Power amplifier breakdown maintenance

Time:2013-11-28 | Read:592 | Posted: Site Editor
HI - FI stereo and common faults of the AV amplifier has the machine doesn't work, no sound output, light tone, big noise, distortion, noise, etc. 
Here are all kinds of failure maintenance and repair skills. 
The machine doesn't work 
The machine doesn't work after fault show the power amplifier without any display, the function keys are failure, also do not have any sound, like an unpowered when a sample. 
When maintenance should first check the power supply circuit. Available multimeter measuring power plug on both ends of the direct current resistance (power switch should be connected), should be normal when hundreds of ohm resistance. If the measured value of many smaller, and the power transformer serious fever, shows that the primary windings of power transformer has a local short circuit; If measured value is infinite, should check whether the fuse fusing, transformer primary winding between open circuit, the power cord and plug do you have any break. Some machine increased the temperature protection device, in the primary windings of power transformer by the current fuse (usually installed inside the power transformer, the transformer exterior insulation paper can be seen to remove), damage can also make it power transformer primary circuit broken. 
If the power plug on both ends of the resistance is normal, electric measurement of the output voltage power supply circuit is normal. For the system control microprocessor or logic control circuit of the amplifier, should inspect the control circuit of power supply voltage (usually a + 5 v). 
If no + 5 v voltage, should measure the three-terminal voltage regulator integrated circuit 7805 input voltage is normal, if the input voltage is not normal, should check the rectifier, filter circuit. If the input voltage of 7805 normal, and no. 5 v output voltage or low voltage, can disconnect the load see if + 5 v voltage can return to normal. If + 5 v voltage is normal, the fault in the load circuit; If + 5 v voltage is not normal, the fault itself in 7805. 
If the system control circuit of + 5 v power supply voltage is normal, should check the microprocessor clock and reset signal is normal, keying and display driver circuit is damaged. 
No sound output 
Silent failure performance for each function key operation, have corresponding status display, but no signal output. 
Maintenance has a protection circuit amplifier, should look after startup protection relay can suck. If the relay action, whether should point measuring amplifier circuit output voltage offset, over-current detection voltage is normal. If the midpoint abnormal voltage output voltage deviation or over-current detection, power amplification circuit fault, should check whether the positive and negative power supply is normal. If the asymmetry of positive and negative voltage, load of positive and negative power supply circuit can be disconnected, is to determine the power circuit itself has the fault caused by abnormal or amplifier circuit. If the positive and negative power supply is normal, should check each tube is damaged in the power amplifier circuit. 
If the amplifier output voltage and over current detection circuit middle voltage are normal, and protection relay not suck, failure in protection circuit, should check relay drive integrated circuit or drive pipe is damaged, the detection circuit is normal. If the relay contact that can suck up close, but no sound output, should be first to check whether the speaker is normal, whether the relay contact contact good, whether action noise circuit. 
If some of these are normal, reoccupy interference method to check fault is in the power amplifier circuit levels after or before. Multimeter is R x 1 block, will be red pens and grounding, black pens and fast after tapping the amplifier circuit input end, if the speaker in the strong "raise price" sound, fault in the first stage amplifier circuit; If the speaker has no reaction, the fault in the stage amplifier circuit. 
For not using the integrated circuit amplifier circuit protection circuit of the peripheral (usually in integrated circuit internal thermal protection), can measure the first power supply voltage is normal or not. If the voltage is normal, and then check with interference method: in the signal input end of the power amplifier IC to dc discontinuous signal, if the speaker has the strong "raise price" sound, power amplifier integrated circuit is normal, the fault in the first stage amplifier circuit; Without "raise price" sound, and check the relevant peripheral components are normal, the fault in the power amplifier integrated circuit itself. 
