Introduction to Common Fault Inspection and Maintenance Techniques of Digital Multimeters
For a faulty instrument, it is necessary to first check and determine whether the fault phenomenon is common (all functions cannot be measured) or individual (certain functions or certain ranges), and then deal with the situation according to the specific circumstances and solve the problem accordingly.
If all ranges cannot work, the power supply circuit and the A/D converter circuit should be focused on for inspection. When checking the power supply part, remove the stack battery, press the power switch, connect the positive test lead to the negative terminal of the power supply of the measured meter and the negative test lead to the positive terminal of the power supply (for a digital multimeter). Set the switch to the diode measurement range. If the displayed value is the forward voltage of the diode, it indicates that the power supply part is in good condition. If there is a large deviation, it means that there is a problem with the power supply part. If there is an open circuit, focus on checking the power switch, battery leads, etc. If there is a short circuit, the open-circuit method needs to be adopted, gradually disconnecting the components that use the power supply, and focusing on checking the operational amplifier, timer, A/D converter, etc. In the case of a short circuit, generally more than one integrated component is damaged. The inspection of the A/D converter can be carried out simultaneously with the basic meter, which is equivalent to the DC meter head of an analog multimeter. The specific inspection methods are as follows:
(1) Turn the range of the measured meter to the lowest DC voltage range.
(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 meter, measure between the V+ pin and the COM pin, and compare the measured value with its typical value to see if they match.
(3) Measure the reference voltage of the A/D converter. The reference voltage of commonly used digital multimeters at present 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 value when the input is zero. 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 instrument should display "00.0" or "00.00".
(5) Check the fully lit strokes of the display. Short-circuit the TEST pin of the test terminal with the positive power supply terminal V+ to make the logic ground become a high potential, and all digital circuits stop working. Since a DC voltage is applied to each stroke, all strokes are lit. For a four-digit display meter, it will show "1888", and for a five-digit display meter, it will show "18888". If there is a phenomenon of missing strokes, check whether there is poor contact or disconnection between the corresponding output pins of the A/D converter and the conductive adhesive (or connection wires), and the display.
