Knowledge on the use and maintenance of digital multimeter calibrator

　The digital multimeter calibrator is used to calibrate multimeters and various meter heads below level 1. If used improperly, the meter being tested and the internal circuit of the instrument will be damaged. Therefore, it is necessary to test it carefully during use to extend the service life of the instrument.

　　1. Precautions for use

　　1. The instrument is suitable for working in an environment with an ambient temperature of 0℃-40℃ and a relative humidity below 80%. The input power supply is preferably stabilized, otherwise the fluctuation or excessive high or low of the external power supply, or even sometimes close to the upper and lower limits of the use range, will damage the power circuit of the instrument.

　　2. After turning on the power switch, let the instrument preheat for 30 minutes. First, turn the two output adjustment knobs counterclockwise to the minimum position. According to the type and range of the instrument to be tested, turn the item selection switch, range switch, and frequency switch (400Hz for DC gear) to the required position. Press the "output" button, the red light will turn on, and slowly adjust the "coarse adjustment" knob clockwise to test the output.

　　3. After each range or item is tested, the output "coarse adjustment" and "fine adjustment" should be turned counterclockwise to the minimum position before it is allowed to switch to other ranges or other items, so as to avoid damaging the measured meter or testing the damaged instrument output due to excessive output voltage or current.

　　4. Do not short-circuit the output line at any voltage level, and do not open the output line at any current output level to avoid damaging the instrument.

　　5. Do not use beyond the range of the instrument. For example, do not use 5V to 5.5V.

　　6. During calibration, if the current level circuit of the meter under test is open, fine-tune the current output "coarse adjustment", and the meter under test and the instrument display have no response, do not turn the knob up to avoid damaging the instrument. When the load is heavy, slowly turn up the "coarse adjustment", and the meter under test and the instrument display do not change much, and stop the calibration immediately.

　　It is best not to use the 7.5A and 10A levels to the full range output. When used near the upper limit, the output indication time should not be too long, which can easily damage the instrument.

　　2. Repair

　　1. The amplifier circuit (see figure)

　　takes DO30-C as an example. The fault phenomenon is that when adjusting the coarse and fine adjustment, the meter head has no display and there is no output in each gear. Check that the 5A fuse is burned out, the reason is that the power amplifier is damaged. When checking each component, it is necessary to disconnect it from the external circuit for inspection to avoid mutual influence and failure to check the true situation.

　　Pull out the amplifier board from the instrument and first check the BG1 and BG2 high-power tubes, which are generally damaged. Then check the components on the amplifier board, especially the two medium-power tubes BG1-4, BG1-5, the push tube BG1-3 and the potentiometer W1-3. After repair, check that the BG1 and BG2 shells should be short-circuited to the ground, and the base of BG1-4 and the base of BG1-5 cannot be open. After the amplifier board is checked correctly, insert it into the instrument. Turn on the power supply, do not turn on the output button first, and check the voltage at each point according to the schematic diagram. Adjust the DC static operating point of the amplifier: Use the minimum DC voltage setting of the multimeter to measure the midpoint voltage between the emitter resistors of the two medium-power tubes. If it is less than 0.01V (the pointer does not move or moves slightly), it is normal. If it is abnormal, adjust W1-1.

　　Set the project switch to AC, 60Hz, 5V, turn on the output button, and the output transformer is connected to the midpoint of the amplifier. The voltage on R4 and R5 is generally about 0.15V. If the deviation is too large, it means that the center voltage is too high or other faults. Slowly open the coarse adjustment and touch the heat of the two high-power tubes to see if it is evenly heated. If the contrast is too large, adjust W1-3, which is the dynamic center point of the amplifier.

　　Set the project to DC, 400Hz, 10A at the beginning, add a short circuit line to the output terminal, adjust the output to 10A, and adjust W1-2 if it is not enough. It cannot be adjusted too much. Although the amplifier's amplification factor is increased to increase the current, it will reduce the feedback depth and cause instability. The above projects are completed, indicating that the power amplifier is normal.

　　2. The fault phenomenon of the oscillation circuit (see the figure above)

　　is that there is no output or the output is unstable in all the voltage and current gears. First check the coarse and fine adjustment potentiometers. You can use a multimeter to check whether the resistance value can change smoothly to judge whether it is good or bad. Check whether the frequency conversion switch is in good contact. If it is good, check the oscillation board. Slightly move the potentiometer on the board to see if there is any change and whether it starts to oscillate. The most likely part of the oscillation board to be 　　damaged is the operational amplifier 5G23A.

　　3. Power supply circuit The fault phenomenon is that the power supply fuse burns when the machine is turned on. The bridge rectifier OSZ is the most likely to be damaged. 　　4. High-voltage rectifier board 　　The fault phenomenon is that the DC gear 50V, 500V, and IkV are abnormal, and the output is adjusted, but there is no display or insufficient display. The most likely part to be damaged is the capacitor. You can use the BX10K gear of the multimeter to check the charging and discharging of the capacitor. If the resistance value is small after the discharge is stable, it means that the capacitor has leaked and is damaged. 　　The above is the repair of common faults of digital multimeter calibrators.