Let's see how to debug a new circuit board fault

For a circuit designed with a new PCB, some difficulties are often encountered in debugging, especially when the board is relatively large and there are many components, and there is often no way to start. But if we master a set of reasonable debugging methods, we will get twice the result with half the effort. For the new PCB just brought back, we should first roughly observe whether there are problems on the board, such as obvious cracks, short circuits, open circuits, etc. If necessary, check whether the resistance between the power supply and the ground wire is large enough.

Then there is the installation of components. If you are not sure to ensure the normal operation of independent modules, you'd better not install them all, but part by part (for small circuits, you can install them all at once), so that it is easy to determine the fault range and avoid having no way to start when encountering problems. Generally speaking, the power supply part can be installed first, and then power on to detect whether the power supply output voltage is normal. If you are not sure when powering on (even if you are sure, you are also recommended to add a fuse, just in case), you can consider using an adjustable regulated power supply with current limiting function. Preset the overcurrent protection current, then slowly increase the voltage of the stabilized voltage power supply, and monitor the input current, input voltage and output voltage. If there are no over-current protection problems during the upward adjustment and the output voltage is normal, the power supply is OK. Otherwise, disconnect the power supply, find the fault point, and repeat the above steps until the power supply is normal.

Next, gradually install other modules. After each module is installed, power on and test it. Follow the above steps when powering on to avoid burning components due to overcurrent caused by design errors or/and installation errors.

There are several ways to find faults:

① Measuring voltage method. First, check whether the voltage of the power supply pin of each PCB chip is normal, then check whether the reference voltages are normal, and whether the working voltage of each point is normal. For example, when the general silicon triode is turned on, the BE junction voltage is about 0.7V, while the CE junction voltage is about 0.3V or less. If the BE junction voltage of a triode is greater than 0.7V (except for special triodes, such as Darlington transistors), the BE junction may be open.

② Signal injection method. Add the signal source to the input terminal, and then measure the waveform of each point backward in order to see if it is normal, so as to find the fault point. Sometimes we also use simpler methods, such as holding a pair of tweezers to touch the input terminals at all levels to see if the output terminals react, which is often used in audio, video and other amplification circuits (but it should be noted that this method cannot be used for circuits with hot backplanes or PCB circuits with high voltage, otherwise it may cause electric shock). If there is no reaction at the level before the collision, but there is reaction at the level after the collision, it indicates that the problem is at the level before the collision, and the inspection shall be focused.

③ Of course, there are many other ways to find the fault point, such as looking, listening, smelling, touching, etc. "Look" is to see if PCB components have obvious mechanical damage, such as crack, burn black, deformation, etc; "Listening" is to listen to whether the working sound is normal, for example, something that should not be loud is ringing, the place that should be loud is silent or the sound is abnormal; "Smell" means to check whether there is any smell, such as the smell of burning, the smell of capacitor electrolyte, etc. An experienced electronic maintenance personnel is very sensitive to these smells; "Touch" means to test whether the temperature of PCB board is normal, such as too hot or too cold. Some power devices will generate heat when they work. If they feel cold, it can basically be judged that they are not working. However, if the place that should not be hot is hot or the place that should be hot is too hot, it is not good. General power triodes, voltage stabilizing chips, etc., can work under 70 degrees. What is the concept of 70 degrees? If you press your hand on it for more than three seconds, it means that the temperature is below 70 degrees (please try to touch it first, and don't burn your hand).