For a faulty instrument, the first thing to check and judge is the common phenomenon (all functions can not be measured), or personality (individual function or individual range), and then distinguish the situation, symptomatic solution.
1. If all files cannot work, check the power supply circuit and A/D converter circuit. When inspecting the power supply section, remove the laminate battery, press the power switch, connect the meter to the negative power meter with a positive meter, and connect the power supply to the negative meter (for digital multimeters). Switch to the secondary measurement file. The positive voltage of the diode indicates that the power supply is good. If the deviation is large, it indicates that there is a problem with the power supply. If an open circuit occurs, check the power switch and battery lead. If a short circuit occurs, you need to use the disconnection method, 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, there is generally no damage to an integrated component. Check that the A/D converter can be performed at the same time as the basic meter, which is equivalent to the DC meter of the analog multimeter. The specific inspection method is as follows:
(1) The range of the meter to be measured goes to the lowest level of the DC voltage;
(2) Measure whether the A/D converter operating voltage is normal. According to the type of A/D converter used in the table, whether the measured value matches its typical value corresponds to the V+ pin and COM pin.
(3) The reference voltage of the A/D converter is measured. The reference voltage of the commonly used digital multimeter is generally 100mV or 1V, that is, the DC voltage between VREF+ and COM is measured. If the deviation is 100mV or 1V, an external potentiometer can be used. Make adjustments.
(4) Check the number of inputs to zero, and short-circuit the positive terminal IN+ and negative terminal IN- of the A/D converter so that the input voltage Vin=0 and the meter displays “00.0†or “00.00â€.
(5) Check the display's full bright strokes. The TEST pin of the test terminal is shorted to the positive power supply terminal V+, so that the logic goes high, and all digital circuits stop working. Since DC voltage is applied to each stroke, all pen strokes are displayed as "1888" in the alignment table and "18888" as the alignment table. If there is a lack of strokes, check the A/D converter's corresponding output pin and conductive adhesive (or wiring), and check whether there is poor contact and disconnection between the monitor and the monitor.
2. If there is a problem with individual files, the A/D converter and the power supply are working properly. Because the DC voltage and resistance file share a set of voltage divider resistors; AC and DC current sharing shunts; AC voltage and AC current share a set of AC/DC converters; others such as Cx, HFE, F, etc. are composed of different independent converters. . Understand the relationship between them, and then according to the power diagram, it is easy to find the fault site. If the measurement of small signal is inaccurate or the displayed digital jitter is large, check whether the contact of the range switch is good.
3. If the measurement data is unstable and the value is always accumulatively increased, shorting the input of the A/D converter and displaying the data is not zero, it is generally caused by poor performance of the 0.1μF reference capacitor.
Based on the above analysis, the basic sequence of digital multimeter repair should be: digital table head → DC voltage → DC current → AC voltage → AC current → resistance file (including buzzer and check the positive pressure drop of the diode) → Cx → HFE , F, H, T, etc. However, it cannot be over-machined. Some obvious problems can be dealt with first. However, when making adjustments, you must follow the above procedure.
In short, a fault multimeter, after proper detection, must first analyze the location of possible faults, and then find the fault location according to the circuit diagram for replacement and repair. Because digital multimeters are more sophisticated instruments, replacement components must use components with the same parameters. In particular, when replacing A/D converters, it is imperative to use integrated blocks that are strictly screened by the manufacturer. Otherwise, errors will occur and the required quantities will not be met. Accuracy. Newly-changed A/D converters also need to be inspected according to the methods described above, and must not be trusted for new ones.
