Common Faults and Repair Methods of Class H Power Amplifier Modules

Class H power amplifiers achieve high efficiency through multi-level voltage rail switching and are commonly used in applications such as professional audio and broadcast transmission. Common faults in Class H power amplifier modules involve modules such as the power supply, amplifier tubes, and control circuits. The following is a systematic troubleshooting and repair guide.

I. Common Fault Classification and Repair
1. No Output (No Sound)
Possible Causes:
Power Fault: No output from the main power supply, blown fuse
Protection Circuit Action: Overcurrent/Overheating/DC Offset Protection Triggered
Signal Path Interruption: Failed input coupling capacitor, oxidized relay contacts
Repair Steps:
Check the power supply:
Measure the main power supply voltage (±VCC, see if it's normal, e.g., ±60V).
Check the fuse (replace with a slow-blow fuse of the same specification).
Check the protection circuit:
Reset the protection circuit (e.g., disconnect the AC power supply for 30 seconds and restart).
Check the DC offset at the output (should be <±100mV; otherwise, the amplifier tubes may be damaged). Check the signal path:
Use an oscilloscope to trace the input signal to the final power amplifier tube.

2. Output distortion (clipping/noise)
Possible causes:
Voltage rail switching failure: Boost circuit abnormality (e.g., damaged IR2110 driver IC)
Power amplifier tube imbalance: Mismatched complementary pair (NPN/PNP) tube parameters
Negative feedback failure: Open feedback resistor (e.g., 22kΩ) or leaking capacitor
Repair steps:
Test voltage rail switching:
Check the boost MOSFET (e.g., IRF540) and drive signal (use an oscilloscope to view the PWM waveform).
Pairing power amplifier tubes:
Replace tubes from the same batch (beta value difference <10%) and adjust the bias voltage if necessary.
Check the feedback network:
Measure the feedback loop impedance (e.g., the resistance from the output to the inverting input).

3. Frequent overheat protection triggering
Possible causes:
Poor cooling: Fan stalled/dry thermal paste
Excessive quiescent current: Bias circuit misalignment (e.g., poor contact of variable resistor)
Load short circuit: Abnormal speaker impedance (e.g., <2Ω)
Repair steps:
Check the cooling system:
Clean the air ducts and replace the thermal paste (thermal conductivity ≥ 5W/m·K).
Adjust the quiescent current:
Adjust the bias potentiometer (usually to a final current of 50-100mA).
Measure the load impedance:
Disconnect the load and use a multimeter to measure the speaker impedance (to ensure it matches the nominal value).

4. Repeated relay energization
Possible causes:
Protection circuit malfunction: Capacitance degradation of the detection capacitor (e.g., 100μF)
Relay contact erosion: High current causes increased contact resistance (>0.5Ω)
Repair steps:
Replace the detection capacitor:
Replace the power supply filter capacitor and protection circuit capacitor (preferably 105°C temperature-resistant).
Cleaning the relay:
Use contact cleaner or replace the relay.

II. Typical Repair Cases
Case 1: Fuse Blows Immediately Upon Power-Up
Fault Point: Rectifier Bridge Breakdown (e.g., KBPC3510)
Solution: Replace the rectifier bridge and check for short-circuited filter capacitors.
Case 2: High-Frequency Output Oscillation
Fault Point: Open-Circuited Compensation Capacitor (e.g., 100pF)
Solution: Connect a 47pF ceramic capacitor in parallel with the feedback resistor.
Case 3: Voltage Rail Switching Delay
Fault Point: Damaged LM393 Comparator
Solution: Replace the IC and adjust the voltage divider resistor (to ensure the switching threshold is correct).

III. Preventive Maintenance Recommendations