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Ten Methods for Fault Diagnosis of Instruments

by:Sure     2021-08-15
Instrumentation circuit maintenance has always been an indispensable part in electronics companies. Because only through it can the original unqualified products finally leave the factory. However, maintenance is also the most complicated part of an electronics company. Because it not only needs to apply a lot of electronic expertise, but sometimes also needs a wealth of field experience. The following is my personal summary of repairs and instrumentation circuit repairs in electronics companies has always been an indispensable part. Because only through it can the original unqualified products finally leave the factory. However, maintenance is also the most complicated part of electronics companies. Because it not only needs to apply a lot of electronic expertise, but sometimes also needs a wealth of field experience. Let me share with interested friends the maintenance experience I personally summarized over the years. 1. Percussion hand pressure method    often encounters the phenomenon of good and bad when the instrument is running, most of this phenomenon is caused by poor contact or weak welding. For this situation, percussion and hand pressure can be used.   The so-called 'knock' is to tap the plug-in board or component lightly with a small rubber flap or other knocking objects to see if it will cause an error or shutdown failure on the part that may cause failure. The so-called 'hand pressing' means that when a fault occurs, the components, plugs and sockets that are inserted after the power is turned off are pressed firmly by hand, and then the power is turned on to try whether the fault will be eliminated. If you find that it is normal to tap the case, and then it is not normal, it is best to re-plug all the connectors firmly and try again. If it is unsuccessful, you have to find another way. 2. The observation method uses vision, smell and touch. Sometimes, damaged components will change color, blistering or burnt spots; burned components will produce some special smell; short-circuited chips will be hot; virtual soldering or desoldering can also be observed with the naked eye . 3. Elimination method The so-called elimination method is a method to determine the cause of the fault by plugging in some plug-in boards and components in the machine. When a plug-in board or device is removed and the meter returns to normal, it means that the fault has occurred there. 4. The replacement method requires two instruments of the same model or sufficient spare parts. Replace a good spare part with the same component on the faulty machine to see if the fault is eliminated. 5. The comparison method requires two instruments of the same model, and one of them is operating normally. To use this method, you must have the necessary equipment, such as a multimeter, an oscilloscope, etc. According to the nature of comparison, there are voltage comparison, waveform comparison, static impedance comparison, output result comparison, current comparison, etc. The specific method is: let the faulty instrument and the normal instrument run under the same conditions, and then detect the signals at some points and then compare the two sets of measured signals. If there are differences, you can conclude that the fault is here. This method requires maintenance personnel to have considerable knowledge and skills. 6. Temperature rise and fall method. Sometimes, the instrument will work for a long time, or when the working environment temperature is high in summer, it will malfunction. Turn it off and check it is normal, stop it for a while and turn it on again. It will malfunction after a while. This phenomenon is due to the poor performance of individual ICs or components, and the high temperature characteristic parameters fail to meet the index requirements. In order to find out the cause of the failure, the heating and cooling method can be used. The so-called cooling is to wipe anhydrous alcohol with cotton fiber on the part that may be malfunctioning when a malfunction occurs to cool it down, and observe whether the malfunction is eliminated. The so-called warming is to artificially raise the ambient temperature, for example, use an electric soldering iron to put it close to the suspicious part (note that the temperature must not be raised too high to damage the normal components) and try to see if the fault occurs. 7. Shoulder riding method Riding shoulder method is also called parallel method. Put a good IC chip on the chip to be inspected, or connect a good component (resistance capacitor, diode, triode, etc.) in parallel with the component to be inspected, and keep good contact. If the fault is caused by an open circuit or Reasons such as poor contact can be ruled out by this method. 8. Capacitor bypass method When a certain circuit produces a strange phenomenon, such as a display disorder, the capacitor bypass method can be used to determine the part of the circuit that is probably malfunctioning. Connect the capacitor across the power and ground terminals of the IC; connect the transistor circuit across the base input or collector output, and observe the effect on the failure phenomenon. If the input terminal of the capacitor bypass is invalid and the fault disappears when the output terminal is bypassed, it is determined that the fault has occurred in this stage of the circuit. 9. State adjustment method Generally speaking, before the fault is determined, do not touch the components in the circuit casually, especially the adjustable devices, such as potentiometers. However, if you take pre-reference measures (for example, make a position mark or measure a voltage or resistance value before touching it), you can still allow touching if necessary. Maybe the fault will be eliminated sometimes after the change. 10. Isolation method The fault isolation method does not require the same type of equipment or spare parts for comparison, and it is safe and reliable. According to the fault detection flow chart, segmentation and encirclement gradually narrow the fault search range, and then with signal comparison, component exchange and other methods, the fault will generally be found quickly.
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