Solutions to Common Amplifier Problems
1. Entire Unit Not Working
Symptoms: No display after power-on, all function keys unresponsive, no sound, as if unplugged.
First, check the power circuit. Measure DC resistance across the power plug (switch ON). Normal: several hundred ohms. If resistance is much lower and the power transformer overheats, the primary circuit likely has a short. If resistance is infinite, check fuses, primary winding continuity, and power cord/plug connections. Some units have thermal fuses inside the transformer primary; damaging them opens the circuit.
If plug resistance is normal, measure power circuit output voltages. For amps with microprocessors or logic control, check the +5V supply. If missing, check the 7805 regulator input. If input abnormal, check rectifier/filter circuits. If 7805 input normal but output missing/low, disconnect the load. If +5V returns, the load is faulty. If still abnormal, replace the 7805. If +5V is normal, check the microprocessor clock/reset signals, and key/display driver circuits.
2. Excessive Noise
Noise types: Hum, crackling, induced noise, white noise.
First, identify if noise originates from preamp or power amp. Disconnect pre/power interconnects. If noise reduces significantly, the preamp is faulty; otherwise, the power amp.
Hum: Low, monotonous 100Hz buzz. Primarily caused by poor power supply filtering. Check rectifier, filter, and regulator components. Decoupling capacitors failing in pre/power stages can cause similar low-frequency oscillation noise.
Induced Noise: Complex, harsh AC noise. Caused by poor grounding of switches/potentiometers or unshielded signal cables in the preamp.
Crackling: Intermittent "crack/pop." In preamp: Check input jacks, switches, potentiometers for poor contact; coupling capacitors for cold solder or leakage. In power amp: Check oxidized relay contacts; input coupling capacitors for leakage or poor contact. Soft failure in differential input or constant current transistors can also cause spark-like noise.
White Noise: Continuous "hiss." Caused by noisy transistors/FETs/op-amps in pre/power input stages. Replace suspected components.
3. Low Volume
"Low volume" means reduced gain or output power due to attenuation or reduced amplification in the signal path. First, verify source and speakers using substitution. Then check switches and potentiometers for volume restoration.
If sources/speakers are good, determine if fault is in preamp or power amp. For one channel weak, swap its preamp output to the other channel's power amp input. If the weak channel remains, the power amp is faulty; otherwise, the preamp.
Power amp causes: Insufficient output power or inadequate gain. Inject a suitable signal (e.g., connect a recorder's speaker output directly to the power amp input, varying recorder volume to observe amp output changes).
If output is loud enough with increased input, power is sufficient, but gain is low. Check for increased contact resistance in relay contacts, reduced input coupling capacitor value, increased isolation resistor value, reduced/open negative feedback capacitor value, or increased/open negative feedback resistor value.
If output distorts without significant volume increase with increased input, output power is insufficient. First, check positive/negative supply voltages (if only one channel weak, skip for now), degraded power transistors/ICs, or increased emitter resistor value.
Preamp causes: Faulty switches/potentiometers causing low volume are easily spotted visually (clean/replace). Suspect coupling capacitors can be paralleled with same-value caps. Degraded transistors/op-amps can be replaced. Faulty negative feedback components also reduce gain.
Mixer operation requires knowledge; lacking it makes operation difficult and sound quality poor. Learning is crucial for improving mixer output.
1. Harsh Sound: Cause: Excessive high-frequency energy; Solution: Attenuate 6KHz.
2. Irritating Sound: Cause: Unpleasant highs; Solution: Attenuate 8K-10KHz.
3. Fuzzy Sound: Cause: Excessive ultra-high-frequency energy; Solution: Attenuate 12K-16KHz.
4. Dry Microphone, Tiring Singing: Cause: Insufficient midrange energy; Solution: Boost 800Hz band.
5. Hard Bass: Cause: Excessive upper bass energy; Solution: Attenuate 125-160Hz.
6. Flabby Bass: Cause: Excessive low bass energy; Solution: Raise the subwoofer crossover frequency.
Symptoms: No display after power-on, all function keys unresponsive, no sound, as if unplugged.
