Three cases of klystron high power amplifier maintenance
The high power amplifier is an important broadcasting device of the earth station. Its function is to amplify the high-frequency signal output by the up-converter into a high-power microwave signal that can meet the needs of satellite reception. At present, there are three types of klystron (KH-PA), traveling wave tube (TWTA) and all-solid state high power amplifier (SS-PA). Among them, the klystron high power amplifier has the advantage of large output power. Due to the need for safe broadcasting of programs, the State Administration of Radio, Film and Television requires that all upstream stations must be equipped with high-power klystron high-power amplifiers. For this reason, our station purchased the CPI GEN IV series 3 000W klystron high-power amplifiers in 2005 and has been used as a host. . The GEN IV series is a new generation of klystron high power amplifier. The power amplifier is composed of high voltage power supply, radio frequency components, control system and air cooling system. Its main feature is the use of multi-collector pole klystron (MSDC), and can be based on the output The size of the power automatically adjusts the tube bundle voltage and the filament voltage to reduce the power dissipation and extend the service life. Another feature is the use of a distributed micro-processing system, CAN internal bus, and a unified user interface. It is compatible with a variety of flexible communication methods and is convenient for remote monitoring. The standby machine still uses the American VAR IAN ZJ-2700M klystron high power amplifier purchased in the early stage of the station construction in 1994. VZJ- 2700M klystron high power amplifier is mainly composed of radio frequency amplification system, power supply system, control, monitoring, protection system, wind cooling system. Its transmission frequency range is 5. 85 ~ 6. 45 GH z, with 24 available channels, limiting gain 77 dB, rated output power 3 kW, 1 dB bandwidth 45MH z. Because the VAR IAN high power amplifier has a long service life and the performance of the klystron decreases, the maximum actual output power can only reach 1 600W. This model has long been discontinued. The biggest difficulty in maintenance is that there are no spare parts and no after-sales service. If the machine breaks down, it can only be repaired by itself. If the parts are damaged, only another disassembled part can be used. In recent years, VARIAN ZJ-2700M high power amplifier has two failures, and CPI GEN IV high power amplifier has a low wind pressure failure. The following describes the analysis and processing of the three failures. (1) Failure phenomenon: VAR IAN ZJ- 2700M klystron high-power amplifier starts showing EaterOvervoltage (filament voltage overvoltage), the whole machine does not work. Analysis and treatment: Under normal circumstances, the filament voltage is 6 V, the filament current is 7 A, and the equivalent resistance of the klystron according to the equivalent circuit principle is 0.857 Ω. According to the overvoltage protection warning prompt, the filament voltage should be higher than the normal value by 10% at this time (warning when the filament voltage is 10% higher). Due to the protection of the machine system due to excessive voltage, we cannot read the actual value of the filament voltage, and the actual value is very helpful to determine the fault. So we use the instantaneous reading method to obtain this important data. The specific method is to cancel the alarm instantly, read the actual value of the filament voltage, and the instantaneous high voltage will not cause damage to the equipment, and it can help us read the important data. The effect is good. Through this method, we read the actual value of the filament voltage of 8.3 V, which is 38% higher than the actual voltage. At this time, the equivalent resistance is 1.186 Ω, which is an increase of 0.329 Ω from the normal equivalent resistance of 0.857 Ω. According to experience, the resistance of wires and nodes can be neglected at low current and high resistance, and the resistance of wires and contacts must be considered at high current and low resistance, because the actual equivalent resistance only increases by 0.329 Ω, We must first consider whether the added resistance of 0.329 Ω is caused by the increase in the contact resistance. The specific measures are to check whether the contact is loose or rusty, and then remove all the connection contacts between the filament disk and the klystron and wipe them with alcohol. After the alcohol volatilizes, re-tighten the original contacts, and then restart the machine to check, the alarm disappears. Summary: In the case of low voltage and high current, pay attention to whether the contacts and terminals are in good contact. Due to the high current of the high power amplifier filament, the high power amplifier terminal should be regularly checked for loose rust. (2) Failure phenomenon: VAR IAN ZJ- 2700M klystron high power amplifier remote control panel has no display, and the received spectrum and carrier are normal. Analysis and processing: The normal reception spectrum and carrier does not affect the broadcast. We judge that the klystron host is not faulty, and the fault is in the remote control terminal. Remove the control terminal from the console, open the upper panel to check the circuit board, and find that the circuit board has no black components. When the power supply is under load, use a multimeter DC 20 V to measure the output voltage of the power supply is zero. Normally, it