The indication of the multimeter DC current is normal when the range is small, and there is no indication when the range is large. Repair the fault
Reason: The shunt resistor with a large range (500mA, 1A) is short-circuited. This is because the long-range shunt resistor works under a high current state for a long time, causing the insulation layer of the resistance wire of the shunt resistor to burn out. In addition, the operation error will also cause the shunt to burn out, for example, when measuring the AC voltage, the range selection switch of the multimeter is mistakenly turned on the DC current gear.
Repair: Replace the burned out shunt resistor. If you don't have a wiring diagram at hand. I don’t know the resistance value of the burnt resistor. Two wires can be drawn from the two solder joints of the damaged resistor and connected to the output terminal of the Z×21 type DC resistance box. Adjust the buttons of the resistance box until the meter indicates normal. Write down the value of the resistance box, this value is the resistance value of the shunt resistor that was burned out. Amateur electronic hobbyists generally do not have the Z×21 DC resistance value. At this time, a digital multimeter or a multimeter with higher sensitivity can also be used to measure the burned out shunt resistor. Since the shunt resistor is generally made of manganese steel wire, in order to avoid the inaccurate reading caused by the short circuit caused by the burning of the insulating lock, the resistance wire on the shunt resistor can be removed, stretched and placed on an insulating board or dry measured on the desktop. Of course, if there is a multimeter circuit diagram, you can know the size of the resistance value by consulting the circuit diagram.
