Fault inspection and elimination of multimeters
1. Faults and troubleshooting of DC current mode
Use a highly accurate milliampere meter or a fault free multimeter as a standard meter, and connect it in series with the fault meter to measure a DC current. If the reading on the fault gauge is much larger than the standard gauge, it indicates an open circuit in the shunt resistor. If there is no reading, the fault meter conversion switch can be placed in the lowest DC voltage range (such as 2.5V), and the voltage of a new dry battery can be directly measured. If there is still no reading, it is an open circuit in the meter head circuit; If there is a reading and the indicated value is greater than 1.6V, it is an open circuit of the shunt resistor.
Exclusion method: If it is an open circuit in the header circuit, the cause of the open circuit should be identified. If there is an open circuit in the shunt resistor, the instrument back cover needs to be opened to check each shunt resistor one by one (usually a wound resistor), and replace the damaged shunt resistor.
2. Faults and troubleshooting of DC voltage gear
Firstly, if the readings of each gear are too high, it indicates that the multimeter has been dampened, causing the resistance value of the voltage divider resistor to decrease; Exclusion method: Dry the multimeter.
The second issue is that the readings for each gear are relatively small, with higher maximum ranges resulting in more frequent readings. The reason is that the voltage divider resistor changes value; Exclusion method: Replace the resistor with a new one.
The third is normal at low range and the pointer does not move at high range. The reason is that the voltage divider resistor used for high range is open circuit; Exclusion method: Replace the open circuit voltage divider resistor.
3. Faults and troubleshooting of AC voltage gear
There is no reading for each gear. Generally, it is caused by damage to the rectifier components or open circuit in the measurement circuit; Exclusion method: Replace the damaged diode, identify the open circuit in the circuit and repair it.
Secondly, there are readings for each gear, but the readings decrease by half. The reason is that half of the rectifier circuit in the multimeter using a full wave rectifier circuit fails and becomes a half wave rectifier circuit; Exclusion method: Replace the damaged rectifier diode.
4. Faults and troubleshooting of resistance gear
Under normal circumstances, when the transfer switch is set to the R × 1k position and the two probes are short circuited, turn the ohm zero adjuster and the pointer should smoothly move to the zero ohm position. Then turn the switch to each gear of R × 100, R × 10, and R × 1 in sequence, and the pointer should gradually deviate from the zero ohm position, but eventually all can be adjusted to the zero ohm position.
The pointer of the ohm zero adjuster cannot be adjusted to zero, and it is even worse in Rx1 mode. The battery voltage has dropped below 1.5V; troubleshooting method: replace the battery with a new one.
Secondly, if the ohm zero adjuster is misaligned or the pointer jumps during the adjustment process, it is caused by a fault in the ohm zero potentiometer; Exclusion method: Replace or repair the ohm zero potentiometer.
Thirdly, some gear readings are inaccurate. Usually caused by changes in the shunt resistance of this gear; Exclusion method: Replace the variable shunt resistor.
The fourth issue is that each crotch is inaccurate or has no readings. The reason is caused by an open circuit in the battery circuit, an open circuit in the current limiting resistor, or a change in value; Exclusion method: Connect the battery circuit or replace the current limiting resistor.
Number five is that the measured value of a certain gear is 10 times, 100 times, or 1000 times larger than the actual value. The reason is caused by the open circuit of the shunt resistor in this gear; Exclusion method: Connect the shunt resistor.
