Lesson Notes By Weeks and Term v3 - Senior Secondary 3
Diagnosis and Repair of Colour TV Receiver
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Diagnosis and Repair of Colour TV Receiver
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Subject: Radio Television And Repairs
Class: Senior Secondary 3
Term: 2nd Term
Week: 3
Theme: Workshop Practice And Maintenance
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Identifysymptoms and faults commonto each stageof a colour TVset. Differentiatebetween staticand dynamiccolorconvergenceand operatethe colour.
the screen. If the beams do not converge properly, the image will appear blurry, fringed with colour, or have colour misregistrations, especially noticeable around edges of objects and text.
Static Colour Convergence: Definition: Refers to the convergence of the electron beams at the centre of the screen.
Purpose: To achieve a pure white image at the centre and ensure all three beams meet at a single point.
Adjustment: Primarily achieved using magnetic rings (purity magnets, convergence magnets) mounted on the neck of the CRT, just behind the deflection yoke.
Procedure (for CRT TVs):
1. Switch on the TV and display a stable crosshatch or dot pattern (using a pattern generator or service mode).
2. Adjust the purity magnet rings (usually two tabs or rings) by rotating and sliding them to make the centre white dots or crosshatch lines appear as sharp, single white points without colour fringes.
3. The goal is to superimpose the Red, Green, and Blue dots/lines perfectly in the centre.
4. Use non-metallic tools to avoid magnetic interference.
Dynamic Colour Convergence: Definition: Refers to the convergence of the electron beams at the edges and corners of the screen. Static convergence only corrects the centre; dynamic convergence ensures proper alignment across the entire display area.
Purpose: To achieve accurate colour reproduction and sharpness across the full screen, compensating for the curvature of the CRT face.
Adjustment: Typically involves internal circuitry (convergence ICs, coils, and potentiometers) within the TV. The deflection yoke often has dedicated convergence coils. These adjustments are usually more complex and require specific service alignment procedures, often found in the service manual. Procedure (for CRT TVs, advanced):
1. Display a crosshatch pattern.
2. Access the service mode or dedicated convergence adjustment potentiometers (if available).
3. Adjust controls (e.g., pincushion, linearity, corner convergence) to align the R, G, B lines at the top, bottom, and corners of the screen. This often involves adjusting current waveforms to the convergence coils.
Tools for Convergence: Crosshatch/Dot Pattern Generator (essential for visual alignment). Non-metallic alignment tools (for purity/convergence magnet adjustments). Service Manual (for dynamic convergence potentiometers/software adjustments).
D. Operating Colour Controls User-accessible colour controls on a TV remote or menu include: Colour (Saturation): Adjusts the intensity or vividness of the colours.
Tint (Hue): Adjusts the overall colour balance, shifting towards green or magenta. This control is more prominent in NTSC systems (common in some regions) but less critical in PAL systems (common in Nigeria) as phase errors are automatically corrected. A. Fundamentals of Colour TV Operation (Brief Recap) A colour television receiver processes signals through several distinct stages to produce a full-colour moving image and accompanying sound. Understanding these stages is crucial for systematic fault diagnosis.
The main stages include:
1. RF/IF Stage (Tuner and Intermediate Frequency): Selects and amplifies the desired broadcast signal, then converts it to an intermediate frequency (IF) for easier processing.
2. Video Detector/Amplifier: Extracts the video information from the IF signal and amplifies it.
3. Luminance (Y)
Stage: Processes the brightness (black and white) information of the picture.
4. Chrominance (Colour)
Stage: Processes the colour (hue and saturation) information, separating it into Red, Green, and Blue (RGB) colour difference signals.
5. Sync Separator Stage: Extracts horizontal and vertical synchronisation pulses to keep the picture stable.
6. Vertical Sweep (Deflection)
Stage: Generates the vertical scanning signal for the electron beam, moving it from top to bottom of the screen.
7. Horizontal Sweep (Deflection)
Stage: Generates the horizontal scanning signal, moving the electron beam from left to right across the screen. It also generates the High Voltage (EHT).
8. High Voltage (EHT)
Stage: Produces the extremely high voltage required to accelerate the electron beams in the Cathode Ray Tube (CRT).
