Lesson Notes By Weeks and Term v3 - Senior Secondary 2
Electronic Repairs
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Electronic Repairs
Download the Lessonotes Mobile Nigeria 2025 app for faster lesson access on Android and iPhone.
Subject: Radio Television and Electrical Work
Class: Senior Secondary 2
Term: 1st Term
Week: 5
Theme: Workshop Practice And Maintenance
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This topic introduces students to the fundamental principles and practical techniques involved in diagnosing and repairing faults in common electronic devices. In Nigeria, the ability to repair electronic gadgets is a valuable skill, fostering self-reliance, reducing electronic waste, and creating numerous entrepreneurial opportunities in communities where new device acquisition can be costly. Understanding electronic repairs is crucial for students aspiring to careers as technicians, engineers, or entrepreneurs in the electronics sector, contributing significantly to the national economy and local service provision.
Specific Learning Objectives:
This section covers the core knowledge required for understanding and performing electronic repairs. A. Introduction to Electronic Repair and Troubleshooting Electronic repair involves identifying, isolating, and rectifying faults in electronic circuits and devices. Troubleshooting is the systematic process of finding the cause of a fault. B. Common Electronic Components and Their Functions A foundational understanding of components is critical for effective repair.
Resistors: Limit current, divide voltage. Identified by colour codes (e.g., brown-black-red-gold = 1kΩ, 5% tolerance).
Capacitors: Store electrical energy, filter AC signals, block DC. Types include electrolytic (polarized, marked with '+' and '-'), ceramic, and film. Values in microfarads (μF), nanofarads (nF), picofarads (pF).
Diodes: Allow current flow in one direction (rectification). Identified by a band marking the cathode.
Examples: Rectifier diodes (e.g., 1N4007), Zener diodes (for voltage regulation).
Transistors: Amplify current or act as electronic switches.
Types: Bipolar Junction Transistors (BJTs) and Field-Effect Transistors (FETs). Identified by part numbers (e.g., BC547, 2N3904).
Integrated Circuits (ICs): Miniature electronic circuits on a single chip, performing complex functions. Identified by part numbers (e.g., 555 timer, LM741 op-amp).
Inductors: Store energy in a magnetic field, filter signals. Found in power supplies and RF circuits.
Switches: Control the flow of current (ON/OFF).
Fuses: Overcurrent protection devices. A thin wire melts and breaks the circuit if current exceeds a safe limit.
C. Essential Electronic Repair Tools and Their Usage Multimeter (Digital and Analog): Voltage Measurement (DC/AC): Connect probes in parallel across the component/source. Select appropriate range.
Current Measurement (DC/AC): Connect probes in series with the circuit. Select appropriate range.
Caution: Incorrect connection can damage the multimeter.
Resistance Measurement: Measure resistance of components when power is off and component is isolated from the circuit.
Continuity Test: Used to check for unbroken paths (short circuits or open circuits). A beep indicates continuity.
Diode Test: Checks if a diode is functional and determines forward voltage drop.
Soldering Iron: Used to melt solder for making electrical connections.
Temperature Control: Important for different types of solder and components. Typically 300-400°
C. Tip Cleaning: Essential for good heat transfer; use a damp sponge or brass wool.
Solder Wire: An alloy (e.g., 60/40 tin/lead or lead-free) with a low melting point, used to create electrical and mechanical bonds. Rosin flux core helps in cleaning surfaces.
Desoldering Pump (Solder Sucker): Used to remove molten solder from a joint for component replacement.
Desoldering Braid (Solder Wick): Copper braid used to absorb molten solder.
Wire Strippers: For removing insulation from wires without damaging the conductors. Pliers (Long Nose, Side Cutters): For holding, bending, and cutting wires. Screwdriver Set (Philips, Flathead, Torx): For opening device enclosures and securing components.
Magnifying Glass/Lamp: For inspecting fine details on circuit boards.
Anti-static Wrist Strap: Prevents damage to sensitive components from electrostatic discharge (ESD).
Heat Gun/Blower: For heat-shrink tubing, BGA rework (advanced). D. Systematic Troubleshooting Procedures A logical approach minimises guesswork and time.
1. Observe (Visual Inspection): Look for obvious signs of damage: Burnt components (resistors, ICs, transformers). Swollen capacitors (especially electrolytic ones, indicating failure). Cracked circuit board traces. Loose connections or dry solder joints (dull, cracked appearance). Corrosion or liquid damage. Blown fuses (discoloured glass, broken filament).
Example: A TV that won't power on might have a swollen capacitor in the power supply section or a blown fuse.
2. Check for Simple Things First: Is the device plugged in? Is the wall socket working? Are batteries inserted correctly and charged (for portable devices)? Is the power switch ON?
Example: A remote control not working might simply need new batteries.
3. Sense (Smell, Hear, Touch): Smell: Burning odour (e.g., from burnt components).
Hear: Hissing, popping, buzzing sounds (e.g., from faulty transformers, arcing).
Touch: Overheating components (careful, can be very hot!). Always test touch safely.
