Ten methods for diagnosing instrument faults

　　Instrument and meter circuit maintenance has always been an indispensable part of electronic companies. Because only through it can the originally unqualified products finally leave the factory. However, maintenance is also the most complicated part of electronic companies. Because it not only requires a lot of electronic professional knowledge, but also sometimes requires rich field experience. The following is my personal maintenance experience summarized over the years to share with interested friends.

　　1. Tap hand pressure method

　　It is common to encounter the phenomenon that the instrument works well and sometimes not. This phenomenon is mostly caused by poor contact or cold soldering. For this situation, you can use the knocking and hand pressure method.

　　The so-called "knocking" is to gently knock the plug-in board or component with a small rubber hammer or other knocking objects on the part that may cause the fault , to see if it will cause an error or shutdown fault. The so-called "hand pressing" is to turn off the power and press the plugged components, plugs and sockets firmly by hand when a fault occurs, and then turn on the machine to see if the fault can be eliminated. If it is found that it is normal to knock the case once, but it is not normal when knocked again, it is best to re-insert all the connectors and try again. If the brain-wracking method does not work, you have to find another way.

　　2. Observation method

　　Use your sense of sight, smell, and touch. Sometimes, damaged components will change color, bubble, or have burnt spots; burnt devices will produce some special smells; short-circuited chips will get hot; and cold solder joints or loose solder joints can be observed with the naked eye.

　　3. Elimination method

　　The so-called elimination method is to determine the cause of the fault by plugging in and out some plug-in boards and components in the machine. When the instrument returns to normal after a plug-in board or component is removed, it means that the fault occurred there.

　　4. Replacement method

　　It is required to have 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. Contrast method

　　It requires two instruments of the same model, and one of them is in normal operation. This method also requires necessary equipment, such as a multimeter, oscilloscope, etc. According to the nature of the 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, then detect the signals at some points and compare the two groups of signals. If there is a difference, it can be concluded that the fault is here. This method requires maintenance personnel to have considerable knowledge and skills.

　　6. Heating and cooling method

　　Sometimes, the instrument will fail when it works for a long time or when the working environment temperature is high in summer. It will be normal when it is turned off and checked, and it will be normal when it is turned on again after a while, but it will fail again after a while. This phenomenon is caused by the poor performance of individual ICs or components, and the high temperature characteristic parameters cannot meet the index requirements. In order to find out the cause of the failure, the temperature rise and fall method can be used.

　　The so-called cooling means that when a fault occurs, use cotton fiber to wipe anhydrous alcohol on the part that may have a fault to cool it down and observe whether the fault is eliminated. The so-called heating means artificially raising the ambient temperature, such as putting a soldering iron close to the suspicious part (be careful not to raise the temperature too high to damage normal components) to see if the fault occurs.

　　7. Shoulder riding

　　The shoulder-riding method is also called the parallel method. Place a good IC chip on the chip to be checked, or connect good components (resistors, capacitors, diodes, transistors, etc.) in parallel with the components to be checked to maintain good contact. If the fault is caused by an open circuit or poor contact inside the device, this method can be used to eliminate it.

　　8. Capacitor bypass method

　　When a circuit produces a strange phenomenon, such as a chaotic display, the capacitor bypass method can be used to determine the circuit part that is probably faulty. Connect the capacitor across the power supply and ground of the IC; for transistor circuits, connect it across the base input or collector output, and observe the effect on the fault phenomenon. If the capacitor bypass input terminal is ineffective but the fault phenomenon disappears when bypassing its output terminal, it is determined that the fault occurs in this level of circuit.

　　9. State Adjustment Method

　　Generally speaking, before the fault is determined, do not touch the components in the circuit at will, especially the adjustable components, such as potentiometers, etc. However, if you take reference measures in advance (for example, mark the position or measure the voltage or resistance value before touching), you can still touch it if necessary. Maybe the fault will be eliminated after the change.

　　10. Isolation

　　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, segmentation and encirclement gradually narrow the fault search range, and combined with signal comparison, component exchange and other methods, the fault location can generally be found quickly.