Repair and maintenance of digital multimeter
1. Maintenance
The digital multimeter is a precision electronic instrument, do not change the circuit at will, and pay attention to the following points:
⒈Do not connect higher than 1000V DC voltage or higher than 700V AC RMS voltage.
2. Do not connect the voltage source when the function switch is in the Ω and position.
3. Please do not use this watch when the battery is not installed or the back cover is not tightened.
2. Repair method
Digital multimeters have high sensitivity and accuracy, and their applications are found in almost all enterprises. However, due to the multi-factors of its failures, and the randomness of the problems encountered, there are not many rules to follow. Some repair experiences accumulated in the actual work are sorted out for reference by colleagues engaged in this major.
When looking for faults, you should start from the outside and then the inside, first the easy ones and then the difficult ones, break them down into parts, and make breakthroughs in key points. The methods can be roughly divided into the following categories:
1. Feeling method
Use the senses to directly judge the cause of the fault. Through visual inspection, you can find such as disconnection, desoldering, short circuit, broken fuse tube, burned components, mechanical damage, copper foil lifting and fracture on the printed circuit, etc. ; You can touch the temperature rise of the battery, resistors, transistors, and integrated blocks, and you can refer to the circuit diagram to find out the reason for the abnormal temperature rise. In addition, by hand, you can also check whether the components are loose, whether the integrated circuit pins are firmly inserted, and whether the transfer switch is stuck; you can hear and smell whether there are abnormal sounds and odors.
2. Voltage measurement method
By measuring whether the working voltage of each key point is normal, the fault point can be quickly found. Such as measuring the working voltage and reference voltage of the A/D converter.
⒊Short circuit method
In the method of checking the A/D converter mentioned above, the short-circuit method is generally used. This method is often used when repairing weak and micro-electric instruments.
⒋ circuit breaking method
Disconnect the suspicious part from the whole machine or unit circuit. If the fault disappears, it means that the fault is in the disconnected circuit. This method is mainly suitable for the situation where there is a short circuit in the circuit.
5. Measuring element method
When the fault has been narrowed down to one or several components, it can be measured online or offline. If necessary, replace it with a good one. If the fault disappears, the component is broken.
3. Repair skills
For a faulty instrument, first check and judge whether the fault phenomenon is common (all functions cannot be measured) or individual (individual function or individual range), and then distinguish the situation and solve it symptomatically.
1. If all gears fail to work, focus on checking the power supply circuit and A/D converter circuit. When checking the power supply part, you can remove the laminated battery, press the power switch, connect the positive test lead to the negative of the power supply of the meter under test, and the negative test lead to the positive power supply (for digital multimeters), and switch to the diode measurement position. If the forward voltage of the diode is higher, it means that the power supply part is good. If the deviation is large, it means that there is a problem with the power supply part. If there is an open circuit, focus on checking the power switch and battery leads. If there is a short circuit, you need to use the open circuit method to gradually disconnect the components that use the power supply, and focus on checking the operational amplifier, timer and A/D converter. In the event of a short circuit, generally more than one integrated component is damaged. Checking the A/D converter can be carried out simultaneously with the basic meter, which is equivalent to the DC meter head of the analog multimeter. The specific checking method:
⑴The range of the meter under test is turned to the lowest level of DC voltage;
⑵Measure whether the working voltage of the A/D converter is normal. According to the A/D converter model used in the table, corresponding to V+ pin and COM pin, whether the measured value is consistent with its typical value.
(3) Measure the reference voltage of the A/D converter. The reference voltage of a commonly used digital multimeter is generally 100mV or 1V, that is, measure the DC voltage between VREF+ and COM. If it deviates from 100mV or 1V, it can be adjusted by an external potentiometer.
(4) Check the display number with zero input, short-circuit the positive terminal IN+ and the negative terminal IN- of the A/D converter to make the input voltage Vin=0, and the meter displays "00.0" or "00.00".
⑸ Check the full brightness strokes of the display. Short the test terminal TEST pin and the positive power supply terminal V+, make the logic ground become high potential, and all digital circuits stop working. Because DC voltage is added to each stroke, all the strokes are bright and the alignment table shows "1888", and the alignment table shows "18888". If there is a lack of strokes, check whether there is poor contact or disconnection between the corresponding output pin of the A/D converter and the conductive glue (or connection), and the display.
2. If there is a problem with individual files, it means that the A/D converter and the power supply are working normally. Because DC voltage and resistance files share a set of voltage dividing resistors; AC and DC current share a shunt; AC voltage and AC current share a set of AC/DC converters; others such as Cx, HFE, F, etc. are composed of independent different converters . Understand the relationship between them, and then according to the power supply diagram, it is easy to find the fault location. If the measurement of small signals is inaccurate or the displayed number jumps greatly, then focus on checking whether the contact of the range switch is good.
3. If the measurement data is unstable and the value always increases cumulatively, short-circuit the input terminal of the A/D converter and the displayed data is not zero, it is generally caused by the poor performance of the 0.1μF reference capacitor.
According to the above analysis, the basic sequence of digital multimeter repair should be: digital meter head → DC voltage → DC current → AC voltage → AC current → resistance file (including buzzer and checking the positive voltage drop of the diode) → Cx → HFE , F, H, T, etc. But don't be too mechanical. Some obvious problems can be dealt with first. However, when adjusting, the above procedures must be followed.
