Repair methods for digital multimeters
Digital instruments have high sensitivity and accuracy, and their applications are found in almost all enterprises. However, due to the fact that there are many factors in the failure, and the randomness of the problems encountered is large, there are not many rules to follow, and the repair is difficult. Therefore, I have sorted out some repair experience accumulated in many years of work practice for the reference of colleagues engaged in this profession.
1. Repair method
When looking for faults, you should start from the outside and then the inside, first easy and then difficult, break the whole into parts, and make breakthroughs in key points. The methods can be roughly divided into the following categories:
1. 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 out components, mechanical damage, and copper foil warping on the printed circuit. You can touch the temperature rise of batteries, resistors, transistors, and integrated blocks, and you can refer to the circuit diagram to find out the reason for 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.
2. Voltage measurement method: measure whether the working voltage of each key point is normal, and find out the fault point quickly. Such as measuring the working voltage and reference voltage of the A/D converter.
3. Short-circuit method In the method of checking the A/D converter mentioned above, the short-circuit method is generally used. This method is often used when repairing weak and micro-electric instruments.
4. 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.
5. Measuring component method When the fault has been reduced to a certain place or several components, 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, first check and judge whether the fault phenomenon is common (all functions cannot be measured) or individual (individual function or individual range), and then distinguish the situation and solve it symptomatically.
1. 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 remove 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), and switch to the diode measurement position. 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 carried out simultaneously with the basic meter, which is equivalent to the DC meter head of the analog multimeter. The specific checking method:
(1) The range of the meter under test is turned to the lowest gear 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, whether the measured value is consistent with its typical value.
(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 an external potentiometer Make adjustments.
(4) Check the display number whose input is zero, 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 the 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 any poor contact or disconnection between the corresponding output pin of the A/D converter and the conductive glue (or connection), and the display.
2. If there is a problem with individual files, it means that the A/D converter and the power supply are working normally. Because DC voltage and resistance files share a set of voltage dividing resistors; AC and DC current share a shunt; AC voltage and AC current share a set of AC/DC converters; others such as Cx, HFE, F, etc. are composed of independent different converters . Understand the relationship between them, and then according to the power supply diagram, it is easy to find the fault location. If the measurement of small signals is inaccurate or the displayed number jumps greatly, then focus on checking whether the contact of the range switch is good.
3. If the measurement data is unstable, and the value always increases cumulatively, short the input terminal of the A/D converter, and the displayed data is not zero, it is generally caused by the performance of the reference capacitance of 0.1μF** .
According to the above analysis, the basic sequence of digital multimeter repair should be: digital meter head→DC voltage→DC current→AC voltage→AC current→resistance gear (including buzzer and checking the positive voltage drop of diode)→Cx→HFE , F, H, T, etc. But don't be too mechanical. Some obvious problems can be dealt with first. However, when adjusting, the above procedures must be followed.
In short, for a faulty multimeter, after proper testing, it is first necessary to analyze the possible location of the fault, and then find the fault location according to the circuit diagram for replacement and repair. Because the digital multimeter is a relatively precise instrument, components with the same parameters must be used for replacement components, especially for the replacement of A/D converters, the integrated blocks that have been strictly screened by the manufacturer must be used, otherwise errors will occur and the required components will not be met. Accuracy. The newly replaced A/D converter also needs to be checked according to the method mentioned above, and it must not be trusted because of its newness.
At present, there are many domestic manufacturers of digital multimeters, and the quality is also good or bad. It is not easy to find out the quality problems of double-sided copper clad boards during repairs. When the insulation strength of the resin board is not enough, it is mainly manifested in the large error when measuring high voltage, and it must be distinguished from the resistance change of the voltage dividing resistor when repairing. In this case, it is best to use the open circuit method to find the fault point. The burnt and carbonized parts should be cleaned up to meet the insulation requirements. When the input signal cannot be input due to the breakage of the transition hole caused by the double-sided connection, it is easy to be confused with the phenomenon of the changeover switch and it is difficult to separate. This kind of fault should use the short-circuit method to find the fault point.
