Introduction to Ten Methods for Fault Diagnosis of Multimeters and Other Instruments
1. Knocking hand pressure method
It is common to encounter the phenomenon of instruments running on and off, which is mostly caused by poor contact or virtual soldering. For this situation, tapping and hand pressing methods can be used.
The so-called "tapping" refers to gently tapping the plug-in board or component with a small rubber hammer or other striking object to see if it will cause errors or shutdown faults in the parts that may cause faults. The so-called "hand pressure" refers to when a malfunction occurs, turning off the power and re pressing the inserted components, plugs, and sockets with your hands, and then turning them on again to see if it will eliminate the malfunction. If you find that knocking the casing is normal, but knocking it again is not normal, it is best to re plug all the joints and try again. If it is troublesome and unsuccessful, you have to find another way.
2. Observation method
Utilize visual, olfactory, and tactile senses. Sometimes, damaged components may discolor, blister, or show burnt spots; Burnt devices may produce some special odors; Short circuited chips will heat up; Virtual or detached solder joints can also be observed with the naked eye.
3. Exclusion method
The so-called troubleshooting method is to determine the cause of a malfunction by plugging and unplugging some plug-in boards and components inside the machine. When a certain plug-in board or component is unplugged and the instrument returns to normal, it indicates where the fault occurred.
4. Substitution method
Require 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 has been eliminated.
5. Comparative method
Two instruments of the same model are required, and one of them is operating normally. To use this method, it is necessary to have necessary equipment, such as a multimeter, 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 to operate the faulty instrument under the same conditions as the normal instrument, then detect signals at certain points and compare the two sets of signals. If there are differences, it can be concluded that the fault is located here. This method requires maintenance personnel to have considerable knowledge and skills.
6. Temperature rise and fall method
Sometimes, when the instrument works for a long time or when the working environment temperature is high in summer, it will malfunction. After shutting down and checking, it will be normal. After stopping for a period of time and then turning on again, it will be normal, and after a while, it will malfunction again. This phenomenon is caused by the poor performance of individual ICs or components, and the inability of high-temperature characteristic parameters to meet the requirements. To identify the cause of the malfunction, the temperature rise and fall method can be used.
The so-called cooling refers to using cotton fibers to wipe anhydrous alcohol on the area where the malfunction may occur, to cool it down and observe whether the malfunction has been eliminated. The so-called temperature rise refers to artificially raising the ambient temperature, such as placing an electric soldering iron close to a suspicious area (be careful not to raise the temperature too high to damage normal components) to see if a malfunction has occurred.
7. Shoulder riding technique
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 to be inspected and maintain good contact. If the fault is caused by internal open circuits or contact , this method can be used to eliminate it.
8. Capacitor bypass method
When a certain circuit experiences strange phenomena, such as display confusion, the capacitor bypass method can be used to determine the approximate faulty part of the circuit. Connect the capacitor across the power and ground terminals of the IC; Cross connect the transistor circuit at the base input or collector output and observe the impact on the fault phenomenon. If the capacitor bypass input terminal is invalid and the fault disappears when bypassing its output terminal, it is determined that the fault is in this * * circuit.
9. State adjustment method
Generally speaking, before the fault is determined, do not touch the components in the circuit casually, especially adjustable devices such as potentiometers. However, if multiple reference measures are taken in advance (such as marking the position or measuring the voltage or resistance value before triggering), triggering is still allowed if necessary. Perhaps sometimes the malfunction will be eliminated after the change.
10. Isolation method
The fault isolation method does not require comparison of equipment or spare parts of the same model, and is safe and reliable. According to the fault detection flow chart, the fault search scope is gradually reduced by dividing and surrounding, and then combined with signal comparison, component exchange and other methods, the fault will generally be found quickly.
