Lesson Notes By Weeks and Term v3 - Senior Secondary 3
Diagnosis and Repair of Black and White (Monochrome) TV Receiver
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Diagnosis and Repair of Black and White (Monochrome) TV Receiver
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Subject: Radio Television And Repairs
Class: Senior Secondary 3
Term: 3rd Term
Week: 4
Theme: Workshop Practice And Maintenance
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State the procedures for TV repairs. In terpretservicemanuals and Circuitdiagrams. Identify the symptoms and clear faultscommon to tuner, IF amp,Video amp,CRT and synchronizingstages. Operatevariouselectronicinstruments to clear faults ina TV set.
A systematic approach is crucial for efficient and effective TV repair. This procedure minimizes guesswork and ensures thorough diagnosis.
Initial Inspection and Fault Analysis: Customer Interview: Gather information from the TV owner (what happened, when did it start, specific symptoms, any prior attempts at repair).
Visual Inspection: Check for obvious physical damage (burnt components, broken wires, loose connections, bulging capacitors, signs of overheating, dust accumulation).
Power-Up Test (with caution): Connect to power (preferably through a series incandescent lamp for safety, especially with unknown faults) and observe symptoms (no power, distorted picture/sound, specific lines, smell of burning).
Verify Basic Functions: Check power cord, fuse, power switch. Referencing Service Manuals and Circuit Diagrams: Locate the specific model's service manual. Understand the block diagram to isolate the faulty stage. Use the schematic diagram to trace signals, identify components, and check voltage points.
Fault Localization to a Specific Stage: Based on observed symptoms and knowledge of TV stages, narrow down the fault to a particular section (e.g., no sound but picture suggests audio section fault). Use test equipment (multimeter, oscilloscope) to confirm signal presence or absence at input/output points of suspected stages.
Component Level Diagnosis: Once a stage is identified, systematically test components within that stage (resistors, capacitors, diodes, transistors, ICs, coils). Check supply voltages to the stage. Look for open circuits, short circuits, or leaky components.
Repair and Replacement: Desolder faulty components using appropriate tools. Replace with exact equivalents or suitable substitutes. Pay attention to polarity (capacitors, diodes, transistors, ICs) and wattage (resistors). Ensure clean solder joints and proper connections.
Testing and Adjustment: After repair, perform initial power-up carefully. Verify the fault is cleared. Check overall TV performance (picture quality, sound, stability). Perform any necessary adjustments (e.g., focus, screen voltage, alignment) as specified in the service manual.
Final Quality Check and Customer Handover: Run the TV for an extended period to check for intermittent faults. Clean the TV casing. Explain the repair to the customer. Service manuals are invaluable tools. Students must understand their structure and content.
Contents of a Service Manual: General Information: Safety precautions, specifications, block diagram.
Alignment Procedures: Instructions for adjusting various circuits.
Troubleshooting Guide: A flowchart or table of common symptoms and possible causes.
Circuit Diagrams (Schematics): Detailed circuit drawings with component values, test points, and voltage readings.
Exploded Views and Parts List: For mechanical assembly and ordering replacement parts.
Waveform Diagrams: Expected signal shapes at specific test points.
Understanding Circuit Diagram Symbols: Resistor (R): Zigzag line (US) or rectangle (IEC). Value in Ohms (Ω), Kilo-ohms (kΩ), Mega-ohms (MΩ).
Capacitor (C): Two parallel lines (ceramic, electrolytic - one curved for negative). Value in Farads (F), microfarads (μF), nanofarads (nF), picofarads (pF).
Inductor (L)/Coil: Looping lines. Value in Henries (H), millihenries (mH), microhenries (μH).
Diode (D): Triangle with a bar. Allows current one way.
Transistor (Q): NPN/PNP symbols (circle with arrows). Used for amplification or switching.
Integrated Circuit (IC): Rectangle with pins. Often labeled with IC number.
Transformer (T): Two coils separated by lines (iron core). Used for voltage step-up/down, isolation.
Cathode Ray Tube (CRT): Specialized vacuum tube symbol.
Switches (S): Various types (SPST, DPDT).
Power Supply (PS): Rectifier, filter, regulator circuits.
Ground (GND): Three horizontal lines decreasing in length. Reference point (0V).
Connectors (J/P): Multi-pin connectors for inter-board connections.
Tracing Signal Paths: Start from the antenna input and follow the signal through the tuner, IF amplifier, video detector, video amplifier, to the CRT. For sound, branch off from the IF stage to the audio amplifier and speaker. For power, trace from the AC input through the power supply to all stages. For deflection, follow signals from sync separator to horizontal/vertical oscillators and output stages to deflection coils.
