Repair Techniques for Digital Multimeters
1. If all gears cannot work, the focus should be on checking the power circuit and A/D converter circuit. When checking the power supply, remove the stacked battery, press the power switch, connect the positive lead to the negative power supply of the measured meter, and connect the negative lead to the positive power supply (for a digital multimeter). Turn the switch to the measurement position of the secondary transistor. If the display shows the positive voltage of the secondary transistor, it indicates that the power supply is good. If the deviation is large, it indicates that there is a problem with the power supply.
If an open circuit occurs, focus on checking the power switch and battery leads. If a short circuit occurs, it is necessary to use the circuit breaker method to gradually disconnect the components using the power supply, with a focus on checking operational amplifiers, timers, and A/D converters. If a short circuit occurs, it usually damages more than one integrated component.
The A/D converter can be checked simultaneously with the basic meter, which is equivalent to the DC meter head of an analog multimeter. The specific inspection method is:
1) Turn the range of the measured meter to the lowest level of DC voltage;
2) Measure whether the working voltage of A/D converter is normal. According to the A/D converter model used in the table, corresponding to the V+pin and COM pin, do the measured values match their typical values.
3) Measure the reference voltage of A/D converter. At present, the reference voltage of 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 with zero input, short circuit the positive terminal IN+and negative terminal IN - of the A/D converter, so that the input voltage Vin=0, and the instrument displays "00.0" or "00.00".
5) Check the full bright strokes on the monitor. Short circuit the test pin at the testing end to the positive power supply terminal V+, so that the logic ground becomes high potential and all digital circuits stop working. Due to the DC voltage applied to each stroke, the alignment meter displays "1888" and the alignment meter displays "18888" when all strokes are lit. If there is a lack of stroke, check the corresponding output pin of the A/D converter and the conductive adhesive (or wiring), as well as whether there is poor contact or disconnection between the A/D converter and the display.
2. If there are issues with individual gears, it indicates that both the A/D converter and power supply are working properly. Because the DC voltage and resistance range share a set of voltage divider resistors; AC and DC current sharing shunt; AC voltage and AC current share a set of AC/DC converters; Other components such as Cx, HFE, F, etc. are composed of independent different converters. Understanding the relationship between them and based on the power diagram, it is easy to locate the faulty part. If the measurement of small signals is not accurate or the displayed number jumps excessively, the focus should be on checking whether the contact of the range switch is good.
3. If the measurement data is unstable and the value always accumulates, and the input terminal of the A/D converter is short circuited, and the displayed data is not zero, then it is generally 0.1 μ Caused by poor performance of F's reference capacitor.
Based on the above analysis, the basic repair sequence for a digital multimeter should be: digital meter head → DC voltage → DC current → AC voltage → AC current → resistance range (including buzzer and checking positive voltage drop of the secondary tube) → Cx → HFE, F, H, T, etc. But it should not be too mechanical. Some obvious problems can be addressed first. But when conducting calibration, it is necessary to follow the above procedure.
