First, repair methods
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Looking for the fault should first outside after inside, first easy after difficult, break the whole into pieces, key breakthrough. Its methods can be roughly divided into the following:
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1. The sensory method can judge the cause of the fault directly by the senses. Through the appearance inspection, it can find out such as wire break, welding failure, wire short circuit, fuse tube break, burnt components, mechanical damage, copper foil warping and breaking on the printed circuit, etc. The temperature rise of the battery, resistor, transistor and integrated block can be touched, and the cause of abnormal temperature rise can be found by referring to the circuit diagram. In addition, the hand can also check whether the components are loose, whether the integrated circuit pin is inserted firmly, and whether the transfer switch is cassette; You can hear and smell a different sound, odor.
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2. The voltage measurement method measures whether the working voltage of each key point is normal, which can quickly find the fault point. Such as measuring the operating voltage of A\/D converter, reference voltage, etc.
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3. Short circuit method In the above mentioned inspection A\/D converter method is generally used in the short circuit method, which is used more in the repair of weak current and micro-electric instruments.
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4. The open circuit method interrupts the suspicious part from the whole machine or unit circuit. If the fault disappears, it indicates that the fault is in the disconnected circuit. This method is mainly suitable for the case of short circuit.
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5. Component measurement method When the fault has been reduced to a place or several components, it can be measured online or offline. If necessary, replace with a good component. If the fault disappears, it indicates that the component is broken.
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6. Interference method uses human body induced voltage as interference signal to observe the change of liquid crystal display, which is often used to check whether the input circuit and the display part are intact.
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Two, repair skills
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A faulty instrument should first check 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.
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1. If all files do not work, focus on checking the power circuit and A\/D converter circuit. When checking the power supply, you can lower the laminated battery, press the power switch, use the positive pen to connect the meter to the negative power supply, and the negative pen to the positive power supply (for a digital multimeter), and switch to the secondary tube measurement level. If the positive voltage of the secondary tube is displayed, it indicates that the power supply is good; if the deviation is large, it indicates that the power supply is faulty. If there is an open circuit, check the power switch and battery leads. If there is A short circuit, it is necessary to use the open circuit method to gradually disconnect the components using the power supply, focusing on checking the operational amplifier, timer and A\/D converter. If there is a short circuit, it usually damages more than one integrated component. The A\/D converter can be checked at the same time as the basic meter, which is equivalent to the DC meter head of the analog multimeter. The specific check method is:
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(1) The range of the meter to be measured is switched to the lowest DC voltage;
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(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 the V pin and COM pin, the measured value is compared with its typical value.
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(3) To measure the reference voltage of A\/D converter, the reference voltage of the commonly used digital multimeter is generally 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 an external potentiometer. (4) Check the display number of zero input, short-circuit the positive end IN of A\/D converter and the negative end IN, so that the input voltage Vin=0, the instrument displays \"00.0" Or \"00.00" .
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(5) Check the display for full bright strokes. Short-connect the TEST pin of the test end to the positive power supply end V, so that the logic becomes a high potential, and all digital circuits stop working. Because each stroke is added to the DC voltage, so all stroke bright counterpoint display \"1888" , the counterpoint table shows \"18888" . If there is A missing stroke phenomenon, check the A\/D converter corresponding output pin and conductive adhesive (or wire), and whether there is poor contact and broken line between the display.
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2. If there is A problem with individual files, it means that the A\/D converter and the power supply are working properly. Because DC voltage and resistance file share a set of divider resistance; Ac-dc current sharing shunt; AC voltage and AC current share a set of AC\/DC converter; Others such as Cx, HFE, F, etc., are composed of separate 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 signals is not accurate or the display of digital jump is large, the focus is on checking whether the contact of the range switch is good.
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3. If the measurement data is unstable, and the value always increases cumulatively, the input end of the A\/D converter is short-circuited, and the display data is not zero, it is generally caused by the poor performance of the 0.1μF reference capacitor.
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According to the above analysis, the basic order of digital multimeter repair should be: digital meter head → DC voltage → DC current → AC voltage → AC current → Resistance (including the buzzer and check the positive pressure drop of the secondary tube)→Cx→HFE, F, H, T, etc. But also not too mechanical, some obvious problems can be seen, can be dealt with first. However, in the adjustment, you must follow the above procedures.
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In short, a fault multimeter, after proper detection, first of all to analyze the possible parts of the fault, and then according to the line diagram to find the fault location for replacement and repair. Because the digital multimeter is A more sophisticated instrument, the replacement of components must use the same parameters of the components, especially the replacement of A\/D converter, must use the manufacturer's strictly screened integration block, otherwise there will be errors and not reach the required accuracy. The new A\/D converter also needs to be checked according to the method described above, and it must not be believed because of the new.
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At present, there are many manufacturers producing digital multimeters in China, and the quality also has advantages and disadvantages. The quality problem of double-sided clad copper plate is not easy to find in repair. When the insulation strength of the resin board is not enough, the error is mainly manifested in the measurement of high voltage, and the resistance value change should be distinguished from that of the partial voltage resistor when repaired. In this case, it is best to use the open circuit method to find the fault point. The burned carbonized part should be cleaned to meet the insulation requirements. When the signal cannot be input due to the double-sided connection due to the break of the transition hole, it is easy to be confused with the phenomenon of poor transfer switch and difficult to separate, such faults should be found by short circuit method.
Disassemble the dial, find the fault point, correct the deformed part or replace the parts. After cleaning, apply a small amount of lubricating silicone grease to the rotating part and install it.
1. Check the fault of the digital multimeter, first of all, we should check and determine whether the fault phenomenon is with common (such as all files can not be measured), but also with personality (such as only the current file can not be measured), for all
2. The minimum DC voltage file of the digital multimeter (that is, DC 200mV file) is the basic file of the three-and-a-half digital multimeter, and the rest of the files are mostly expanded on this basis, so when the instrument is repaired
3. The basic DC voltage does not return to zero. Generally, due to the dirty near the voltage divider resistor, the resistor should be scrubbed around to make it back to zero, and then the DC voltage source input 1V voltage for calibration, and adjust the DC potentiometer during calibration
The gear switch of the multimeter is a circuit board, above the copper sheet, if the peripheral meter is used for a long time, the copper sheet of it will wear, and I will be in poor contact to a certain extent, and it will not be used
Use an eraser to remove contact oxides, and use flat nose pliers to adjust contacts to keep them in the same plane to prevent virtual connections.