First, check the power circuit. Measure DC resistance across the power plug (switch ON). Normal: several hundred ohms. If resistance is much lower and the power transformer overheats, the primary circuit likely has a short. If resistance is infinite, check fuses, primary winding continuity, and power cord/plug connections. Some units have thermal fuses inside the transformer primary; damaging them opens the circuit.
If plug resistance is normal, measure power circuit output voltages. For amps with microprocessors or logic control, check the +5V supply. If missing, check the 7805 regulator input. If input abnormal, check rectifier/filter circuits. If 7805 input normal but output missing/low, disconnect the load. If +5V returns, the load is faulty. If still abnormal, replace the 7805. If +5V is normal, check the microprocessor clock/reset signals, and key/display driver circuits.
2. Excessive Noise
Noise types: Hum, crackling, induced noise, white noise.
First, identify if noise originates from preamp or power amp. Disconnect pre/power interconnects. If noise reduces significantly, the preamp is faulty; otherwise, the power amp.
Hum: Low, monotonous 100Hz buzz. Primarily caused by poor power supply filtering. Check rectifier, filter, and regulator components. Decoupling capacitors failing in pre/power stages can cause similar low-frequency oscillation noise.
Induced Noise: Complex, harsh AC noise. Caused by poor grounding of switches/potentiometers or unshielded signal cables in the preamp.
Crackling: Intermittent "crack/pop." In preamp: Check input jacks, switches, potentiometers for poor contact; coupling capacitors for cold solder or leakage. In power amp: Check oxidized relay contacts; input coupling capacitors for leakage or poor contact. Soft failure in differential input or constant current transistors can also cause spark-like noise.
White Noise: Continuous "hiss." Caused by noisy transistors/FETs/op-amps in pre/power input stages. Replace suspected components.
3. Low Volume
"Low volume" means reduced gain or output power due to attenuation or reduced amplification in the signal path. First, verify source and speakers using substitution. Then check switches and potentiometers for volume restoration.
If sources/speakers are good, determine if fault is in preamp or power amp. For one channel weak, swap its preamp output to the other channel's power amp input. If the weak channel remains, the power amp is faulty; otherwise, the preamp.
Power amp causes: Insufficient output power or inadequate gain. Inject a suitable signal (e.g., connect a recorder's speaker output directly to the power amp input, varying recorder volume to observe amp output changes).
If output is loud enough with increased input, power is sufficient, but gain is low. Check for increased contact resistance in relay contacts, reduced input coupling capacitor value, increased isolation resistor value, reduced/open negative feedback capacitor value, or increased/open negative feedback resistor value.
If output distorts without significant volume increase with increased input, output power is insufficient. First, check positive/negative supply voltages (if only one channel weak, skip for now), degraded power transistors/ICs, or increased emitter resistor value.
Preamp causes: Faulty switches/potentiometers causing low volume are easily spotted visually (clean/replace). Suspect coupling capacitors can be paralleled with same-value caps. Degraded transistors/op-amps can be replaced. Faulty negative feedback components also reduce gain.
Mixer operation requires knowledge; lacking it makes operation difficult and sound quality poor. Learning is crucial for improving mixer output.
1. Harsh Sound: Cause: Excessive high-frequency energy; Solution: Attenuate 6KHz.
2. Irritating Sound: Cause: Unpleasant highs; Solution: Attenuate 8K-10KHz.
3. Fuzzy Sound: Cause: Excessive ultra-high-frequency energy; Solution: Attenuate 12K-16KHz.
4. Dry Microphone, Tiring Singing: Cause: Insufficient midrange energy; Solution: Boost 800Hz band.
5. Hard Bass: Cause: Excessive upper bass energy; Solution: Attenuate 125-160Hz.
6. Flabby Bass: Cause: Excessive low bass energy; Solution: Raise the subwoofer crossover frequency.