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Fault diagnosis method of instrument

Date:2019/7/25 520 times Size:Large medium Small

1. Percussion hand pressing method


When we use the instrument, we often encounter the phenomenon that the instrument is good or bad when it is running. The vast majority of this phenomenon is caused by poor contact or false welding. In this case, the method of knocking and hand pressing can be used.


The so-called "knocking" is to gently tap the plug-in board or components with a small rubber hammer or other knocking objects on the parts that may have faults, to see if it will cause errors or shutdown faults. The so-called "hand pressing" means that when a fault occurs, after turning off the power supply, the inserted parts, plugs and seats are pressed firmly by hand again, and then the machine is turned on to try whether the fault will be eliminated. If it is found that the knock on the casing is normal, and then the knock is abnormal, it is better to re plug all the connectors firmly and try again. If it is not successful, we have to find another way.


2. Observation method


Use vision, smell, touch. Sometimes, the damaged components will change color, blister or appear scorched spots; the burnt components will produce some special smell; the short-circuit chips will get hot; the false soldering or desoldering can be observed with naked eyes.


3. Exclusion method


The so-called elimination method is to judge the cause of the fault through some plug-in boards and devices in the plug-in machine. When a plug-in board or device is removed and the instrument returns to normal, the fault occurs there.


4. Substitution method


Two instruments of the same type or sufficient spare parts are required. Replace a good spare part with the same component on the faulty machine to see if the fault is eliminated.


5. Contrast method


It is required to have two instruments of the same model, and one of them is in normal operation. Using this method also has the necessary equipment, for example, 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: let the faulty instrument and the normal instrument operate under the same conditions, and then detect the signals of some points, and then compare the two groups of measured signals. If there is any difference, it can be concluded that the fault is here. This method requires maintenance personnel to have considerable knowledge and skills.


6. Ascending and descending temperature method


Sometimes, when the instrument works for a long time, or when the temperature of the working environment is high in summer, it will break down, and the shutdown check is normal. After a period of time, it will turn on again, and then it will break down again. This phenomenon is due to the poor performance of individual IC or components, and the high temperature characteristic parameters can not meet the index requirements. In order to find out the cause of the fault, the temperature rise and fall method can be used.


The so-called cooling is to use cotton fiber to wipe anhydrous alcohol on the parts where the fault may occur, so as to reduce the temperature and observe whether the fault is eliminated. The so-called warming is to artificially raise the ambient temperature, such as using an electric soldering iron to close to the suspicious part (pay attention not to raise the temperature too high to damage the normal device) to see if the fault occurs.


7. Shoulder riding


Shoulder riding method is also called parallel method. A good IC chip is installed on the chip to be inspected, or good components (resistance, capacitance, diode, triode, etc.) are connected in parallel with the components to be inspected to keep good contact. If the fault is caused by the internal open circuit or poor contact of the components, this method can be used to eliminate it.


8. Capacitance bypass method


When a strange phenomenon occurs in a certain circuit, such as display confusion, the capacitance bypass method can be used to determine the fault circuit. The capacitor is connected across the power supply and ground of the IC, and the transistor circuit is connected across the base input or collector output to observe the influence on the fault phenomenon. If the capacitor bypass input is invalid and the fault disappears when its output is bypassed, the fault will appear in this stage.


9. State adjustment method


Generally speaking, before the fault is determined, do not touch the components in the circuit, especially the adjustable devices, such as potentiometers. However, if multiple reference measures are taken in advance (for example, make position mark or measure voltage or resistance value before touching), touching is allowed when necessary. Maybe after the change, sometimes the fault will be eliminated.


10. Isolation method


The fault isolation method does not need the same type of equipment or spare parts for comparison, and is safe and reliable. According to the flow chart of fault detection, the scope of fault search is gradually reduced by dividing and encircling, and then combined with signal comparison, component exchange and other methods, the fault can be found quickly.


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