Digital multimeter commonly used fault checking and repair skills
A faulty instrument should first check and identify the fault phenomenon is common (all functions can not be measured), or individuality (individual functions or individual ranges), and then differentiate between the situation, the right solution.
1. If all the gear can not work, should focus on checking the power supply circuit and A/D converter circuit. Check the power supply part, you can remove the laminated battery, press the power switch, with a positive pen connected to the measured table power supply negative, negative pen connected to the power supply positive (for the digital multimeter), the switch is hit to the secondary tube measurement file, if the display is the secondary tube positive voltage, it means that the power supply part is good, if the deviation is large, then the power supply part of the problem. If there is an open circuit, focus on checking the power switch and battery leads, etc.. If there is a short circuit, it is necessary to use the disconnect method to gradually disconnect the components that use the power supply, focusing on checking the operational amplifier, timer and A/D converter, etc.. If a short circuit occurs, it usually damages more than one integrated component. Check the A/D converter can be carried out at the same time with the basic table, equivalent to the DC head of the analog multimeter, the specific check method:
(1) the range of the measured table to the lowest DC voltage gear;.
(2) Measure whether the A/D converter operating voltage is normal. According to the table used in the A/D converter model, corresponding to the V + feet and COM feet, the measured value and its typical value compared to whether it is consistent.
(3) Measure the reference voltage of the A/D converter, the current commonly used digital multimeter reference voltage is generally 100mV or 1V, that is, to measure the DC voltage between VREF + and COM, if it deviates from the 100mV or 1V, it can be adjusted by an external potentiometer.
(4) Check the input zero display, the positive terminal IN + and negative terminal IN - short A/D converter, so that the input voltage Vin = 0, the meter displays "00.0" or "00.00".
(5) Check the full brightness of the display strokes. The test terminal TEST foot and the positive power supply terminal V + short, so that the logic ground into a high potential, all digital circuits stop working. Because each stroke is added to the DC voltage, so all strokes bright counterpoint table shows "1888", counterpoint table shows "18888". If there is a lack of strokes phenomenon, check the A/D converter corresponding to the output pin and conductive rubber (or line), and the display between the poor contact and disconnection.
2. If there is a problem with individual files, the A/D converter and power supply part of the normal operation. DC voltage, resistance file because of a common set of voltage divider resistor; AC and DC current shunt; AC voltage and AC current with a common set of AC/DC converter; other 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 site. If the measurement of small signal inaccuracy or display figures jump big, then focus on checking the range switch contact is good.
3. If the measurement data is unstable, and the value is always increasing cumulatively, shorting the input of the A/D converter, the display data is not zero, it is generally caused by the poor performance of the 0.1μF reference capacitor.
According to the above analysis, the basic order of repair of the digital multimeter should be: digital table head → DC voltage → DC current → AC voltage → AC current → resistance file (including the buzzer and check the positive voltage drop of the secondary tube) → Cx → HFE, F, H, T and so on. However, it should not be overly mechanical, and some problems that can obviously be seen can be dealt with first. However, when performing tuning, then be sure to follow the above procedures.
General In short, a faulty multimeter, after proper testing, the first thing to do is to analyze the possible parts of the fault, and then according to the wiring diagram to find the fault location for replacement and repair. Because the digital multimeter is a more precise instrument, the replacement components must be used with the same parameters of the components, especially the replacement of the A/D converter, must be used by the manufacturer after rigorous screening of the integrated block, otherwise there will be errors and can not achieve the required accuracy. The new A/D converter, also need to check the method described earlier, not because of the new and believe it.
At present, the domestic production of digital multimeter manufacturers are many, the quality is also good and bad, the quality of the double-sided compound copper board, in the repair is not easy to find. Resin plate insulation strength is not enough, mainly in the measurement of high voltage error is large, the repair should be differentiated from the change in resistance value of the voltage divider. When encountering this situation, it is best to use the disconnection method to find the point of failure. The burnt and carbonized part should be cleaned up to meet the insulation requirements. Encountered by the double-sided connecting line due to the transition hole fracture caused by the inability to input signals, easy to confuse with the phenomenon of bad transfer switch and difficult to separate, this type of failure is preferable to use the short-circuit method to find the point of failure.
