly grounded and that you turn it off whenever you are not using it. You also want to give the soldering iron its own electrical socket as overloading a socket can blow fuses or cause other safety issues. Protect Your Eyes It is a good idea to wear protective eyewear when soldering as there can be some splatter. You also should wear high heat resistant gloves if you need to hold anything into place. If you donâ&#x20AC;&#x2122;t have gloves, use needle nose pliers to hold things in place. If you are going to do a lot of soldering then a heat resistant apron can also protect your clothing. Wash Your Hands You are suppose to wash your hands before eating and after using the bathroom and it is also important to wash your hands after soldering. Solder contains lead, which is toxic if it is absorbed through the skin or ingested somehow. You should make sure you wash your hands and workstation thoroughly to avoid a contamination or lead that may be present.

DEE3052 ELECTRONIC EQUIPMENT REPAIR

CHAPTER 2: TEST EQUIPMENT Introduction In this unit, itâ&#x20AC;&#x2122;s explain the equipments use for electricity and electronic instruments testing such as multimeter, oscilloscope, transistor tester, audio generator and colour TV test pattern generator. Multimeter i. Analogue Multimeter (VOM- Volt, Ohm dan Miliampere) MOV can perform various measurement such as voltage DC/AC, current measurement, resistance, Capacitance, diods and transistors. a. 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. R1

+88.8 Volts

MULTIMETER DCV RANGE

Figure 2.1: Measuring DC/AC Voltage b. 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:R1

+88.8 Amps

MULTIMETER DC mA

Cut Circuit

Figure 2.2: Current Measurement ( milliampere)

DEE3052 ELECTRONIC EQUIPMENT REPAIR

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

X10K X1K Zero adj.

X100 X10 X1

Figure 2.3: Measuring Resistance

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

X10K X1K

X100 X10 X1

Figure 2.4: Forward-bias

X100 X10 X1

Figure 2.5: reverse-bias

Small-signal diode

: 2 to 10 ohm

Small-signal diode

: High or ∞

Rectifier diode

: 2 to 10 ohm

Rectifier diode

: High or ∞

Zener diode

: 2 to 10 ohm

Zener diode

: High or ∞

DEE3052 ELECTRONIC EQUIPMENT REPAIR

e. 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 PNP and NPN transistor, the measurement obtained with an ohmmeter is as shown below:-

High or α

2 - 10Ω

High or α

B 2 - 10Ω

2 - 10Ω High or α

High or α

R is Red Probe and B is Black Probe

Figure 2.6: PNP Transistor

Figure 2.7: NPN Transistor

Resistance for Base – Collector = 2 - 10Ω (forward bias) Resistance for Base – Emitter = 2 - 10Ω (forward bias)

If Base is Red Probe = PNP If Base is Black Probe = NPN

Resistance for Emitter – Base = High or α (reverse bias) Resistance for Collector – Base = High or α (reverse bias)

Resistance Collector-Base > Emitter - Base

Figure 2.8: Type of transistor 11

DEE3052 ELECTRONIC EQUIPMENT REPAIR

ii. Digital Multimeter The digital multimeter, DMM, is one of the most common items of test equipment used in the electronics industry today. While there are many other items of test equipment that are available, the multimeter is able to provide excellent readings of the basic measurements of amps, volts and ohms. In addition to this the fact that these digital multimeters use digital and logic technology, means that the use of integrated circuits rather than analogue techniques, enables many new test features to be embedded in the design. As a result, most of today's digital multimeters incorporate many additional measurements that can be made.

Figure 2.9: Digital multimeter DMMs are able to offer as standard the basic measurements that would typically include:  Current (AC/DC)  Voltage (AC/DC)  Resistance  Capacitance  Continuity (buzzer) Oscilloscope Oscilloscopes are one of the most versatile items of test equipment. Oscilloscopes provide a graphical view of the waveforms within a circuit and this gives a particularly useful view of what is happening within a circuit. This makes them an essential item of test equipment for use within electronics design, production test and also for use within service organizations. Oscilloscopes fall into a variety of categories. The biggest distinction is whether they are analogue or digital, but within the digital oscilloscope arena there are ordinary digital oscilloscopes, digital storage oscilloscopes, digital phosphor oscilloscopes, and digital sampling oscilloscopes. 12

DEE3052 ELECTRONIC EQUIPMENT REPAIR

Analogue oscilloscope

Figure 2.10: Analogue oscilloscope The analogue oscilloscope is the original type of oscilloscope. As the name implies it uses analogue techniques throughout to create the pattern on the display. Typically they use a cathode ray tube where the voltages on the x and y plats cause a dot on the screen to move. In the horizontal direction this is controlled by the time base, whereas in the vertical direction the deflection is proportional to the signal input. Essentially the signal is amplified and applied to the Y plates of the cathode ray tube using analogue technology. 

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 left-right on the screen and to change the Xaxis 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.

ii. Digital oscilloscope / Digital Storage Oscilloscope, DSO The concept behind the digital oscilloscope is somewhat different to an analogue scope. Rather than processing the signals in an analogue fashion, this type of scope converts the signal into a digital format using an analogue to digital converter and then processes the signals digitally. There are several different types of digital oscilloscope that can be encountered. With many manufacturers trying to gain an edge on their competitors, new names tend to be developed to try to bring over the new levels of functionality and features. The boundary between these two types of oscilloscopes has become very indistinct in recent years. Originally the storage scopes, DSO had additional memory to enable the storage of waveforms. Digital oscilloscopes are now the major form of oscilloscope that is available on the test equipment market. These scopes may be referred to as digital oscilloscope or digital

DEE3052 ELECTRONIC EQUIPMENT REPAIR

storage oscilloscope. Although these two names used to indicate separate types of instrument, they are often used interchangeably.

Figure 2.11: Digital oscilloscope

The original digital storage scopes had analogue input stages, and then converted the signals into a digital format to enable them to be stored in digital memory. As technology progressed, the storage facility was retained but the whole oscilloscope become digital. Signal Generator Signal generators that produce waveforms in the audio spectrum are used by technicians to aid in repairing audio equipment. There are several different types of signal generator. Although they all generate electronic signals and waveforms, the different signal generator types are used for different applications and to develop different types of electronic signal. The different types of signal generator also have very different designs because of the circuits required to realize their requirements. As a result the different types of signal generator have very different levels of capability and functionality. Audio signal generator As the name implies this type of signal generator is used for audio applications. Signal generators such as these run over the audio range, typically from about 20 Hz to 20 kHz and more. They are often used in audio measurements of frequency response and for distortion measurements. As a result they must have a very flat resp0pnse and also very low levels of harmonic distortion. i.

RF signal generator As the name indicates, this type of signal generator is used to generate RF or radio frequency signals. An RF signal generator may use a variety of methods to generate the signal. Analogue signal generator types used free running oscillators, although some used frequency locked loop techniques to improve stability. However most RF signal generators use frequency synthesizers to provide the stability and accuracy needed. Both phase locked loop and direct digital synthesis techniques may be used. Radio frequency signal generators (RF signal generators) are a particularly useful item of test equipment widely used in RF microwave design and test applications. ii.

DEE3052 ELECTRONIC EQUIPMENT REPAIR

Transistor Tester Transistor testers are instruments for testing the electrical behavior of transistors and solidstate diodes. There are three types of transistor testers each performing a unique operation. i.

