What is the main maintenance of the frequency function of the multimeter?
What is the main maintenance of the frequency function of the multimeter?
1. Repair method When looking for faults, you should start from the outside and then the inside. The methods can be roughly divided into the following categories:
The sensory method directly judges the cause of the fault by means of the senses. Through visual inspection, it can be found such as disconnection, desoldering, short circuit, broken fuse tube, burnt components, mechanical damage, copper foil on the printed circuit, and Fracture, etc.; you can touch the temperature rise of the battery, resistors, transistors, and integrated blocks, and you can refer to the circuit diagram to find out the cause of the abnormal temperature rise. In addition, by hand, you can also check whether the components are loose, whether the integrated circuit pins are firmly inserted, and whether the transfer switch is stuck; you can hear and smell whether there are abnormal sounds and odors.
Voltage measurement method Measures whether the working voltage of each key point is normal, and can quickly find out the fault point. Such as measuring the working voltage and reference voltage of the A/D converter.
Short-circuit method The short-circuit method is generally used in the inspection method of the A/D converter mentioned above, and this method is often used in repairing weak and micro-electric instruments.
Open circuit method Disconnect the suspicious part from the whole machine or unit circuit. If the fault disappears, it means that the fault is in the disconnected circuit. This method is mainly suitable for the situation where there is a short circuit in the circuit.
Detecting element method When the fault has been reduced to a certain place or several elements, it can be measured online or offline. If necessary, replace it with a good one. If the fault disappears, the component is broken. 6. Interference method Use the human body induced voltage as the interference signal to observe the changes of the liquid crystal display, which is often used to check whether the input circuit and the display part are intact.
2. Repair skills For a faulty instrument, you should first check and judge whether the fault phenomenon is common (all functions cannot be measured) or individual (individual functions or individual ranges), and then distinguish the situation and solve it symptomatically.
If all gears fail to work, focus on checking the power circuit and A/D converter circuit. When checking the power supply part, you can take off the laminated battery, press the power switch, connect the positive test lead to the negative of the power supply of the meter under test, and the negative test lead to the positive power supply (for digital multimeters), switch to the diode measurement position, if it shows If the forward voltage of the diode is higher, it means that the power supply part is good. If the deviation is large, it means that there is a problem with the power supply part. If there is an open circuit, focus on checking the power switch and battery leads. If there is a short circuit, you need to use the open circuit method to gradually disconnect the components that use the power supply, and focus on checking the operational amplifier, timer and A/D converter. In the event of a short circuit, generally more than one integrated component is damaged. Checking the A/D converter can be performed simultaneously with the basic meter, which is equivalent to the DC meter head of the analog multimeter.
Specific inspection method:
(1) The range of the meter under test is turned to the lowest level of DC voltage;
(2) Measure whether the working voltage of the A/D converter is normal. According to the A/D converter model used in the table, corresponding to V+ pin and COM pin, compare the measured value with its typical value to see if it is consistent.
(3) Measure the reference voltage of the A/D converter. The reference voltage of the 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, you can use the external potentiometer Make adjustments.
(4) Check the displayed number when the input is zero, and 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 meter displays "00.0 or "00.00.
(5) Check the full brightness strokes of the display. Short the test terminal TEST pin and the positive power supply terminal V+, make the logic ground become high potential, and all digital circuits stop working. Because DC voltage is added to each stroke, all strokes are bright and the alignment table shows "1888, and the alignment table shows "18888. If there is a lack of strokes, check whether there is poor contact or disconnection between the corresponding output pin of the A/D converter and the conductive glue (or wiring), and the display.
