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Instrumentation circuit maintenance has always been an indispensable part of electronic companies. Because only through it can the original substandard products finally leave the factory. However, servicing instrumentation is also one of the most complex parts of an electronics company. Because it not only requires a lot of electronic expertise, but also sometimes extensive field experience.

1. Percussion hand pressure method

It is often encountered that the instrumentation is good or bad when it is running, and most of this phenomenon is caused by poor contact or virtual welding. In this case, tapping and hand pressing can be used.

The so-called "knock" is to tap the plug-in board or component lightly through a small rubber cockroach head or other percussion object to see if it will cause an error or downtime. The so-called "hand pressure" means that when a fault occurs, after turning off the power, press the plugged parts, plugs and sockets firmly by hand again, and then start the machine again to try whether the fault will be eliminated. If you find that tapping on the casing is normal, and hitting it again is abnormal, it is best to re-insert all the connectors and try again.

2. Observation method

Use sight, smell, touch. Sometimes, damaged components will discolor, blister or have burnt spots; burnt components will produce some special odor; shorted chips will become hot; virtual soldering or desoldering can also be observed with the naked eye. .

3. Exclusion method

The so-called elimination method is a method of judging the cause of the fault by plugging in some plug-in boards and devices in the machine. When the instrument returns to normal after a plug-in board or device is removed, it means that the fault occurs there.

4. Substitution 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. 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 requires the 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 and so on.

The specific method is: let the faulty instrument and the normal instrument run under the same conditions, and then detect the signals of some points and then compare the two groups of signals measured. If there is a difference, it can be concluded that the fault is here. This method requires the maintenance personnel to have considerable knowledge and skills.

6. Heating and cooling method

Sometimes, the instruments and meters work for a long time, or when the temperature of the working environment is high in summer, they will fail. The shutdown and inspection are normal. After a period of time, the startup is normal, and the failure occurs again after a while. This phenomenon is due to the poor performance of individual ICs or components, and the high temperature characteristic parameters do not meet the index requirements. In order to find out the cause of the failure, the heating and cooling method can be used.

The so-called cooling is to use cotton fiber to wipe the anhydrous alcohol on the part that may fail to cool down when the failure occurs, and observe whether the failure is eliminated. The so-called temperature rise is to artificially raise the ambient temperature. For example, use an electric soldering iron to approach the suspicious part (be careful not to raise the temperature too high to damage the normal device) to see if the fault occurs.

7. Shoulder riding

The shoulder riding method is also called the parallel method. Put a good IC chip on the chip to be checked, or connect good components (resistor capacitors, diodes, transistors, etc.) in parallel with the components to be checked, and maintain good contact. If the fault comes from the internal open circuit of the device or Reasons such as poor contact can be ruled out by this method.

8. Capacitor bypass method

When a certain circuit produces a relatively strange phenomenon, such as the confusion of the display, the capacitor bypass method can be used to determine the probably faulty circuit part. 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 phenomenon. If the failure phenomenon disappears when the capacitor bypass input terminal is invalid and its output terminal is bypassed, it is determined that the fault occurs in this stage of the circuit.

9. State adjustment method

Generally speaking, before the fault is determined, do not touch the components in the circuit casually, especially the adjustable devices, such as potentiometers. However, if the double reference measures are taken in advance (for example, the position is marked or the voltage value or resistance value is measured before being touched), it is still allowed to be touched if necessary. Maybe after the change sometimes the glitch will go away.

10. Isolation Law

The fault isolation method does not require the same type of equipment or spare parts to be compared, and is safe and reliable. According to the fault detection flow chart, the division and encirclement gradually narrow the fault search range, and then cooperate with methods such as signal comparison and component exchange, and generally the fault of the instrument will be quickly found.