First, repair method:
1. Sensory observation
Relying on the senses to directly judge the cause of the failure, through the appearance inspection, if you can find such as broken wire, welding can be seen by the naked eye, it is the best situation, then you can touch the temperature rise of the battery, resistor, transistor, integrated block, according to the circuit diagram to find out the cause. In addition, the hand can also check whether the components are loose, whether the integrated circuit pin is inserted firmly, and whether the transfer switch is cassette; You can hear and smell a different sound, odor.
Of course, this method only applies to digital multimeters, for strong electricity, or I hope you pay attention to safety, life can not come again.
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2. Measure voltage and short circuit
Measuring whether the working voltage of each key point is normal can quickly find the fault point. Such as measuring the operating voltage of A\/D converter, reference voltage, etc. The short-circuit method is generally used in the inspection of A\/D converter, which is used more in the repair of weak current and micro-electric instruments.
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3. Open circuit method
The suspicious part is interrupted from the whole machine or unit circuit. If the fault disappears, it indicates that the fault is in the disconnected circuit. This method is mainly suitable for the case of short circuit.
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4. Measuring components
When the fault has been narrowed down to one place or a few components, it can be measured online or offline. If necessary, replace with a good component. If the fault disappears, it indicates that the component is broken.
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5. Interference method
The human body induced voltage is used as a interference signal to observe the change of the liquid crystal display, which is often used to check whether the input circuit and the display part are intact.
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Two, repair skills:
1. If all files do not work, focus on checking the power circuit and A\/D converter circuit.
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2. If an open circuit occurs, check the power switch and battery leads.
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3. If there is a short circuit, it is necessary to use the open circuit method to gradually disconnect the components using the power supply, and focus on checking the operational amplifier,
Timer and A\/D converter, etc. In this case, more than one integrated component is usually damaged. The A\/D converter can be checked at the same time as the basic meter, which is equivalent to the DC meter head of the analog multimeter. The specific inspection methods are as follows:
The reference voltage of the current commonly used digital multimeter is generally 100mV or 1V, that is, the DC voltage between VREF+ and COM is measured. If it deviates from 100mV or 1V, it can be adjusted by an external potentiometer.
④ Check the display number of zero input, short-circuit the positive end of the A\/D converter IN+ and the negative end IN-, so that the input voltage Vin=0, the instrument displays \"00.0\" or \"00.00\".
⑤ Check the display for full bright strokes. Short-circuit the TEST pin of the test end and the positive power supply end V+, so that the logic becomes a high potential, and all digital circuits stop working. Because DC voltage is added to each stroke, all strokes are bright to display \"1888\" and \"18888\". If there is A missing stroke phenomenon, check the A\/D converter corresponding output pin and conductive adhesive (or wire), and whether there is poor contact and broken line between the display.
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4. If there is A problem with individual files, it means that the A\/D converter and the power supply are working properly. Because DC voltage and resistance file share a set of divider resistance; Ac-dc current sharing shunt; AC voltage and AC current share a set of AC\/DC converter; Others such as Cx, HFE, F, etc., are composed of separate different converters. Understanding the relationship between them, and then according to the power diagram, it is easy to find the fault location. If the measurement of small signals is not accurate or the display of digital jump is large, the focus is on checking whether the contact of the range switch is good.
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5. If the measurement data is unstable, and the value always increases cumulatively, short-circuit the input end of the A\/D converter, and the display data is not zero, it is generally caused by the poor performance of the reference capacitor of 0.1μF.
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