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 Electrical Work
Class: Senior Secondary 3
Term: 2nd Term
Week: 4
Theme: Workshop Practice And Maintenance
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Identifysymptoms and faults commonto each stageof a colour TVset. Differentiatebetween staticand dynamiccolorconvergenceand operatethe colour.
This section provides a detailed explanation of the core concepts required for diagnosing and repairing colour TV receivers, focusing on common faults, their symptoms, and the principles of colour convergence. A. Understanding the Colour TV Block Diagram (Recap) A colour TV receiver is typically divided into several functional stages. Understanding these stages is fundamental to fault diagnosis.
1. Power Supply Unit (PSU): Supplies various DC voltages to all other stages.
2. RF/IF Section (Tuner & IF Amplifier): Selects and amplifies the desired broadcast signal, then converts it to a fixed Intermediate Frequency (IF).
3. Video Detector/Processor: Demodulates the IF signal to extract the composite video signal.
4. Audio Section: Separates the audio signal from the video, amplifies it, and drives the loudspeaker.
5. Luminance (Y)
Channel: Processes the brightness information (monochrome signal).
6. Chrominance (C)
Channel: Processes the colour information (hue and saturation).
7. Sync Separator: Extracts horizontal and vertical synchronizing pulses from the composite video signal.
8. Vertical Deflection Section: Generates a saw-tooth waveform to drive the vertical deflection coils, sweeping the electron beam vertically.
9. Horizontal Deflection Section: Generates a saw-tooth waveform and high voltage (EHT) for the Horizontal Deflection Coils and Picture Tube anode respectively, sweeping the electron beam horizontally.
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0. High Voltage (EHT)
Section: Generates extremely high voltage (typically 20-30 kV) required to accelerate the electron beams in the Cathode Ray Tube (CRT).
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1. CRT (Picture Tube): Displays the image by emitting electron beams that strike phosphors on the screen. B. Common Faults, Symptoms, and Probable Component Failures per Stage The ability to correlate symptoms with specific stages and components is paramount for effective troubleshooting. | TV Stage | Common Fault Symptoms | Probable Component Failures | | :----------------------- | :------------------------------------------------------ | :-------------------------------------------------------------------------------------------------------------------------------- | |
1. Power Supply Unit (PSU) | - No power (dead set) | - Blown fuse, faulty rectifier diodes, open-circuited filter capacitors (bulged/leaking), shorted switching transistor/IC, faulty power transformer. | | | - Dim picture, reduced raster, distorted sound | - Partially shorted rectifier diode, dried-up filter capacitors, leaky/open regulator transistor/IC. | | | - Intermittent power, flickering | - Loose connections, cold solder joints, intermittent components (e.g., thermal cracks in resistors, dry-joints). | |
2. RF/IF Section (Tuner & IF Amp) | - Snowy picture (poor signal) | - Faulty tuner (RF amplifier, local oscillator), weak IF amplifier, open antenna terminal, poor antenna connection. | | | - No picture, only snow (no channels) | - Faulty tuner, IF amplifier stage failure, open antenna. | | | - Weak picture, no sound (or vice versa) | - Faulty IF IC, misaligned IF stages, specific component failure in RF/IF paths. | |
3. Video Detector/Processor | - No picture, sound okay | - Faulty video detector diode, video amplifier transistor/IC failure, open video coupling capacitor. | | | - Negative picture, weak or washed-out picture | - Faulty video detector, incorrect bias on video amplifier, leaky coupling capacitor. | |
4. Audio Section | - No sound, picture okay | - Faulty audio amplifier IC, open speaker, faulty audio output transistor, open audio coupling capacitor, dry joints. | | | - Distorted sound, humming | - Faulty audio amplifier IC, leaky filter capacitors in audio power supply, speaker coil damage, misaligned sound IF. | | | - Intermittent sound | - Cold solder joints, intermittent volume control, loose connections, internal cracks in audio IC. | |
5. Luminance (Y) Channel | - No brightness (no picture, no raster) | - Faulty video output transistor/IC, open coupling capacitor to CRT cathode, faulty CRT, faulty ABL (Automatic Brightness Limiter) circuit. | | | - Poor contrast, washed-out picture (no brightness control) | - Faulty video amplifier, leaky capacitor in Y channel, ABL circuit fault. | |
6. Chrominance (C) Channel | - No colour, but picture okay (black and white) | - Faulty chrominance IC, crystal oscillator failure, open colour killer transistor. | | | - Wrong colours, colour streaks, excessive colour | - Faulty chrominance decoder IC, misaligned chroma circuits, leaky capacitors, colour burst circuit issues. | | | - Rainbow effect/colour noise | - Faulty colour burst amplifier, faulty 3.58MHz/4.43MHz crystal, poor signal. | pure white lines/dots.