4. Test (Using Multimeter and other tools): Voltage Checks: Check incoming power supply voltage. Check voltage at key test points (e.g., regulated outputs from power supply unit, IC supply pins).
Example:* If a 5V Is the power switch ON?
Example: A remote control not working might simply need new batteries.
3. Sense (Smell, Hear, Touch): Smell: Burning odour (e.g., from burnt components).
Hear: Hissing, popping, buzzing sounds (e.g., from faulty transformers, arcing).
Touch: Overheating components (careful, can be very hot!). Always test touch safely.
4. Test (Using Multimeter and other tools): Voltage Checks: Check incoming power supply voltage. Check voltage at key test points (e.g., regulated outputs from power supply unit, IC supply pins).
Example: If a 5V regulator output is significantly lower or absent, the regulator or its surrounding components might be faulty.
Continuity Checks: Test fuses for continuity. Test switches for continuity in ON/OFF positions. Test PCB traces for breaks.
Resistance Checks: Check resistance of resistors (compare with colour code or schematic). Check for short circuits between power rails and ground (very low resistance). Component Testing (in-circuit or out-of-circuit): Diodes: Use diode test mode on multimeter. Good diode shows a voltage drop in one direction and open circuit in the reverse.
Capacitors: Check for shorts. More advanced meters can test capacitance. Visually inspect for swelling or leakage.
Transistors: Use diode test mode or special transistor test function to check junctions.
E. Basic Repair Techniques Component Replacement:
1. Identify the faulty component.
2. Desolder the component: Apply heat to one joint, melt solder, then quickly suck it with the desoldering pump or wick it with braid. Repeat for all pins.
3. Remove the component.
4. Clean the PCB holes using a desoldering pump or wick.
5. Insert the new component, ensuring correct polarity (for polarized capacitors, diodes, ICs).
6. Solder the new component: Heat the component lead and the PCB pad simultaneously with the soldering iron. Apply solder to the heated joint, allowing it to flow around the lead and pad. Remove solder, then remove iron. A good solder joint is shiny, cone-shaped, and smooth. Trim excess leads.
Track Repair: If a PCB track is broken:
1. Scrape off the solder mask from the broken ends of the track to expose the copper.
2. Solder a thin insulated wire across the break, or bridge with a dab of solder if the break is very small.
3. Example: Repairing a broken track on a charger circuit board due to physical stress.
Cold Solder Joint Repair: Reheat the joint, apply a tiny bit of fresh solder, and allow it to cool properly. F. Safety Precautions in Electronic Repair Always disconnect power before working on electronic circuits. Capacitors can retain charge even after power is off – discharge them safely using a resistor. Work in a well-ventilated area when soldering (solder fumes are harmful).
Wear appropriate PPE: Safety glasses to protect from flying debris or molten solder. Use anti-static precautions (wrist strap, anti-static mat) when handling sensitive components, especially ICs. Avoid touching hot components (soldering iron tip, power resistors, heat sinks). Handle sharp tools (cutters, pliers) carefully. * Know where the first aid kit and fire extinguisher are located.
Teacher Activities: Introduction (10 minutes): Teacher introduces the topic "Electronic Repairs" by asking students about common electronic problems they encounter at home (e.g., phone charger not working, radio silent, TV refusing to come on). Teacher explains the relevance of repair skills in Nigeria for job creation, reducing waste, and resourcefulness. Key Concepts Explanation & Demonstration (30 minutes): Teacher uses a chart or projector to display common electronic components (resistors, capacitors, diodes, transistors, ICs) and explains their functions using simple analogies. Teacher demonstrates the proper use of a multimeter for voltage, continuity, and resistance checks on simple components (e.g., a battery, a fuse, a resistor). Teacher demonstrates basic soldering and desoldering techniques using a practice PCB or scrap board. Emphasise safety. Teacher explains the systematic troubleshooting steps (Observe, Check Simple Things, Sense, Test) using a faulty device (e.g., a non-functional power adapter or an old radio). Practical Application - Component Identification & Tool Handling (30 minutes): Teacher provides various discarded electronic circuit boards (e.g., from old radios, TVs, phone chargers) and sets of basic repair tools (multimeters, soldering irons, desoldering pumps, pliers, screwdrivers). Teacher guides students on how to safely open device casings and identify different components on the circuit board. Teacher ensures students properly hold and manipulate tools under supervision. Troubleshooting Scenario Discussion (15 minutes): Teacher presents a simple fault scenario (e.g., "A car radio suddenly stopped producing sound, but the display is still on. What could be wrong and how would you investigate?"). Teacher facilitates a class discussion on potential causes and troubleshooting steps, drawing on the systematic approach taught.
Safety Reinforcement (5 minutes): Teacher reiterates all safety precautions, especially electrical safety, proper tool handling, and fume ventilation during soldering.
Student Activities: Participate in Discussion: Students share experiences with faulty electronic devices and ask questions during the introduction.
Component Identification: Students observe and identify common electronic components on provided circuit boards, matching them with their names and functions.
Tool Familiarisation: Students handle various repair tools under teacher guidance, practicing safe manipulation.
