Maintenance methods and skills of digital multimeter
Digital meters have high sensitivity and accuracy, and are used in almost all enterprises. However, due to the multiple factors of its failure, and the randomness of the problems encountered, there are not many rules to follow, and it is difficult to repair. Therefore, I have sorted out some repair experience accumulated in practical work for many years for the reference of colleagues engaged in this profession.
1. Repair method
Finding faults should be done first from the outside before the inside, from the easy to the difficult. The methods can be roughly divided into the following categories:
(1) The sensory method directly judges the cause of the failure by means of the senses. Through the visual inspection, it can be found such as disconnection, de-soldering, short circuit of the wire, broken fuse tube, burned out components, mechanical damage, and copper foil on the printed circuit. Lifting and breaking, etc.; the temperature rise of batteries, resistors, transistors, and integrated blocks can be touched, and the cause of abnormal temperature rise can be found out by referring to the circuit diagram. In addition, you can also check by hand whether the components are loose, whether the integrated circuit pins are firmly inserted, and whether the transfer switch is cassette; you can hear and smell whether there is any abnormal sound or smell.
(2) Voltage measurement method To measure whether the working voltage of each key point is normal, the fault point can be found quickly. Such as measuring the working voltage of the A/D converter, the reference voltage, etc.
(3) Short-circuit method The short-circuit method is generally used in the method of checking the A/D converter mentioned above. This method is used more when repairing weak and micro-electrical instruments.
(4) Circuit breaking method Disconnect the suspicious part from the whole machine or unit circuit. If the fault disappears, it means that the fault is in the disconnected circuit. This method is mainly suitable for short circuits in the circuit.
(5) Measuring component method When the fault has been reduced to a certain place or a few components, it can be measured online or offline. If necessary, replace with good components. If the fault disappears, the components are broken.
(6) Interference method The human body induced voltage is used as the interference signal to observe the changes of the liquid crystal display, which is often used to check whether the input circuit and the display part are in good condition.
2. Repair skills
For a faulty instrument, first check and determine whether the fault phenomenon is common (all functions cannot be measured) or individual (individual functions or individual ranges), and then distinguish the situation and solve the problem.
(1) If all gears fail to work, focus on checking the power supply circuit and the A/D converter circuit. When checking the power supply part, you can remove the laminated battery, press the power switch, connect the positive test lead to the negative power supply of the meter under test, and connect the negative test lead to the positive power supply (for digital multimeters), switch the switch to the diode measurement gear, if the display shows If it is the forward voltage of the diode, it means that the power supply part is good. If the deviation is large, it means that there is a problem with the power supply part. If there is an open circuit, focus on checking the power switch and battery leads. If there is a short circuit, you need to use the circuit breaking method to gradually disconnect the components that use the power supply, focusing on checking operational amplifiers, timers, and A/D converters. If a short circuit occurs, more than one integrated component is generally damaged. The A/D converter can be checked at the same time as the basic meter, which is equivalent to the DC meter of an analog multimeter. The specific inspection method is as follows:
(2) The range of the meter under test is turned to the lowest level of DC voltage;
(3) Measure whether the working voltage of the A/D converter is normal. According to the A/D converter model used in the table, corresponding to the V+ pin and COM pin, compare the measured value with its typical value.
(4) Measure the reference voltage of the A/D converter. The reference voltage of the commonly used digital multimeters is generally 100mV or 1V, that is, the DC voltage between VREF+ and COM is measured. If it deviates from 100mV or 1V, an external potentiometer can be used. Make adjustments.
(5) Check the displayed number of input zero, short-circuit the positive terminal IN+ of the A/D converter with the negative terminal IN-, so that the input voltage Vin=0, the meter displays "00.0" or "00.00".
(6) Check the full bright stroke of the display. Short-circuit the test terminal TEST pin with the positive power supply terminal V+ to make the logic ground become a high potential, and all digital circuits stop working. Because DC voltage is applied to each stroke, all strokes are bright and the alignment table displays "1888", and the alignment table displays "18888". If there is a lack of strokes, check the corresponding output pin of the A/D converter and the conductive glue (or connection), and check whether there is poor contact and disconnection between the A/D converter and the display.
3. If the measurement data is unstable, and the value always increases cumulatively, the input terminal of the A/D converter is short-circuited, and the displayed data is not zero, which is generally caused by the poor performance of the 0.1μF reference capacitor.
According to the above analysis, the basic repair sequence of the digital multimeter should be: digital meter head→DC voltage→DC current→AC voltage→AC current→resistance gear (including buzzer and checking diode positive voltage drop)→Cx→HFE , F, H, T, etc. But it should not be too mechanical. Some obvious problems can be dealt with first. But when making adjustments, be sure to follow the above procedure.
In short, a faulty multimeter, after proper testing, must first analyze the possible parts of the fault, and then find the fault location according to the circuit diagram for replacement and repair. Because the digital multimeter is a relatively precise instrument, the replacement components must be replaced by components with the same parameters, especially the replacement of the A/D converter, must use the integrated block that has been strictly screened by the manufacturer, otherwise there will be errors and will not meet the requirements. Accuracy. The newly replaced A/D converter also needs to be checked according to the method described above, and must not be trusted because of the new one.
At present, there are many domestic manufacturers of digital multimeters, and the quality is also good and bad. It is not easy to find the quality problems of double-sided copper-clad laminates during repairs. When the insulation strength of the resin board is not enough, it is mainly manifested in the large error when measuring the high voltage, and the repair should be distinguished from the resistance change of the voltage divider resistor. In this case, it is best to use the open circuit method to find the fault point. The burnt and carbonized parts should be cleaned up to meet the insulation requirements. When the input signal cannot be input due to the breakage of the transition hole of the double-sided connection, it is easy to be confused with the bad transfer switch and difficult to separate. For this kind of fault, the short-circuit method should be used to find the fault point.
