Lesson Notes By Weeks and Term v3 - Senior Secondary 2
Fault Finding and Repairs in Radio Receiver
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Fault Finding and Repairs in Radio Receiver
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Subject: Radio Television And Repairs
Class: Senior Secondary 2
Term: 1st Term
Week: 8
Theme: Workshop Practice And Maintenance
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Diagnose faultby using faultfinding piecesof equipmentand logicaltrouble shootingprocedure. Identify specificcomponentsresponsible for fault. Correct the fault by replacing faultycomponents. Align the RFand IF stages ofa radio setusing the necessaryequipments and tools.
IF signals. Multimeter (with sensitive DC voltage range): Used to measure the DC voltage across the detector or the audio output level to indicate maximum signal.
Non-metallic Alignment Tools: Essential for adjusting the ferrite slugs within IF transformers and trimmer capacitors without detuning.
5. General Alignment Procedure (AM Radio Example):
A. IF Alignment (Typically performed first):
1. Disconnect the radio's antenna.
2. Connect the RF signal generator to the mixer input or the first IF amplifier input (often through a small capacitor).
3. Set the signal generator to the radio's IF frequency (e.g., 455 kHz) and modulate it with an audio tone (e.g., 400 Hz).
4. Connect a multimeter (DC voltage range) across the detector output or the AGC (Automatic Gain Control) line, or monitor the audio output level.
5. Adjust the slugs of each IF transformer (T3, T2, T1 – working backwards from the detector) using a non-metallic tool until the multimeter shows maximum DC voltage or the audio output is loudest. Repeat adjustments until no further improvement is observed.
B. RF Alignment:
1. Connect the RF signal generator to the radio's antenna input.
2. Oscillator Tracking: Set the signal generator to a high-frequency point (e.g., 1400 kHz for AM). Tune the radio receiver to the same frequency. Adjust the oscillator trimmer capacitor (C-osc) for maximum output. Set the signal generator to a low-frequency point (e.g., 600 kHz). Tune the radio receiver to the same frequency. Adjust the oscillator coil (L-osc) for maximum output. Repeat these steps until maximum output is achieved at both high and low ends without further adjustment.
3. RF/Antenna Tracking: Set the signal generator to the high-frequency point (e.g., 1400 kHz). Adjust the antenna trimmer capacitor (C-ant) for maximum output. Set the signal generator to the low-frequency point (e.g., 600 kHz). Adjust the antenna coil (L-ant) for maximum output. Repeat until satisfactory. Important
Note: FM alignment is similar but uses a higher IF frequency (10.7 MHz) and usually involves a 'wobbulator' and oscilloscope for a visual response curve. For SS2, focus is often on conceptual understanding and basic AM alignment. Worked Examples (Conceptual Troubleshooting): Example 1: "No Power / Dead Set" Radio Symptom: A radio receiver, after being accidentally dropped, is completely dead. No lights, no sound.
Troubleshooting Steps:
1. Visual Inspection: First, carefully inspect the power cord for cuts or loose connections. Look inside the radio for any obvious physical damage like a detached speaker wire, a visibly burnt component (e.g., a resistor, capacitor, or diode), or loose connections on the PCB.
2. Fuse Check: Disconnect the radio from the power source. Locate the fuse (usually near the AC input). Use a multimeter on the continuity or resistance (Rx1) range. A good fuse will show continuity (low resistance), while a blown fuse will show an open circuit (infinite resistance). If blown, replace it with the correct rating.
3. Power Switch: If the fuse is good, test the power switch. With the radio still disconnected, set the multimeter to continuity mode. Test the switch terminals in both ON and OFF positions. It should show continuity when ON and open when OFF.
4. Power Supply Output: If the above are good, reconnect the radio and carefully measure the DC voltage output of the power supply section (after the transformer and rectifier/filter circuit). Compare with the expected voltage (e.g., 9V, 12V DC). If voltage is absent or very low, focus on the rectifier diodes, filter capacitors, or voltage regulator.
