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 Electrical Work
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.
problematic stage.
3. Consult Service Manuals and Circuit Diagrams: Purpose: Service manuals provide schematics, block diagrams, component layouts, voltage charts, waveforms, and troubleshooting guides specific to the TV model.
Interpretation: Block Diagram: Shows the overall architecture and interconnections of different stages, useful for isolating faulty sections.
Schematic Diagram: Detailed circuit showing all components, their values, and interconnections within each stage. Used for component- level troubleshooting.
Voltage Charts: Specify expected DC voltages at key test points, vital for power supply and amplifier stage checks.
Waveforms: Illustrate expected AC signal shapes at various points, essential for oscillator and signal path verification using an oscilloscope.
Example (Nigerian context): A technician in Enugu trying to fix an old "AVS" or "Grundig" B/W TV might find a service manual (digital or hardcopy) to identify the correct component values for replacement, e.g., identifying the part number for a faulty horizontal output transistor.
4. Isolate the Faulty Stage: Based on the symptoms and block diagram, determine which stage is most likely faulty.
Example: No picture, no sound, no power light -> Power Supply Unit (PSU) or primary circuit.
Example: Sound present, no picture -> Video amplifier, CRT, or EHT supply.
Example: Picture rolls horizontally -> Horizontal sync or oscillator circuit.
Example: No channels tuning -> Tuner or IF amplifier.
5. Component Level Troubleshooting (within the isolated stage): Voltage Measurements: Use a multimeter to check DC voltages at key points (e.g., power rails, transistor bias points, IC supply pins) against the service manual's voltage chart. Deviations indicate problems.
Resistance Measurements: With power off, check component resistances (resistors, coils) and continuity of tracks and fuses.
Capacitor Checks: Visually inspect for bulging/leaking. Use a capacitance meter or ESR meter for more accurate testing.
Diode/Transistor Checks: Use multimeter's diode test function to check junctions for shorts or open circuits. Signal Tracing/Injection (using oscilloscope/signal generator): Signal Tracing: Inject a known signal at the input of a stage and trace its path through the stage using an oscilloscope. If the signal disappears or is distorted, the fault is between the last good point and the first bad point.
Signal Injection: Inject a test signal (e.g., video signal, audio signal) at the output of a stage and move towards the input. If the TV responds correctly from a certain point onwards, the fault is before that point.
Component Substitution: For difficult-to-diagnose components, temporarily replace with a known good part (if available and safe).
6. Repair/Replacement: De-solder faulty components carefully. Clean the PCB area. Solder new, equivalent components using appropriate wattage soldering iron and lead-free solder (if specified). Ensure correct orientation (polarised capacitors, diodes, ICs).
Safety: Always discharge large capacitors (especially in PSU and EHT section) before touching components. Wear safety glasses.
7. Testing and Adjustment: After repair, test the TV. Check for stable operation, correct picture geometry, brightness, contrast, and sound quality. * Make any necessary adjustments (e.g., focus, brightness, vertical/horizontal size) using service controls or potentiometers.
8. Final Inspection: Reassemble the TV, ensuring all connections are secure and no tools are left inside.
C. Common Faults, Symptoms, and Clearing Methods by Stage: | Stage | Common Faults | Symptoms | Clearing Methods | | :---------------- | :----------------------------------------------- | :--------------------------------------------------------------------------- | :----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | Power Supply | Blown fuse, faulty rectifier diodes, bulging filter capacitors, faulty voltage regulator, dry joints. | No power, no picture, no sound; sometimes 'click' sound, LED blinks briefly. |
1. Check/replace fuse.
2. Test rectifier diodes (multimeter diode test).
3. Replace bulging/leaking capacitors.
4. Check DC voltages at regulator output.
5. Resolder dry joints. | | Tuner | Faulty RF amplifier, local oscillator, mixer, loose antenna connection. | No channels, snowy/noisy picture on all channels, weak signal. |
1. Check antenna connection/cable.
2. Verify supply voltages to tuner module.
3. Resolder dry joints within tuner.
4. If integrated, replace tuner module (often a sealed unit). | | IF Amplifier | Faulty transistors/ICs, open/shorted tuned circuits, power supply issues to IF stage. | Weak, snowy, or distorted picture; sometimes no picture but sound is present (if visually damaged or primary open: Replace FB
T. Step 5: Check Video Amplifier Output: Procedure: If EHT is present and CRT filament glows, but still no picture, the video signal path is suspect. Power ON. Use an oscilloscope to trace the composite video signal from the video detector output, through the video amplifier, up to the CRT cathode/grid input.
Expected: Presence of amplified video signal (complex waveform, typically 0.5-5Vp-p). If no/weak/distorted signal at video amp output: Faulty video amplifier stage. Check DC supply voltages to the video amplifier IC or transistors. Test/replace the video amplifier IC or discrete components (transistors, resistors, capacitors).
Remedy (based on the example): Let's assume the fault was a completely blank screen, good sound, glowing filament, but no EHT. The technician found the Horizontal Output Transistor (HOT) to be shorted between collector and emitter.
