One, power amplifier machine does not work
The machine does not work the fault performance for electricity amplifier without any display, the function keys are failure, no sound, like not energized. Maintenance should first check the power supply circuit. Use multimeter to measure the power supply plug at each end of the DC resistance value (power switch should be switched on ), the normal should be hundreds of ohmic resistance value. If the measured resistance is small, and the power supply transformer serious heating, power transformer primary circuits are local short circuit; if the measured resistance is infinite, should check the fuse is fused, the transformer primary winding open circuited, power line and the plug between any disconnection. Some machines have increased temperature protection device, in the power transformer in the primary circuit for temperature fuse ( usually installed in the power transformer, the transformer exterior insulation paper can be removed to see), it is damaged will make the power transformer primary circuit open circuit.
If the power supply plug ends of resistance to normal, electricity measurement power supply circuit output voltage is normal. The microprocessor for system control and logic control circuit of the amplifier, should focus on the examination of the control circuit power supply voltage ( usually +5V) and whether it is normal.
If no +5V voltage, should be measured three terminal voltage regulator IC 7805 input voltage is normal, if the input voltage is not normal, should check the rectifier, filter circuit. If the 7805 input voltage is normal, while the output terminal of no ten 5V voltage or low voltage, disconnecting the load at +5V voltage can return to normal. If the +5V voltage is normal, then the fault in the load circuit; if the +5V voltage is not normal, then the fault in 7805 itself.
If the system control circuit of +5V power supply voltage is normal, should check the microprocessor clock and reset signal is normal, keying and display drive circuit is damaged.
Two, no sound output power amplifier
Silent malfunction appears to be operating the function keys, the corresponding status display, but no output signal.
Maintenance of protection circuit of the amplifier, should look after boot protection relay can attract. If the relay action, should be measured circuit output voltage is shifted, the midpoint of over current detection voltage is normal. If the point offset of the output voltage or current voltage abnormality, illustrate the power amplification circuit is faulty, should check the positive, negative power supply is normal or not. If positive, negative voltage asymmetry, can be positive, negative power supply to the load circuit is disconnected, so as to judge the power circuit itself is not normal or power amplifier circuit fault caused by. If positive, negative power supply is normal, should check the power amplifier circuit in the amplifier tube is damaged.
If the amplifier circuit output voltage and overcurrent detection of midpoint voltage were normal, while the protection relay is not operating, the fault in the protection circuit, should check the relay driver integrated circuit or the drive pipe is damaged, the detection circuit is normal. If the relay contacts can attract, but no sound output, should first check the speaker is normal or not, whether good contact of the relay contact, the squelch circuit whether action.
If the parts are normal, then the signal interference method for checking the fault in the power amplifier stage or the front stage circuit. A multimeter R × 1 block, the red one to the black one, fast contact after the input end of the amplification circuit, if the speaker has a strong " Titicaca " sound, indicating failure in the forestage amplifying circuit; if the speaker has no reaction, then the fault in the post amplifier.
For not using peripheral protection circuit for integrated circuit amplifier circuit ( usually in the integrated circuit has internal thermal protection), to measure the voltage of power supply is normal or not. If the power supply voltage is normal, then the signal interference method to check: in the integrated circuit and the signal input end of joined DC intermittent signal, if the speaker has a strong " Titicaca " sound, that integrated circuit is normal, fault in the forestage amplifying circuit; if there is no " Titicaca " sound, and check the peripheral component is also normal, then the fault in power amplifier integrated circuit itself.
Electron tube power amplifier without sound output, also should check the power supply, the filament is bright, shell and tube temperature is normal. If the filament is not bright, shell and tube is very cool, should check the power amplifier tube filament and anode voltage is normal or not. If the voltage is not normal, further examination of the power circuit, when necessary, disconnect the power load circuit, to determine the fault of the power circuit or load short circuit. If the voltage is normal, the volume potentiometer center head joined DC intermittent interference signal, if there is a strong reaction, after amplifying circuits are normal, fault in the forestage amplifying circuit; conversely, failure in post amplifier. Respectively at the push tube grid and the input amplifier tube grid interference signal is added, in which level plus interference signal has no reaction, the stage behind the circuit does not work properly. To the suspicious elements (such as electronic tube ) available substitution maintenance.
With Dolby surround decoding functions of the AV amplifier, if Dolby surround sound state of each channel are silent and elbow through state main sound normal, the power circuit under normal circumstances, Dolby was usually surround decoding circuit or system control circuit not working properly. If the surround sound and direct connection mode of each channel are silent, should check the system control circuit, signal selection circuit and total volume control circuit.
Three, the power amplifier tone light ( output voice )
The so-called tone light fault, refers to the audio signal in the amplification process, as a result of a stage put a lot of changes or in a link is attenuated, so that the gain of the amplifier to decrease or output power of small.
Maintenance, should first check the signal source and the speaker is normal, can replace approach to examination. Then check the various types of switches and control potentiometer, see volume can change.
