Maintenance of multimeter circuit faults:
Fault 1: The input fuse 250V/0.5A is burnt out, causing no current input and all gears cannot be used. This fault is considered the simplest in the repair table. Due to incorrect use, using the current gear to measure voltage or using the resistance gear to measure voltage can cause the input fuse to be burnt out, resulting in no current input fault, The maintenance method is very simple. Disassemble the multimeter and test the input fuse. It is found that the fuse has been burnt out. Replace it with a fuse of the same model and specification. Repair the fault. The fuse of the 47 type meter has a specification of 250V/0.5A
Fault 2: Two rectifier diodes in the half wave rectification circuit of the rectifier in the AC voltage range have breakdown or open circuit phenomenon. We know the measurement principle of the pointer multimeter in the AC voltage range: the measurement range is expanded by connecting internal resistors in series to divide the voltage. The half wave rectification circuit rectifies the AC signal into a DC signal and flows it through the meter head for measurement, Since the head of the pointer multimeter is a DC Ammeter, which cannot flow through the AC signal, it is necessary to add a half wave rectifier circuit in the AC voltage range as a rectifier to convert the AC signal into a DC signal and flow through the meter head for measurement. Therefore, once the AC power is measured, it must be rectified once through the rectifier diode.
If AC power is measured for a long time, the rectifier diode will often be impacted by AC signals and will encounter open or short circuits, which is inevitable. Therefore, if you encounter a malfunction where the AC voltage range of the pointer multimeter cannot be used, first check the safety tube. Because the safety tube is burned out, it will cause no current input, and the meter needle will definitely not move. If the safety tube is normal and the AC voltage range is still unusable, You should first check the voltage divider in the AC voltage range to see if there is any increase or decrease in the resistance value of the voltage divider.
If the AC voltage range is still unusable after replacing it first and ruling out any damage to the voltage divider, the focus should be on checking the forward and reverse resistance values of the two rectifier diodes in the half wave rectifier circuit in the AC voltage range to prevent the breakdown or open circuit of the rectifier diodes from causing the malfunction of the AC voltage range unusable. If there is a breakdown or open circuit in the pipe, replace the same type of pipe to repair the fault urgently.
Fault 3: Inaccurate measurement of the resistance gear and the inability to use the resistance gear. Due to the low voltage of the 9V and 1.5V batteries, the measurement was not accurate. The battery was replaced for repair, and the resistance gear was unable to measure. Due to broken wires or solder joints in the wiring between the 9V and 1.5V batteries, the fault was checked and repaired. The standard resistance of the resistance gear was open, causing the resistance gear to be unusable.
Fault 4: The contact between the brush and the circuit board is loose, causing a certain gear or other gear to be unable to measure, and the pointer to be unable to deflect. The maintenance method is simple. Remove the multimeter, reinstall the brush, and clean the circuit board before installing the brush. This fault is very simple, caused by poor contact between the brush and the circuit board.
Fault 5: The shunt resistor in the current range burns out. We know that the shunt resistor values in the current range are relatively small, ranging from a few hundred ohms to several tens of ohms. If the current range is used incorrectly to measure voltage, it is equivalent to a short circuit power source, which generates a short circuit current and directly burns out the shunt resistor. This should be noted.
Fault 6: The resistance gear is not zeroed correctly. Check whether the battery voltage is low and the battery is normal. Check the wiring. The wiring is normal. In the future, the resistance gear is not zeroed correctly. Check the standard resistance. If the standard resistance is normal and the zeroing is not correct, adjust the Potentiometer by measuring the deflection current of the meter head. If the zeroing is still not possible, it is the fault of the ohmic zeroing circuit. First check the Potentiometer. If the Potentiometer is normal, check the wiring of the Potentiometer, If the wiring is normal, finally check the adjustable resistance connected to the Potentiometer. The fault may be caused by the increased or decreased value of the adjustable resistance. After checking, replace it and repair it.
Fault 7: The AC and DC voltage divider resistors are burned out. We know that the resistance values of voltage divider resistors are relatively high, such as DC 1000V connected to 10M divider resistors 500V connected to 5M 250V connected to 4M 50V connected to 800K 10V connected to 150K. The resistance values of AC voltage divider resistors 1000V connected to 2M 500V connected to 1M 250V connected to 800K are relatively high and are not easily damaged. If the voltage divider resistors below 10V range are used incorrectly, use a gear below 10V range to measure high voltage, 1： The input fuse will definitely be burnt out. 2: The voltage divider resistor is easy to burn out, and this should be checked carefully.
