1. Power failure inspection To repair instruments and […]
1. Power failure inspection
To repair instruments and meters, you must first have a certain circuit foundation and theoretical knowledge of electronic circuits. You must understand the correct use and operation of commonly used test instruments, know how to check the causes of instrument failures, and follow scientific working procedures on this basis.
(1) Carefully read the instruction manual of the instrument, understand its working principle, understand the circuit diagram and the conditions of various accessories and spare parts.
(2) Observe whether the switch knob, dial, indicator, etc. on the instrument panel are loose, and whether the fuse is in good condition.
(3) Open the cover of the instrument housing and observe whether the internal components, power transformer connections, etc. are burnt, moldy, or fall off.
While performing power-off inspection, not only can the internal cleaning of the instrument be done well, but more importantly, various obvious fault locations can be found for faster judgment and troubleshooting. For example: when the electron tube leaks, there is a milky white substance in the tube. When the electrolytic capacitor is broken down and deteriorated, most of the electrolyte will flow out or the shell will burst. When the resistor is damaged, it will turn black or burnt. When diodes, transistors, and voltage regulator tubes are broken down, the shell will turn black, burnt, and burst. When the transformer burns out, there is a burning smell and traces. Therefore, power-off inspection is an important step in the maintenance of instrumentation. But it should be pointed out: pay special attention when repairing, not just replace the damaged device, but you should check the circuit diagram of the instrument to find out the location and reason of the damaged device in order to analyze the real cause of the damage. For example, in the maintenance of SP2305 gas chromatograph, when the carrier gas is not turned on (violating the operating regulations) before starting the machine, the tungsten wire of the thermal conductivity cell is burned, and then there is no bridge current. With this method, the fault location will be detected only by the multimeter when the power is cut off. This is what we often encounter in maintenance. During the maintenance process, some found that the components in the circuit were desoldered and poor contact, which would affect the normal operation of the circuit. Therefore, the power-off inspection method is relatively intuitive and effective.
2. Power transmission inspection
If the problem is not found in the power failure inspection, the "power transmission" should be used to check, and further find the specific part of the instrument failure, so as to replace the component that caused the failure. In order to avoid the expansion of instrument failures and facilitate repeated observations, a stepwise pressurized power supply method can be used. When checking the power supply, must pay attention to whether the indicator light is on? Whether the fuse, transformer, and rectifier are working properly. If any abnormality is found, the power supply should be turned off immediately and the voltage of the voltage regulator should be adjusted back to 0V. If you can’t see the damaged components clearly, you can also start up and gradually increase the voltage for observation. The purpose of this is to prepare for the following power transmission inspections. If there are no abnormal phenomena at this time, then each part of the circuit of the instrument can be charged for measurement. The measurement sequence is generally: rectified input voltage-stabilized output voltage-various levels of working current, voltage and static operating point voltage, from which to find the fault location, the method is as follows.
2.1 Measuring voltage method
According to the principle diagram of the instrument, measure its corresponding voltage value at its various voltage points. If the measured voltages at various places are very different from those listed in the instruction manual, it is the fault. If the measured voltage is not much different from the instruction manual, it means that the static operation of the circuit is normal. This method is very effective whether in electron tubes, transistors or integrated circuits.
2.2 Simple interference method
This method is especially suitable for the recorder and digital processor of the analytical instrument. When you hit the signal input end with a small screwdriver, observe whether the pointer moves. Using this method, it can be carried out step by step under the power supply situation, and the collision is gradually made from the latter stage to the forward. The level encountered is not reflected, indicating which level the fault is.
2.3 Signal injection method
Use different external signal sources as the detection signal, and use the terminal indicator of the machine to show the result of the test. That is, use the external corresponding signal source to inject from the input terminal of the terminal indicator of the instrument to be repaired, and then move to the previous circuit in sequence, inject the test signal to the input terminal of each circuit, and observe whether the terminal indicator of the instrument responds. To determine the location of the fault and analyze the cause of the fault. According to which level the signal is injected into, if the terminal indicator does not respond, which level is the fault. This is also a relatively simple and effective method.
2.4 Oscilloscope method
This is mainly used in dynamic measurement, that is, when the static measurement of the repaired instrument is normal, and the fault location is not found, dynamic measurement should be used. Use the oscilloscope to observe the schematic diagram step by step, and analyze the waveform at the relevant location, and you can also find the exact location of the fault. This method is often used in the electronic amplification part of the overhaul of automated instruments, and it is very successful. However, this method requires maintenance personnel to be familiar with the circuit principle and waveform changes of the instrument being repaired, and requires proficiency in operation and the correct use of an oscilloscope to observe various signals with different changing laws.
2.5 Device substitution method
When the approximate range of the instrument failure cannot be determined, the unit replacement or single board replacement method can be used. Be careful not to disassemble the components in the circuit, especially the precision instruments, and it is not allowed to disassemble them randomly. You can use devices of the same model, specification, and structure first. Unit plug-in parts to temporarily replace the questionable part, and observe its influence on the failure. If the fault disappears, it indicates that the replaced part is faulty. This method is particularly suitable for electronic tube circuits or printed circuit board circuits and integrated circuits.
2.6 Step by step disconnect method
The circuit composition of some instruments is more complicated, involving more components, and they influence each other or have influence in many aspects. Some instrument failures are manifested at a certain level, but the real part of the failure is at the next level. For example, in the counting circuit, because the input impedance of the next stage has too much effect on the load of the previous stage, the counting is stopped. When overhauling this type of instrument, you can separate all levels from back to front until the fault is found. When using this method, you must choose the location of the disconnection point. Only when the disconnection is done reasonably can the fault location be quickly determined. At the same time, it is necessary to prevent the voltage from being too high or too low in other parts due to the disconnection of a certain part.
When the fault location and the cause of the fault are found out, the power can be cut off to deal with the fault. Pay attention to:
(1) The power must be cut off, and the power must be shut down for processing;
(2) When components need to be replaced, they should be as good as the original components of the same model and specifications. If not, components with similar specifications as the same machine can be used instead. Try not to change the original circuit structure, so as not to affect the electrical performance of the instrument;
(3) When soldering field effect transistors or integrated circuits, be sure to prevent leakage and static electricity from damaging the components. Therefore, the electric soldering iron is required to be well grounded, otherwise it should be cut off and soldered;
(4) When the power transformer is damaged and needs to be rewinded, it is necessary to find out the data of each winding of the original transformer, and try to keep the original parameters.