3. Safety note: Always discharge the CRT anode before touching internal components, and be aware of high voltage.
Dynamic Convergence: Purpose: Ensures the electron beams converge accurately across the entire screen, especially towards the edges and corners. Static convergence only corrects the centre.
Method: Achieved using dynamically driven convergence coils and potentiometers, usually located on the deflection yoke or on a dedicated convergence board. These circuits generate varying magnetic fields that compensate for the curvature of the CRT screen and the geometric distortion inherent in scanning.
Procedure (General):
1. After static convergence, display a crosshatch or dot pattern.
2. Adjust various potentiometers (e.g., Red Horizontal, Blue Vertical, Green Pincushion, etc.) for each colour, typically found on the convergence board or accessed via a service menu, to align the R, G, B lines/dots towards the edges and corners of the screen.
3. The goal is to achieve pure white lines/dots from the centre to all edges and corners of the screen.
4. This often involves interacting with the TV's service mode (if a newer model) or manual adjustments on older sets. C.
4. Tools and Test Patterns for Convergence: - Test Patterns: Crosshatch pattern (most common), dot pattern, white raster. These are typically generated by a pattern generator or available through the TV's service menu. - Service Manual: Crucial for identifying specific adjustment points and procedures for a particular TV model. - Non-metallic alignment tools: For adjusting potentiometers (to avoid electrical shock and unintended magnetic interference). - Screwdrivers: For accessing internal components. D. Using the Cathode Ray Oscilloscope (CRO) in TV Diagnosis The CRO is an invaluable tool for TV repair, allowing technicians to visualize waveforms at various points in the circuit.
Signal Tracing: By connecting the CRO probe at different stages (e.g., input to IF amplifier, output of video detector, input to horizontal/vertical deflection circuits), a technician can observe if the expected waveform is present and has the correct amplitude and shape. Absence of a waveform, distorted waveform, or incorrect amplitude indicates a fault in the preceding stage.
Voltage Measurement (AC/DC): While a multimeter provides simple voltage readings, a CRO can show AC ripple on DC lines, which can indicate faulty filter capacitors in the power supply.
Frequency Measurement: Can be used to check oscillator frequencies (e.g., horizontal oscillator, colour burst crystal). Troubleshooting
Examples: No horizontal sweep: Check the horizontal oscillator waveform. If absent or distorted, the oscillator stage is faulty.
Rolling picture: Check vertical sync pulse at the output of the sync separator. If absent or weak, the sync separator is faulty.
No sound: Trace the audio signal from the video detector to the audio amplifier. Identify where the signal is lost or becomes distorted.
Example Scenario: "No Colour" Fault Symptom: The TV displays a clear black and white picture, but no colour appears, even on colour broadcasts.
Diagnosis Steps:
1. Verify input signal: Ensure the source (DVD player, cable box, antenna) is outputting a colour signal.
2. Check colour settings: Ensure the colour saturation control is not set to minimum.
3. Identify probable stage: This symptom points directly to the Chrominance (C) Channel. The Luminance (Y) channel and audio are working, as is deflection.
4. Component Check: Colour Killer Circuit: This circuit disables colour when a colour burst signal is not detected (e.g., during a B&W broadcast). If it's faulty, it might always be "killing" the colour. 3.58MHz (or 4.43MHz PAL)
Crystal Oscillator: This crystal provides the reference frequency for colour decoding. If faulty, the colour decoder cannot function.