Circuit Tester A circuit tester is used to check whether a transistor which has previously been performing properly in a circuit is still operational. The transistor's ability to "amplify" is taken as a rough index of its performance. This type of tester indicates to a technician whether the transistor is dead or still operative. The advantage of this tester is that the transistor does not have to be removed from the circuit.

ii. Service type transistor testers These devices usually perform three types of checks:  Forward-current gain, or beta of transistor.  Base-to-collector leakage current with emitter open (ico)  Short circuits from collector to emitter and base. Some service testers include a go/no-go feature, indicating a pass when a certain hfe is exceeded. These are useful, but fail some functional but low h fe transistors. Some also provide a means of identifying transistor elements, if these are unknown. The tester has all these features and can check solid-state devices in and out of circuit. Transistor hfe varies fairly widely with Ic, so measurements with the service type tester give readings that can differ quite a bit from the hfe in the transistor's real life application. Hence these testers are useful, but can't be regarded as giving accurate real-life hfe values. iii. Laboratory-standard transistor tester or Analyzer This type of tester is used for measuring transistor parameters dynamically under various operating conditions. The readings they give are absolute. Among the important characteristics measured are:  Icbo collector current with emitter open (Common base)  ac beta (Common emitter)  Rin (Input resistance) Transistor testers have the necessary controls and switches for making the proper voltage, current and signal settings. A meter with a calibrated "good" and "bad" scale is on the front. In addition, these transistor testers are designed to check the solid-state diodes. There are also testers for checking high transistor and rectifiers.

DEE3052 ELECTRONIC EQUIPMENT REPAIR

Color TV Test Pattern Generator A test signal generator generates test patterns, and other useful test signals, for troubleshooting and analyzing television systems. These devices are generally intended for offline 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. Multi burst, 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 2.12: Samples of test pattern generator

DEE3052 ELECTRONIC EQUIPMENT REPAIR

CHAPTER 3: DIAGNOSIS TECHNIC Introduction Any component parts that are inter connected together to make a complete system have 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 questions must be answer:i. What type of equipment 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 technique is to be used. Analyzing and fault finding techniques To find fault for an electronic equipment, we must rely on itâ&#x20AC;&#x2122;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 timewasting. The techniques can be line up as follows:i. Symptom Finding ii. Printed Circuit Tracing iii. Signals Injections iv. Voltage Measurement v. Resistance Measurement or Continuity vi. Physical Fault Detection a. Symptom Finding Every an equipment normally displays some symptom to give indication of the faults. The troubleshooter must be well versed in the system and functions of the equipment in order to determine the fault. But for the users, they can only recognize a fault such as by the sound output of a radio either no sound, up normal sound etc. Below is the example of fault and functions for some equipment:-

1 2 3 4

Equipment Color TV Radio Oscilloscope Audio Generator

Symptom picture no color no sound no display no output

Table 3.1: System finding 17

DEE3052 ELECTRONIC EQUIPMENT REPAIR

b. 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:a. Determine the input and output b. Determine the power source and earth c. Determine the numbering of the components Printed circuit tracing can be done either from the input to output or vice-versa. Below is an example of the printed circuit (pre amp printed circuit). GND

Vcc

Figure 3.1: Pre amp printed circuit & with component label For a simple circuit such as audio amplifier, pre amplifier, tone circuit and etc, tracing the whole circuit is needed (pre amp printed circuit with component label). Vcc C1

R4 C4

to switch selector

R2 C2 Q1

Mic

Figure 3.2: Pre amplifier schematic diagram 18

DEE3052 ELECTRONIC EQUIPMENT REPAIR

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 digits of the number represent the block grouping of the components. c. Signal Injection After isolating the fault area such as audio amplifier, signal injection and tracing is applied to this area. Function generator is one of the equipment 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:1. Identify the amplifier blocks 2. Identify the supply voltage to prevent from over-current. 3. Adjust a suitable signal of 1KHz or 400Hz from the function generator 4. 5

RF, IF and Detector Volum e

Pre Amplifier

Tone control

Driver Amp

Driver & Power Amp

Figure 3.3: Audio amplifier block diagram

1. Check the speaker to determine whether it is functioning or not. It can be done by multimeter using X1â&#x201E;Ś range.

Figure 3.4: Speaker Testing 2. If the speaker is functioning, then signal can be injected at the input of power Amplifier

DEE3052 ELECTRONIC EQUIPMENT REPAIR Vcc

R9 C28 C7 R2

R7 T7

T8 Q4

1nF

SPEAKER

VR3

Q3 82u

R13 C13

Rth

Figure 3.5: Push-Pull Amplifier Circuit

3. If normal, place the audio generator probe at the driver input i.e before or after C2. Observe the output 4. If normal, check the tone circuit. The output will be small or the tone cannot be adjusted if this circuit is faulty. R22 Vcc

C20 C12 R18

R20 C3

C4 VR1

from detector

to driver amp

R13

C1 Q1 VR2

R19

R21

C13

Figure 3.6: Pre-Amplifier Circuit 5. If the circuit is normal, inject signal at the pre-Amp input. Level the function generator to obtained an undistorted output signal. 20

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d. 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 meant the supply is not normal. If the supply voltage is normal, voltage must be measured around the active component i.e transistor Q1. +12V C20 R18

R20 C3 Output

Vc Vdc

Q1 Input

Multimeter _ +

Ve R19

R21

Figure 3.7: 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. 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 21

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Example 3 If VB = 0.1V Answer: i. BE Q1 shorted or ii. R1 open Example 4 If VE = 0V Answer: i. C2 shorted or ii. E Q1 open e. Resistance Measurement Technique The resistance measurement technique can be measured outside from the circuit. The power supply must be ‘OFF’. This method is used for components with known resistance such as resistor, diodes, transistors etc. 1. Resistor Resistor can be measured in the circuit with the lower of resistance. The resistor gives an infinity or higher resistance when resistor is in open circuit. 2. Fuse The suitable ohm range for checking continuity of fuse is X1Ω. If the resistance is low, the fuse is normal but if the resistance is infinity or high value, the fuse is blow. 3. Inductor A normal inductor, the resistance of is low. The suitable ohm range of multimeter is X1Ω. If the resistance of inductor is 0 Ohm when used x1Ω range, the inductor is shorted. When the resistance of inductor infinity or high value (use X10KΩ range), the inductor in open circuit. 4. Transformer The continuity or resistance of transformer primary and secondary coil is low when using X1Ω range. If the resistance is 0Ω (X1Ω range), the coil is shorted. If the resistance is infinity (X10KΩ range), the coil is opened circuit.

Primary

Secondary Coil/inductor

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5. Relay The two terminal of relay is coil or inductor. The resistance of coil is low when using X1Ω range. If the resistance is 0Ω (X1Ω range), the coil is shorted. If the resistance is infinity (X10KΩ range), the coil is opened circuit. NC is normally close. NO is normally open. 6. Switch A simple ON-OFF switch, the two terminals are either connected together or disconnected from each other. The suitable range of ohm range to check continuity between two terminals of switch is X1Ω. If the resistance is 0Ω, the switch is connected. If the resistance is infinity, the switch is disconnected 7. Diode and zener diode The same applied to measuring diode which including checking on it’s biasing. If diode is forward bias the resistance is low and if diode is reverse bias the resistance is infinity or high. If the resistance of diode is 0 Ohm when used x1Ω range, the diode is shorted. 8. Bipolar Transistor A normal transistor, the resistance of Base-Collector and Base-Emitter is low. If the resistance of Base-Collector and Base-Emitter is 0 Ohm when used x1Ω range, the transistor is shorted. If the resistance of Base-Collector and Base-Emitter is infinity or high Ohm when used x10KΩ range, the transistor is opened. 9. Silicon Control Rectifier (SCR) or Thyristor The thyristor or silicon control rectifier, SCR is a device that has a number of unusual characteristics. It has three terminals: Anode, cathode and gate, reflecting thermionic valve / vacuum tube technology. As might be expected the gate is the control terminal while the main current flows between the anode and cathode. The resistance measurement of SCR is the same like the resistance of diode. The forward bias between terminals Anode – Cathode is low resistance using X1Ω range. If the resistance is 0 Ω, the thyristor is shorted. The reverse bias between terminals Anode – Cathode is infinity or high using X10KΩ range.

