Repairing and Maintaining Digital Multimeters
maintain
A digital multimeter is a precision electronic instrument. Do not change the circuit arbitrarily, and pay attention to the following points:
1. Do not connect to a DC voltage higher than 1000V or an AC RMS voltage higher than 700V
2. Do not connect the voltage source when the function switch is in the Ω and positions
3. Do not use this meter when the battery is not properly installed or the back cover is not tightened
Repair method
Digital multimeter meters have high sensitivity and accuracy, and their applications are almost universal in all enterprises. However, due to the multifactorial nature of its faults and the high randomness of encountered problems, there are not many rules to follow. Therefore, some repair experience accumulated in practical work will be compiled for reference by colleagues in this field.
Finding faults should start from the outside and then from the inside, from the easy to the difficult, break them into parts, and focus on breakthroughs. The methods can be roughly divided into the following:
1. Sensory method
By relying on the senses to directly determine the cause of the fault, through visual inspection, it can be found that such as wire breakage, desoldering, short circuit to grounding, broken fuse tubes, burnt components, mechanical damage, copper foil warping and breakage on printed circuits, etc; You can touch the temperature rise of the battery, resistor, transistor, and integrated block, and refer to the circuit diagram to identify the cause of abnormal temperature rise. In addition, you can also check by hand whether the components are loose, whether the integrated circuit pins are securely inserted, and whether the transfer switch is stuck; Can be heard and smelled for any abnormal sounds or odors.
2. Voltage measurement method
Measuring whether the working voltage of each key point is normal can quickly identify the fault point. For example, measuring the working voltage and reference voltage of the A/D converter.
3. Short circuit method
The short circuit method is generally used in the inspection of A/D converters mentioned earlier, which is more commonly used in repairing weak and micro electrical instruments.
4. Circuit breaking method
Disconnect the suspicious part from the entire machine or unit circuit. If the fault disappears, it indicates that the fault is in the disconnected circuit. This method is mainly suitable for situations where there is a short circuit in the circuit.
5. Measurement element method
When the fault has narrowed down to a certain location or several components, it can be measured online or offline. If necessary, replace with good components. If the fault disappears, it indicates that the component is damaged.
3. Repair Techniques
For a faulty instrument, the first step is to check and distinguish whether the fault phenomenon is common (all functions cannot be measured) or individual (individual functions or ranges), and then distinguish the situation and solve the problem accordingly.
If all gears cannot work, the power supply circuit and A/D converter circuit should be checked with emphasis. When checking the power supply, remove the stacked battery, press the power switch, connect the positive lead to the negative power supply of the measured meter, and connect the negative lead to the positive power supply (for a digital multimeter). Turn the switch to the measurement position of the secondary transistor. If the display shows the positive voltage of the secondary transistor, it indicates that the power supply is good. If the deviation is large, it indicates that there is a problem with the power supply. If an open circuit occurs, focus on checking the power switch and battery leads. If a short circuit occurs, it is necessary to use the circuit breaker method to gradually disconnect the components using the power supply, with a focus on checking operational amplifiers, timers, and A/D converters. If a short circuit occurs, it usually damages more than one integrated component. The A/D converter can be checked simultaneously with the basic meter, which is equivalent to the DC meter head of an analog multimeter. The specific inspection method is:
(1) Turn the range of the measured meter to the lowest level of DC voltage;
⑵ Measure whether the working voltage of the A/D converter is normal. According to the A/D converter model used in the table, corresponding to the V+pin and COM pin, whether the measured values match their typical values.
⑶ Measure the reference voltage of the A/D converter. The reference voltage of commonly used digital multimeter is generally 100mV or 1V, which means measuring the DC voltage between VREF+and COM. If it deviates from 100mV or 1V, it can be adjusted through an external potentiometer.
(4) Check the display number with zero input, short circuit the positive terminal IN+and negative terminal IN - of the A/D converter, so that the input voltage Vin=0, and the instrument displays "00.0" or "00.00".
(5) Check the full bright strokes on the display. Short circuit the test pin at the testing end to the positive power supply terminal V+, so that the logic ground becomes high potential and all digital circuits stop working. Due to the DC voltage applied to each stroke, the alignment meter displays "1888" and the alignment meter displays "18888" when all strokes are lit. If there is a lack of stroke, check the corresponding output pin of the A/D converter and the conductive adhesive (or wiring), as well as whether there is poor contact or disconnection between the A/D converter and the display.
2. If there is a problem with individual gears, it indicates that the A/D converter and power supply are working properly. Because the DC voltage and resistance range share a set of voltage divider resistors; AC and DC current sharing shunt; AC voltage and AC current share a set of AC/DC converters; Other components such as Cx, HFE, F, etc. are composed of independent different converters. Understanding the relationship between them and based on the power diagram, it is easy to locate the faulty part. If the measurement of small signals is not accurate or the displayed number jumps excessively, the focus should be on checking whether the contact of the range switch is good.
If the measurement data is unstable and the value always accumulates, and the input terminal of the A/D converter is short circuited, and the displayed data is not zero, then it is generally 0.1 μ Caused by poor performance of F's reference capacitor.
Based on the above analysis, the basic repair sequence for a digital multimeter should be: digital meter head → DC voltage → DC current → AC voltage → AC current → resistance range (including buzzer and checking positive voltage drop of the secondary tube) → Cx → HFE, F, H, T, etc. But it should not be too mechanical. Some obvious problems can be addressed first. But when conducting calibration, it is necessary to follow the above procedure.
In short, a faulty multimeter, after appropriate testing, first needs to analyze the possible location of the fault, and then find the fault location according to the circuit diagram for replacement and repair. Due to the fact that a digital multimeter is a more precise instrument, when replacing components, it is necessary to use components with the same parameters, especially when replacing A/D converters. It is necessary to use integrated blocks that have been strictly selected by the manufacturer, otherwise errors may occur and the required accuracy may not be achieved. The newly replaced A/D converter also needs to be checked according to the method mentioned earlier, and must not be trusted due to its novelty.
