Circuit boards electrical faults analysis and detection method

Electronic circuit board troubleshooting can generally be detected by input to the output sequence or by the reverse method of output to input.

Various kinds of good and bad electrical faults may include the following situations:

1. Poor contact: Poor contact between the circuit board and the slot, when the cable is broken inside and not conductivity, poor contact between the plug and the terminal, and virtual welding of the components.

The solution to this type of fault is to carefully inspect the suspected connector to see if there is any obvious oxidation or poor contact, scrape the oxidized metal contact point, adjust the position of the contact point, and re-dial the test contact after treatment. .

Second, the signal is disturbed: For digital circuits, in a certain situation,  fault will be presented. It is possible that the interference is too large, which affects the control system to make an error, and the individual component parameters or the overall performance parameters of the circuit board have changed, so that the anti-interference ability tends to a critical point, and thus a failure occurs.

This type of fault focuses on checking whether the equipment is well grounded. Using the test pencil to check whether the equipment casing is energized, or using the meter to measure whether the outer casing has a higher voltage on the earth, generally below 1V. If it is above 10V, it is suspected that the grounding is good or not .

Third, the thermal stability of components is not good: from a large number of maintenance practices, the thermal stability of the first electrolytic capacitor is not good, followed by other capacitors, transistors, diodes, ICs, resistors and so on.

1. This kind of fault usually occurs or disappears with the change of the boot time of the machine. The essence is that the fault changes with the temperature change of a faulty component. For example, the fault caused by the aging of the electrolytic capacitor is generally a fault when it is just energized, and the fault disappears after being energized for a period of time, that is, there is a fault in the cold machine, and there is no fault in the heat engine. The essence is that the capacitance of the aging electrolytic capacitor changes with temperature, and the capacity is small when the temperature is low, resulting in poor filtering, the circuit board cannot work normally, and the temperature of the electrolytic capacitor rises as the power is turned on, and the capacity increases. , the filter condition is satisfied, and the fault disappears again.

2. The thermal stability fault is a soft fault. It is not easy to directly detect and determine the faulty component during maintenance. However, the inspection scope can be narrowed by artificially raising or cooling the suspected component. The suspected component can be heated with a hair dryer or a heat gun, and the suspected component can be cooled by using a cotton swab dipped in alcohol. The quality of the capacitor is easily discriminated using the VI curve test.

Fourth, the circuit board has moisture, dust, etc.: moisture and dust will be conductive, have a resistance effect, and in the process of thermal expansion and contraction, the resistance value will also change, this resistance value will be related to other components When the effect is strong, the circuit parameters will be changed to make the fault occur.

This type of fault can be solved by cleaning the circuit  board. It is recommended to wash the circuit board with wash water or directly with clean water and then blow dry thoroughly with a hair dryer. Alcohol is not recommended because it is easy to leave some white matter on the circuit board after cleaning.

Fourth, the software is also one of the considerations: many parameters in the circuit are adjusted by software, the margin of some parameters is adjusted too low, which is in a critical range, when the machine operating conditions meet the reason for the software to determine the fault, then the alarm will appear.

Fault detection and judgment method:

Method 1. Direct observation: When the circuit is faulty, the instrument measurement is not immediately used, but the naked eye is used to find the abnormal part of the circuit. The direct observation method is divided into no power and power detection.

The no power inspection is to check whether the level and polarity of the power supply voltage meet the circuit requirements; the polarity of the electrolytic capacitor and the pin position of the two transistors, and the pin position of the integrated circuit have problems of virtual soldering, mis-welding and crossover; Is there any unreasonable layout in the wiring; when the printed circuit board is printed, there is no line breakage; the resistor and the capacitor have obvious burning problems.

The power-on inspection is mainly to observe whether the components have overheating, smoke and obvious burnt smell, and whether there is any problem such as high-voltage ignition between the electron tube and the filament of the oscillating tube.

Method 2: Multimeter detection: The multimeter detection is mainly to check the static working point. The power supply system, the triode, the integrated block and the resistance value and the DC working state of the electronic circuit can be detected by a multimeter. Check to see if the value is normal.

Method 3: Signal tracing method: In a complicated circuit, a waveform can be observed by inputting a signal at the input end, and then observing the waveform and the amplitude value from the front stage to the back stage or from the back stage to the previous stage level by the oscilloscope. Finally look at which level has an exception.

Method 4: Contrast method: The contrast method is more intuitive, mainly by comparing the parameters of the suspected fault circuit with the same circuit with normal working condition, to find out whether there is a value with a large parameter gap, and then analyzing the cause of the fault, finally judgment fault location.

Method 5: Replacement method: For an electronic circuit board whose fault is not obvious, when an intuitive judgment or a suspected fault point cannot be made, the existing same component can be used for replacement, and the fault judgment direction can be shortened by replacing whether the observation circuit changes.

Method 6: Bypass inspection method: If there is parasitic oscillation in the circuit, then a capacitor of a certain capacity can be used to connect the capacitor between the place to be inspected or the reference grounding point, and then observe whether the oscillation exists or not, if the oscillation disappears , indicating that the oscillation is generated in the circuit of the front stage or nearby. If not, move backwards and continue to look for checkpoints. The choice of capacitors should be careful not to oversize the bypass capacitors, and it is better to eliminate the unfavorable signals.

Method 7: Short-circuit inspection method: The short-circuit inspection method is that we actively create a temporary short circuit to make some circuits short-circuit.

Method 8: Open circuit check method: The short circuit method mentioned above is used to check the open circuit is the most effective. Similarly, the short circuit check with the open circuit method is also the most effective. The thinking of the open circuit check method is similar to the previous methods, and is used to exclude suspected points and shorten the range. Assume that the regulated power supply is connected to a faulty circuit. At this time, the output circuit is too large. Then, if we turn off a certain part of the circuit in turn, then observe the current output of the circuit to judge the branch where the fault occurs.