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Use Skills and Maintenance Methods of Digital Multimeter
1. Repair method
Looking for faults should start from outside to inside, first from easy to difficult, turn the whole into zero, and break through the key points. Its methods can be roughly divided into the following categories:
1. Sensory method can directly judge the cause of the fault by sensory organs. Through appearance detection, it can discover such things as broken wires, de-welding, short circuit, broken fuse tubes, burned components, mechanical damage, copper foil warping and breaking on printed circuit, etc. It can touch the temperature rise of batteries, resistors, transistors and integrated blocks, and can refer to electricity. The road map finds out the cause of abnormal temperature rise. In addition, the hand can also detect whether the component is loose, whether the integrated circuit foot pipe is firmly inserted, whether the switch is taped, and whether the noise and odor can be heard and smelled.
2. Measuring the working voltage of each key point by voltage measuring method can find out the fault point quickly. Such as measuring the working voltage and reference voltage of A/D converter.
3. Short-circuit method is generally used in the detection of A/D converters mentioned above. This method is used more in repairing weak and micro-electric instruments.
4. Circuit-breaking method interrupts 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 case of short circuit in the circuit.
5. Measuring element method can measure faults on-line or off-line when they have been reduced to one or more components. If necessary, replace it with good components. If the failure disappears, it means that the components have been damaged.
6. The interference method uses the induced voltage of human body as interference signal to observe the change of liquid crystal display. It is often used to detect whether the input circuit and display part are in good condition.
Two, repair techniques
For a fault instrument, we should first detect and distinguish whether the fault phenomenon is common (all functions can not be measured), or personality (individual functions or individual ranges), and then distinguish the situation and solve the problem symptomatically.
1. If all gears are not working, the power supply circuit and A/D converter circuit should be detected. When testing the power supply part, the stacked battery can be removed, the power switch can be pressed, the positive meter pen is used to connect the negative power supply of the measured meter, and the negative meter pen is used to connect the positive power supply (for the digital multimeter). The switch hits the measuring file of the secondary tube. If the positive voltage of the secondary tube is displayed, the power supply part is good. If the deviation is large, the power supply part has. Problem. If there is an open circuit, focus on the detection of power switches and battery leads. If there is a short circuit, it is necessary to adopt the circuit breaking method, gradually disconnect the components using the power supply, focusing on the detection of operational amplifiers, timers and A/D converters. If there is a short circuit, it usually damages more than one integrated component. The detection of A/D converter can be carried out simultaneously with the basic table, which is equivalent to the DC head of analog multimeter.
(1) The measuring range of the meter is transferred to the lowest level of DC voltage;
(2) Measure whether the working voltage of A/D converter is normal. According to the type of A/D converter used in the table, corresponding to V-foot and COM-foot, the measured value is consistent with its typical value.
(3) To measure the reference voltage of A/D converter, the common reference voltage of digital multimeter is 100mV or 1V, that is, to measure the DC voltage between VREF and COM. If it deviates from 100mV or 1V, it can be adjusted by external potentiometer.
(4) Detecting the display number with zero input, and connecting the positive and negative IN-of A/D converter to make the input voltage Vin = 0. The instrument shows "00.0" or "00.00".
(5) Detect the full bright strokes of the display. The TEST foot of the test end is short-connected with the V of the positive power supply end, so that the logic becomes high potential, and all the digital circuits stop working. Because DC voltage is added to each stroke, all strokes show "1888" on the bit table and "18888" on the bit table. If there is a lack of strokes, check whether there is bad contact and disconnection between the output foot of A/D converter and the conductive adhesive (or connection) and the display.
2. If there are problems with individual gears, the A/D converter and power supply are working normally. Because DC voltage and resistance gear share a set of voltage divider resistance; AC and DC current share a shunt; AC voltage and AC current share a set of AC/DC converter; other such as Cx, HFE, F are composed of independent different converters. Understanding the relationship between them, and then according to the power diagram, it is easy to find the fault location. If the measurement of small signal is inaccurate or the display digital jump is large, the key point is to check whether the contact of the range switch is good.