Analysis and Repair of Common Faults of Multimeter
Analysis and Repair of Common Faults
When analyzing and repairing the common faults of the digital multimeter, first open the meter and carefully check whether the various components on each (PCB) board have burnt out or desoldering. After eliminating such problems, check whether the various output voltages of the power supply (PCB) board are normal. The following introduces the analysis and repair methods of common faults mentioned in Section 2
1. The resistance cannot be measured
This kind of failure is usually a hard failure caused by component damage. Most of its faults appear in the protection and measurement circuit and the adjustment and correction circuit. After eliminating the faults of the protection and measurement circuit, check repeatedly the resistance accuracy adjustment and correction circuit of each file composed of TL006 operational amplifier and adjustment resistor. When it is determined that the calibration resistors are normal, this fault is caused by the damage of the TL006 operational amplifier, and the fault is eliminated after replacing the TL006 operational amplifier.
2. All levels of resistance, voltage and current cannot be measured
This kind of failure is also a hard failure caused by damaged components. Usually people think that it is caused by damaged components in the power supply or drive display (fPCB). The author found that the power supply (PCB1) board and drive display (PCB4) were damaged during years of repair work. Boards are not easily damaged. This kind of failure is caused by the damage of components in the data sampling circuit composed of A/D converter. At this time, as long as the damaged TI 7135 h/b converter is replaced, the failure can be eliminated.
3. The input test leads are short-circuited and the displayed number does not return to zero. When the AC voltage is measured on the millivolt block, the indicated value jumps wildly. This fault is a soft fault. Usually it is very difficult to find and eliminate the cause of the fault. According to the experience of many years of repair work, it was found that this failure was caused by the contact oxidation of the relay in the input measurement circuit, resulting in poor contact. At this time, carefully clean the contacts of the relay. It is an effective method to eliminate this kind of failure.