Chrominance Decoder IC: The main chip responsible for processing colour information.
Associated components: Resistors, capacitors, transistors around the chroma IC.
5. Troubleshooting with CRO: Check for the presence of the 3.58MHz (or 4.43MHz) oscillation at the crystal pins. * Trace the chrominance signal into and out of the chrominance decoder IC. Poor contrast, washed-out picture (no brightness control) | - Faulty video amplifier, leaky capacitor in Y channel, ABL circuit fault. | |
6. Chrominance (C) Channel | - No colour, but picture okay (black and white) | - Faulty chrominance IC, crystal oscillator failure, open colour killer transistor. | | | - Wrong colours, colour streaks, excessive colour | - Faulty chrominance decoder IC, misaligned chroma circuits, leaky capacitors, colour burst circuit issues. | | | - Rainbow effect/colour noise | - Faulty colour burst amplifier, faulty 3.58MHz/4.43MHz crystal, poor signal. | |
7. Sync Separator | - Picture rolls vertically (vertical hold fault) | - Faulty vertical sync separator, leaky capacitors in vertical sync path, incorrect vertical oscillator frequency. | | | - Picture tears horizontally (horizontal hold fault) | - Faulty horizontal sync separator, leaky capacitors in horizontal sync path, incorrect horizontal oscillator frequency. | |
8. Vertical Deflection | - Horizontal line across the screen (no vertical deflection) | - Faulty vertical output IC/transistor, open vertical deflection coils, open coupling capacitor to deflection coils, power supply issues to vertical stage. | | | - Reduced height, fold-over at top/bottom | - Leaky/dried-up filter capacitor in vertical output stage, faulty vertical linearity components, faulty vertical output IC. | |
9. Horizontal Deflection | - Vertical line across the screen (no horizontal deflection) | - Faulty horizontal output transistor (HOT), open horizontal deflection coils, faulty flyback transformer, lack of drive from horizontal oscillator. | | | - Reduced width, fold-over on sides | - Faulty horizontal output stage, leaky capacitors in horizontal correction circuits, power supply issues to horizontal stage. | | | - No raster, high voltage arc/smell | - Faulty flyback transformer (FBT), shorted HOT, faulty EHT rectifier diode. | |
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0. High Voltage (EHT) / CRT | - No raster, sound okay (dark screen) | - Faulty flyback transformer, faulty HOT, open EHT rectifier, faulty focus/screen grid resistors, faulty CRT. | | | - Dim picture, poor focus | - Faulty focus/screen grid resistors, faulty CRT, incorrect EHT voltage, faulty focus/screen potentiometers on FBT. | | | - Arcing inside CRT or FBT, burning smell | - Faulty FBT, cracked CRT neck, internal short in CRT, dust/moisture on EHT components. | C. Colour Convergence and Alignment Colour CRTs use three electron guns (Red, Green, Blue) that must converge at a single point on the screen to strike their corresponding phosphor dots. Misalignment results in colour fringing or blurred images. C.
1. What is Colour Convergence? It is the process of making sure the three electron beams (Red, Green, Blue) from the CRT's guns meet precisely at each intended pixel on the screen. If they don't converge correctly, coloured outlines or halos appear around objects in the picture. C.
2. Why is Convergence Necessary? - Manufacturing tolerances of CRT and deflection yoke. - Earth's magnetic field interference. - Ageing of components. - Impact from moving the TV set. C.
3. Types of Colour Convergence: Static Convergence: Purpose: Ensures the three electron beams converge accurately at the centre of the screen. This is the primary adjustment and must be done first.
Method: Achieved using permanent magnets (purity magnets, convergence rings) mounted on the neck of the CR
T. These magnets create a magnetic field that slightly steers the electron beams.
Procedure (General):
1. Switch to a test pattern (typically a white crosshatch or dot pattern).
2. Adjust the purity magnets (usually a set of rings that can be rotated and moved) to bring the Red, Green, and Blue lines/dots together at the exact centre of the screen, creating pure white lines/dots.
