Introduction to repair skills of digital multimeter
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) 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.
(3) 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.
(4) Check the displayed number of input zero, short-circuit the positive terminal IN+ of the A/D converter with the negative terminal IN-, 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 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.
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 diagram, it is easy to find the fault location. If the measurement of the small signal is inaccurate or the displayed number 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 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.
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.
The digital multimeter is a relatively precise instrument. The replacement components must be replaced by components with 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. Spend. 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.
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. This kind of fault should use the short-circuit method to find the fault point.