should be 5 V. It is judged that the remote control board power supply has failed. Continue to check forward. When the power bridge rectifier circuit is checked, two diodes are short-circuited. After replacing the same type of diode, the 5 V voltage output is normal. After restarting, the high power amplifier remote control panel returns to normal. Summary: This is a typical DC power output failure. Generally, the whole machine cannot be turned on without a display on the panel. There are many reasons for the failure. Problems with rectifier diodes, filter capacitors, and control circuits may cause this failure, depending on the actual situation. treat. (3) Trouble phenomenon: CPI GENIV klystron power amplifier has a 'LOW A IRFLOW FAULT' failure, which causes the high power amplifier RF output to be suppressed. Analysis and treatment: When the klystron power amplifier is working, the temperature is high, and it is necessary to ensure that sufficient exhaust air is supplied to the internal klystron for heat dissipation. There is an air pressure switch on the rear of the GEN IV power amplifier to monitor the air volume of the power amplifier. When the air volume is normal, the air pressure switch is normally open. When the air volume is reduced, the air pressure switch becomes normally closed.The power amplifier control system will suppress The RF output protects the internal klystron. Analysis shows that the LOW A IRFLOW FAULT failure may be a fan failure or too much dust in the air inlet filter, resulting in insufficient air flow. Check the fan and clean the dust filter and the cavity of the whole machine. The fault still exists. The fault occurred shortly after installation, and the analysis may be caused by the unreasonable initial setting of the air pressure switch. After adjusting the air pressure switch setting, the fault was eliminated after restarting, and no similar fault occurred after a period of observation. Compared with other upstream equipment, the failure rate of high power amplifiers is higher, the maintenance involves more areas, and the danger is also greater. Therefore, the maintenance of high power amplifiers has always been the focus of our station's maintenance. The klystron has a slightly higher failure rate than the all-solid high-power amplifier, and the maintenance difficulty is relatively increased, which requires higher ambient temperature and humidity. For key equipment such as high power amplifiers, our point of view has always been to prevent prevention. We should pay attention to observe the operation status of equipment, understand its performance characteristics, and do basic maintenance work for high power amplifiers to ensure their normal and smooth operation. The maintenance of klystron high-power amplifier must first ensure that the technical indicators of high-power discharge gas performance and mechanical performance meet the requirements; secondly, the system equipment configuration must be adjusted reasonably to extend the service life of the equipment; thirdly, the failure can be quickly eliminated to minimize the loss of failure. Based on years of experience, high power amplifier maintenance work includes: (1) Pay attention to maintaining the environmental temperature and humidity of the high-power amplifier room. According to experience, high-temperature and high-humidity environments in summer are the most likely to cause high power amplifier failures, especially the failure rate of high-voltage devices is significantly increased. Therefore, special attention should be paid to maintaining the temperature and humidity of the operating environment of the equipment in summer. (2) Pay attention to the dustproof of the high-power amplifier room, regularly clean the high-power amplifier dust filter, check the fan and fan operation, if the fan is not running smoothly or there is a noise, it should be replaced in time. (3) Record the main parameter display of the high-power amplifier panel on a daily basis, regularly analyze and sort the changes in the main parameter of the high-power amplifier and various alarm information, predict the possible faults and parts based on these information, and detect and adjust the parts that may occur And maintenance, and timely replacement of parts. (4) Measure the main technical indicators and test points of the high power amplifier, analyze and find the reasons for the changes in the indicators and test data, and make targeted adjustments or take appropriate emergency measures. High power amplifier failures account for a large proportion of total earth station failures. In daily maintenance, you should pay attention to accumulating experience. You must periodically open the chassis to clean and remove dust inside the machine. In other situations, especially check whether the power system connector screws are tight, ensure that the power system electrical connectors are in good contact, and the line insulation is good. When the klystron power amplifier is turned on, the filament resistance must maintain accurate rated values. Too high or too low will cause uneven emission on the cathode surface and accelerate cathode aging. Focus on checking whether the high-voltage capacitors, transformers, contactors and other components in harsh working conditions are abnormal, and whether various protective devices are sensitive and effective. Test the rated output power, gain and gain frequency response of the power amplifier once a year. Accurately analyze and deal with faults when they occur, to prevent equipment damage or fault enlargement caused by improper handling.