Ten Methods for Fault Diagnosis of Instruments and Meters

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Circuit repair for instruments and meters has always been an essential part of electronics companies. Because only through it can products that are otherwise unqualified eventually make it out of the factory. However, maintenance is also the most complex part in electronics companies. Because it requires not only extensive knowledge of electronics but sometimes also rich on-site experience. I would like to share with friends who are interested the repair experience I have accumulated over the years. 1. Tap and pressure method: It is common to encounter situations where the instrument operates well at times and poorly at other times; this phenomenon is mostly caused by poor contact or weak soldering. For this situation, tapping and hand pressure methods can be used. 　　The so-called “tapping” involves gently tapping the circuit board or components with a small rubber mallet or another striking tool on the areas where failures might occur, to see if this will trigger errors or shutdowns. The so-called “manual pressing” involves, when a fault occurs, turning off the power supply and then manually pressing the connected components, as well as the plugs and sockets, firmly together, before turning the power on again to see if the fault is resolved. If it works when the casing is tapped once but stops working when tapped again, it’s best to reseat all connections firmly before trying again; if that doesn’t work and proves troublesome, another solution will have to be found. 2. Observation method Utilize vision, smell, and touch. Sometimes, damaged components will change color, develop bubbles, or show burnt spots ； Burnt components emit a distinct odor ； Short-circuited chips will get hot ； Solder defects or poor solder joints can be observed with the naked eye. 3. Elimination method The so-called elimination method is a technique for determining the cause of a fault by inserting and removing various circuit boards and components within the device. When the instrument returns to normal after removing a certain plug-in board or component, it indicates that the fault lies there. 4. Replacement method: Two instruments of the same model are required, or there must be sufficient spare parts available. Replace a good spare part with the same component on the faulty machine to see if the fault is resolved. 5. Comparison method: Two instruments of the same model are required, one of which is operating normally. To use this method, necessary equipment is also required, such as a multimeter, oscilloscope, etc. Based on the nature of comparison, there are voltage comparison, waveform comparison, static impedance comparison, output result comparison, current comparison, etc. 　　The specific method is to operate the faulty instrument and the normal instrument under the same conditions, then measure the signals at certain points and compare the two sets of measured signals; if there are differences, it can be concluded that the fault lies here. This method requires maintenance personnel to have considerable knowledge and skills. 6. Temperature rise and fall method 　　Sometimes, after the instrument has been in operation for a long time, or when the operating temperature is high in summer, faults may occur. Turning it off for inspection shows that everything is normal; after waiting for a while and turning it on again, it works fine, but the fault reappears after some time. This phenomenon is caused by the poor performance of individual ICs or components, with their high-temperature performance parameters failing to meet the specified requirements. To determine the cause of the fault, the temperature rise and fall method can be used. 　　The so-called cooling method involves, when a fault occurs, using cotton fibers to apply anhydrous alcohol to the area where the fault might occur in order to cool it down, and then checking whether the fault is resolved. By \"raising the temperature,\" it means artificially increasing the ambient temperature; for example, using a soldering iron to heat up the suspicious area (be careful not to raise the temperature too much, as this could damage normal components) to see if the fault occurs. 7. Shoulder Riding Method The shoulder riding method is also known as the parallel method. Place a good IC chip on top of the chip to be inspected, or connect good components (resistors, capacitors, diodes, transistors, etc.) in parallel with the component under inspection, ensuring good contact. If the fault is caused by an internal open circuit or poor contact within the device, this method can be used to resolve it. 8. Capacitor bypass method When a circuit exhibits unusual behavior, such as a distorted display, the capacitor bypass method can be used to determine the approximate section of the circuit that is faulty. Connect a capacitor across the IC’s power and ground terminals ； A transistor circuit is connected across the base input or collector output to observe the effect on fault phenomena. If the input terminal connected to the capacitor in bypass mode is ineffective, but the fault disappears when its output terminal is bypassed, it can be determined that the fault lies in this stage of the circuit. 9. Status adjustment method Generally speaking, before the nature of the fault is determined, one should not casually touch the components in the circuit, especially those that are adjustable, such as potentiometers. However, if reference measures are taken in advance (such as marking the position or measuring voltage or resistance values before making contact), contact is still allowed when necessary. Perhaps the faults will disappear sometimes after the change. 10. Isolation method The fault isolation method does not require devices or spare parts of the same model for comparison, and it is safe and reliable. According to the fault detection flowchart, the search area for the fault is gradually narrowed down through segmentation and enclosure, and by combining methods such as signal comparison and component swapping, the location of the fault can generally be identified quickly.