If the above sections were normal, should judge the fault before or after level circuit. To one channel of a tone is light, the front stage circuit output signal switching input to another channel after the circuit, if the speaker sound size unchanged, then the fault in the circuit; conversely, the fault in the circuit.
Post amplifier caused by sound light, are mainly insufficient output power and gain enough for two reasons. Available due to the input signal (such as the recorder output to the loudspeaker signal directly add to the level of the amplifier circuit. The input end of tape recorders, change the volume, to observe the changes of the amplifier output ) method to determine which causes. If increase after the input signal, the output voice is big enough, illustrate the amplifier output power enough, just gain reduction, should focus on the examination of relay contacts there contact resistance increases, the input coupling capacitor capacity decreases, isolation resistance increases, the negative feedback capacitance smaller or open circuit, negative feedback resistance increase or open phenomenon. If increase after the input signal, the output voice distortion, the volume is not increased significantly, note the output stage amplifier of power shortage, should first check amplifier positive, negative supply voltage is too low ( if only one channel audio and light, can not check the power supply ), power tube or the performance of the integrated circuit is worse, the emitter resistor is large.
The front stage circuit switch, potentiometer of a tone is light, with a visual inspection of easier to find, can be carried out for cleaning or replacement. If the suspicion of a signal coupling capacitor failure, can be used with the value capacitor in parallel with the operational integrated circuit; amplifier tube or bad performance, can also be used substitution method of inspection. In addition, negative feedback element has a problem, will cause the circuit gain fall.
In four, the output of the power amplifier noise
The amplifier noise hum, crackling, induction noise and white noise.
Maintenance, should first determine the noise comes from the front or from the rear stage circuit. The former, after the signal connection plug removed, if the noise is significantly reduced, indicating failure in the front stage circuit; conversely, after the circuit fault in.
AC noise refers to a sense, monotonous and stable 100Hz AC hum, is mainly caused by poor power supply filtering, should focus on the examination power rectifying, filtering and voltage stabilizing element is damaged. Ago, later stage amplifying circuit power terminal of the decoupling capacitor fault or failure, may also produce a similar sound exchange frequency oscillation noise.
Induction noise is more complex and dissonant sound exchange, primarily the front stage circuit of switch, potentiometer bad ground or signal line shielding due to bad.
Burst sound is uninterrupted " crack", " KaKa " sound, at the front stage circuit, should check the signal input plug and socket, switch, potentiometer and whether the bad contact, coupling capacitor without cold solder joint, leakage current. Post amplifier should check whether the relay contact oxidation, the input coupling capacitor have leakage or the bad contact. In addition, after the circuit of differential input tube or a constant current tube soft breakdown, can produce a spark " KaKa " noise.
White noise is a random continuous " rustle" sound, usually by pre, post amplifier in the input stage transistor, field effect transistor or operational amplifier integrated circuit performance created by the bad background noise, maintenance, available with the specifications of the element substitution test.
In five, the output of the power amplifier distortion
Distortion failure is a stage work point offset or amplifier push-pull output stage work caused by asymmetric. Maintenance, according to the output power amplifier and distortion of the changes, to judge the specific fault position.
Electronic tube amplifier and distortion at the output power decreases (sound, light ) should check whether the power amplifier in a tube aging, the working point is right or output transformer partial short circuit caused the imbalance; if the distortion at the output power is large, negative feedback circuit of resistance value, capacitor failure or cathode self-bias bypass capacitor caused by short circuit.
Transistor amplifier if the distortion with volume remarkably increased, should check the push a transistor operating point is offset ( usually occurred in the absence of protection circuit for power amplifier in) or feedback circuit capacitance distortion; if both volume size have distortion, then the fault in the preceding stage amplifying circuit, should check the discharge pipe. Work point has no offset.
Integrated circuit amplifier working abnormal voltage or power amplifier integrated circuit internal damage, can also cause distortion ( without a protection circuit of the machine).
Six, the power amplifier howling
Howling fault is in circuit self-excitation caused by, and divided into low frequency whistle and high-frequency howling.
Low frequency whistle refers to the lower frequency of " puff " or " beep", is usually the result of the power filter or decoupling due to bad ( the whistle at the same time is often accompanied by a sound exchange ), should check the power filter capacitor, voltage regulator and decoupling capacitor is open-circuited or failure, to increase the resistance of electric source. Power amplifier integrated circuit performance is bad, will also appear low whistle fault, the integrated circuit operating temperature will be very high.
High-frequency howling frequency is higher, is usually the amplifying circuit high frequency vibration damping capacitor failure or front stage amplifier integrated circuit performance variation induced by. In the later stage amplifying circuit for eliminating vibration decoupling capacitor and capacitor or small capacitance to check. In addition, negative feedback element damage, variable value or unsoldering, can also cause high frequency positive feedback and high-frequency howling.