Multimeter Practice (Supervised): Students practice using the multimeter to measure voltage from a battery, test continuity of a wire, and measure resistance of a resistor. Soldering/Desoldering Practice (Supervised): Students practice basic soldering and desoldering techniques on a practice PCB or designated soldering kits, focusing on creating good, shiny joints and cleanly removing components.
Troubleshooting Brainstorm: Students engage in group discussions to brainstorm potential faults and troubleshooting steps for given scenarios.
Note Taking: Students record key concepts, tool usage, and safety precautions in their notebooks.
Question 1: A student is trying to repair a faulty power adapter for a small DC fan. The fan is not turning on. The student observes a swollen component on the adapter's circuit board. a) Which component is most likely swollen? b) What is the function of this component, and why would its swelling indicate a fault? c) What measurement can be taken to confirm if the power adapter is producing output voltage?
Solution 1: a) Electrolytic capacitor. These are common in power supply circuits and are prone to swelling or bulging when they fail, usually due to internal pressure from overheating or overvoltage. b) The primary function of an electrolytic capacitor in a power adapter is filtering and smoothing the DC output voltage. It helps to convert the pulsating DC from the rectifier into a stable, ripple-free DC voltage required by the fan. Swelling indicates that the capacitor has failed (lost capacitance, developed high Equivalent Series Resistance - ESR, or shorted internally), leading to poor filtering, high ripple, or even complete loss of output, hence the fan not turning on. c) To confirm if the power adapter is producing output voltage, use a multimeter set to DC Voltage (VDC) mode. Connect the positive probe to the adapter's positive output terminal and the negative probe to the negative output terminal. Read the voltage displayed. If the voltage is significantly lower than the specified output (e.g., 12V for a 12V adapter) or zero, it confirms a power supply fault.
Question 2: While troubleshooting a non-responsive car radio, a technician notices that the radio's fuse is completely dark inside, and the metallic filament is broken. a) What does a dark, broken fuse filament indicate? b) What is the immediate next step after identifying a blown fuse? c) Why is it not advisable to simply replace the blown fuse with one of a higher current rating?
Solution 2: a) A dark, broken fuse filament indicates that the fuse has blown due to an overcurrent condition, typically a short circuit or a severe overload downstream in the circuit. The darkness is often caused by the heat generated during the blowing process, vaporizing the fuse element. b) The immediate next step is to investigate the cause of the overcurrent before replacing the fuse. Simply replacing it without identifying and fixing the underlying fault will likely result in the new fuse blowing again immediately, potentially causing further damage. The investigation would involve checking for short circuits in the radio's power input section or any connected components. c) Replacing a blown fuse with one of a higher current rating is highly inadvisable and dangerous. Fuses are safety devices designed to protect the circuit and device from damage due to excessive current. A higher-rated fuse would allow more current to flow than the circuit is designed to handle, potentially leading to overheating of wires, components burning out, or even fire.
Question 3: A student is attempting to solder two wires to a new switch for a domestic lighting circuit. After soldering, the joints appear dull, lumpy, and do not seem to hold the wires firmly. a) Describe what kind of solder joint this is, and why it is undesirable. b) What are two common causes for this type of solder joint? c) What steps should the student take to rectify this situation and ensure a good joint?
Solution 3: a) This describes a "cold solder joint". It is undesirable because it creates a poor electrical connection (high resistance, intermittent connection) and a weak mechanical bond. Such a joint can easily fail, leading to device malfunction or safety hazards. b) Two common causes for a cold solder joint are: Insufficient heat: The soldering iron tip was not hot enough, or it did not adequately heat both the wire and the switch terminal simultaneously. The solder did not flow properly.
Movement during cooling: The components or wires were moved before the solder had fully solidified, disrupting the crystalline structure of the cooling solder. c)
To rectify this and ensure a good joint: * Desolder the existing cold joint cleanly using a desoldering pump or braid to remove all the old
Entrepreneurship and Local Employment: Proficiency in electronic repairs directly translates into viable self-employment opportunities for Nigerian youths. Local technicians repair mobile phones, televisions, radios, power inverters, and chargers, providing essential services in communities where purchasing new electronics is often unaffordable. This skill helps reduce unemployment and stimulates local micro-economies. For instance, a student mastering these skills can set up a small repair shop in their community, offering affordable services for common household electronics. Environmental Sustainability (E-waste Reduction): Repairing electronic devices rather than discarding them contributes significantly to reducing electronic waste (e-waste). Nigeria faces a growing challenge with e-waste management. By extending the lifespan of devices, students contribute to a cleaner environment, conserve natural resources used in manufacturing new electronics, and reduce the toxic chemicals leaching from landfills. This aligns with global efforts towards a circular economy.
Community Service and Empowerment: The knowledge of electronic repairs can be integrated into community outreach programs. Students can volunteer to repair basic electronic devices for elderly community members or for schools and community centres, providing valuable service while gaining practical experience. For example, repairing faulty public address systems in churches or mosques, or restoring old school computers. This fosters a sense of social responsibility and practical application of learned skills.