Digital multimeter has high sensitivity and accuracy, and its application is almost all enterprises. However, due to the multi-factors of its failure and the randomness of encountering problems, there are not many rules to follow, and it is difficult to repair. Therefore, I will sort out some repair experience accumulated in the practical work for many years, for the reference of colleagues engaged in this profession.

1. Repair method of digital multimeter

Finding faults should be done first, then first, then first, then hard. The methods can be roughly divided into the following types:

1. The sensory method directly judges the cause of the fault by virtue of the senses. Through visual inspection, it can find such as broken wire, unsoldering, short circuit of the bonding wire, broken fuse tube, burned out component, mechanical damage, copper foil warping on the printed circuit Starting and breaking, etc .; you can touch the temperature rise of the battery, resistor, transistor, and integrated block. You can refer to the circuit diagram to find out the cause of the abnormal temperature rise. In addition, you can also check whether the components are loose, whether the pins of the integrated circuit are plugged in firmly, and whether the transfer switch is stuck; you can hear and smell for abnormal sounds and odors.

2. The voltage measurement method measures whether the working voltage of each key point is normal, and can quickly find the fault point. Such as measuring the working voltage and reference voltage of the A / D converter.

3. Short-circuit method The short-circuit method is generally used in the A / D converter inspection method mentioned above. This method is often used when repairing weak and micro-electric instruments.

4. The disconnection method disconnects the suspicious part from the complete machine or unit circuit. If the fault disappears, it means that the fault is in the disconnected circuit. This method is mainly suitable for the case where there is a short circuit in the circuit.

5. Measuring component method When the fault has been reduced to a certain place or several components, it can be measured online or offline. If necessary, replace it with a good component. If the fault disappears, the component is damaged.

6. The interference method uses the human body induced voltage as the interference signal to observe the change of the liquid crystal display. It is often used to check whether the input circuit and the display part are intact.

Second, digital multimeter repair skills

For a faulty instrument, you should first check and determine whether the fault phenomenon is common (all functions cannot be measured) or personality (individual functions or individual ranges), and then distinguish the situation and solve the symptom.

1. If all files are not working, you should focus on checking the power supply circuit and the A / D converter circuit. When checking the power supply, you can remove the laminated battery, press the power switch, connect the power supply of the meter under test with a positive test lead, and connect the power supply of the negative test lead (for digital multimeter). It is the forward voltage of the diode, which means that the power supply part is good. If the deviation is large, it means that there is a problem in the power supply part. If there is an open circuit, focus on checking the power switch and battery leads. If a short circuit occurs, you need to use the open circuit method to gradually disconnect the components that use the power supply, focusing on checking the operational amplifier, timer and A / D converter. In the event of a short circuit, 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 head of an analog multimeter. The specific check method is:

(1) The range of the meter under test is turned to the lowest DC voltage;

(2) 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, whether the measured value is consistent with its typical value.

(3) The reference voltage of the A / D converter is measured. 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, it can be passed through an external potentiometer. Make adjustments.

(4) Check the display number of input zero, short-circuit the positive terminal IN + and the negative terminal IN- of the A / D converter to make the input voltage Vin = 0, and the meter displays "00.0" or "00.00".

(5) Check the full-bright stroke of the display. Short-circuit the TEST pin of the test terminal to the positive power supply terminal V +, so that the logic ground becomes a high potential, and all digital circuits stop working. Because DC voltage is added to each stroke, all strokes show "1888" in the alignment table and "18888" in the alignment table. If there are missing strokes, check the corresponding output pin of the A / D converter and the conductive glue (or connecting line), whether there is poor contact and disconnection with the display.

2. If there is a problem with individual files, it means that the A / D converter and the power supply are working normally. Because DC voltage and resistance files share a set of voltage divider resistors; AC and DC current share a shunt; AC voltage and AC current share a set of AC / DC converters; others such as Cx, HFE, F, etc. are composed of independent different converters . Understand the relationship between them, and then according to the power supply diagram, it is easy to find the fault location. If the measurement of small signals is inaccurate or the displayed digital jump is large, it is important to check whether the range switch is in good contact.

3. If the measured data is unstable and the value always increases cumulatively, short-circuiting the input end of the A / D converter and indicating that the data is not zero is generally caused by poor performance of the 0.1μF reference capacitor.

According to the above analysis, the basic sequence of the repair of the digital multimeter should be: digital watch head → DC voltage → DC current → AC voltage → AC current → resistance file (including buzzer and check 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, you must follow the above procedure.

In short, a faulty multimeter, after proper detection, must first analyze the location where the fault may occur, and then find the fault location according to the circuit diagram for replacement and repair. Because the digital multimeter is a more precise instrument, the replacement components must use the same parameters, especially the replacement of the A / D converter, the manufacturer must strictly select the integrated block, otherwise errors will occur and the required Accuracy. The newly replaced A / D converter also needs to be checked according to the method described above, and must not be doubted by the new one.

At present, there are many domestic manufacturers of digital multimeters, and the quality also has advantages and disadvantages. The quality problems of double-sided copper composite boards are not easy to find during repairs. When the insulation strength of the resin board is not enough, it mainly shows that the error is large when measuring high voltage, and it needs to be distinguished from the change of the resistance value of the voltage dividing resistor during repair. In this case, it is best to use the open circuit method to find the fault point. The burned and carbonized parts should be cleaned up to meet the insulation requirements. When encountering the inability to input signals due to the breakage of the double-sided connection due to the transition hole, it is easy to be confused with the poor phenomenon of the transfer switch and it is difficult to separate. This type of fault should use the short-circuit method to find the fault point.

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