(Technical Post) Digital Multimeter Repair Methods and Techniques
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 range), 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, and), 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 of the good, if the deviation is large, it means that 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 using 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 as the basic table, equivalent to the DC head of the analogue multimeter, the specific check method:
(1) The range of the measured table is turned 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, measure the DC voltage between VREF + and COM, if it deviates from the 100mV or 1V, can be adjusted by an external potentiometer.
(4) check the input is zero display, the positive terminal of the A/D converter IN + and the negative terminal IN - short, so that the input voltage Vin = 0, the instrument shows "00.0" or "00.00".
(5) Check the full brightness of the display strokes. The test terminal TEST feet and 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 para table display "1888", para table display "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, it means that the A/D converter and the power supply part are working properly. Because the DC voltage, resistance file share a set of voltage divider resistor; AC and DC current share a common shunt; AC voltage and AC current share a set of AC/DC converter; others, such as Cx, HFE, F, etc. are composed of independent different converters. Understanding 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 signals is inaccurate or the display figure jumps a lot, then focus on checking whether the contact of the range switch 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 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 are clearly visible can be dealt with first. However, when performing tuning, then the above procedure must be followed.
