Methods and techniques for the repair of digital multimeters
1, Repair method:
Finding faults should start with the outside and then the inside, starting with the easy and then the difficult, breaking down the whole into small parts and focusing on breakthroughs. The methods can be roughly divided into the following categories:
Sensation method relies on sensory perception to directly determine the cause of faults. Through visual inspection, it can detect issues such as broken wires, desoldering, short circuits in grounding, broken fuse tubes, burnt components, mechanical damage, copper foil warping and breakage on printed circuits, etc; You can touch the temperature rise of the battery, resistor, transistor, and integrated block, and refer to the circuit diagram to find the cause of the abnormal temperature rise. In addition, you can also check by hand whether the components are loose, whether the integrated circuit pins are firmly inserted, and whether the conversion switch is stuck; You can hear and smell any unusual sounds or odors.
2. The voltage measurement method measures whether the working voltage of each key point is normal, which can quickly identify the fault point. Measure the operating voltage, reference voltage, etc. of the A/D converter.
3. Short circuit method is generally used in the inspection of A/D converters mentioned earlier, and this method is more commonly used in repairing weak and micro electrical instruments.
4. The circuit breaking method interrupts the suspicious part from the entire machine or unit circuit. If the fault disappears, it indicates that the fault is in the disconnected circuit. This method is mainly suitable for situations where there is a short circuit in the circuit.
When the fault has narrowed down to a certain or several components, it can be measured online or offline using the component measurement method. If necessary, replace with good components. If the fault disappears, it indicates that the component is broken.
6. Interference method uses human induced voltage as an interference signal to observe the changes in LCD display, commonly used to check whether the input circuit and display part are intact.
Repair techniques:
(1) Switch the range of the tested meter to the low 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 table, corresponding to the V+pin and COM pin, 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 lit up and the alignment table displays "1888" and "18888". If there is a missing stroke, check whether there is poor contact or wire breakage between the corresponding output pin of the A/D converter and the conductive adhesive (or wiring), and between the A/D converter and the display.
