Inspection and repair skills of tube power amplifier

by:Gisen     2020-06-20
1. Measurement before power on For a faulty machine, through understanding and observation, if you can not determine the location and location of the fault, you must not rush to power, the following measurements should be taken. 1. Measure the resistance between the DC high voltage circuit and the bottom line, and check whether the high voltage circuit is shorted to ground or whether there is an open circuit in the bleeder resistance. If the short circuit between the high-voltage circuit and the ground is not excluded and the power is turned on directly, the rectifier tube or the power transformer may be easily burned. Some amplifiers have an open discharge resistor, which will cause the curtain voltage to be too high, the power tube screen will increase and the screen will become red or damaged, and even affect the power supply part, causing the fault to expand. The most vulnerable component in the circuit is the 5Z3P, the rectifier screen and the cathode will meet. 2. Measure the output circuit to understand the output load. If no load (speaker) is connected, the secondary transformer load impedance of the output transformer is equal to the maximum, and the impedance reflected to the primary is also maximum. In this way, when the power is turned on, as long as a little signal current passes through the primary of the output transformer, extremely high signal voltage will be generated at both ends of the primary winding, which is easy to break down the output transformer or cause the internal flashover damage of the power tube, causing the fault to expand. The method of measuring the output circuit is to use the multimeter RX1 file, connect the two test leads to the primary winding of the output transformer to the two ends of the power tube screen, one test pen is fixed, and the other test pen touches on and off. , The surface output circuit is normal; if there is no sound, the surface output circuit is open or short-circuited. There is no sound from the loudspeaker, it is not necessarily the loudspeaker or its wire is open circuit, it may also be the output transformer primary winding is open circuit. 3. Measure the resistance between the AC incoming circuit and understand whether there is serious leakage or collision between the incoming circuit and the ground to prevent the chassis from being electrified after being energized. It is accidentally punctured during inspection, causing damage to the machine or hurting people, or Leave hidden dangers to users. The resistance between the primary circuit of the power transformer and the ground should be as large as possible, at least not less than 10MΩ. If it is lower than 10MΩ, it indicates that the insulation layer between the primary winding and the core is broken or short-circuited to ground at a certain point. 2. Measurement after power-on The power amplifier stage is the main load of the DC power supply of the amplifier. Some of its failures are very harmful to the power supply part. Therefore, the amplifier should measure the power amplifier stage as follows immediately after power-on. 1. Measure the screen voltage of the power tube to see if it is consistent with the voltage value required by the machine. It is not normal if it is too high or too low, so please pay attention. 2. Measure the power grid voltage to see if it is consistent with the voltage value required by the machine of this model. If it is too high, it will be too low to be normal. The measurement of the bias voltage of the power tube grid needs to be carried out in the following three steps, namely: measuring the cathode voltage or fixed negative pressure; measuring the transmission of the bias voltage, and measuring whether the grid has a leakage positive voltage. III. Analysis of power amplifier level measurement 1, measured screen. Curtain grid voltages are lower. At this time, the fault is in the power supply part. Because if the power supply part is normal, only when the screen and curtain grid currents of the power tube are relatively large, will the load supplied by the power supply be too heavy, and the output DC high voltage will be reduced. Now the cathode voltage is also low. Obviously the current of the screen and curtain grid is small instead of large, and the load supplied by the power supply is light rather than heavy, so the fault can only be in the power supply part. If the AC input voltage is normal, the cause of the high current drop may be the failure or opening of the first filter capacitor at the output of the rectifier. If this is the reason, replace it with a capacitor in parallel with it, and the high voltage will rise to the normal value. In addition, it is also possible that the fault is severe aging of the rectifier tube, the full-wave rectifier becomes a half-wave rectifier or the rectifier filament voltage is too low. 2. The two power tube screens are measured. The curtain grid voltage is low, and the cathode voltage is zero or almost zero. The reason is that the self-sufficient grid is biased between the cathode and the ground. Because the