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f. Fault Finding by Physical Technique It involves a few senses such as visual observation, hearing, smell and taste or touch. i.

ii.

iii.

Visual Observation By observation, we can locate the faulty area such as burning trace, loose connection, broken etc. 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 his sound, or else yield the burning smell if over heated. Taste This method only applied to fault such as broken components, dry joints, dislodge and etc.

Identify the terminal of component i.

Diode

Figure 3.8: Diode ii. Bipolar transistor

Figure 3.9: Bipolar transistor

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iii. Silicon Control Rectifier (SCR)

Figure 3.10: Bipolar transistor iv. Uni Junction Transistor (UJT)

Figure 3.11: Uni junction transistor v.

Field Effect Transistor (FET)

Figure 3.12: Field effect transistor 25

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vi. Diac

Figure 3.13: Diac vii. Triac

Figure 3.14: Triac viii. Integrated Circuit (IC)

Figure 3.15: Integrated circuit

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CHAPTER 4: POWER SUPPLY A power supply is a device that supplies electric power to an electrical load. The term is most commonly applied to electric power converters that convert one form of electrical energy to another, though it may also refer to devices that convert another form of energy (mechanical, chemical, solar) to electrical energy. A regulated power supply is one that controls the output voltage or current to a specific value; the controlled value is held nearly constant despite variations in either load current or the voltage supplied by the power supply's energy source. Every power supply must obtain the energy it supplies to its load, as well as any energy it consumes while performing that task, from an energy source. Depending on its design, a power supply may obtain energy from:    

Electrical energy transmission systems. Common examples of this include power supplies that convert AC line voltage to DC voltage. Energy storage devices such as batteries and fuel cells. Electromechanical systems such as generators and alternators. Solar power.

A power supply may be implemented as a discrete, stand-alone device or as an integral device that is hardwired to its load. Examples of the latter case include the low voltage DC power supplies that are part of desktop computers and consumer electronics devices. Types of power supply a. Linear regulated dc power supply b. Switching mode dc power supply Linear dc power supply Basic linear regulated dc power supply consists of the following parts: i. ii. iii. iv. v.

Protector Transformer Rectifier Filter Regulator Protector

Transformer

Rectifier

Filter

Regulator

240Vac

Vout

Figure 4.1: Linear regulated dc power supply block diagram 27

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i. Protector The basic component of protector is fuse. A fuse is a type of low resistance resistor that acts as a sacrificial device to provide overcurrent protection, of either the load or source circuit. Its essential component is a metal wire or strip that melts when too much current flows through it, interrupting the circuit that it connects. Short circuits, overloading, mismatched loads, or device failure are the prime reasons for excessive current. Fuses are an alternative to circuit breakers. A fuse interrupts excessive current ("blows") so that further damage by overheating or fire is prevented. Wiring regulations often define a maximum fuse current rating for particular circuits. Overcurrent protection devices are essential in electrical systems to limit threats to human life and property damage. ii. Transformer A transformer can be defined as a static device which helps in the transformation of electric power in one circuit to electric power of the same frequency in another circuit. The voltage can be raised or lowered in a circuit, but with a proportional increase or decrease in the current ratings. The main principle of operation of a transformer is mutual inductance between two circuits which is linked by a common magnetic flux. A basic transformer consists of two coils that are electrically separate and inductive, but are magnetically linked through a path of reluctance.

Transformer construction

Schematic symbol

Figure 4.2: Transformer 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 28

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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. Circuits design to determine the function and efficiency of the work. Common circuit design is divided into three, namely: a. Half-wave rectifier b. Full-wave rectifier c. Full-wave bridge rectifier Half-wave rectifier Referring to figure below, a single diode will produce a half-wave output and relatively low average voltage. Rectifier is mainly used for ac voltage with a higher frequency as the output of the transformer high voltage (Flyback transformer - FBT) in circuit television or output mode-switching supply transformer as it is easier and cheaper. Vp

Vp D1

RL Input

Output

Figure 4.3: Half-wave rectifier circuit Full-wave rectifier For twin diodes can also produce a full wave. But the design of the circuit as shown in figure below must be combined with the center tap transformer. Vp

240Vac 50Hz D2 RL

Output Output

Figure 4.4: Full-wave rectifier circuit 29

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Full-wave bridge rectifier Bridge rectifier is very popular, in order to produce a full wave output. In addition to easy installation, it also saves space. Sometimes there is also cubic form as in figure below. It does not require a transformer center tap.

Figure 4.5: Bridge Full-wave rectifier circuit iv. Filter Usually component 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 'motor boating' or 'humming'. Apart from short and open, filters regularly also occur 'leak'. This will cause the current unstable. Referring as shown below, 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 became small. Simplest filter is a single capacitor filter.

Input

Output

Figure 4.6: Filter circuit and output wave

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Another filter that is often used to avoid interference and noise is T or Ď&#x20AC; filters as shown below. Both have the same function, just only design that distinguishes it. 1/2 L

1/2 L

1/2 C

T filter

1/2 C

Ď&#x20AC; filter Figure 4.7: T filter and Ď&#x20AC; filter

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 is 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: a. Single diode regulator ( zener diode ) b. Transistor regulator c. Integrated circuit (IC) regulator Single diode regulator

+16V

Unstable input

+12V

Dz=12V

Figure 4.8: Single diode regulator circuit With features such zener diode, the output voltage at the same level, i.e + 12V, even if the input voltage varies. Simple circuit in figure as shown above 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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Transistor regulator One of the simplest implementations of this concept is to use a single pass transistor in the form of an emitter follower configuration, and a single Zener diode drive by a resistor from the unregulated supply. This provides a simple form of feedback system to ensure the zener voltage is maintained at the output, albeit with a voltage reduction equal to the base emitter junction voltage - 0.6 volts for a silicon transistor It is a simple matter to design a series pass voltage regulator circuit like this. Knowing the maximum current required by the load, it is possible to calculate the maximum emitter current. This is achieved by dividing the load current, i.e. transistor emitter current by the Î&#x2019; or hfe of the transistor. Series pass transistor

Resistor drops Voltage to Required level

Load

Zener voltage output +0.6V

Figure 4.9: Simple series transistor The Zener diode will generally need a minimum of around 10mA for a small Zener to keep its regulated voltage. The resistor should then be calculated to provide the base drive current and the minimum Zener current from a knowledge of the unregulated voltage, Zener voltage and the current required. [(Unregulated voltage - Zener voltage) /current]. A small margin should be added to the current to ensure that there is sufficient room for margin when the load, and hence the transistor base is taking the full current. The power dissipation capacity for the Zener diode should be calculated for the case when the load current, and hence the base current is zero. In this case the Zener diode will need to take the full current passed by the series resistor. 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

The 78xx (sometimes L78xx, LM78xx, MC78xx...) is a family of self-contained fixed linear voltage regulator integrated circuits. The 78xx family is commonly used in electronic circuits 32

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requiring a regulated power supply due to their ease-of-use and low cost. For ICs within the family, the xx is replaced with two digits, indicating the output voltage (for example, the 7805 has a +5V output, while the 7812 produces +12V). The 78xx line are positive voltage regulators: they produce a voltage that is positive relative to a common ground. There is a related line of 79xx devices which are complementary negative voltage regulators (for example 7905 has a 5V output). 78xx and 79xx ICs can be used in combination to provide positive and negative supply voltages in the same circuit. 78xx ICs have three terminals and are commonly found in the TO220 form factor, although smaller surface-mount and larger TO3 packages are available. These devices support an input voltage anywhere from a few volts over the intended output voltage, up to a maximum of 35 to 40 volts depending on the make, and typically provide 1 or 1.5 amperes of current (though smaller or larger packages may have a lower or higher current rating).

