Solutions to common failures of power amplifiers

What should I do if there is a problem with the power amplifier? Gap is like a line, although not everyone can solve it, but knowing some related knowledge will make you easy to solve. The following describes three common faults and maintenance methods of the power amplifier.

1. The whole machine does not work

The failure of the whole machine does not show any display after the power is turned on, all function keys are invalid, and there is no sound, just like when the power is not turned on.
The power supply circuit should be checked first during maintenance. A multimeter can be used to measure the DC resistance value at both ends of the power plug (the power switch should be turned on). Normally, there should be a resistance value of hundreds of ohms. If the measured resistance value is much smaller, and the power transformer is seriously heated, it means that the primary loop of the power transformer has a local short circuit; if the measured resistance value is infinite, it should be checked whether the fuse is blown, the transformer primary winding is open, the power cord and the plug Is there any disconnection between them? Some machines have added a temperature protection device, and a current fuse is connected to the primary circuit of the power transformer (usually installed inside the power transformer, you can see it by removing the insulation paper outside the transformer), it will also make the power transformer primary after damage The circuit is open.
If the resistance at both ends of the power plug is normal, you can power on to measure whether the output voltage of the power circuit is normal. For amplifiers using system-controlled microprocessors or logic control circuits, the focus should be on checking whether the supply voltage (usually + 5V) of the control circuit is normal. If there is no + 5V voltage, it should be measured whether the input voltage of the three-terminal voltage regulator integrated circuit 7805 is normal. If the input voltage is abnormal, the rectifier and filter circuit should be checked. If the voltage at the 7805 input terminal is normal and there is no 10V or low voltage at the output terminal, you can disconnect the load to see if the + 5V voltage can return to normal. If the + 5V voltage is normal, the fault is in the load circuit; if the + 5V voltage is still abnormal, the fault is in the 7805 itself. If the + 5V power supply voltage of the system control circuit is normal, you should check whether the clock and reset signal of the microprocessor are normal, and whether the key control and display drive circuits are damaged.

Second, the noise is large

The noise of the amplifier includes hum, pop, induced noise and white noise.
When overhauling, you should first determine whether the noise comes from the previous stage or from the latter stage circuit. The signal connection plugs of the front and rear stages can be removed. If the noise becomes significantly smaller, it means that the fault is in the front stage circuit; otherwise, the fault is in the rear stage circuit. The hum refers to the low-pitched, monotonous and stable 100Hz AC hum, which is mainly caused by the poor filtering of the power supply. It should focus on checking whether the power rectification, filtering and voltage stabilizing components are damaged. The virtual welding or failure of the decoupling capacitors at the power supply terminals of the front and rear amplifier circuits will also produce a low-frequency oscillation noise similar to hum.
Induced noise is a complex and harsh AC sound, mainly caused by poor grounding of the transfer switch and potentiometer in the pre-stage circuit or poor shielding of the signal wiring. The cracking sound refers to the intermittent "slap" and "click" sound. In the pre-stage circuit, check whether the signal input plug is in poor contact with the socket, transfer switch, potentiometer, etc., and the coupling capacitor is not welded or leaked. The post-amplifier circuit should check whether the relay contact is oxidized, whether the input coupling capacitor has leakage or poor contact. In addition, the soft breakdown of the differential input tube or the constant current tube in the post-stage circuit will also produce a "click" noise similar to sparks. White noise refers to the irregular continuous "rustle" sound, which is usually the background noise caused by the poor performance of the input stage transistors, field effect transistors or op amp integrated circuits in the front and rear amplifier circuits. Replacement of components with specifications.

3. Light volume

The so-called light fault refers to the audio signal in the process of amplification and transmission, due to a certain amplification stage change or attenuation in a certain link, so that the amplifier's gain decreases or the output power becomes smaller. When overhauling, you should first check whether the signal source and speakers are normal, and you can use alternative methods to check. Then check all kinds of transfer switches and control potentiometers to see if the volume can be increased.
If the above parts are normal, it should be judged whether the fault is in the pre-stage or post-stage circuit. For a channel with light sound, the signal output from the front-end circuit can be exchanged and input to the back-end circuit of the other channel. If the sound volume of the speaker does not change, the fault is in the back-end circuit; otherwise, the fault is in the front-end circuit Circuit. The sound caused by the post-amplifier circuit is mainly due to insufficient output power and insufficient gain. You can use the appropriate method to increase the input signal (for example, the signal output from the recorder to the speaker is directly added to the input end of the power amplifier circuit of the rear stage, change the volume of the recorder, and observe the change in the output of the amplifier) to determine what caused the cause.
If the output sound is large enough after the input signal is increased, it means that the output power of the power amplifier is sufficient, but the gain is reduced. It should be focused on checking whether the contact resistance of the relay contact increases, the input coupling capacitance decreases, and the isolation resistance increases. Negative feedback capacitor capacity becomes small or open circuit, negative feedback resistance increases or is open. If the input signal is increased, the output sound is distorted, and the volume does not increase significantly, indicating that the output power of the post-stage amplifier is insufficient. You should first check whether the positive and negative power supply voltages of the amplifier are low (if only one channel is light , It is not necessary to check the power supply), whether the performance of the power tube or the integrated circuit has deteriorated, and whether the resistance of the emitter has become larger. The sound caused by the transfer switch and potentiometer in the pre-stage circuit is light, and it is easier to find by visual inspection, and it can be cleaned or replaced. If it is suspected that a signal coupling capacitor has failed, it can be tested in parallel with the same value capacitor; the performance of the amplifier tube or op amp integrated circuit is poor, and it can also be checked by the substitution method. In addition, problems with negative feedback components can also cause the circuit gain to decrease.