There are generally the following ways to find PCB faults:
①Measurement Voltage Method
The first thing to confirm is whether the voltage of the power supply pins of each chip is normal, and then check whether the various reference voltages are normal, and whether the working voltage of each point is normal, etc. For example, when a general silicon triode is turned on, the BE junction voltage is around 0.7V, while the CE junction voltage is around 0.3V or less. If the BE junction voltage of a triode is greater than 0.7V (except for special triodes, such as Darlington tubes, etc.), it may be that the BE junction is open.
②Signal Injection Method
Add the signal source to the input terminal, and then measure the waveform of each point in turn to see if it is normal, so as to find the fault point. Sometimes we also use simpler methods, such as holding a tweezers by hand, to touch the input terminals of all levels to see if the output terminals respond, which is often used in audio, video and other amplification circuits (but pay attention that the method cannot be used for circuits with high voltage or high voltage circuits, otherwise it may cause electric shock). If there is no response at the previous level, but there is a response at the next level, it means that the problem lies in the previous level and should be checked.
③ Of course, there are many other ways to find fault points, such as looking, listening, smelling, touching, etc.
“Looking” is to see if the components have obvious mechanical damage, such as cracking, burning black, deformation, etc.
“Listening” is to listen to whether the working sound is normal, such as some things that should not be sounding, the place that should sound is not sounding or the sound Abnormal, etc.
“Smelling” is to check whether there is any odor, such as the smell of burning, the smell of capacitor electrolyte, etc. For an experienced electronic maintenance personnel, they are very sensitive to these smells.
“Touching” is to use hand to test whether the temperature of the device is normal, such as too hot, or too cold. Some power devices will heat up when they work. If they feel cool to the touch, it can basically be judged that they are not working. But it doesn’t work if the place that shouldn’t be hot is hot or the place that should be hot is too hot. General power transistors, voltage regulator chips, etc., work under 70 degrees is completely no problem. What kind of concept is 70 degrees? If you press your hand up and you can hold it for more than three seconds, it means that the temperature is probably below 70 degrees (be careful to touch it tentatively first, and don’t burn your hand).
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