Common Digital Multimeter Troubleshooting and Repair Techniques
For a faulty instrument, the first step is to check and distinguish whether the fault phenomenon is common (all functions cannot be measured) or individual (individual functions or individual ranges), and then differentiate the situation and solve it accordingly.
If all gears cannot work, the power supply circuit and A/D converter circuit should be checked carefully. When checking the power supply, remove the stacked battery, press the power switch, connect the positive probe to the negative power supply of the tested meter, and connect the negative probe to the positive power supply (for a digital multimeter). Turn the switch to the diode measurement mode. If the display shows the positive voltage of the diode, 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 there is an open circuit, focus on checking the power switch and battery leads. If a short circuit occurs, it is necessary to use the circuit breaking method to gradually disconnect the components that use the power supply, with a focus on checking the operational amplifier, timer, and A/D converter. If a short circuit occurs, it usually damages more than one integrated component. Checking the A/D converter can be performed simultaneously with the basic meter, equivalent to the DC meter head of an analog multimeter. Specific inspection methods:
(1) Turn the range of the tested meter to the lowest DC voltage level;
(2) Measure whether the operating voltage of the A/D converter is normal. According to the model of the A/D converter used in the table, corresponding to the V+and COM pins, compare the measured values with their typical values to see if they match.
(3) Measure the reference voltage of the A/D converter. The commonly used digital multimeter reference voltage is generally 100mV or 1V, which measures 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 negative terminal IN - of the A/D converter to make the input voltage Vin=0, and the instrument will display "00.0" or "00.00".
(5) Check the fully lit strokes on the monitor. Short circuit the test terminal TEST pin to the positive power terminal V+, causing the logic ground to become high and all digital circuits to stop working. Due to the direct current voltage applied to each stroke, all strokes are illuminated and the alignment table displays "1888" and "18888". If there is a missing stroke phenomenon, check whether there is poor contact or disconnection between the output pin of the A/D converter and the conductive adhesive (or wiring), and the display.
If there are problems with individual files, 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/DC current sharing shunt; AC voltage and AC current share a set of AC/DC converters; Others such as Cx, HFE, F, etc. are composed of independent converters. By understanding their relationship and referring to the power diagram, it is easy to locate the faulty part. If the measurement of small signals is inaccurate or the displayed numbers fluctuate greatly, it is important to check whether the contact of the range switch is good.
If the measurement data is unstable and the values always accumulate, and the input terminal of the A/D converter is short circuited, and the displayed data is not zero, it is generally caused by poor performance of the 0.1 μ F reference capacitor.
