The visual inspection method is to judge the cause of the malfunction by the sense of the senses (hand, eye, ear, nose) without relying on electrical measurement without power or power. Can be divided into non-power observation method and power observation method.
1) Method Step 1 In the case of no power, first observe the switches, knobs, dials, sockets, binding posts, indicating meters and display devices, power cords, fuses, plugs, etc. on the panel of the repaired electrical appliance. Loose, misplaced, stuck, broken, etc.
2 Open the cover of the cabinet and observe the internal components, components, plug-ins, circuit wiring, power transformer, exhaust fan, etc., whether there is burnt discoloration, liquid leakage deformation, mold, breakdown, etc. Loose, broken, desoldered, poor contact and other issues. Check the copper foil and solder joints on the printed circuit board for goodness, and pull out some components and parts by hand to check them.
(3) In the case where the above problem cannot be found by the non-power-on observation method, the power-on observation is performed. That is, in the case of power-on, observe whether there is fire, smoke, odor, abnormal sound, touch the transistor, integrated circuit, etc. Whether it is hot, if there is an abnormal phenomenon, immediately shut down, find the fault location. For the sake of safety, it should be observed by step-by-step energization, that is, using an autotransformer, a series AC ammeter and a parallel AC voltmeter to gradually pressurize the repaired appliance to the rated working voltage, and observe whether there is any abnormality in the machine. It is necessary to immediately power off the repair. If the pressure is not large, the indication value of the AC ammeter has increased significantly. This indicates that there is a short circuit fault in the meat. The output of the autotransformer should be adjusted to zero volt immediately, the power supply should be disconnected, and then the rectifier of the device should be divided. The circuit, the power supply branch or the plug-in unit with doubtful separation, and then the step-by-step pressure test, when the circuit or unit with short-circuit fault is separated, the indication of the AC ammeter should return to normal, otherwise the power transformer has a short-circuit fault.
During the step-by-step energization observation process, if the AC voltage output from the autotransformer is close to the rated operating voltage of the device [usually (1 ± 0.1) × 200 V], and the indication value of the AC voltmeter does not exceed the normal value, then the instrument There is no short circuit inside.
2) Precautions 1 The current meter range I0 in the power-on observation method can be selected according to the following formula:
I0 ≥ 2 × P / U
Where P is the power consumption of the electrical equipment; U is the operating voltage.
2 If you can't simply replace the damaged components during repair, you should further check the circuit schematic diagram and assembly drawing of the repaired electrical equipment, find out the circuit parts of the damaged components and their functions, and analyze the real cause of electrical damage. And the extent to which it may be affected. In this way, we can find out the various causes of the fault and repair it together. Otherwise, the cause of the fault is not eliminated. When the electrical equipment is powered on, it will inevitably cause the updated components to be damaged immediately.
3 When replacing damaged components, in principle, components of the same type and specifications should be used. Especially for circuits with precision requirements, the performance parameters of the replaced components should be tested to ensure the electrical parameters of the damaged components. .
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