Voltage and Waveform Measurements: Schematics often indicate typical DC voltages at transistor terminals (base, collector, emitter) and IC pins. Deviations indicate faults. Waveform diagrams show expected AC signals at key test points (e.g., oscillator outputs, IF signals, sync pulses). An oscilloscope is used to verify these. This section details common faults within each major stage of a monochrome TV receiver.
A. Tuner Stage (RF Amplifier, Mixer, Local Oscillator)
Function: Selects the desired TV channel, amplifies the weak RF signal, mixes it with a local oscillator signal to produce a fixed Intermediate Frequency (IF).
Common Faults:
1. No picture, no sound (or very weak/snowy picture): Often indicates no output from the tuner.
Symptoms: Screen might be completely blank or show heavy snow (noise).
Diagnosis: Check supply voltage to the tuner module. Check for broken antenna lead. Use a signal generator to inject an IF signal into the IF stage to bypass the tuner and check if the IF/Video stages are working.
Repair: Clean tuner contacts, check for cold solder joints, replace faulty components (transistors/diodes within tuner) if serviceable, or replace the entire tuner module.
2. Poor reception on all channels (or some channels): Symptoms: Snowy picture, weak sound, ghosting.
Diagnosis: Check antenna connection, coaxial cable condition. Verify tuner supply voltage is stable.
Repair: Re-adjust fine-tuning control (if external). Replace tuner if internal components are faulty.
3. Channel drift: Symptoms: Picture and sound slowly fade or change channels.
Diagnosis: Faulty local oscillator component (e.g., capacitor, transistor, varactor diode).
Repair: Replace faulty components or tuner module.
B. Intermediate Frequency (IF)
Amplifier Stage Function: Amplifies the IF signal (which contains both picture and sound information) to a sufficient level for detection, provides selectivity. Includes Automatic Gain Control (AGC) circuitry.
Common Faults:
1. Weak, distorted picture and/or sound: Symptoms: Dark picture, poor contrast, weak sound, sometimes distorted.
Diagnosis: Check supply voltage to IF IC/transistors. Check AGC voltage. Check for faulty bypass capacitors or open resistors.
Repair: Replace faulty components (IF amplifier transistors/ICs, associated resistors, capacitors). Re-align IF transformers if necessary (requires specialized tools and expertise).
2. No picture, but good sound (or vice versa): Indicates a fault in the video or audio detector after the IF stage, or a specific IF stage component affecting only one signal.
Symptoms: Screen is blank or snowy, but sound is present; or picture is present, but no sound.
Diagnosis: For no picture but sound, focus on the video detector, video amplifier, or picture IF stage. For no sound but picture, focus on the audio detector, audio IF, or audio amplifier.
Repair: Replace specific components in the affected path.
3. Horizontal lines/streaks (or 'hum bars'): Symptoms: Horizontal bars that slowly roll up/down the screen.
Diagnosis: Poor filtering in the power supply section supplying the IF stage, or faulty decoupling capacitors within the IF stage.
Repair: Check and replace power supply filter capacitors or IF stage decoupling capacitors.
C. Video Amplifier Stage Function: Amplifies the detected video signal to drive the CRT's cathode and grid, controlling brightness and contrast.
Common Faults:
1. No picture (raster present, no video): Symptoms: Screen lights up (raster), but no image, just a blank grey/white screen.
Diagnosis: Check video detector output. Check supply voltage to video amplifier. Test video amplifier transistors/ICs for open/short circuits. Check coupling capacitors.
Repair: Replace faulty video amplifier transistors/IC, resistors, or capacitors.
2. Weak/Dim picture (poor contrast): Symptoms: Picture is faint, lacks brightness and contrast, even with controls maxed out.
Diagnosis: Partially faulty video amplifier, low gain transistor, leaky capacitor. Check contrast control circuit.
Repair: Replace video amplifier components, check associated resistors.
3. Negative picture (image looks like a photographic negative): Symptoms: Bright areas appear dark, dark areas appear bright.
Diagnosis: Usually caused by a shorted transistor in the video amplifier that inverts the signal, or a fault in the DC restorer circuit (if present).
Repair: Test and replace video amplifier transistors.
4. Picture too bright/blooming: Symptoms: Picture is excessively bright, details are lost, picture may expand or "bloom" at high brightness.
Diagnosis: Faulty brightness control circuit, over-amplification in video amp, incorrect CRT screen or G2 voltage.
Repair: Check brightness control pot, associated resistors/capacitors. Check CRT anode and G2 voltages.
D. CRT (Cathode Ray Tube) * Function: Converts the electrical video signal into a transistor in the video amplifier that inverts the signal, or a fault in the DC restorer circuit (if present).
Repair: Test and replace video amplifier transistors.
4. Picture too bright/blooming: Symptoms: Picture is excessively bright, details are lost, picture may expand or "bloom" at high brightness.
Diagnosis: Faulty brightness control circuit, over-amplification in video amp, incorrect CRT screen or G2 voltage.