At present, there are many domestic manufacturers of digital multimeters, and their quality is also good and bad. The quality of double-sided copper composites is not easy to find in repairs. When the dielectric strength of the resin plate is not enough, the main error is that when measuring high voltage, it is necessary to distinguish from the change of the resistance value of the voltage divider resistance during repair. In this case, it is best to use the open circuit method to find fault points. The part that burns out carbonization should be cleaned and the insulation requirement must be achieved. When the signal cannot be inputted due to the breakage of the transition hole caused by the double-sided connection, it is easy to be confused with the phenomenon of poor transfer switch and difficult to separate. Such faults should be short circuited to find the fault point. (end)
1. If all files cannot work, check the power supply circuit and A/D converter circuit. When inspecting the power supply section, remove the laminate battery, press the power switch, connect the meter to the negative power meter with a positive meter, and connect the power supply to the negative meter (for digital multimeters). Switch to the secondary measurement file. The positive voltage of the diode indicates that the power supply is good. If the deviation is large, it indicates that there is a problem with the power supply. If an open circuit occurs, check the power switch and battery lead. If a short circuit occurs, you need to use the disconnection method, 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, there is generally no damage to an integrated component. Check that the A/D converter can be performed at the same time as the basic meter, which is equivalent to the DC meter of the analog multimeter. The specific inspection method is as follows:
(1) The range of the meter to be measured goes to the lowest level of the DC voltage;
(2) Measure whether the A/D converter operating voltage is normal. According to the type of A/D converter used in the table, whether the measured value matches its typical value corresponds to the V+ pin and COM pin.
(3) The reference voltage of the A/D converter is measured. The reference voltage of the commonly used digital multimeter is generally 100mV or 1V, that is, the DC voltage between VREF+ and COM is measured. If the deviation is 100mV or 1V, an external potentiometer can be used. Make adjustments.
(4) Check the number of inputs to zero, and short-circuit the positive terminal IN+ and negative terminal IN- of the A/D converter so that the input voltage Vin=0 and the meter displays “00.0†or “00.00â€.
(5) Check the display's full bright strokes. The TEST pin of the test terminal is shorted to the positive power supply terminal V+, so that the logic goes high, and all digital circuits stop working. Since DC voltage is applied to each stroke, all pen strokes are displayed as "1888" in the alignment table and "18888" as the alignment table. If there is a lack of strokes, check the A/D converter's corresponding output pin and conductive adhesive (or wiring), and check whether there is poor contact and disconnection between the monitor and the monitor.
2. If there is a problem with individual files, the A/D converter and the power supply are working properly. Because the DC voltage and resistance file share a set of voltage divider resistors; AC and DC current sharing shunts; AC voltage and AC current share a set of AC/DC converters; others such as Cx, HFE, F, etc. are composed of different independent converters. . Understand the relationship between them, and then according to the power diagram, it is easy to find the fault site. If the measurement of small signal is inaccurate or the displayed digital jitter is large, check whether the contact of the range switch is good.
3. If the measurement data is unstable and the value is always accumulatively increased, shorting the input of the A/D converter and displaying the data is not zero, it is generally caused by poor performance of the 0.1μF reference capacitor.
Based on the above analysis, the basic sequence of digital multimeter repair should be: digital table head → DC voltage → DC current → AC voltage → AC current → resistance file (including buzzer and check the positive pressure drop of the diode) → Cx → HFE , F, H, T, etc. However, it cannot be over-machined. Some obvious problems can be dealt with first. However, when making adjustments, you must follow the above procedure.
In short, a fault multimeter, after proper detection, must first analyze the location of possible faults, and then find the fault location according to the circuit diagram for replacement and repair. Because digital multimeters are more sophisticated instruments, replacement components must use components with the same parameters. In particular, when replacing A/D converters, it is imperative to use integrated blocks that are strictly screened by the manufacturer. Otherwise, errors will occur and the required quantities will not be met. Accuracy. Newly-changed A/D converters also need to be inspected according to the methods described above, and must not be trusted for new ones.
At present, there are many domestic manufacturers of digital multimeters, and their quality is also good and bad. The quality of double-sided copper composites is not easy to find in repairs. When the dielectric strength of the resin plate is not enough, the main error is that when measuring high voltage, it is necessary to distinguish from the change of the resistance value of the voltage divider resistance during repair. In this case, it is best to use the open circuit method to find fault points. The part that burns out carbonization should be cleaned and the insulation requirement must be achieved. When the signal cannot be inputted due to the breakage of the transition hole caused by the double-sided connection, it is easy to be confused with the phenomenon of poor transfer switch and difficult to separate. Such faults should be short circuited to find the fault point. (end)
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