9. Audio Stage: Processes the sound signal extracted from the IF signal, amplifying it to drive the loudspeaker.
1
0. Power Supply Stage: Provides various DC voltages required for all stages of the TV. B. Symptoms and Faults Common to Each Stage | Stage | Common Symptoms (Faults) | Probable Component Failures | | :-------------------- | :-------------------------------------------------------- | :-------------------------------------------------------------------------------------------------------------------------------- | | Power Supply | No power (dead set), Intermittent power, Reduced picture size, Hum in audio/video, Overheating. | Rectifier diodes, Filter capacitors (electrolytic), Voltage regulator ICs/transistors, Power transformer, Fuses, Thermistors. | | RF/IF (Tuner & IF)| Snowy picture, Weak/fuzzy picture, No channels, Poor sound, Interference patterns. | Tuner unit, IF transformers, IF amplifier ICs/transistors, Resistors, Capacitors. | | Video Detector/Amp| Weak/faint picture, No picture (raster OK), Negative picture, Smearing/ghosting, Poor contrast. | Video detector diode, Video amplifier transistors/ICs, Coupling capacitors, Resistors. | | Luminance (Y) | No brightness, Poor contrast, Washed out picture, Incorrect black/white levels. | Delay line, Luminance amplifier ICs/transistors, Associated passive components. | | Chrominance (Colour)| No colour (black and white picture only), Weak/faint colour, Incorrect/shifted colours, Colour flickering, Colour blooming. | Colour killer circuit, Bandpass amplifier, Colour demodulators, Colour difference amplifiers, ICs, Crystals, Capacitors. | | Sync Separator | Picture rolling vertically, Picture tearing/shaking horizontally, Picture skewed or distorted. | Sync separator ICs/transistors, Integrating/differentiating networks (RC components), Capacitors, Diodes. | | Vertical Sweep | Single horizontal line across the screen, Reduced vertical height, Folded picture (top or bottom), Picture jittering vertically. | Vertical oscillator ICs/transistors, Vertical output ICs/transistors, Vertical deflection coils (yoke), Electrolytic capacitors in vertical circuit, Diodes, Resistors. | | Horizontal Sweep | Single vertical line across the screen, No High Voltage (EHT), Excessive width, Raster bloom (picture expands and blurs), Arcing. | Horizontal oscillator ICs/transistors, Horizontal Output Transistor (HOT), Flyback Transformer (FBT), Damper diode, Deflection coils (horizontal part), High voltage capacitors. | | High Voltage (EHT)| Dim picture, No raster, Arcing, Focus problems, Screen not lighting up. | Flyback Transformer (FBT), EHT rectifier diode, Focus/screen potentiometers, Picture tube (CRT) itself. | | Audio Stage | No sound, Distorted sound, Weak sound, Hum in sound, Intermittent sound. | Audio detector ICs/diodes, Audio amplifier ICs/transistors, Coupling capacitors, Volume control, Loudspeaker. | C. Colour Convergence Colour convergence is the process of ensuring that the three electron beams (Red, Green, Blue) fired from the electron guns within a CRT converge precisely at the same point on the screen. If the beams do not converge properly, the image will appear blurry, fringed with colour, or have colour misregistrations, especially noticeable around edges of objects and text.
Static Colour Convergence: Definition: Refers to the convergence of the electron beams at the centre of the screen.
Purpose: To achieve a pure white image at the centre and ensure all three beams meet at a single point.
Adjustment: Primarily achieved using magnetic rings (purity magnets, convergence magnets) mounted on the neck of the CRT, just behind the deflection Teacher Activities: Introduction (10 minutes): Begin by reviewing the basic block diagram of a colour TV receiver from previous lessons. Ask students to identify key stages. Introduce the concept of fault diagnosis as a systematic approach to identifying and rectifying malfunctions. Discuss the importance of TV repair skills in Nigeria for employment and entrepreneurship.
Explanation of Key Concepts (40 minutes): Present the detailed table of common symptoms, associated stages, and probable component failures. Use a projector or clearly written chart. For each major stage (e.g., Power Supply, Vertical Sweep, Horizontal Sweep, Chrominance), describe typical fault symptoms, explaining why they occur based on the stage's function. Use examples relevant to typical Nigerian homes (e.g., a TV that suddenly shows "only a straight line," or "no colour despite being on a colour channel"). Explain colour convergence, differentiating between static and dynamic. Demonstrate static convergence using an actual CRT TV (if available) or detailed diagrams/videos. Emphasise the use of a crosshatch pattern and non-metallic tools. Explain the role of user-operable colour controls (colour, tint). Practical Demonstration & Discussion (30 minutes): If a faulty CRT TV is available with a known, easily demonstrable fault (e.g., a horizontal line or no colour), power it on and ask students to identify the symptom. Guide a class discussion on which stage is likely faulty and what components might be involved. If a functional CRT TV is available, demonstrate the effect of misaligned static convergence and then correct it while students observe. Discuss the safety precautions when working with high voltage in TVs.