cathode and the ground are shorted, the power tube has no grid bias (fixed grid bias) The open or short circuit of the power supply also makes the power tube without grid bias), which makes the screen current and curtain grid current very large, causing the load supplied by the power supply to increase and the DC high voltage output by the power supply to decrease. If such a fault occurs, the power supply should be cut off immediately, otherwise the power tube will be damaged due to too much screen current, and the screen will burn red quickly, even harming the power supply part. 3. The two power tube screens were measured. Curtain grid and cathode voltages are very high. The cause of the failure is that the cathode circuit is open. After the cathode is open, the screen and curtain grid power supply is zero or very small because there is no loop. The DC voltage supplied by the power supply is also increased due to the reduction of the load. . Screen pole. The curtain grid current is zero, and the cathode voltage is also zero, but when measuring the cathode circuit, because the circuit in the multimeter communicates the cathode with the ground, the trace current will deflect the pointer greatly through the multimeter circuit, and the voltage value it points will be The voltage is higher than the normal cathode, in fact there is no voltage drop in the cathode. This kind of fault makes the power amplifier not work, the power supply load is light, and the DC high voltage is much higher than the normal load. If the withstand voltage of the filter capacitor is insufficient or the quality is poor, it may be broken down, so the power supply should be cut off as soon as possible. 4. The measured voltage of the screen electrode and curtain grid is too high, and the cathode voltage is too low. In this case, it may be that only one of the two power tubes works. If the single tube works, the load supplied by the power supply will be reduced due to the reduction of the total current, so that the mainstream voltage output by the power supply will be higher. Therefore, the measured screen pressure and curtain pressure are high. However, the cathode voltage is not reduced by half due to the single-tube operation. This is because the reduction of negative bias voltage will inevitably increase the current of the screen and curtain grid of the working tube, which in turn causes the increase of the cathode voltage. Therefore, the cathode voltage during single-tube operation will not be halved, and will only be about 3 to 5V lower. When the single-tube power amplifier stage with fixed grid bias voltage works, the negative bias voltage is not affected, but due to the high screen voltage and curtain grid voltage, especially the high screen voltage, the screen flow of the working power tube will increase. Large, single-tube work is easy to make a working tube seriously aging, its performance is similar to single-tube work, the screen flow of two power tubes should be measured separately to judge. 5. The measured screen voltage is too high, and the screen voltage and cathode voltage are low. The reason may be that the buck resistance of the curtain grid (usually an adjustable wire-wound resistor) is adjusted to be greater than the required resistance value, or the self-replacement resistance is greater than the required resistance value. Because the voltage drop resistance of the curtain grid is too large, the voltage of the curtain grid will inevitably be reduced, and the screen current will also be reduced. The screen current is reduced, and the load of the power supply is reduced, so that the screen voltage is high. The effect of the screen pressure on the screen flow is more obvious. If the screen pressure is too high, the screen will be very red: if the screen pressure is too low, it will reduce the output power of the amplifier. Therefore, the screen voltage should be adjusted to Specified value. The power amplifying part and the power supply part are the most critical and most important part of the tube amplifier, and are also the parts with the highest faults. A megaphone passes the above measurement before and after power-on. It is difficult to find out where the fault is. Troubleshooting for tube amplifier Playing a tube amplifier has accumulated some troubleshooting methods. You can refer to it for your own opinions, only for my humble opinion! If the old burners in the altar have better experience, they can share together: 1: Many amplifiers have left and right channels with different sounds It is recommended that friends turn off the machine, swap the left and right channel tubes, and try again after booting: or replug the link cable: or replace the tube. 2: After the machine snapped, the power can no longer work normally after power off At this time, friends should not worry, first open the fuse holder and check whether the fuse is burnt. Normally, every machine has spare fuses, and replace the bad ones. Just restart the machine: if it is not turned on, it may be that the rectifier is broken, and replace it with another one.
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