Figure 4.10: Fixed regulator IC ii. Regulator integrated circuit Op-Amp As linear regulators, the LM317 and LM337 are used in DC to DC converter applications. Linear regulators inherently draw as much current as they supply. When this current is multiplied by the voltage difference between input and output, a significant amount of heat resulted. Therefore the use of an LM317 commonly also requires a heat sink. For large voltage differences, the energy lost as heat can ultimately be greater than that provided by the circuit. This is the trade-off for using linear regulators which are a simple way to provide a stable voltage with few additional components. The alternative is to use a switching voltage regulator which is usually more efficient but has a larger footprint and requires a larger number of associated components. In packages with a heat-dissipating mounting tab, such as TO-220, the tab is connected internally to the output pin which may make it necessary to electrically isolate the tab or the heat sink from other parts of the application circuit. Failure to do this may cause the circuit to short.

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Figure 4.11: Op-Amp regulator IC iii.

Regulator integrated circuit 723

Based regulator transistor series and Op Amp, a more cohesive is design that has been made of the voltage regulator 723 as shown in figure below. 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. 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 is not exceeding 400 mA. If you need more current, modifying the external series transistor should be added as shown below:

Figure 4.12: Internal circuit uA723 34

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Switching Mode Power Supply A switched-mode power supply (switching-mode power supply, switch-mode power supply, SMPS, or switcher) is an electronic power supply that incorporates a switching regulator to convert electrical power efficiently. Like other power supplies, an SMPS transfers power from a source, like mains power, to a load, such as a personal computer, while converting voltage and current characteristics. Unlike a linear power supply, the pass transistor of a switching-mode supply continually switches between low-dissipation, full-on and full-off states, and spends very little time in the high dissipation transitions, which minimizes wasted energy. Ideally, a switched-mode power supply dissipates no power. Voltage regulation is achieved by varying the ratio of on-to-off time. In contrast, a linear power supply regulates the output voltage by continually dissipating power in the pass transistor. This higher power conversion efficiency is an important advantage of a switched-mode power supply. Switched-mode power supplies may also be substantially smaller and lighter than a linear supply due to the smaller transformer size and weight. Switching regulators are used as replacements for linear regulators when higher efficiency, smaller size or lighter weight are required. They are, however, more complicated; their switching currents can cause electrical noise problems if not carefully suppressed, and simple designs may have a poor power factor.

Vin AC

Vout rectifier

Switching/voltage divider

Driver/ oscillator

Rectifier/simple filter

Comparator/ voltage starter

Voltage reference

Figure 4.13: Simple block diagram of a switching-mode power supplies

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Figure 4.14: simple schematic diagram

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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CHAPTER 5: AMPLIFIER 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 class of an amplifier is determined by their functions. i.

Class A

Input

Output

Amplifier

Figure 5.1: Class A amplifier Class A is an amplifier that produces a full output signal of  360. Example is an audio preamp ii. Class B

Output

Input Class B

Figure 5.2: Class B amplifier Produce ½ cycle of the signal i.e  180. Class B amplifier is usually comes with twin amplifier + ve and – ve to produce an output signal of 360. The advantage is that it can produce two times the gain of class A. Example is push-pull power amplifier and complementary symmetry.

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iii.

Class C Class C amplifier produce signals in pulse i.e the output is less than 180ď&#x201A;°. Example is IF and RF amplifiers.

< 180Âş Class C Amplifier Input

Output

Figure 5.3: Class C amplifier Audio amplifier block diagram As being mentioned earlier, every audio block has certain stages to produce the better controlled and audio qualities. Example : Selector

Mic Input

Pre Amp

switch

Tone Control

Driver /Pre Amp 2

Powe r Amp

Pre Amp Pre

Aux

Amp

Figure 5.4: Audio Amplifier Block diagram

Audio PrePhono - 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:

Vcc C1

R4 C3

to switch selector

R2 C2 Q1

Mic

Figure 5.5: Audio Pre-Amplifier

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

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Vcc 1.5M

5.6K 15nF

12K

0.22uF

12K

1nF

50k

0.22uF

TR3

50k

150pF

Input 100pF

180K

150

180pF

TREBLE

VOLUME

0.1uF

50k

2.2K

BASS

50k

BALANCE

Figure 5.6: Pre Amplifier for tone Control

Tone Control Treble - is to control high frequency audio or high pitch sound. This circuit is based on high pass filter. Bass

- is to control low frequency audio that is designed by low pass filter to control low pitch sound.

Volume - control the overall loudness of the audio i.e by reducing or increasing the 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. Power Amplifier There are few types of power amplifier circuits i.e:i. ii. iii. iv.

Single Ended Amplifier Push â&#x20AC;&#x201C; Pull amplifier Complementary Symmetry Integrated circuit

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Faults at the power amplifiers i. 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 integrated circuit will feed a bias signal to drive the transistor to work in Class A.

T351

C353

C352

C351 SPEAKER

Q301

R2 C707

Figure 5.7: Single-ended Amplifier The input signal is fed to base of Q301 and travel through T351 that acts as ac loads. The attenuated signal is further reduce to match the speaker impedance. C352 is used to adjust the treble sound. Probable faults C707- open C353-open/short Q301-short/open T351- open/short Resistor

- Echo sound and distorted output - no sound/disconnected supply - no sound/disconnected supply - no sound - any open resistor - no sound

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ii. Push Pull Amplifier

Vcc

R9 C28 C7 R2

R7 T7

T8 Q4

1nF

SPEAKER

VR3

Q3 82u

R13 C13

Rth

Figure 5.8: Push-Pull Amplifier This circuit 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 â&#x20AC;&#x201C; ve signal. Both bases of the Q4 and Q5 become the commons for R7, D3 and Rth. Vcc

R9 C28 C7 R7 T7

T8 Q4

SPEAKER

Rth

Figure 5.9: Push-Pull Amplifier +ve signal 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 each biasing through diode D3.

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For -ve signal, B winding will allow the current to flow into base of Q5 transistor and A winding is cutoff to earth.

Vcc

R9 C28 C7 R7 T7

T8 Q4

SPEAKER

Rth

Figure 5.10: Push-pull Amplifier â&#x20AC;&#x201C;ve signal

The same applies to T8 whereby the +ve and -ve signal flows through primary winding a C and D respectively. Probable Faults Transistor Q4 or Q5 - short/open, amplitude distortion Resistor R8 & R6 burn - open- amplitude distortion T7 and T8 - short/open â&#x20AC;&#x201C; no sound Biasing resistor - cross-over distortion iii. 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 different types that compliment each other.

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iv. Coupling Transformer Circuit R616 TR25 TH

601

R620

R617

C621

C618 TR24 T605

R610

R611

R618 TR26

R613 TH

602 C619

R619

SPEAKER

R621

C620

R612

R614

C622

R622

Figure 5.11: 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 â&#x20AC;&#x201C;ve signal, B winding will operates. v. Direct Biasing

R231

+12V

R230

Q205

R226 R227

C240

C244

R213 C243

SPEAKER

R214

C239 Q204

Q206

Input C241

R228

R229

C242

Figure 5.12: Direct biasing circuit 44

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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. vi. Diodes biasing +12V R1 C1 TR1

Input D2 C2 TR2

Figure 5.13: Diode biasing 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. ii.

iii.

iv.