Repair: Check brightness control pot, associated resistors/capacitors. Check CRT anode and G2 voltages.
D. CRT (Cathode Ray Tube)
Function: Converts the electrical video signal into a visible image using an electron beam that scans across a phosphorescent screen. Requires high voltage (EHT).
Common Faults:
1. No raster (screen completely dark, but sound present): Symptoms: Screen remains black even with brightness turned up.
Diagnosis: Check CRT filament for glow (it should glow orange/red). Check for presence of EHT (Extra High Tension/voltage) at the anode cap (use high voltage probe, extreme caution). Check screen (G2) and focus voltages. Check video amplifier output to CRT cathode.
Repair: Replace faulty CRT (expensive, often not practical for older B&W TVs). Check flyback transformer for EHT generation. Check filament supply.
2. Dim picture (poor emission): Symptoms: Picture is very dark and lacks brightness, even at max brightness.
Diagnosis: Weak CRT emission (cathode has lost its ability to emit electrons efficiently).
Repair: Use a CRT reactivator (if available) to attempt to rejuvenate the CRT (temporary fix), otherwise replace CRT.
3. Blurry/Out of focus picture: Symptoms: Image is not sharp, edges are soft or indistinct.
Diagnosis: Faulty focus control potentiometer or associated components (e.g., focus voltage divider resistors). Incorrect focus voltage supplied to the CR
T. Repair: Adjust focus control. Test and replace faulty focus components.
4. Raster present, but very fine white horizontal line (collapsed vertical scan): Symptoms: A very thin, bright horizontal line across the center of the screen.
Diagnosis: Fault in the vertical deflection stage.
Repair: Refer to Synchronizing Stage (vertical deflection).
5. Raster present, but very fine white vertical line (collapsed horizontal scan): Symptoms: A very thin, bright vertical line across the center of the screen.
Diagnosis: Fault in the horizontal deflection stage.
Repair: Refer to Synchronizing Stage (horizontal deflection).
E. Synchronizing Stage (Sync Separator, Horizontal/Vertical Oscillators, Deflection Amplifiers)
Function: Extracts horizontal and vertical sync pulses from the video signal to control the electron beam's sweeping motion (raster). Horizontal and vertical deflection circuits generate signals to drive the deflection coils.
Common Faults:
1. Picture rolls vertically: Symptoms: The entire picture scrolls up or down continuously.
Diagnosis: Fault in the vertical sync separator or vertical oscillator/deflection stage. The vertical hold control might be unable to stabilize the picture.
Repair: Check vertical sync components (capacitors, resistors) for value changes. Check vertical oscillator and output stage components (ICs, transistors, coupling capacitors, diodes, vertical output transformer). Adjust vertical hold.
2. Picture tears horizontally (or horizontal pull): Symptoms: The picture appears to be ripped or torn horizontally, often appearing as diagonal lines or unstable segments.
Diagnosis: Fault in the horizontal sync separator or horizontal oscillator/Automatic Frequency Control (AFC) circuit. The horizontal hold control might be ineffective.
Repair: Check horizontal sync components. Check AFC components (diodes, resistors, capacitors). Check horizontal oscillator components. Adjust horizontal hold.
3. No vertical deflection (single horizontal line across the screen): Symptoms: A very bright, thin horizontal line across the middle of the screen. Prolonged display can damage the CR
T. Diagnosis: Complete failure of the vertical output stage (e.g., open vertical output transistor/IC, open deflection coil, loss of supply voltage to vertical output).
Repair: Check supply voltage to vertical output IC/transistor. Test and replace vertical output IC/transistor. Check continuity of vertical deflection coils. Check coupling capacitors.
4. No horizontal deflection (single vertical line across the screen): Symptoms: A very bright, thin vertical line across the middle of the screen. Prolonged display can damage the CR
T. Diagnosis: Complete failure of the horizontal output stage (e.g., open horizontal output transistor, open flyback transformer winding, open horizontal deflection coil, loss of supply voltage to horizontal output). *
Community Electronics Repair Services: Students can establish small-scale TV repair businesses in their local communities, providing affordable repair options for old B&W TVs which are still common in many rural and semi-urban Nigerian households. This directly addresses local needs, reduces waste, and generates income.
Vocational Training and Employment: The skills acquired are directly applicable to careers as electronics technicians, either in dedicated repair shops, electronics manufacturing/assembly plants (if available), or as independent contractors. This aligns with Nigeria's drive for skill acquisition and youth empowerment. Technological Literacy and Problem Solving: Understanding the internal workings of a TV fosters a deeper understanding of electronics and complex systems. This problem-solving mindset is invaluable, whether students pursue further studies in engineering, become entrepreneurs, or simply need to fix household appliances. It encourages innovation and a "fix-it" mentality rather than immediate replacement, which is economical and environmentally sound in the Nigerian context. ---