How to judge the DC voltage/current block failure of the mechanical multimeter and how to repair it?
1. DC current block fault
1) When measuring, the pointer has no deflection.
The fault is mostly:
The meter head circuit is open circuit, so that the current is equal to zero.
The meter head shunt resistor is short-circuited, so that most of the current can't flows throught the meter head.
The terminal is unwelded, so that the meter is in the meter No current flows.
2) Some ranges are not available or the error is large.
Caused by a shunt resistor open, shorted or variable.
3) The measurement error is large
Because of the shunt resistance is variable (the resistance value becomes large, resulting in a positive error error; the resistance value becomes small, resulting in a negative error tolerance).
4) The indication is irregular and the range is difficult to control.
The reason is mostly the position change switch position is tilted (adjust the position, it can be solved after the installation is correct).
2. DC voltage block fault
1) The pointer is not deflected and the indication is always zero.
The reason is that the voltage divider is additionally broken or the meter is broken.
2) The error is large.
The reason is that the increase of the resistance of the additional resistor causes the positive error of the indication value to be out of tolerance, and the decrease of the resistance value causes the negative error of the indication value to be out of tolerance.
3) The positive error is out of tolerance and is severe as the voltage range becomes larger.
The voltage circuit components in the meter are damp and leak, the circuit components or other components leak, and the printed circuit board is contaminated by moisture, moisture, breakdown, electric shock and carbonization. When repairing, scrape off the charred fiberboard, remove the dust, clean the circuit with alcohol and then dry. In severe cases, use a knife to cut the circuit board between the copper box and the copper foil, so that the insulation is good.
4) The pointer is deflected when not energized, and is more obvious when the range is small.
The cause of the failure is that the voltage measurement circuit forms a leakage circuit with the built-in battery due to moisture and serious pollution. The treatment method is the same as above.