Repair: Replaced the faulty HOT with an equivalent new component (e.g., BU208D, D1555). Also inspected the surrounding components (driver transformer, damper diode, capacitors) for secondary damage. Resoldered any dry joints found in the horizontal section. * Test:** Powered ON. Observed a raster. Injected a test signal or tuned to a channel. Verified picture and sound. (e.g., video input, audio input) to test stages downstream.
Alignment: For RF/IF stages (less common for B/W TV repair today, but historically used).
4. Soldering Iron/Station: Function: Joins electronic components by melting solder.
Use in TV Repair: Desoldering faulty components and soldering in replacements. Proper technique (correct temperature, clean tip, minimal heat application) is critical to prevent damage.
5. Desoldering Tools (Solder Sucker, Desoldering Braid): Function: Removes solder to extract components.
Use in TV Repair: Safely removing components without damaging the PC
B. Worked
Example: Diagnosing a "No Picture, Good Sound" Fault Scenario: A customer brings in an old "Binatone" black and white T
V. The symptom is: "The TV produces clear sound, but the screen is completely blank. No raster, no picture." Analysis and Steps:
1. Initial Observation: Power ON: Yes, sound is heard. This immediately tells us the Power Supply Unit (PSU) and the audio section are likely functional.
No raster: This suggests a problem in the video signal path to the CRT, the CRT itself, the EHT (Extremely High Tension) supply, or the horizontal deflection circuit (which also generates EHT).
2. Isolate the Faulty Stage: Since sound is present, the tuner and IF amplifier stages are likely working, as both picture and sound IF signals pass through them initially. The problem is downstream of the IF stage, affecting only the picture.
Likely candidates: Video Detector, Video Amplifier, Horizontal Deflection (loss of EHT), CRT (no emission or short), or high voltage section of the Flyback Transformer (FBT).
3. Consult Service Manual (Hypothetical): Locate the block diagram. See that the video detector feeds the video amplifier, which then drives the CRT. The horizontal deflection generates EHT from the FBT. Locate the schematic for the video amplifier and horizontal deflection stages. Look for key test points for voltages and waveforms.
4. Component Level Troubleshooting: Step 1: Check CRT Filament: Procedure: Power OFF. Visually inspect the CRT neck from the back. The filament should glow faintly orange/red when the TV is powered ON. If not, use a multimeter on continuity/resistance range to check the filament pins (usually two larger pins on the CRT base) for continuity.
Expected: Continuity (low resistance, ~1-5 ohms).
If no glow/open circuit: Faulty CRT filament.
Remedy: Replace CRT (often not economical).
Step 2: Check EHT Voltage: Procedure: This requires extreme caution and a high voltage probe with an oscilloscope or a specialized EHT meter. (Teacher note: For students, this step might be simulated or demonstrated only due to high voltage risks). Place the probe tip near the anode cap of the CR
T. Expected: Typically 10-15kV DC for B/W TVs.
If no EHT: Problem in the Horizontal Output Transistor (HOT), Flyback Transformer (FBT), or their associated drive circuits. Proceed to check the HOT and FB
T. Step 3: Check Horizontal Output Transistor (HOT): Procedure: Power OFF. Locate the HOT (a large power transistor, usually on a heatsink, near the FBT). Use a multimeter diode test function to check the junctions between its collector, emitter, and base. Check for shorts or open circuits.
Expected: Diode drop readings similar to a standard transistor.
If shorted/open: Replace HO
T. Step 4: Check Flyback Transformer (FBT): Procedure: Visually inspect the FBT for cracks, burn marks, or signs of arcing. With power OFF, use a multimeter to check the primary winding continuity. While difficult to test internal shorts without specialized equipment, visual cues and HOT failure often point to FBT issues.
Expected: Low resistance continuity on primary winding.
If visually damaged or primary open: Replace FB
T. Step 5: Check Video Amplifier Output: Procedure: If EHT is present and CRT filament glows, but still no picture, the video signal path is suspect. Power ON. Use an oscilloscope to trace the composite video signal from the video detector output, through the video amplifier, up to the CRT cathode/grid input.
Expected: Presence of amplified video signal (complex waveform, typically 0.5-5Vp-p). If no/weak/distorted signal at video amp output: Faulty video amplifier stage. * Check DC supply voltages to the video amplifier Check DC voltages at regulator output.