3. Safety note: Always discharge the CRT anode before touching internal components, and be aware of high voltage.
Dynamic Convergence: Purpose: Ensures the electron beams converge accurately across the entire screen, especially towards the edges and corners. Static convergence only corrects the centre. * Method: Achieved using dynamically driven convergence coils and potentiometers, usually located on the deflection yoke or on a dedicated convergence board. These circuits generate varying magnetic fields that compensate for the curvature of the CRT screen and the geometric distortion inherent in scanning. This section outlines practical activities for both teachers and students to facilitate understanding and skill development.
Teacher Activities: Introduction and Recap (10 mins): Begin by reviewing the basic block diagram of a colour TV and the function of each major stage. Emphasize the importance of systematic fault diagnosis.
Demonstration of Fault Symptoms (20 mins): If available, use a faulty colour TV set (or visual aids like diagrams/videos) to demonstrate common fault symptoms (e.g., a horizontal line, no raster, snowy picture, no sound, no colour). Explain the visible symptoms and ask students to infer the likely affected stage.
Explanation of Fault Diagnosis (30 mins): Explain in detail the common faults associated with each TV stage, their symptoms, and the probable component failures (as detailed in Section 2B). Use a fault chart or flowchart approach to guide students through the logical steps of diagnosis. Emphasize safety precautions, especially when dealing with high voltages inside a CRT T
V. Explanation of Colour Convergence (25 mins): Explain the concept of static and dynamic colour convergence, detailing why it's needed and how each type corrects specific misalignments. Demonstrate (using diagrams, videos, or a TV with service mode/convergence rings if available) the procedure for adjusting static and dynamic convergence. Show examples of correct and incorrect convergence using test patterns (crosshatch/dots).
Demonstration of CRO Usage (15 mins): Briefly demonstrate how a CRO can be used for signal tracing and waveform analysis in a TV circuit (e.g., checking a horizontal sync pulse, an audio signal, or power supply ripple). If a TV cannot be opened, use a simulated circuit or pre-recorded CRO waveforms.
Guided Practice Facilitation (10 mins): Lead students through guided practice questions, encouraging discussion and peer learning. Assignment of Independent Practice (5 mins): Assign independent practice questions for homework or further in-class work.
Student Activities: Observation and Note-Taking: Actively observe teacher demonstrations and take detailed notes on fault symptoms, probable causes, and convergence procedures.
Active Participation in Discussions: Engage in class discussions, asking questions, and contributing to the identification of faults based on presented symptoms. Practical Identification (if equipment available): If a faulty TV set is available, students can visually inspect it and identify components, or trace signal paths on a simplified schematic. Practice adjusting convergence rings or potentiometers under strict supervision, if a working TV is available.
Analysis of Scenarios: Work in groups or individually to analyze given fault scenarios and propose diagnostic steps and probable component failures.
Simulated Troubleshooting: Use simplified block diagrams to "trace" signals and locate potential fault points.
Problem Solving: Attempt guided and independent practice questions, applying the learned concepts.
The skills and knowledge acquired from this topic have significant real-world implications, particularly within the Nigerian context.
Entrepreneurship and Job Creation: Many households and small businesses in Nigeria rely on functional electronics but may not afford brand new replacements. A skilled TV repair technician can establish a repair shop in a local market, residential area, or provide mobile repair services. This directly addresses local demand, creates self-employment opportunities, and contributes to the informal economy. For example, a technician in a Lagos market repairing old CRT TVs that are still widely used, earning a livelihood.
Cost Savings and Resourcefulness: Repairing a faulty colour TV is often significantly cheaper than purchasing a new one, especially for older CRT models still prevalent in many Nigerian homes. This skill enables individuals to save money for themselves and their families, or for their clients, by extending the life of existing appliances. It promotes resourcefulness and reduces the financial burden on consumers. Electronic Waste (E-waste) Reduction and Environmental Sustainability: Electronic waste is a growing environmental concern globally, including Nigeria. By diagnosing and repairing TVs instead of discarding them, individuals contribute to reducing the volume of e-waste in landfills. This practice aligns with principles of a circular economy and promotes environmental sustainability within local communities. A technician in Kano or Port Harcourt who can fix old TVs helps prevent hazardous materials from polluting the environment.