Output Transistor: Open/short – distorted output or not functioning 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. 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. Diode Biasing: For certain design, biasing diode is used to ensure the phase output is 180. If faulty diode, should be replaced by the bias of the transistor driver.

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

Figure 5.14: Amplitude distortion 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. Phase Distortion

Figure 5.15: Phase distortion 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.

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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. Frequency Distortion

Figure 5.16: Frequency distortion Frequency distortion describes the condition where different frequencies within the amplifierâ&#x20AC;&#x2122;s bandwidth are amplified by different amounts. The gain over the bandwidth is no longer flat. Under these 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. Cross-over Distortion

Figure 5.17: Cross-over distortion This fault is commonly occurs 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.

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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.

Figure 5.18: Phase distortion

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 48

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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 5.19: PA system block diagram Category PA Systems These systems can be categorized into two parts: i. High level of system ii. Low level system High Level System High level of public address systems is used for the area to be covered by the 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. PA system basic components i. ii. iii. iv. v. vi.

Input Devices Mixer Equalizer Crossovers Amplifiers Speakers

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.

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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.

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 those of the input device (microphone or DI box). Crossovers Crossovers are specialized filters that split an input signal into a few different output signals based on the specific frequency range that a speakerâ&#x20AC;&#x2122;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. Amplifiers Amplifiers convert the electrical signal used amongst the sound system components into a much more powerful one to drive the loudspeakers.

Figure 5.20: PA system schematic diagram

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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 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 6: DOMESTIC ELECTRONIC APPLIANCES

AM Radio

Figure 6.1: AM Radio block diagram Antenna / Aerial Accept signal wave from the transmitter and choose frequency is wanted. Frequency value want to be selected is 540 KHz until 1600 Hz Rf Amplifier Strengthen wave received and selected by antenna Local Oscillator Produce frequency a value sinus wave. Frequency value produce is higher 455KHz than the value accepted frequency by antenna. Mixer Mix frequency who come from amplifier radio frequency with frequency from local oscillator and produce frequency a value whose name Intermediate frequency (IF). Process made one new value from two stated frequency call action SUPERHETERODYNE. IF Amplifier Strengthen frequency among those accepted from mixer. Amplifier type in IF amplifier is tuned where transformer IF tuned to frequency 455Khz

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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 ac to voltage at that is required by device receiver 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. General Radio Operating 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. 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 signal to be amplified and further double by power amplifier and speaker. Front end receive comprise from the division RF's amplifier, mixer and local oscillator. User can tune up to frequency is wanted inside 540 KHz-1600 KHz circle On the part local oscillator mixer will generate frequency that more 455 KHz from frequency received. Mixer is used to get frequency of Intermediate Frequency (IF) to the value 455 KHz. Example: Frequency receiver signals =1 MHz Frequency LO

= 1 MHz + 455 KHz = 455 KHz

Frequency different

= LO freq - RF freq = 1.455 MHz â&#x20AC;&#x201C; 1 MHz =455Khz

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This blending gospel known as the principle heterodyning Gospel heterodyning 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 tuned circuit 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. 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. Antenna

RF Amp

Mixer

IF Amp

Limiter

Frequency Demodulator

AF Power Amplifier Speaker

Local Oscillator

AFC

Figure 6.2: FM radio block diagram The FM band covers 88MHz - 108MHz. There are signals from many radio transmitters in this band inducing signal voltages in the antenna. 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 FM band. The selected frequency is 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 54

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from the mixer is the intermediate frequency (IF). The IF is a fixed frequency of 10.7MHz. 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 limiter and demodulator. The limiter clips the top and bottom of the FM signal to remove any AM signal or static that was picked up. This prevents the FM discriminator from seeing those unwanted signals and inadvertently converting them to audio, scrambling the intended audio signal. Simply put, the limiter blocks interference and static making FM reception very clean. This circuit recovers the audio signal and discards the RF. carrier. Some of the audio is fed back to the oscillator as an AUTOMATIC FREQUENCY CONTROL (AFC) 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. 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. Advantages of frequency modulation There are many advantages to the use of frequency modulation. These have meant that it has been widely used for many years, and will remain in use for many years. ď&#x201A;ˇ

Resilient to noise One of the main advantages of frequency modulation that has been utilized by the broadcasting industry is the reduction in noise. As most noise is amplitude based, this can be removed by running the signal through a limiter so that only frequency variations appear. This is provided that the signal level is sufficiently high to allow the signal to be limited.

ď&#x201A;ˇ

Resilient to signal strength variations In the same way that amplitude noise can be removed, so too can any signal variations. This means that one of the advantages of frequency modulation is that it does not suffer audio amplitude variations as the signal level varies, and it makes FM ideal for use in mobile applications where signal levels constantly vary. This is provided that the signal level is sufficiently high to allow the signal to be limited.

ď&#x201A;ˇ

Does not require linear amplifiers in the transmitter As only frequency changes are required to be carried, any amplifiers in the transmitter do not need to be linear.

DEE3052 ELECTRONIC EQUIPMENT REPAIR 

Enables greater efficiency than many other modes The use of non-linear amplifiers, e.g. class C, etc means that transmitter efficiency levels will be higher - linear amplifiers are inherently inefficient.

Disadvantages of frequency modulation There are a number of dis-advantages to the use of frequency modulation. Some are can be overcome quite easily, but others may mean that another modulation format is more suitable. 

Requires more complicated demodulator One of the minor dis-advantages of frequency modulation is that the demodulator is a little more complicated, and hence slightly more expensive than the very simple diode detectors used for AM. Also requiring a tuned circuit adds cost. However this is only an issue for the very low cost broadcast receiver market.



Some other modes have higher data spectral efficiency Some phase modulation and quadrature amplitude modulation formats have a higher spectral efficiency for data transmission that frequency shift keying, a form of frequency modulation. As a result, most data transmission system use PSK and QAM.



Sidebands extend to infinity either side The sidebands for an FM transmission theoretically extend out to infinity. To limit the bandwidth of the transmission, filters are used, and these introduce some distortion of the signal.

TV System The word television means “to see at a distance”. In our practical TV broadcasting system, the visual information you see in the scene is converted to an electric signal which is transmitted to the receiver. The electrical variations that correspond to changes in light values form the video signal. At the receiver, the video signal is used to reassemble the image on the screen of the picture tube, in monochrome television, the picture is reproduced in black and white and shades of grey. In colour television, all the natural colours are added as combinations of red, green, and blue in the main parts of the picture.

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CRT TV systems

Figure 6.3: Basic CRT TV block diagram Antenna A television antenna, or TV aerial, is an antenna specifically designed for the reception of overthe-air broadcast television signals, which are transmitted at frequencies from about 41 to 250 MHz in the VHF band, and 470 to 960 MHz in the UHF band in different countries. Television antennas are manufactured in two different types: "indoor" antennas, to be located on top of or next to the television set, and "outdoor" antennas, mounted on a mast on top of the owner's house. The most common types of antennas used are the dipole ("rabbit ears") and loop antennas, and for outdoor antennas the yagi and log periodic. 57