5. Resolder dry joints. | | Tuner | Faulty RF amplifier, local oscillator, mixer, loose antenna connection. | No channels, snowy/noisy picture on all channels, weak signal. |
1. Check antenna connection/cable.
2. Verify supply voltages to tuner module.
3. Resolder dry joints within tuner.
4. If integrated, replace tuner module (often a sealed unit). | | IF Amplifier | Faulty transistors/ICs, open/shorted tuned circuits, power supply issues to IF stage. | Weak, snowy, or distorted picture; sometimes no picture but sound is present (if sound IF is separate). |
1. Check supply voltage to IF IC/transistors.
2. Check signal path with oscilloscope/signal injection.
3. Test/replace faulty transistors/ICs.
4. Check continuity of tuned coils. | | Video Amplifier | Faulty transistors/ICs, open/shorted components (resistors, capacitors). | No picture, dark/faint picture, low contrast, negative picture, picture with retrace lines. |
1. Check supply voltage to video amp.
2. Trace video signal from detector to CRT.
3. Test/replace video amplifier IC or discrete components (transistors, resistors).
4. Check DC restoration circuit components. | | CRT (Picture Tube) | Low emission, shorted elements, broken filament, internal arcing. | No raster, very dim picture, single bright dot, coloured patches (on colour CRTs, but can manifest as weird brightness on B/W). |
1. Check CRT filament continuity (multimeter).
2. Check EHT voltage.
3. Use a CRT restorer/rejuvenator (if available, for low emission).
4. If shorted, replace CRT (expensive, often not cost-effective for B/W). | | Horizontal Deflection | Faulty horizontal output transistor (HOT), flyback transformer (FBT), horizontal oscillator, damper diode. | Horizontal line only, no picture (no raster), picture shrunk horizontally, folded picture (vertical lines). |
1. Check HOT (often fails due to heat/overload).
2. Check FBT for cracks or arcing.
3. Check horizontal oscillator frequency/output.
4. Verify EHT voltage (generated by FBT).
5. Check damper diode.
6. Inspect for dry joints in the horizontal section. | | Vertical Deflection | Faulty vertical output IC, vertical oscillator, filter capacitors, deflection coils. | Horizontal line only, picture shrunk vertically, folded picture (horizontal lines). |
1. Check vertical output IC.
2. Check vertical oscillator frequency/output.
3. Replace electrolytic capacitors in vertical stage (common cause of vertical collapse).
4. Check continuity of vertical deflection coils. | | Sync Separator | Faulty IC/transistor, input signal issues. | Picture rolls horizontally, picture rolls vertically, picture tears diagonally. |
1. Check input signal to sync separator.
2. Check supply voltage.
3. Test/replace sync separator IC/transistor.
4. Check associated filter capacitors. |
D. Operation of Various Electronic Instruments:
1. Digital Multimeter (DMM): Function: Measures DC/AC voltage, DC/AC current, resistance, continuity, and often capacitance and diode test.
Use in TV Repair: Voltage: Check power supply rails (e.g., +12V, +24V, +110V), bias voltages on transistors/ICs.
Resistance: Check resistors, fuses (continuity), deflection coil continuity.
Continuity: Check for open circuits (broken tracks, wires) or short circuits.
Diode Test: Test rectifier diodes, damper diodes, individual transistor junctions.
2. Oscilloscope: Function: Displays waveforms (voltage vs. time). Essential for observing signal shapes, frequencies, and amplitudes.
Use in TV Repair: Signal Tracing: Observe video signal, horizontal/vertical sync pulses, oscillator waveforms, audio signals.
Troubleshooting Oscillators: Verify correct frequency and waveform of horizontal/vertical oscillators.
EHT Check (with High Voltage Probe): Measure the Extremely High Tension (EHT) voltage at the CRT anode (very high voltage, requires special probe and extreme caution).
3. Signal Generator: Function: Produces various test signals (sine wave, square wave, video test patterns).
Use in TV Repair: Signal Injection: Inject a known signal at various points in the TV (e.g., video input, audio input) to test stages downstream.
Alignment: For RF/IF stages (less common for B/W TV repair today, but historically used).
4. Soldering Iron/Station: Function: Joins electronic components by melting solder.
Use in TV Repair: Desoldering faulty components and soldering in replacements. Proper technique (correct temperature, clean tip, minimal heat application) is critical to prevent damage.
5. Desoldering Tools (Solder Sucker, Desoldering Braid): Function: Removes solder to extract components. * Use in TV Repair: Safely removing components without damaging the PCB. *Worked
Example: Diagnosing a "No
Entrepreneurship and Employment (Community Impact): Students with these diagnostic and repair skills can establish small businesses or work as technicians in electronics repair shops across Nigeria. Many households, especially in rural and semi-urban areas, still rely on older B/W TV sets. The ability to repair these appliances locally provides a vital service, creates income opportunities for the technician, and saves consumers money compared to buying new (and often more expensive) colour TVs. This skill directly addresses youth unemployment and fosters self-reliance. Waste Reduction and Sustainable Practices (Environmental Impact): Repairing rather than discarding faulty TV sets contributes significantly to reducing electronic waste (e-waste). E-waste is a growing environmental concern in Nigeria, with improper disposal leading to soil and water pollution. By extending the lifespan of appliances, students contribute to a more sustainable consumption model, reducing the burden on landfills and conserving resources that would otherwise be used to manufacture new products. Basic Electronic Problem-Solving and Critical Thinking: The systematic approach to TV repair teaches valuable critical thinking and problem-solving skills applicable to many other fields, not just electronics. Students learn to observe symptoms, formulate hypotheses, test them systematically, and identify root causes – a mindset valuable in various technical and even non-technical professions in Nigeria and beyond. This can be integrated with problem-solving in other subjects like Physics or even everyday life challenges.