Example 2: "No Sound, but Powers On" Radio Symptom: A radio powers on, display lights up, but there is absolutely no sound from the speaker. * Troubleshooting Steps:
1. Speaker Check: First, visually inspect the speaker cone for tears. Then, disconnect the speaker from the amplifier and use a multimeter on the resistance (Rx1) range to check its coil continuity. A good speaker typically has a low resistance (e.g., 4-16 Ohms). An open circuit means a faulty speaker coil. If the speaker is good, connect a known good audio source (e.g., phone with music) to the speaker to confirm its Symptom: A radio powers on, display lights up, but there is absolutely no sound from the speaker.
Troubleshooting Steps:
1. Speaker Check: First, visually inspect the speaker cone for tears. Then, disconnect the speaker from the amplifier and use a multimeter on the resistance (Rx1) range to check its coil continuity. A good speaker typically has a low resistance (e.g., 4-16 Ohms). An open circuit means a faulty speaker coil. If the speaker is good, connect a known good audio source (e.g., phone with music) to the speaker to confirm its functionality.
2. Audio Amplifier Input: If the speaker is good, the problem is likely in the audio amplifier stage or earlier. Use a signal injector to inject an audio tone (e.g., 1kHz) directly at the input of the audio power amplifier IC or transistor stage. If sound is heard from the speaker, the audio power amplifier and speaker are likely functional, indicating the fault lies earlier (e.g., pre-amplifier, volume control, detector, IF).
3. Volume Control: Test the volume control potentiometer for continuity and smooth resistance change. A dirty or faulty potentiometer can cause intermittent or no sound.
Example 3: "Poor Reception / Weak Stations" Radio Symptom: A radio turns on, produces static or white noise, but can only weakly pick up local strong stations, or struggles to tune into any station clearly. * Troubleshooting Steps:
1. Antenna Check: Inspect the antenna for physical damage, breaks, or poor connections. For telescopic antennas, extend and ensure segments are intact. For internal ferrite rod antennas, check for cracks or detached windings.
2. RF/IF Alignment (Suspicion): If physical checks yield no obvious fault, and major repairs (especially involving RF/IF components) were recently done, suspect misalignment. Proceed with the alignment procedure using a signal generator and multimeter as described in section F.
3. Component Check (RF/IF Stages): If alignment doesn't resolve the issue or isn't feasible, focus on checking components in the RF, Mixer, Local Oscillator, and IF amplifier stages. This would involve checking supply voltages, and possibly testing transistors/ICs in these stages, or looking for open/shorted coils/capacitors. the waveform at various stages. Identify where the signal is lost or distorted.
4. Component Testing: Once a faulty stage is identified, test individual components within that stage.
Resistors: Measure resistance with a multimeter (power OFF). Compare with marked value (tolerance).
Capacitors: Electrolytic: Visual inspection for bulging/leakage. Can be tested for shorts/opens with a multimeter on resistance range (will show charging/discharging then open circuit if good). For more accurate test, use an ESR (Equivalent Series Resistance) meter.
Non-electrolytic: Test for shorts/opens with multimeter.
Diodes: Test for forward and reverse bias using multimeter diode test function (should conduct in one direction, block in the other).
Transistors: Test for junctions (Base-Emitter, Base-Collector) similar to diodes. Can also be tested in-circuit with power OFF for shorts.
Integrated Circuits (ICs): Often tested by checking supply voltages, input/output signals (with oscilloscope), or by direct replacement (last resort).
Coils/Transformers: Check for continuity with multimeter (open circuits are common).
D. Fault Finding Equipment
1. Multimeter (Digital & Analog): Functions: Measures AC/DC voltage, AC/DC current, resistance (Ohms), continuity (for shorts/opens), diode test, sometimes capacitance and frequency.
Usage: Essential for checking power supply, component integrity, and signal presence.
2. Signal Generator / Injector: Function: Generates various frequencies (audio, RF, IF) to inject into different stages of the radio, helping to trace the signal path and identify where the signal is lost.
3. Oscilloscope: Function: Displays waveforms, allowing technicians to visualize signal shapes, amplitudes, and frequencies at various points in the circuit. Crucial for advanced signal tracing and alignment.
