Maintenance and repair skills of industrial instruments

1 overhaul

As long as proper maintenance steps are taken, scientific maintenance methods are followed, the technical performance of the instrument is understood, the working principle of the circuit is well studied, the basic measurement methods are familiar, and the fault inspection skills are mastered, it is not difficult to find out its regularity.

2 power failure inspection

To repair instruments and meters, one must first have a certain basic circuit and theoretical knowledge of electronic circuits, know how to use and operate common test instruments correctly, know how to check the causes of instrument failures, and follow scientific work procedures on this basis.

(1) Carefully read the instruction manual of the instrument to understand its working principle, understand the circuit diagram and the status of various accessories and spare parts.

(2) Observe whether the switch knobs, dials, indicators, etc. on the instrument panel are loose, and whether the fuse is intact.

(3) Open the cover of the instrument and meter shell, and observe whether the internal components, power transformer connection, etc. are burnt, moldy, or falling off.

While performing power-off inspection, not only can the cleaning work inside the instrument be done well, but more importantly, various obvious fault locations can be found, so as to judge and eliminate faults faster. For example: when the electronic tube leaks, there is a milky white substance in the tube. When the electrolytic capacitor breaks down and deteriorates, most of the electrolyte flows out or the shell bursts.

When the resistor is damaged, it will turn black or burnt. When diodes, triodes, and Zener tubes break down, the shells will also turn black, burnt, and burst. When the transformer burns out, there is a burnt smell and traces. Therefore, the power-off inspection is an important step in the maintenance of instruments and meters.

However, it should be pointed out that special attention should be paid during maintenance. It is not enough to simply replace the damaged device, but to further check the circuit diagram of the instrument to really find out the location and cause of the damaged device, so as to analyze the real cause of the damage.

For example, in the SP2305 gas phase maintenance, when the carrier gas is not turned on before starting up (violating the operating regulations), the tungsten wire of the thermal conduction cell is blown out, and there is no bridge current at this time. Careful inspection will find that the thermally conductive tungsten wire is blown. Using this method, only a multimeter will be used to measure the fault location when the power is off, which is what we often encounter in maintenance. During the maintenance process, some found that the components in the circuit were desoldered and had poor contact, which would affect the normal operation of the circuit. Therefore, the power-off inspection method is relatively intuitive and effective.

3 power transmission inspection

If no problem is found during the power-off inspection, power-on inspection should be used to further find the specific location of the instrument failure, so as to replace the component that caused the failure. In order to avoid the expansion of instrument failures and to facilitate repeated observations, the method of gradually increasing the power supply can be used. When checking power transmission, be sure to pay attention to whether the indicator light is on or not, whether the fuse, transformer, and rectifier are working normally. If any abnormal phenomenon is found, the power should be turned off immediately, and the voltage of the voltage regulator should be adjusted back to 0V. If the damaged components cannot be seen clearly, you can turn it on again and gradually increase the voltage for observation. The purpose of this is to prepare for the following transmission inspection. If there are no abnormal phenomena at this time, the live measurement of each part of the circuit of the instrument can be performed. The measurement sequence is generally: rectified input voltage, stabilized output voltage, working current, voltage and static operating point voltage at all levels, and find out where the fault is. The method is as follows.

3.1 Measuring voltage method

According to the schematic diagram of the instrument, measure the corresponding voltage values of the voltage points at all levels. If the voltage measured everywhere is very different from that 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 approach works well whether in tubes, transistors or integrated circuits.

3.2 Simple interference method

This method is especially suitable for high-end recorders and digital processors. When a small screwdriver is used to bump the signal input terminal, observe whether the pointer moves. Using this method, it can be carried out step by step under the condition of power transmission, and the collision is carried out step by step from the rear stage to the front. If there is no reflection on which stage it encounters, it means that the fault is on that stage.

3.3 Injection signal method

Use the external difference as the detection signal, and use the terminal indicator of the machine to indicate the test result. That is to use the corresponding external signal source to start injecting from the input end of the instrument terminal indicator to be repaired, and then advance to the previous circuit in sequence, inject the test signal to the input end of each level of circuit, and observe whether the instrument terminal indicator responds at the same time To determine the location of the fault and analyze the cause of the fault. Depending on which stage the signal is injected into, if the terminal indicator does not respond, the fault is on that stage. This is also a relatively simple and effective method.

3.4 Oscilloscope method

This is mainly because dynamic measurement is often used, that is, dynamic measurement should be used when the static measurement of the repaired instrument is normal and no fault location is found. Use the oscilloscope to observe the schematic diagram step by step, and analyze the waveform of the relevant position, and you can also find the exact location of the fault. This method is often used in the electronic amplification part of the maintenance automation instrument, 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 skilled operation and correct use of an oscilloscope to observe various signals with different changing laws.

3.5 Device Substitution Method

When the approximate scope of instrument failure cannot be determined, unit replacement or single board replacement can be used. Note that it is best not to disassemble the components in the circuit, especially precision instruments, let alone dismantle them casually. Devices of the same model, same specification, and same structure can be used first. Plug-in parts of the unit are used to temporarily replace the part in question and observe its effect on the failure. If the fault disappears, it indicates that the replaced part is faulty. This method is especially suitable for electronic tube circuits or printed circuit board circuits, integrated circuits.

3.6 Step-by-step disconnection method

The circuit composition of some instruments and meters is relatively complex, involving many components and components, and they affect each other or have influence in many ways.

Some instrument failures appear 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, the stages can be separated from the back to the front until the fault is found. When using this method, it is necessary to 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 due to disengagement of a certain part.

4 Troubleshooting

After the faulty part of the instrument and meter being overhauled and the cause of the fault are found out, the power can be cut off to deal with the fault.

In the process of processing, we must pay attention to: (1) the power supply must be cut off, and the power supply must be shut down for processing; (2) when components need to be replaced, try to use qualified components of the same model and specifications as the original components. If not available, components with similar specifications to 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 welding field effect tubes or integrated circuits, be careful to prevent damage to components caused by leakage and static electricity. Therefore, the electric soldering iron is required to be well grounded, otherwise the power should be cut off for welding; (4) When the power transformer is damaged and needs to be rewound, it is necessary to find out the data of each winding of the original transformer, and try to keep the original parameters.

In short, all instruments and meters must be restored to normal functions after troubleshooting, and maintenance records must be made as required, so as to provide a basis for the long-term use of the instruments. For maintenance personnel, it is also an opportunity to combine theory and practice, and they should do their due diligence