Repair Methods and Techniques for Digital Multimeters
Digital instruments 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 encountering problems, there are not many rules to follow, making repair difficult. Therefore, I have compiled some of the repair experience I have accumulated through years of practical work for reference by colleagues in this field. The capacitive voltage divider high-voltage measurement system is suitable for measuring pulse high voltage, lightning high voltage, and power frequency high voltage, and is an alternative to high-voltage static voltage meters.
Repair method:
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. The voltage measurement method measures whether the working voltage of each key point is normal, which can quickly identify the fault point. For example, measuring the working voltage and reference voltage of the A/D converter.
3. 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. The circuit breaker method disconnects 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. When the fault has narrowed down to a certain location or several components, online or offline measurement can be performed. If necessary, replace with good components. If the fault disappears, it indicates that the component is damaged.
6. Interference method uses human induced voltage as interference signal to observe the changes in LCD display, commonly used to check whether the input circuit and display part are intact.
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 focus should be on checking the power circuit and A/D converter circuit. 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 low DC voltage range;
(2) 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, do the measured values match their typical values.
(3) The reference voltage for measuring A/D converters is generally 100mV or 1V for commonly used digital multimeter, 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 monitor. 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.