4. Soldering Iron & Desoldering Pump/Wick: Function: Essential tools for safely removing and replacing components.
5. Hand Tools: Screwdrivers (Philips, flathead), pliers (long nose, side cutters), tweezers, wire strippers.
6. Non-Metallic Alignment Tools: Function: Used for adjusting RF and IF coils/trimmers. Non-metallic to prevent disturbing the coil's magnetic field and detuning the circuit during adjustment.
E. Replacing Faulty Components
1. Safety First: Disconnect power, discharge large capacitors.
2. Desoldering: Heat the solder joint with the soldering iron while applying the desoldering pump/wick to remove molten solder. Carefully remove the component without damaging the PCB traces.
3. Component Selection: Replace with an identical component or an equivalent with the same specifications (e.g., voltage, capacitance, resistance, power rating, polarity). Observe polarity for electrolytic capacitors, diodes, and ICs.
4. Soldering: Insert the new component into the PCB holes. Bend leads slightly to hold in place. Heat the component lead and PCB pad simultaneously. Apply solder to the heated joint, allowing it to flow smoothly. Remove solder then iron. A good solder joint is shiny and conical. Trim excess leads with side cutters. F. RF and IF Alignment Alignment is the process of adjusting the tunable components (coils, trimmers) in the RF and IF stages to achieve maximum gain, selectivity, and sensitivity.
1. Why it is Necessary: After replacing components in the RF or IF stages. If the radio's reception quality (sensitivity, selectivity) has degraded over time. If the local oscillator frequency has drifted.
2. RF (Radio Frequency)
Stage: The first stage after the antenna, responsible for selecting the desired station and amplifying it. Includes the antenna coil, RF amplifier, mixer, and local oscillator.
3. IF (Intermediate Frequency)
Stage: Converts the high RF signal to a fixed lower frequency (e.g., 455 kHz for AM, 10.7 MHz for FM) for easier amplification and filtering. It comprises IF transformers (IFTs) and IF amplifier stages.
4. Equipment for Alignment: RF Signal Generator: Capable of generating accurate RF and IF signals. Multimeter (with sensitive DC voltage range): Used to measure the DC voltage across the detector or the audio output level to indicate maximum signal.
Non-metallic Alignment Tools: Essential for adjusting the ferrite slugs within IF transformers and trimmer capacitors without detuning.
5. General Alignment Procedure (AM Radio Example): *
A. IF Alignment (Typically performed first):
1. Disconnect the radio's antenna.
2. Connect the RF signal generator to the mixer input or the first IF amplifier input (often through a small capacitor).
3. Set the signal generator to the This section covers the fundamental principles and practical steps involved in fault finding and repairs in radio receivers. A. Introduction to Fault Finding Fault finding, also known as troubleshooting, is a systematic approach to diagnosing the cause of a malfunction in an electronic device. For radio receivers, it involves observing symptoms, using test equipment, and applying logical reasoning to isolate the faulty component or section. B. Common Radio Receiver Faults and Their Symptoms Understanding common faults helps in initial diagnosis.
1. No Power / Dead Set: Radio does not turn on, no lights, no sound.
Possible causes: Blown fuse, faulty power cord, faulty power switch, rectifier diode failure, power supply capacitor failure, short circuit in main power rail.
2. No Sound / No Audio: Radio powers on, but no sound from the speaker. Tuning dial might respond, but no signal audible.
Possible causes: Faulty speaker, open circuit in audio output transformer/stage, faulty audio amplifier IC/transistor, broken signal path from detector, faulty volume control.
3. Weak / Distorted Sound: Low volume even at maximum setting, sound is muffled, crackling, or buzzing.
Possible causes: Aged electrolytic capacitors in audio/power supply stages, faulty audio amplifier components, poor speaker connection, intermittent signal in IF stage, weak power supply.
4. No Reception / Poor Tuning: Radio turns on, sound might be normal (hissing/static), but cannot pick up stations or only picks up very few.
