Inspection methods and operation practices for electronic product maintenance

1, visual inspection

(1) Understand the fault situation

When troubleshooting electronic equipment, do not rush to check the power. First of all, the user should be informed about the use of the electronic device before and after the failure (when the fault occurs at the start-up, or suddenly or gradually occurs during the work, whether there is smoke, burnt smell, flashing, fever; whether the switch is moved before the fault, Knobs, buttons, plug-ins, etc.) and climate and other environmental conditions.

(2) Visual inspection

(a) Check before power-on: Check whether the button, switch and knob are placed correctly; whether the cable or wire plug is loose; whether the printed circuit board copper foil is broken, short-circuited, mildewed, broken, soldered, fired, Yuan Whether the device has deformation, de-soldering, mutual collision, charring, liquid leakage, cracking, etc., whether the fuse is blown or loose, whether the motor, transformer, wire, etc. have burnt smell, broken wire or fire mark; whether the relay coil is good Whether the contact is ablated or the like.

(b) Power-on check: If the check before power-on is normal or the abnormal phenomenon is removed, the power-on check can be performed. During power-on inspection, pay special attention to whether the indicating equipment (such as electric meter, indicator light, fluorescent screen) is normal at the moment of power-on, whether there is smoke or fire in the machine, and the motor casing, transformer, integrated circuit, etc. after power off. Whether it is hot. If it is normal, the measurement can be checked.

2, measurement inspection method

(1) Resistance measurement method

Measurements are generally made with the electrical blocking of the multimeter without power.

Precautions:

(a) When measuring components or circuits that are related to other circuits, pay attention to the parallel effect of the circuit and, if necessary, disconnect one end of the circuit under test;

(b) Measurement. When there is a meter head in the circuit, the meter head should be short-circuited to avoid damage to the meter head;

(c) If there is a large capacitance in the circuit under test, it should be discharged first;

(d) According to the resistance of the measured resistance, the appropriate range should be selected;

(e) Meteor meter for insulation measurement of motors and transformers.

(2) Voltage measurement method

Use the multimeter DC voltage block to detect various DC voltages output from the power supply section; DC voltage of each pole of the transistor to ground; DC voltage of each pin of the integrated circuit; DC voltage of key points.

Maintenance notes:

(a) Correctly select the reference point. In general, the reference point is based on the ground, but the negative and positive terminals of some special circuits are not grounded. The reference point should be referenced to the negative end of the local circuit. ;

(b) Pay attention to the parallel effect of the electric circuit and the influence of the electric meter on the circuit. Sometimes the voltage of a certain component is abnormal, and it is not necessarily that the component is damaged, and it may be caused by the failure of the adjacent component.

(3) Current measurement method

It mainly measures the working current of the whole electronic equipment or the working current in a certain circuit. Current checking tends to reflect the static operation of each circuit more than the resistance check. When measuring the working current of the whole machine, the circuit must be disconnected (or the DC fuse should be removed), and the multimeter current block (select the maximum range) should be connected into the circuit (the multimeter should be connected and then powered); in addition, the electronic can be measured. Device jack current, transistor and integrated circuit operating current, power supply load current, capacitor leakage current, no-load transformer current, overload relay operating current, etc. When measuring home appliances, you must pre-select the range to prevent the meter from being too small and damage the meter.

(4) Waveform measurement method

Measuring the waveform with an oscilloscope and visually checking the dynamic working condition of the circuit is unmatched by other methods.

Precautions:

(a) Select the common point as the oscilloscope ground wire, the ground wire must be in good contact, otherwise the waveform is unstable or the waveform is not visible;

(b) The ground of the device under test must be “cold” (ie isolated from the grid);

(c) The input impedance of the oscilloscope probe should be high, otherwise it will affect the circuit under test;

(d) The oscilloscope input signal should be within the range of the range, otherwise the oscilloscope will be damaged.

3. Interference method

Mainly check for faults that the electronic device exhibits when the appropriate signal is input. The method is to touch the input end of a certain part of the circuit with a simple tool such as a tweezers, a screwdriver, a test lead, etc., and use the clutter in the human body induction or touch as an interference signal to input to each level of the circuit; or use the short circuit method to make the base of the transistor to the ground (continuous or instantaneous) short circuit, while adding these interference signals to the input of the circuit, you can use the multimeter or oscilloscope to measure at the output of the circuit; pay attention to whether there is noise interference on the screen, whether there is noise interference in the speaker, Determine whether the part to be inspected can transmit a signal to determine the fault location. It is best to check forward gradually from the last level.

4, equivalent replacement method

When it is not possible to determine the cause of the fault after roughly determining the fault location, for some components that are difficult to judge (such as partial short circuit of the inductor, poor performance of the integrated circuit, etc.), use other models of the same model or interchangeable Replace the device or component. In the absence of measuring instruments, repairs often use replacement methods to troubleshoot, especially plug-in components are simple and feasible.

note:

(a) Replacement of components should be confirmed to be good, otherwise it will cause misjudgment and detour;

(b) For faults caused by overload, the replacement method should not be used, and it can only be replaced if it is determined that the new components will not be damaged again or protective measures have been taken.

5, comparison method

When repairing a faulty electronic device, if there are two electronic devices, you can use another good electronic device for comparison. The voltage, working waveform, ground resistance, component parameters, etc. of the same part of the two electronic devices are respectively compared to each other, and the fault location can be conveniently determined. In addition, various data of some electronic devices are collected frequently for comparison during maintenance.

6, isolation method

Applicable to circuits that can work independently and interact with each other (such as multi-load parallel arrangement circuit, bifurcation circuit). At this time, each part of a circuit can be disconnected one by one, step by step to narrow down the fault range. For example, when measuring a short circuit to ground, first look at which branch branches meet at this point, and then disconnect each branch one by one or selectively, and short circuit when disconnecting a branch. If it disappears, it means that the short-circuiting element is on this branch. Then along this branch, continue to search by the above method until the short-circuit element is found. Of course, in the process of searching, the branch with a large resistance value connected in series can be ignored.

7, fault deterioration method

For intermittent or random failures, in order to expose the fault, fault deterioration methods such as vibration, edge verification (applying a limit supply voltage), heating (such as baking an integrated circuit with an electric soldering iron), cooling (such as alcohol) may be used. The cotton ball wipes the integrated circuit case), twisting, swaying, etc. of the connector, cable, plug, plug-in unit, etc., but care should be taken to avoid permanent damage.

8, signal tracking method

Use an oscilloscope, logic probe or multimeter to select the correct detection point according to the signal flow, and check whether the resistance, voltage, current, signal waveform, logic level, etc. are normal.

Test points:

(1) Starting from the abnormal detection point along the signal path back to the test;

(2) Firstly, look for the source of the fault in a large range, and then carefully test it in a small range (for the series circuit, it can be inserted from the middle for detection).

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