Repair Methods and Techniques for Digital Multimeters (Technical Tips)
Repair methods
Finding faults should start from the outside and then from the inside, from easy to difficult, break them down into smaller parts, and focus on breakthroughs. The methods can be roughly divided into the following:
1. The sensory method directly judges the cause of the malfunction based on the senses. Through visual inspection, it can be found that faults such as broken wires, desoldering, short circuits in grounding, broken fuse tubes, burnt components, mechanical damage, copper foil lifting and breaking 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 identify the cause of abnormal temperature rise. In addition, you can also check by hand whether the components are loose, whether the integrated circuit pins are securely 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 working voltage and reference voltage of the A/D converter.
3. Short circuit method is generally used in the inspection of A/D converters mentioned earlier, which is more commonly used in repairing weak and micro electrical instruments.
4. The circuit breaker 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.
5. When the fault has narrowed down to a certain location or several components, online or offline measurement can be performed. If necessary, replace with good components. If the fault disappears, it indicates that the component is damaged.
6. Interference method uses human induced voltage as interference signal to observe the changes in LCD display, commonly used to check whether the input circuit and display part are intact.
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 ranges), and then distinguish the situation and solve it accordingly.
If all gears cannot work, the power supply circuit and A/D converter circuit should be carefully checked. When checking the power supply, remove the stacked battery, press the power switch, connect the positive probe to the negative side of the measured meter power supply, and connect the negative probe to the positive side of the power supply (for a digital multimeter). Turn the switch to the measurement position of the diode. 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 breaker method to gradually disconnect the components using the power supply, and focus on checking the operational amplifier, timer, and A/D converter. 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.
