How to break down and repair the gongs

- Nov 12, 2019-

01 machine does not work 

The failure of the whole machine does not work. After the power is turned on, the amplifier has no display, and all the function keys are invalid, and there is no sound, just like when the power is not supplied.

The power circuit should be checked first during maintenance. Use a multimeter to measure the DC resistance at both ends of the power plug (the power switch should be turned on). Normally, there should be hundreds of ohms. If the measured resistance is much smaller and the power transformer is severely heated, it indicates that the primary circuit of the power transformer has a partial short circuit; if the measured resistance is infinite, check whether the fuse is blown, whether the primary winding of the transformer is open, the power cord and the plug There is no disconnection between them. Some machines have added temperature protection devices, and a current fuse is connected to the primary circuit of the power transformer (usually installed inside the power transformer, the insulation paper outside the transformer can be removed), and it will also make the power transformer primary after damage. The circuit is open.

If the resistance of both ends of the power plug is normal, you can measure whether the output voltage of the power supply circuit is normal. For amplifiers that use a system-controlled microprocessor or logic control circuit, it is important to check that the supply voltage (usually +5V) of the control circuit is normal. If there is no +5V voltage, the input voltage of the three-terminal regulator IC 7805 should be measured. If the input voltage is not normal, the rectifier and filter circuits should be checked. If the voltage of the 7805 input terminal is normal, and the output terminal has no voltage of 10V or the voltage is low, you can disconnect the load and 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 supply voltage of the system control circuit is normal, check whether the clock and reset signal of the microprocessor are normal, whether the key control and display drive circuit are damaged.

02 loud noise

The noise of amplifier includes AC noise, burst noise, induced noise and white noise.During maintenance, it is necessary to determine whether the noise comes from the front stage or the back stage circuit. The signal connection plugs of the front and rear stages can be removed. If the noise is obviously reduced, the fault is in the front stage circuit; otherwise, the fault is in the rear stage circuit. AC sound refers to the low, monotonous and stable 100 Hz AC hum, which is mainly caused by the poor filtering of the power supply. It is important to check whether the rectifier, filter and voltage stabilizing elements of the power supply are damaged. When the decoupling capacitor at the power supply end of the front and rear stage amplifier fails to solder or fail, a kind of low-frequency oscillation noise similar to AC sound will also be produced.Induced noise is a complex and harsh AC sound, which is mainly caused by poor grounding of the transfer switch and potentiometer in the previous circuit or poor shielding of the signal wiring. Popping sound refers to the intermittent "crack" and "click" sound. In the front-end circuit, it is necessary to check whether the signal input plug is in poor contact with the socket, transfer switch, potentiometer, etc., and whether the coupling capacitor has faulty welding, leakage, etc. The post amplification circuit shall check whether the relay contact is oxidized, and whether the input coupling capacitance has leakage or poor contact. In addition, the soft breakdown of the differential input or constant current transistor in the later stage circuit will also produce the "KaKa" noise similar to the electric spark. White noise refers to the irregular continuous "sanding" sound, which is usually the background noise caused by the poor performance of input level transistors, field effect transistors or operational amplifier integrated circuits in the front and rear stage amplification circuits. During maintenance, components of the same specification can be used for replacement.

03 light volume

The so-called sound light fault refers to the audio signal in the process of amplification and transmission, because of a large number of changes in amplification stage or attenuation in a certain link, the amplifier's gain or output power decreases. During maintenance, first of all, check whether the signal source and speaker are normal, which can be checked by means of replacement. Then check all kinds of change-over switches and control potentiometers to see if the volume can be increased.

If all the above parts are normal, it should be judged whether the fault is in the front stage or in the back stage circuit. For a channel with light tone, the signal output from its front stage circuit can be exchanged and input to the rear stage circuit of another channel. If the sound size of the speaker is unchanged, the fault is in the rear stage circuit; otherwise, the fault is in the front stage circuit. There are two main reasons for the light tone caused by the post amplifier circuit: insufficient output power and insufficient gain. The input signal can be properly increased (for example, the signal output from the recorder to the speaker can be directly added to the input of the power amplifier circuit of the later stage, the volume of the recorder can be changed, and the change of the power amplifier output can be observed) to determine the cause.

If the output sound is large enough after increasing the input signal, it means that the output power of the power amplifier is sufficient, but the gain is reduced. It is important to check whether the contact resistance of the relay contacts increases, the input coupling capacitance decreases, the isolation resistance increases, the negative feedback capacitance decreases or opens, and the negative feedback resistance increases or opens. If the output sound is distorted and the volume does not increase significantly after the input signal is increased, it means that the output power of the later stage amplifier is insufficient. First, check whether the positive and negative power supply voltage of the amplifier is low (if only one channel is light, it is not necessary to check the power supply), whether the performance of the power tube or integrated circuit is poor, and whether the resistance value of the emitter resistance is large, etc.

The sound caused by the change-over switch and potentiometer in the front-end circuit is light, which can be easily found by visual inspection and can be cleaned or replaced. If it is suspected that a signal coupling capacitor fails, it can be tested in parallel with the same value capacitor; if the performance of the amplifier or the operational amplifier integrated circuit is poor, it can also be checked by substitution. In addition, the negative feedback element has problems, which will also cause the circuit gain to drop.

In the actual operation of the mixer, if you don't have some relevant knowledge, it's quite difficult to operate. At the same time, its sound quality effect is often unsatisfactory. Therefore, in order to improve the output effect of the mixer, it is very important to learn and understand the relevant knowledge of the mixer.1. Harsh sound: cause: high frequency energy is too strong; solution: attenuation 6KHz;

2. The voice has restlessness: cause: high frequency is not smooth; solution: attenuation 8k-1okhz;

3. Sound hairiness: cause: UHF energy is too strong; solution: attenuation 12k-16khz;

4. Dry microphone and tired singing: cause: insufficient medium frequency energy of microphone; solution: increase 800Hz frequency band;

5. Hard Bass: cause: the upper limit energy of subwoofer is too strong; solution: attenuation 125-160hz;

6. Bass dispersion: cause: the low frequency lower limit energy is too large; solution: increase the subwoofer lower limit frequency