No sound output tube amplifiers, also should first check the power supply, watch the filament is bright, shell temperature is normal. If the filament is not bright, shell is very cool, check the power amplifier tube filament and anode voltage is normal or not. If the voltage is not normal, and then step into - check the power supply circuit, load circuit should be power off when necessary, is to determine the power supply circuit fault or load has a short circuit. If the voltage is normal, but in the center of the volume potentiometer head to join dc intermittent interference signal, if has the strong reaction, after stage amplifier circuit is normal, the fault in the first stage amplifier circuit; On the other hand, the failure in the amplifier circuit. Respectively in promoting tube grid and input amplifier tube grid to join the jamming signal, where - no response level and jamming signal, indicated that the level at the back of the circuit is not work properly. Suspicious components (such as tube) available substitution method of maintenance. 
With dolby surround sound decoding function of AV amplifier, if the dolby surround sound be silent to elbow each channel state and the main channel under direct state sound normal, normal operation of the power supply circuit, usually dolby surround sound decoding circuit or system control circuit doesn't work normally. If the surround sound and direct mode of each channel are silent, should check the system control circuit, signal selection circuit and total volume control circuit. 
The sound of light 
Sound light failure, refers to the audio signal in the process of amplification transmission, because of an amplifier stage to put a lot of change, or at some point be attenuation, reduce put amplifier gain or output power decreases. 
Maintenance, you should check whether the signal source and the speaker normally, can be used to replace the way to check. Then check all kinds of switch and control potentiometer, and the volume can be bigger. 
If the above parts are normal, should determine the fault is in the first grade or level in the circuit. To one on one channel tone is light, it can be the front-end circuit of the output signal exchange level after the input to another track circuit, if the voice of the speaker size remains the same, the fault circuit in the level; On the other hand, the fault in the front-end circuit. 
Stage amplifier circuit caused by the light, after have insufficient output power and gain enough two main reasons. Available to properly increase input signals (such as after the tape recorder to the speaker output signal directly added to the input end of the power amplifier circuit, change the volume of the radio recorder, observe the change of the power amplifier output) method to determine what the cause. If increased after the input signal, the output loudly enough, the power amplifier output power enough, just gain is reduced, should inspect relay contact contact resistance increases, the input coupling capacitance decrease, isolation resistance value increase, negative feedback capacitance decreases or open circuit, negative feedback resistance value increases or open circuit. If increased after the input signal, the voice of the output of distortion, and no significant increase in the volume, after that the output power of the amplifier is insufficient, should first check the positive and negative power supply voltage amplifier is low (if only one channel sound light, but do not have to check the power supply), power tube, or integrated circuit performance variation, the emitter resistance tolerance do you have any bigger, etc. 
The front-end circuit switch, potentiometer caused by light, by visual inspection, found that more easily to clean or replace. Suspecting some signal coupling capacitance failure, such as value can be used with the capacitance of the parallel test; Amplifier, or op-amp integrated circuit performance is poor, also can use substitution method to check. In addition, the feedback element has a problem, the circuit gain will also decline. 
Large noise 
The noise of the amplifier are hum, crackle, induction noise and white noise, etc. 
Maintenance and repair, should first determine the noise from the levels before or after from the circuit. Can connect level before and after the signal to remove plug, if noise clear decrescent, specify in the front-end circuit; On the other hand, the fault in the circuit. 
Hum sense refers to hear a low, dull and stable 100 hz ac hum, improper part mainly power supply filter, should inspect the power rectifier, filter and regulator component is damaged. To the level before and after amplifying circuit power supply decoupling capacitors virtual welding or failure, also can produce a kind of low frequency oscillation noise like hum. 
Induced noise is composition is complex and harsh backdrop, mainly is the front-end circuit of signal switch, potentiometer bad earth or improper connection block. 
Crackle of refers to "fire", "kaka" sound, in the first stage circuit, should check the signal input plug and socket, switch, potentiometer, etc is bad contact, coupling capacitance for virtual welding, leakage, etc. After stage amplifier circuit should check whether the relay contact oxidation, the input coupling capacitor have leakage or poor contact. In addition, after the stage differential input pipe or constant current in the circuit soft breakdown, also can produce similar electric spark "kaka" noise. 
Refers to the continuous random white noise "rustling" sound, usually by levels before and after the input amplifier circuit, field effect transistor or op-amp integrated circuit adverse background noise, the performance of maintenance and repair, specifications of the components can be used with the substitution of try.