Activity: Fault Scenario Analysis (20 minutes): Divide students into small groups. Provide each group with 2-3 fault scenarios (written descriptions of TV problems).
Instruct groups to: Identify the likely faulty stage. List 2-3 probable faulty components. Suggest an initial diagnostic step. Have groups present their findings, fostering peer learning and critical thinking.
Student Activities: Actively participate in the review of TV block diagrams. Listen attentively during explanations, taking notes on symptoms, stages, and component failures. Observe teacher demonstrations of fault identification and static convergence. Engage in group discussions and fault scenario analysis, identifying faulty stages and components. Sketch simplified diagrams of a CRT and mark the position of convergence magnets. Practice identifying symptoms from given descriptions and connecting them to specific stages.
Question 1: A customer brings a colour TV with the complaint that there is no sound, but the picture is perfectly normal. a) Which stage of the TV receiver is most likely faulty? b) Name two probable components that might be defective in this stage. c) Suggest one initial diagnostic step the technician would take.
Solution 1: a)
Faulty Stage: Audio Stage. (
Commentary: The normal picture indicates all video and synchronisation stages are working, isolating the fault to the audio path). b)
Probable Components: Audio amplifier IC/transistor, Loudspeaker, Coupling capacitors in the audio path, Volume control potentiometer. c)
Initial Diagnostic Step: Check if the TV is muted or if the volume is set to zero (user error). Check the speaker connections for looseness or damage. Use an audio signal tracer or oscilloscope to check for audio signal presence at different points in the audio circuit, starting from the audio detector output.
Question 2: A technician is trying to repair a colour CRT TV that displays a single, bright horizontal line across the screen. a) Identify the faulty stage responsible for this symptom. b) Explain why this specific symptom appears when this stage fails. c) What is one common cause of failure for this stage in older CRT TVs?
Solution 2: a)
Faulty Stage: Vertical Sweep (Deflection) Stage. (
Commentary: A horizontal line indicates that the electron beam is sweeping horizontally but not moving vertically. This points directly to the vertical deflection system). b)
Explanation: The vertical sweep stage is responsible for moving the electron beam from the top to the bottom of the screen. If this stage fails, the vertical deflection coils in the yoke do not receive the necessary vertical deflection current. Consequently, the electron beam only sweeps horizontally along a single line in the middle of the screen, creating the bright horizontal line. c)
Common Cause of Failure: Failure of the Vertical Output IC/transistor or dried-up/shorted electrolytic capacitors in the vertical deflection circuit.
Question 3: A newly opened electronics repair shop in Lagos receives a colour CRT TV where images appear blurry with noticeable red and blue fringes around objects, especially text. The centre of the screen appears slightly better than the edges. a) What specific problem is the TV experiencing? b) Differentiate between the two main types of adjustments required for this problem, mentioning where each is typically performed on the screen. c) Name two specific tools or patterns needed to correctly adjust this problem.
Solution 3: a)
Problem: Colour Convergence Error (lack of proper convergence). (
Commentary: Colour fringes indicate that the Red, Green, and Blue electron beams are not landing at the same point on the screen). b)
Differentiation: Static Convergence: Adjusted at the centre of the screen. It ensures the R, G, B beams meet precisely at the central point, crucial for a sharp, pure white image there.
Dynamic Convergence: Adjusted at the edges and corners of the screen. It corrects convergence across the entire display area, compensating for the screen's curvature and ensuring sharpness away from the centre. c)
Tools/Patterns: Crosshatch pattern generator (or Dot pattern generator), Non-metallic alignment tools (for purity/convergence magnet adjustment).
Entrepreneurship and Employment: The skills learned in diagnosing and repairing colour TV receivers are directly applicable to starting an independent electronics repair business in local markets or working as a technician for electronic retail stores or service centres across Nigeria. This contributes to reducing youth unemployment and promoting self-reliance.
Cost Savings and Resource Management: By being able to diagnose and repair their own or family members' TV sets, students can save significant costs that would otherwise be spent on purchasing new TVs or hiring external technicians. This fosters a culture of repair and maintenance, promoting sustainable resource management within the community by extending the lifespan of electronic appliances.
Community Service and Skill Transfer: Trained technicians can provide valuable repair services to their local communities, especially in rural areas where access to professional repair shops might be limited. They can also transfer these skills to apprentices, thereby contributing to local capacity building and economic development.