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RF Tuner A television tuner converts a radio frequency analog television or digital television transmission into audio and video signals which can be further processed to produce sound and a picture. Different tuners are used for different television standards such as PAL, NTSC, ATSC, SECAM and others. Analog tuners can tune only analog signals. An ATSC tuner is a digital tuner that tunes digital signals only. Some digital tuners provide an analog bypass. VHF/UHF TV tuners are rarely found as a separate component, but are incorporated into television sets. The IF frequencies for sound and picture are 33.4Mhz and 38.9Mhz. Demodulator/Video Detector They work by detecting emissions from the part of your tally that converts the incoming signal (which is at a high frequency, broadcast able through the air to your aerial) into an intermediate signal (at a lower frequency) that the tally can then convert into pictures and sound. Because the different TV channels are broadcast at different frequencies, the detector can determine which channel is being "down-converted" to pictures and sound. Video Amplifier A wide-Band tube or semiconductor amplifier used in television, radar, oscillographs, and other equipment to amplify video signals before passing them on to a cathode-ray tube. To preserve the shape of the video signals, a video amplifier must amplify them uniformly (with no more than 13 decibelsâ&#x20AC;&#x2122; variation) over a wide frequency pass band (from 10-30 Hz up to 4-6MHz) without appreciable phase distortion. Circuits of one- and two-stage video amplifiers, with a low-value resistor and various combinations of inductance coils, capacitors, and resistors in the load circuit of the gain stage, are the most commonly used. These combinations are selected in such a way as to increase the gain in the low and high frequency regions, thus producing uniform gain and reducing phase distortions over a very wide frequency band. The circuits and amplitude frequency response of a one stage tube video amplifier. Sync Separator Synchronizing pulses added to the video signal at the end of every scan line and video frame ensure that the sweep oscillators in the receiver remain locked in step with the transmitted signal, so that the image can be reconstructed on the receiver screen. A sync separator circuit detects the sync voltage levels and sorts the pulses into horizontal and vertical sync. Horizontal synchronization The horizontal synchronization pulse (horizontal sync HSYNC), separates the scan lines. The horizontal sync signal is a single short pulse which indicates the start of every line. The rest of the scan line follows, with the signal ranging from 0.3 V (black) to 1 V (white), until the next horizontal or vertical synchronization pulse. The format of the horizontal sync pulse varies. In the 525-line NTSC system it is a 4.85 Âľs-long pulse at 0 V. In the 625-line PAL system the pulse is 58

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4.7 Âľs synchronization pulse at 0 V . This is lower than the amplitude of any video signal (blacker than black) so it can be detected by the level-sensitive "sync stripper" circuit of the receiver. The repetition rate of horizontal blanking pulses, therefore ,is the line-scanning frequency of 15,625 Hz. Vertical synchronization Vertical synchronization (Also vertical sync or VSYNC) separates the video fields. In PAL and NTSC, the vertical sync pulse occurs within the vertical blanking interval. The vertical sync pulses are made by prolonging the length of HSYNC pulses through almost the entire length of the scan line. The vertical sync signal is a series of much longer pulses, indicating the start of a new field. The sync pulses occupy the whole of line interval of a number of lines at the beginning and end of a scan; no picture information is transmitted during vertical retrace. The pulse sequence is designed to allow horizontal sync to continue during vertical retrace; it also indicates whether each field represents even or odd lines in interlaced systems (depending on whether it begins at the start of a horizontal line, or mid-way through). The vertical blanking pulses blank out the scanning lines produced when the electron beam retraces vertically from bottom to top in each field. So the frequency of vertical blanking pulses is 50 Hz Chroma/ Colour mixing All colours can be created from a combination of the three primary colours of red, green and blue. The secondary (complementary) colours of cyan, magenta and yellow are created from a combination of two primaries, and white light is perceived from the combination of all three. The human eye perceives colour using three types of receptor whose sensitivities peak in the red, green and blue regions of the spectrum. Other colours are recognized by stimulating more than one type of receptor to a varying degree. Because of this, all colours can be perceived by different combinations of the three primary colours.

Figure 6.4: Colour mixing

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Primary Colour

Complementary Colour

Red

Cyan = blue + green

Green

Magenta = red + blue

Blue

Yellow = red + green

** White = Red + Green + Blue Table 6.1: Colour mixing Sound Systems The 41.25 MHz sound IF carrier is separated from the video in the last IF stage. The signal is then demodulated and converted to 4.5 MHz and amplified in an IF stage. The FM demodulator recovers the audio component, which is further amplified to drive the speaker.

Beam Scanning and Raster Development The television receiver actually reproduces a series of dots. These dots travel at such high speed that the viewer sees the total overall sequence as a picture on the screen. The degree of speed and the intensity of the dots vary. The electron gun produces a stream of electrons which magnetically scan left to right and from top to bottom. Specific phosphors on the screen luminesce when struck by electrons. The scanning system used in television is the interlace system which starts at the top left, scanning the odd lines from left to right and completing 312.5 lines. The interlaced scanning is illustrated in Figure 78. The scanning return from the bottom of the screen back to the top center and completes the scanning of the even lines. Each odd or even set of scanning lines represents a field, and both an odd set and an even set represent a frame. Therefore, there are a total of 625 lines per frame, and the frequency is 30 frames per second. Each time the scanning beam moves from the left side to the right, it must quickly return. This is called horizontal retrace (flyback). When the scanning beam gets to the bottom of the screen, it also must quickly return to the top of the screen. This is called vertical retrace. During retrace, the picture is black. Only during the trace scanning period is a picture visible. The vertical oscillator running at 50 Hz deflects the scanning beam upward. The horizontal oscillator runs at 15 625 Hz and deflects the scanning beam from left to right across the screen.

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Figure 6.5: Beam scanning Composite Video Signal Waveform The three parts of the composite video signal, illustrated in Figure below, are: (1) the camera signal corresponding to the variations of light in the scene; (2) the synchronizing pulses, or sync, to synchronize the scanning; and (3) the blanking pulses to make the retraces invisible.

Figure 6.6: Composite video signal

(a) Camera signal for one horizontal line

(b) H blanking pulse added to camera

signal

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The different types of tv display

Figure 6.7: Type of television display 62

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Figure 6.8: Backlight LED TV block diagram

Television User controls 1. 2. 3. 4. 5. 6. 7. 8. 9.

Power switch: to turn set on and off. Usually combined with volume control Volume control: a control on audio equipment for adjusting the sound level Tone control: a control for adjusting low frequency (bass) and high frequency (treble) VHF channel selector (tuner): to select the VHF channel desired. UHF channel selector: to select the UHF channel desired. Fine tuning: manual controls to compensate for oscillator drift. Contrast control: varies video signal amplitude to control picture contrast. Brightness control: varies bias to control pictures brightness. Sharpness control: varies the voltage on the focus grid to obtain proper beam focus.

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TV Troubleshooting 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. 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. No Picture with Normal Sound (CRT TV) 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. Sound Normal but No Raster (CRT TV) If the receiver lacks a raster, then the fault could exist in the high-voltage 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

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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. 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. No Picture with Normal Sound (LED TV) Maybe the backlight LED problems. Please check the supply to backlight LED Dead Set Like the radio, if a TV set is dead, please 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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CHAPTER 7: ELECTRONIC LABORATORY EQUIPMENTS AUDIO SIGNAL GENERATOR Standard sources of ac energy, both audio frequency (AF) and radio frequency (RF), are often used in the maintenance of electronic equipment. These sources, called AUDIO GENERATORS, are used to test and align all types of transmitters and receivers. They are also used to troubleshoot various electronic devices and to measure frequency. Standard sources of ac energy, both audio frequency (AF) and radio frequency (RF), are often used in the maintenance of electronic equipment.

Figure 7.1: audio generator The function of a audio generator is to produce alternating current (ac) of the desired frequencies and amplitudes with the necessary modulation for testing or measuring circuits. It is important that the amplitude of the signal generated by the signal generator be correct. In many signal generators, output meters are included in the equipment to adjust and maintain the output at standard levels over wide ranges of frequencies. When using the signal generator, you connect the output test signal into the circuit being tested. You can then trace the progress of the test signal through the equipment by using electronic voltmeters or oscilloscopes. In many signal generators, calibrated networks of resistors, called ATTENUATORS, are provided. You use attenuators in signal generators to regulate the voltage of the output signal. Only accurately calibrated attenuators can be used because the signal strength of the generators must be regulated to avoid overloading the circuit receiving the signal.

