Tuesday, January 11, 2011

【 Weak current College 】 digital multimeter and repair methods and repair skills 】

Digital instrument with high sensitivity and accuracy, and its application to all almost all over the enterprise. However, due to its failure to appear is multifactorial, and the randomness of encountering a problem, not many rules to follow, repair difficult. Therefore, I will work in practice over the years has accumulated some experience in repairing, organizing them for engaging in the professional colleagues.

First, the repair method

To find out after the first failure, economic difficulties, decentralised, breakthroughs. Its methods can be divided into the following:

1. feeling method by virtue of the reasons for the failure of the senses direct judgement, by Visual inspection, be found such as broken wires, solder, daxian short-circuit, fuse tube break, burn component, mechanical damage, printed circuit foil tilted and fracture; you can touch up batteries, resistors, transistors, integrated block of temperature rise, refer to the circuit diagram to find out the cause of abnormal temperature rise. In addition, you can check the symbol has become loose, integrated circuit leg is seated, the switch is the cassette; you can hear and smell the odor of sound, there is no different.

2. measure the voltage measuring the key points of the supply voltage is normal, you can quickly identify the point of failure. If logging of the A/D converter voltage, voltage, etc.

3. short circuit in the check in front of A/D converter method in general are short-circuit method, this method of repairing the weak and micro-electric equipment is used more.

4. circuit breaker method to suspicious part of the circuit from the machine or unit, if the break in the fault disappears, said failure is broken circuit. This method is mainly suitable for circuit shorts.

5. test method when a fault has been reduced to a Department or a few components, it can be online or offline measurement. When necessary, the use of symbols to be replaced,if fault disappears, the description element has been bad.

6. interference method use of human induced voltage as the interfering signal, observation of liquid crystal displays, commonly used to check the input circuit and display part is intact.

Second, repair tips

On a faulty meter first you should check and judge the symptom is the common (all features cannot measure) or individual (individual features or individual range), then differences, symptomatic.

1. If all the files will not work, should be focused on checking the power circuit and A/D converter circuit. Check the power part, preferred the following laminated battery, press the power switch is used as the table amount received measured table power negative, negative table pen power (the digital multimeter), switch to the diode measurements, to show the diode forward voltage, power part is good, if large deviation, then the power part of the problem. If an open circuit, check the power switch and battery leads. If a short circuit, you need using method and gradually disconnecting power components, examine the operational amplifier, timer, and A/D converter, etc. If a short circuit, usually more than damage a piece of integrated components. Check A/D converter can and basic tables simultaneously, the equivalent of analogue multimeters DC header, check the method:

(1) to be logging table range go to DC voltage is low;

(2) A/D converter operating voltage is normal. According to table used A/D converter type, the corresponding COM V + feet and legs, measurements and its typical values are compared.

(3) measuring the A/D converters, voltage reference is now commonly used digital multimeter for reference voltage is 100mV or 1V, i.e. measuring the VREF + and DC voltage between the COM, to deviate from the 100mV or 1V, accessible via an external potentiometer to adjust.

(4) check the input to zero the number is displayed, A/D converter is-IN + and negative-IN-the shorted input voltage Vin = 0, the instrument shows "00.0" or "00.00".

(5) check the display of all light strokes. The test-TEST feet and are power-V + shorted, so logically into high potential, all digital circuits to stop working. For each stroke, and a DC voltage, so all of the strokes on the old table shows "1888" bit table shows "18888". If there is the phenomenon of ischemic stroke, check A/D converters corresponding output pin and conductive plastic (or online), and monitor whether poor contact between and breakage.

2. If there is a problem of individual stalls, A/D converter and power components are working properly. Because the DC voltage, resistance stalls with a set of partial pressure resistance; AC and DC current common divider; AC voltage and AC current share a set of AC/DC converter; others, such as Cx, HFE, F, and so is represented by a separate different converters. Understanding the relationships between them, and then under power figure, it is easy to find fault. If inaccurate measurement of low signal or display digital beat large key check range switch contacts are good.

3. If the measurement data, and the value is always a cumulative increase, shorted the A/D converters, display data input is not zero, it is generally datums than 0 μ F capacitor performance caused by poor.

According to the above analysis, digital multimeter repair basic order should be: digital table head ¡ú DC voltage ¡ú DC ¡ú AC voltage → AC current → resistance archives (including the buzzer and check the diode is pressure drop) → Cx → HFE, F, H, T, etc. But also not too mechanical, some obvious problems can be seen, you can handle. But when making adjustments, be sure to follow the procedure described above.

In short, a fault multimeter, after due examination, it is first necessary to analyse the possible failure, and then follow the roadmap found fault location for replacement and repair. Because digital multimeter is a precision instrument, replacing components must use the same parameters, special componentsIs the replacement of A/D converter, be sure to use the manufacturer through strict selection of manifold, will occur error but not reach the required accuracy. The replacement of A/D converter, you need to press the previously described methods, must not result in new and confidence.

At present, the domestic production of the manufacturers of digital multimeter, quality also has good and bad, on both the quality of complex issues, copperplate in repair is not easy to find. Resin plate dielectric strength is not enough time, mainly in the measurement of high voltage error is large, repair and dividing resistors with resistance variation. This is the case, preferably using method, find the point of failure. To burn some of carbonation to clean, insulation requirements. Encountered by the double-sided connections because of fracture transition holes caused no input signal, the easy and transfer switch bad behavior confusing and difficult to separate, such a failure should be short-circuit method to find the point of failure.

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