Common Fault Inspection and Repair Techniques for Digital Multimeters
1. If all gears do not work, the power supply circuit and A/D converter circuit should be checked. When checking the power supply part, you can remove the laminated battery, press the power switch, connect the positive probe to the negative power supply of the meter under test, and the negative probe to the positive power supply (for digital multimeters), and switch to the diode measurement gear. If the diode forward voltage is displayed, 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, it is necessary to use the circuit breaking method to gradually disconnect the components using the power supply, and focus on checking the operational amplifier, timer and A/D converter. If a short circuit occurs, generally more than one integrated component is damaged. The A/D converter can be checked at the same time as the basic meter, which is equivalent to the DC meter head of an analog multimeter. The specific inspection method is:
(1) The range of the meter under test is switched to the lowest DC voltage gear;
(2) Measure whether the working voltage of the A/D converter is normal. According to the A/D converter model used in the table, the corresponding V+ pin and COM pin are compared with the measured value to see if they are consistent with its typical value.
(3) Measure the reference voltage of the A/D converter. The reference voltage of the 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 through an external potentiometer.
(4) Check the display number when the input is zero. Short the positive terminal IN+ and the negative terminal IN- of the A/D converter to make the input voltage Vin=0, and the instrument displays "00.0" or "00.00".
(5) Check the full-bright strokes of the display. Short the test terminal TEST pin with the positive power supply terminal V+ to make the logical ground high potential and all digital circuits stop working. Because a DC voltage is applied to each stroke, all strokes are bright and the alignment table displays "1888". The alignment table displays "18888". If there is a missing stroke phenomenon, check whether there is poor contact and disconnection between the corresponding output pin of the A/D converter and the conductive glue (or connection line) 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 the DC voltage and resistance files share a set of voltage divider resistors; AC and DC currents 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 jumps greatly, 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, the input end of the A/D converter is short-circuited, 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 repair sequence of a digital multimeter should be: digital meter head → DC voltage → DC current → AC voltage → AC current → resistance range (including buzzer and checking the positive voltage drop of the diode) → Cx → HFE, F, H, T, etc. However, it should not be too mechanical. Some obvious problems can be dealt with first. However, when adjusting, it is necessary to follow the above procedures.
In short, for a faulty multimeter, after proper testing, the possible location of the fault should be analyzed first, and then the fault location should be found according to the circuit diagram for replacement and repair. Because the digital multimeter is a more precise instrument, the replacement of components must be carried out with components of the same parameters, especially when replacing the A/D converter, the integrated block that has been strictly screened by the manufacturer must be used, otherwise errors will occur and the required accuracy will not be achieved. The newly replaced A/D converter also needs to be checked according to the method described above, and it must not be trusted because it is new.
