Repair method of digital multimeter
For a faulty instrument, first check and judge whether the fault phenomenon is common (all functions cannot be measured) or individual (individual function or individual range), and then distinguish the situation and solve it symptomatically.
1. If all gears fail to work, focus on checking the power supply circuit and 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 of the power supply of the meter under test, and the negative test lead to the positive power supply (for digital multimeters), and switch to the diode measurement position. If the forward voltage of the diode is higher, 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 open circuit method to gradually disconnect the components that use the power supply, and focus on checking the operational amplifier, timer and A/D converter. In the event of a short circuit, generally more than one integrated component is damaged. Checking the A/D converter can be carried out simultaneously with the basic meter, which is equivalent to the DC meter head of the analog multimeter. The specific checking method:
⑴The range of the meter under test is turned to the lowest level of DC voltage;
⑵Measure whether the working voltage of the A/D converter is normal. According to the A/D converter model used in the table, corresponding to V+ pin and COM pin, whether the measured value is consistent with its typical value.
(3) Measure the reference voltage of the A/D converter. The reference voltage of a commonly used digital multimeter is generally 100mV or 1V, that is, measure the DC voltage between VREF+ and COM. If it deviates from 100mV or 1V, it can be adjusted by an external potentiometer.
(4) Check the display number with zero input, 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".
⑸ Check the full brightness strokes of the display. Short the test terminal TEST pin and the positive power supply terminal V+, make the logic ground become high potential, and all digital circuits stop working. Because DC voltage is added to each stroke, all the strokes are bright and the alignment table shows "1888", and the alignment table shows "18888". If there is a lack of strokes, check whether there is poor contact or disconnection between the corresponding output pin of the A/D converter and the conductive glue (or connection), and 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 dividing 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 number jumps greatly, then focus on checking whether the contact of the range switch is good.
3. If the measurement data is unstable and the value always increases cumulatively, short-circuit the input terminal of the A/D converter and the displayed data is not zero, it is generally caused by the poor performance of the 0.1μF reference capacitor.
According to the above analysis, the basic sequence of digital multimeter repair should be: digital meter head → DC voltage → DC current → AC voltage → AC current → resistance file (including buzzer and checking the positive voltage drop of the diode) → Cx → HFE , F, H, T, etc. But don't be too mechanical. Some obvious problems can be dealt with first. However, when adjusting, the above procedures must be followed.
In short, after a proper inspection of a faulty multimeter, it is first necessary to analyze the possible location 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, components with the same parameters must be used for replacement components, especially for the replacement of A/D converters, the integrated blocks that have been strictly screened by the manufacturer must be used, otherwise errors will occur and the required components will not be met. Accuracy. The newly replaced A/D converter also needs to be checked according to the method mentioned above, and it must not be trusted because of its newness.