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AUDIO SIGNAL GENERATORS produce stable af signals used for testing audio equipment. VIDEO SIGNAL GENERATORS produce signals that include the audio range and extend into the rf range. These signal generators are used to test video amplifiers and other wideband circuits. In both audio and video signal generators, major components include a POWER SUPPLY, an OSCILLATOR , one or more AMPLIFIERS, and an OUTPUT CONTROL. OUTPUT METER OUTPUT OSCILLATOR SECTION

OUTPUT CONTROL AMPLIFIER

(ATTENUATOR)

POWER SUPPLY

Figure 7.2: audio generator block diagram In the audio that produce a beat-frequency, the output frequency is produced by mixing the signals of two separate rf oscillators. One is fixed in frequency, and the other is variable. The difference between the frequencies of the two oscillators is equal to the desired audio frequency. Audio signal generators often include resistance-capacitance (rc) oscillators in which the af is directly produced. In these audio generators, a resistance-capacitance circuit is the frequency-determining part of the oscillator. The frequency varies when either the resistance or the capacitance is changed in value. Most function generators allow the user to choose the shape of the output from a small number of options. Square wave - The signal goes directly from high to low voltage. Sine wave - The signal curves like a sinusoid from high to low voltage. The amplitude control on a function generator varies the voltage difference between the high and low voltage of the output signal. The direct current (DC) offset control on a function generator varies the average voltage of a signal relative to the ground. The frequency control of a function generator controls the rate at which output signal oscillates. On some function generators, the frequency control is a combination of different controls. One set of controls chooses the broad frequency range (order of magnitude) and the other selects the precise 67

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frequency. This allows the function generator to handle the enormous variation in frequency scale needed for signals. The duty cycle of a signal refers to the ratio of high voltage to low voltage time in a square wave signal.

Figure 7.3: MODEL GAG-809/810 GW INSTEK audio generator block diagram The sine-wave signal generated by Wien bridge oscillator circuit is fed through the WAVE FORM selector switch set at the sine-wave position to the AMPLITUDE control, by means of which it is adjusted to any desired voltage. If the waveform switch is in square-wave, the sine-wave signal is shaped into the square wave and the voltage is also adjusted by the AMPLITUDE control. The signal voltage thus adjusted is applied to the output circuit, where its impedance is appropriately converted, and then delivered through an output attenuator to the output terminal. The attenuator provides selectable attenuations 0f 0dB through -50dB in 10dB steps at 600â&#x201E;Ś of output impedance. Wien Bridge Oscillator Circuit The Wien bridge oscillator circuit is composed of the resistance elements, capacity elements and amplifier circuit, which may be switch over 5 range by the FREQ RANGE switch, and the variable capacitor controlled by the FREQUENCY dial. The amplifier circuit for the oscillator circuit is composed of a compound differential amplifier and an output stage. The input circuit is a high input impedance circuit with FET while the amplifier stage is a wide band, high amplification type circuit with PNP transistors featuring high cut-off frequency. The output stage is SEPP circuit using complementary transistors. The output voltage is fed back with positive polarity through the resistance and capacity element to form an oscillating circuit, while it is also fed back with negative polarity through the rectifier and the filter circuit and the variable resistance with FET to stabilize the amplitude. These elements provide means to vary the oscillating frequency continuously over 10 times its frequency on one range, thus determining any desired frequency within the entire frequency range from 10Hz to 1 Mhz.

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Square Wave Shaping Circuit The square wave shaping circuit is a Schmit-trigger circuit in which the sine wave signal from the oscillator circuit is shaped into a square wave. It provides sufficient rising and falling characteristics. Output Circuit The output circuit is a feedback amplifier. It is composed of a differential amplifier, a driver circuit and a SEPP-OCL circuit employing complementary transistors. It converts the impedance of signal from the AMPLITUDE control and amplifies the signal and feeds it to the output attenuator at a low impedance over the range from DC to 1MHz. Output Attentuator The 6-position output attenuator selects attenuations of 0dB to -50dB in 10dB steps. At the 0dB position with the AMPLITUDE control turned fully clockwise, the output voltage ( sine wave at a 600â&#x201E;Ś load ) is more than 5V rms. Power Supply The power supply circuit is powered by AC 100/120/220/230V and delivers DC Âą24V sufficiently stabilized by large capacity smoothing capacitors and two voltage stabilizers. RF SIGNAL GENERATOR Radio frequency signal generators (RF signal generators) are a particularly useful item of test equipment widely used in RF microwave design and test applications. These microwave and RF signal generators come in a variety of forms and with a host of facilities and capabilities. In order to gain the most from any RF signal generator or microwave signal generator, it is necessary to have an understanding of its operation and the capabilities it possesses.

Figure 7.4: RF signal generator

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Types of RF signal generator It is possible to design radio frequency signal generators in a variety of ways. Also with developments that have been made in electronics circuitry over the years, different techniques have evolved. It can be said that there are two forms of signal generator that can be used: Free running RF signal generators These signal generators are rarely used these days as their frequency tends to drift. However they do have the advantage that the signal produced is very clean and does not have the level of noise (phase noise) either side of the main signal that is present on some other radio frequency signal generators. Some signal generators used a form of frequency locked loop to provide a means of adding some frequency stability while still retaining the very low levels of phase noise. Again, these are not common these days because the performance of RF signal generators using frequency synthesizer technology has considerably improved. Synthesized radio frequency signal generators Virtually all radio frequency signal generators used today employ frequency synthesizers. Using this technique enables frequencies to be entered directly from a keypad, or via remote control and it also enables the output signal to be determined very accurately. The accuracy being dependent upon either an internal reference oscillator that can have a very high degree of accuracy, or the signal can be locked to an external frequency reference which can be exceedingly accurate. There are two main techniques that are used within synthesized RF signal generators: ď&#x201A;ˇ

Phase locked loop synthesizer: Phase locked loop synthesizers are used within most RF signal generators as they enable signals to be generated over a wide range of frequencies with a relatively low level of spurious signals. Phase locked loop synthesizer technology is well developed and enables high performance RF signal generators to be produced using them.

ď&#x201A;ˇ

Direct Digital Synthesizer, DDS: Direct digital synthesis techniques may be used in RF signal generators. They enable very fine frequency increments to be achieved relatively easily. However the maximum limit of a DDS is normally much lower than the top frequencies required for the signal generator, so they are used in conjunction with phase locked loops to give the required frequency range.

