Top ten methods for instrumentation fault diagnosis - Solutions - Huaqiang

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Instrumentation circuit maintenance has always been an indispensable part of electronics companies. Because only through it can the original unqualified products finally leave the factory. However, repairs are also the most complex part of an electronics company. Because it not only uses a lot of electronic expertise, but also requires a wealth of field experience. Let me share with my interested friends about my personal maintenance experience over the years.
1. The tapping pressure method often encounters the phenomenon of good and bad operation of the instrument. Most of this phenomenon is caused by poor contact or virtual welding. For this case, tapping and hand pressing can be used.
The so-called "knocking" is to gently knock the board or part through a small rubber hammer or other tapping object to see if it will cause an error or a downtime. The so-called "hand pressure" is when the fault occurs, after the power is turned off, the inserted components and the plug and the seat are again pressed by hand, and then the power is turned on to try to eliminate the fault. If you find that the casing is normal and the knocking is not normal, it is best to re-insert all the joints and try again. If the brain is unsuccessful, you have to find another way.
2. The observation method uses vision, smell, and touch. In some cases, damaged components will change color, blister or burnt spots; burnt devices will produce some special odor; short-circuited chips will be hot; visible or desoldered parts can also be observed with the naked eye. .
3. Exclusion method The so-called elimination method is a method for judging the cause of a fault by plugging in some plug-in boards and devices inside the machine. When the instrument returns to normal after removing a board or device, the fault has occurred there.
4. The replacement method requires two instruments of the same model or sufficient spare parts. Replace a good spare 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 is in normal operation. Use this method to have the necessary equipment, such as a multimeter, oscilloscope, etc. According to the nature of the comparison, voltage comparison, waveform comparison, static impedance comparison, output comparison, current comparison, etc.
The specific method is: let the faulty meter and the normal meter run under the same conditions, and then detect the signals of some points and then compare the two groups of signals measured. If there are differences, it can be concluded that the fault is here. This method requires the maintenance personnel to have considerable knowledge and skills.
6. Lifting and heating method Sometimes, the instrument works for a long time, or when the working environment temperature is high in summer, it will be faulty. The shutdown check is normal. After stopping for a while, the machine is turned on again and normal. After a while, it fails again. This phenomenon is caused by the poor performance of individual ICs or components, and the high-temperature characteristic parameters fail to meet the requirements of the index. In order to find out the cause of the malfunction, the temperature rise and fall method can be employed
The so-called cooling, that is, when the fault occurs, use cotton fiber to wipe the anhydrous alcohol in the part that may be faulty, to cool it, and observe whether the fault is eliminated. The so-called heating is artificially raising the ambient temperature, such as using a soldering iron to close the suspected part (be careful not to raise the temperature too high to damage the normal device) to see if the fault occurs.
7, riding the shoulder method riding the shoulder method is also called parallel method. Place a good IC chip on the chip to be inspected, or connect good components (resistance capacitors, diodes, transistors, etc.) in parallel with the components to be inspected, and keep in good contact. If the fault is caused by an open circuit inside the device or This method can be used to eliminate the cause of poor contact.
8. Capacitor Bypass Method When a certain circuit produces a strange phenomenon, such as when the display is confusing, the capacitor bypass method can be used to determine the part of the circuit that is probably faulty. Connect the capacitor across the power supply and ground of the IC; connect the transistor circuit across the base input or collector output to observe the effect on the fault. If the capacitor bypass input is inactive and the fault disappears when bypassing its output, it is determined that the fault is present in this stage of the circuit.
9, state adjustment method In general, before the fault is not determined, do not touch the components in the circuit, especially adjustable devices, such as potentiometers. However, if a reference is taken in advance (for example, position mark or measured voltage value or resistance value before untouched), it is allowed to be touched if necessary. Maybe after the change, sometimes the fault will be eliminated.
10. Isolation method The fault isolation method does not require the same type of equipment or spare parts for comparison, and is safe and reliable. According to the fault detection flow chart, the segmentation and encirclement gradually narrows the fault search range, and then cooperates with the signal comparison and component exchange methods, and generally finds the fault location.