Method of instrument and instrument failure inspection
Instrumentation circuit repair has always been an indispensable part in electronics companies. Because only through it can the original unqualified products finally leave the factory. However, repair is also the most complex part of an electronics company. Because it not only needs to apply a lot of electronics expertise, sometimes it also requires rich field experience. The following is the maintenance experience that I personally summarized over the years to share with interested friends.
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.
2. Contrast method
Two instruments of the same model are required, and one is operating normally. This method also requires the necessary equipment, such as a multimeter, oscilloscope, and so on. 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 meter and the normal meter run under the same conditions, and then detect the signals at some points and then compare the two sets of measured signals. If there are differences, you can conclude that the fault is here. This method requires considerable knowledge and skills of maintenance personnel.
3. Temperature rise and fall method
Sometimes, when the meter works for a long time or when the temperature of the working environment is high in summer, it will fail. The shutdown check is normal. After a period of time, the meter is turned on again, and then it fails again. This phenomenon is due to the poor performance of individual ICs or components, and the high-temperature characteristic parameters fail to meet the index requirements. In order to find the cause of the failure, the temperature rise and fall method can be used.
The so-called cooling down is to wipe down the alcohol that may be faulty with cotton fiber when the fault occurs, and make it cool down, and observe whether the fault is eliminated. The so-called temperature rise is to artificially increase the ambient temperature, for example, use an electric soldering iron to approach the suspected part (be careful not to raise the temperature too high to damage normal devices) to see if the fault occurs.
4, shoulder riding
The shoulder riding method is also called parallel method. Install a good IC chip on the chip to be inspected, or connect a good component (resistance capacitor, diode, triode, etc.) in parallel with the component to be inspected, and maintain good contact. If the fault is caused by an open circuit in the device or Reasons such as poor contact can be ruled out using this method.
5.Capacitor bypass method
When a circuit has a strange phenomenon, such as when the display is confused, you can use the capacitor bypass method to determine the part of the circuit that is probably faulty. Connect the capacitor across the power and ground terminals of the IC; connect the transistor circuit across the base input or collector output and observe the effect on the failure phenomenon. If the capacitor bypass input is invalid and the output disappears when it bypasses its output, then it is determined that the fault appears in this level of circuit.
6, 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 repeated reference measures are taken in advance (for example, make a position mark or measure the voltage or resistance value before touching), it is still allowed to touch. Maybe the fault will disappear after changing.
The fault isolation method does not require equipment or spare parts of the same type for comparison, and is safe and reliable. According to the fault detection flowchart, segmentation and enclosing gradually narrow down the fault search scope, and then cooperate with methods such as signal comparison and component exchange to generally find the fault quickly.
8, tapping hand pressure
Frequently encountered good and bad phenomena when the instrument is running, most of this phenomenon is caused by poor contact or virtual welding. In this case, knocking and hand pressing can be used.
The so-called "knock" is to gently tap the plug-in board or component with a small rubber hoe or other percussion object to see if it may cause an error or a shutdown failure. The so-called "hand pressure" means that when a fault occurs, the components and plugs and sockets that are plugged in after the power is turned off are pressed again by hand, and the power is turned on to try to eliminate the fault. If you find that the casing is normal, and then it is not normal, it is best to re-plug all the connectors and try again. If you are unsuccessful, you have to think of another way.
Use vision, smell, and touch. In some cases, the damaged components will become discolored, blistering, or burnt spots; burned devices will produce some special odors; short-circuited chips will become hot; and virtual or unsoldering spots can also be observed with the naked eye .
The so-called elimination method is to determine the cause of the fault by removing some plug-in boards and components in the plug-in machine. When the instrument returns to normal after removing a certain plug-in board or device, it indicates that the fault occurred there.