RF signal generator operation In order to understand the operation of a generic microwave or RF signal generator it is useful to understand what is included in terms of a basic block diagram. 70

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Figure 7.5: Block diagram for a generic RF signal generator The diagram shows a very simplified block diagram for an RF / Microwave signal generator. From this, it can be seen that the generator has a few major blocks within it: Oscillator The most important block within the RF signal generator is the oscillator itself. This can be any form of oscillator, but today it would almost certainly be formed from a frequency synthesizer. This oscillator would take commands from the controller and be set to the required frequency. Amplifier The output from the oscillator will need amplifying. This will be achieved using a special amplifier module. This will amplify the signal, typically to a fixed level. It would have a loop around it to maintain the output level accurately at all frequencies and temperatures. Attenuator An attenuator is placed on the output of the signal generator. This serves to ensure an accurate source impedance is maintained as well as allowing the generator level to be adjusted very accurately. In particular the relative power levels, i.e. when changing from one level to another are very accurate and represent the accuracy of the attenuator. It is worth noting that the output impedance is less accurately defined for the highest signal levels where the attenuation is less. Control Advanced processors are used to ensure that the RF and microwave signal generator is easy to control and is also able to take remote control commands. The processor will control all aspects of the operation of the test equipment. 71

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RF signal generator functions Microwave and RF signal generators are able to offer a large variety of functions and facilities these days. These include some that are detailed below: Frequency range Naturally the frequency range of the RF signal generator is of paramount importance. It must be able to cover all the frequencies that are likely to need to be generated. For example when testing a receiver in an item of equipment, be it a mobile phone or any other radio receiver, it is necessary to be able to check not only the operating frequency, but other frequencies where the issues such as image rejection, etc. Output level The output range for an RF and microwave signal generator is normally controlled to a relatively high degree of accuracy. The output within the generator itself is maintained at a constant level and then passed through a high grade variable attenuator. These are normally switch to give the highest degree of accuracy. The range is normally limited at the top end by the final amplifier in the RF signal generator. Modulation Some RF or microwave signal generators have inbuilt oscillators that can apply modulation to the output signal. Others also have the ability to apply modulation from an external source. With modulation formats for applications such as mobile communications becoming more complicated, so the capabilities of RF signal generators have had to become more flexible, some allowing complex modulation formats such as QPSK, QAM and the like. Signal generators that support complex modulation are often referred to as vector signal generators. Sweep On some RF signal generators it is necessary to sweep the signal over a range. Some generators offer this capability. Control There are many options for controlling RF and microwave signal generators these days. While they tend to have traditional front panel controls, there are also many options for remote control. Most items of laboratory bench test equipment come with GPIB fitted as standard, but options such as RS-232, and Ethernet / LXI. Rack technologies where instrument cards are slotted into a rack with other items of test equipment are also popular. The first of these was VXI, but cheaper options such as PXI and PXI express are more widely used. Radio frequency signal generators are a form of electronic test equipment found in virtually every radio frequency design or test laboratory. These signal generators are used wherever an RF signal needs to be supplied to a circuit or unit that is being developed or tested. As such RF signal generators are essential items for RF development and testing. RF signal generators are an essential item of test equipment for any RF design or test laboratory. They enable RF signals to 72

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be generated that enable signals to be fed into RF circuits so that their operation can be viewed when operating under various signal conditions. There are a number of important specifications associated with any RF signal generator. While some are fairly common to all applications, it is necessary to ensure that all the requirements for the signal generator are captured and noted. Frequency range One of the obvious key specifications for any radio frequency signal generator is the frequency range that it covers. When choosing the band required for a signal generator it is necessary to consider all the testing that will be needed. The frequency coverage required for the signal generator may not be just that of the unit under test. For example, when testing radio receivers it is necessary to test their susceptibility to out of band signals at image and other frequencies. These may be will outside the operating frequency range of the unit under test, and the signal generator will need to accommodate these and any other requirements. Harmonics and spurious signals All signal generators produce some level of spurious signals. Harmonics are generally much higher as considerable effort is spent in reducing intermodulation and other non-harmonically related spurious signals. Signal generator power output Another important signal generator specification is its power output. For most RF signal generators, the power output specification is defined in dBm, i.e. dB relative to one milliwatt. POWER DBM

LEVEL

POWER LEVEL MILLIWATTS

100

200 Table 7.1: dBm/milliwatt conversion

Although different signal generators have different output levels, the most common maximum output level is +13 dBm, although whatever the exact maximum level is, it is normally in the range 10 to 100 milliwatt, i.e. 10 to 20 dBm. Power accuracy - relative and absolute For many test scenarios it is necessary for the output of the signal generator to be accurately known. This is because the response of the unit under test is most likely to vary according to the signal generator level. In some circumstances it is likely that the response may be very 73

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sensitive to the signal generator level. As a result the signal generator level accuracy specification is of great importance. There are two elements to the output level accuracy of the signal generator. This is as a result of the way that the output level is controlled. The output of a signal generator generally consists of an attenuator and this gives the ability to vary the output level. Prior to the output attenuator in the signal generator, there is an amplifier with a feedback loop which is used to maintain an accurate fixed level. The accuracy of the attenuator then provides the relative accuracy of the individual steps while the maintained level of the amplifier provides the absolute level accuracy. Phase noise One item that has to be noted on many signal generators these days is the level of phase noise that is produced. The importance arises because many signal generators are fall into the category of a synthesized signal generator. While a synthesized signal generator offers many advantages from exact frequency selection to stability, and high levels of programmability, the issue of phase noise can be a problem in some generators, and the phase noise spec needs to be carefully considered. When making general noise measurements of a system, the phase noise of a signal generator used may affect the measurements. Accordingly it is necessary to know what can be tolerated. The level of phase noise from a radio frequency signal generator will generally fall as the offset from the carrier increases. The actual levels may be given at several points in a specification, and sometimes a plot of the phase noise may be given. Phase noise levels are measured in terms of dBc / Hz. This is the level of noise in a 1 Hertz bandwidth relative to the level of the carrier. As noise is not on a single frequency but distributed over the frequency range, the wider the measurement bandwidth, the more noise is seen. Accordingly it is necessary to specify a bandwidth and 1 Hertz is taken as the standard. Accuracy - short and long term The accuracy of the signal generator is often important. With most RF signal generators using frequency synthesizers, this means that the frequency accuracy is determined by the frequency standard used within the generator. Frequency standards have their accuracy defined with a number of different specifications and these have to be combined in the correct manner to give the overall "accuracy". All accuracy measurements are specified in terms of parts per million (PPM). However there are elements including temperature stability, line voltage stability, ageing (i.e. the steady drift with time over many months of the crystal within the reference standard, etc. These need to be added statistically to gain the overall "accuracy" for the radio frequency signal generator. Modulation formats supported In order that many tests can be undertaken by the signal generator, it is necessary in many instances that the signal can be modulated. In this way real signals can be more fully simulated and the required tests undertaken. Most signal generators have the ability to modulate signals in a variety of ways, some providing greater levels of flexibility than others. As a result it is

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necessary to check the signal generator specifications to ensure that it has the required capabilities. Originally many signal generators had the capability to have amplitude modulation, AM and frequency modulation, FM applied. However with radio and wireless systems using far more advanced forms of modulation, many signal generators have very comprehensive modulation capabilities. Some of these may be provided by the use of additional options. Today a variety of modulation formats may be available in a signal generator. These may include: various forms of phase shift keying, PSK (including BPSK, QPSK, 8PSK, etc) as well as other more complicated modulation formats including quadrature amplitude modulation, QAM (including 16 and 64 point QAM) need to be used. Other modulation types including CDMA and OFDM may also be available. It is necessary to ensure that the radio frequency signal generator being considered is able to offer the required modulation formats. Test equipment calibration interval The calibration interval for any item of test equipment is important, and this is the case for any signal generator, whether it is new or a used signal generator. Test equipment calibration can add a significant amount to the cost of ownership, so it is necessary to consider the calibration interval. For many signal generators the interval will be a year, but if higher degrees of accuracy of any of the signal generator specifications are needed, then shorter time intervals may be needed.

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REFERENCES

1. David A. Bell, Electronic Devices and Circuits.

2. F. F. Mazda, Electronics Engineer's Reference Book (Sixth Edition).

3. Simon M. Sze (Author), Kwok K. Ng (Author), Physics of Semiconductor Devices.

4. www.radio.electronics.com ; May 2015

5. Lenk, John D (1999). Circuit Troubleshooting Handbook. New York: McGraw-Hill.

6. Grob, Bernard (2003). Basic Electronics. New York: Glencoe McGraw-Hill