Possible causes: Faulty antenna, issues in RF amplifier, mixer, local oscillator, or IF stages, misalignment, broken tuning mechanism.
5. Intermittent Faults: The radio works sometimes, then stops, or cuts in and out.
Possible causes: Cold solder joints, loose connections, hairline cracks on PCB, temperature-sensitive components (e.g., transistors, ICs, capacitors).
6. Buzzing / Humming Sound: A low-frequency hum or buzz, often related to the power line frequency (50Hz in Nigeria).
Possible causes: Faulty filter capacitors in the power supply (smoothing), poor grounding, open circuit in rectifier diode. C. Fault Finding Procedures / Logical Troubleshooting Steps A systematic approach is critical to avoid guesswork.
1. Gather Information & Observe Symptoms: Ask the user what happened, when, and what symptoms they observed.
Visually inspect the radio: Look for signs of burning, bulging capacitors, loose wires, physical damage, dust accumulation, signs of insect infestation (e.g., ants, cockroaches causing shorts).
Sensory inspection: Listen for unusual sounds (hissing, crackling), smell for burning components, feel for excessive heat on components.
2. Verify Power Supply: Check power cord: Use a multimeter for continuity.
Check fuse: Use a multimeter for continuity. A blown fuse often indicates a short circuit elsewhere.
Check power switch: Use a multimeter for continuity when switched "ON".
Measure voltage: After the power supply unit, measure DC voltages at various points (e.g., rectifier output, voltage regulator output). Compare with schematic values.
3. Stage-by-Stage Isolation (Signal Tracing): Divide the radio receiver into functional blocks: Antenna, RF amplifier, Mixer, Local Oscillator, IF amplifier, Detector, Audio Pre-amplifier, Audio Power Amplifier, Speaker. Start from the output (speaker) and work backwards towards the input (antenna) or vice versa, depending on the symptom. No sound, power ON: Check speaker, then audio amplifier, then detector, then IF stage. No reception, sound OK: Check antenna, then RF, mixer, oscillator, then I
F. Signal Injection: Use a signal generator/injector to inject an audio signal at the input of the audio amplifier, then the detector, then the IF stage. If the signal is heard at the speaker, the stages after the injection point are likely working.
Signal Tracing (with Oscilloscope): Use an oscilloscope to trace the signal path, observing the waveform at various stages. Identify where the signal is lost or distorted.
4. Component Testing: Once a faulty stage is identified, test individual components within that stage.
Resistors: Measure resistance with a multimeter (power OFF). Compare with marked value (tolerance).
Capacitors: Electrolytic: Visual inspection for bulging/leakage. Can be tested for shorts/opens with a multimeter on resistance range (will show charging/discharging then open circuit if good). For more accurate test, use an ESR (Equivalent Series Resistance) meter.
Non-electrolytic: Test for shorts/opens with multimeter. * Diodes: Test for
Local Entrepreneurship and Job Creation: The skills in fault finding and repair are directly transferable to setting up and running small businesses in electronics repair. In many Nigerian communities, from bustling urban markets like Alaba International Market in Lagos to rural villages, skilled technicians are in high demand to repair household electronics. This knowledge empowers students to become self-employed, providing essential services and creating wealth within their communities rather than relying solely on formal employment. Waste Reduction and Economic Sustainability: Many Nigerians cannot afford to constantly replace faulty electronic devices. The ability to repair radios, rather than discard them, contributes significantly to household budget savings.
Furthermore, it aligns with global efforts towards a circular economy, reducing electronic waste (e-waste) which is a growing environmental concern in Nigeria. Technicians trained in these skills help extend the lifespan of products, making electronics more accessible and sustainable for the average Nigerian. Community Development and Access to Information: Radios remain a crucial source of information, entertainment, and education, especially in remote parts of Nigeria where internet access might be limited. By being able to repair radios, technicians ensure that these vital communication tools remain functional and accessible to communities, thereby promoting literacy, awareness of national issues, and access to educational programs broadcast via radio. This skill helps bridge the information gap in underserved areas.