How to check components mounted on PCB?

The PCB board is an important part of electronic equipment. When checking a faulty PCB board, it should be checked according to its basic principles. First of all, check the circuit connections on the PCB to ensure that each circuit is connected correctly so as not to affect the normal operation of the entire PCB. Secondly, check the electronic components on the PCB to ensure that they are connected correctly so as not to affect the normal operation of the PCB. Finally, check the circuit boards on the PCB to ensure that they are connected correctly so as not to affect the normal operation of the PCB. Through the above inspection, the cause of the faulty PCB board can be determined, to better solve the problem

1. The method of finding the faulty PCB board

a) PCB has many ways to find fault points, such as seeing, listening, smelling, touching, etc.

b) Check whether the components have obvious mechanical damage, such as cracking, blackening, deformation, etc.;

c) Listen to whether the working sound is normal, for example, something that should not be ringing is ringing, the place that should be ringing is not ringing or the sound is abnormal, etc.;

d) Smell whether the PCB has any peculiar smell, such as the smell of burning, the smell of capacitor electrolyte, etc. For experienced electronic maintenance personnel, they are very sensitive to these smells;

e) Use your hands to test whether the temperature of the device is normal, for example, it is too hot or too cold.

The power device will heat up when it is working. If it is cool to the touch, it can be judged that it is not working. But if it's hot where it shouldn't be or if it's too hot where it should be, that won't work either.

General power transistors, voltage regulator chips, etc., are completely fine when working below 70 degrees. What kind of concept are 70 degrees? If you press your hand up and hold it for more than three seconds, it means that the temperature is below 70 degrees (be careful to touch it tentatively first and don't burn your hands)

2. Measure the voltage method to find the faulty PCB board

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 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 tubes, etc.), it may be that the BE junction is open.

3. PCB board debugging method and steps: 

 method:

For the new PCB board just brought back, we first have to roughly observe whether there are any problems on the board, such as whether there are obvious cracks, whether there are short circuits, open circuits, etc. If necessary, you can check whether the resistance between the power supply and the ground wire is large enough.

For a newly designed circuit board, some difficulties are often encountered in debugging, especially when the board is relatively large and there are many components, it is often impossible to start. But if you master a set of reasonable debugging methods, debugging will get twice the result with half the effort.

step:

a) When we just got back the new PCB board, we first need to observe whether there are any problems on the PCB board, such as whether there are obvious cracks, whether there are short circuits, open circuits, etc. If necessary, you can check whether the resistance between the power supply and the ground wire is large enough.

b) Next, gradually install other modules. After each module is installed, power it on and test it. When powering on, follow the above steps to avoid overcurrent and burn-out components due to design errors or/and installation errors.

c) Then it is to install components on the PCB :

A- Mutually independent modules, if you are not sure that they are working properly, do not install them all, but install them part by part (for a relatively small circuit, you can install them all at once), so that it is easy to determine the fault range, To avoid problems when you get it, you have no way to start.

B- You can install the power supply first, and then power it on to check whether the output voltage of the power supply is normal. If you are not very sure when powering on (even if you are sure, it is recommended that you add a fuse just in case), you may consider using an adjustable regulated power supply with a current limiting function.

C- Preset the overcurrent protection current first, then slowly increase the voltage value of the regulated power supply, and monitor the input current, input voltage, and output voltage. If there are no problems such as overcurrent protection during the upward adjustment process, and the output voltage has reached normal, it means that the power supply part is OK. On the contrary, it is necessary to disconnect the power supply, find the fault point, and repeat the above steps until the power supply is normal.

4. Signal injection method to find faulty PCB board

Add the signal source to the input terminal, and then measure the waveform of each point, in turn, to see if it is normal, and to find the fault point.

Sometimes we also use a simpler method, such as holding tweezers in hand and touching the input terminals of each level to see if there is any response at the output terminal, which is often used in audio, video, and other amplification circuits (but be careful, the hot bottom plate This method cannot be used for circuits with high voltage or high voltage, otherwise it may cause electric shock).

If there is no response when you touch the previous level, but there is a response when you touch the last level, it means that the problem lies in the previous level, and you should focus on checking.

5. 9 tips for detecting PCB boards:   

a) Since the vast majority of integrated circuits are directly coupled, do not easily judge that the integrated circuit is damaged. Once a certain circuit is not normal, it may cause multiple voltage changes, and these changes are not necessarily caused by damage to the integrated circuit. It is good to be able to explain the integrated circuit. Because some soft faults will not cause changes in DC voltage.

b) When measuring the DC voltage of the pins of the integrated circuit, a multimeter with an internal resistance greater than 20KΩ/V should be selected, otherwise, there will be a large measurement error for some pin voltages.

c) When you come into contact with special TV or audio equipment, especially those with large output power or don’t know much about the nature of the power supply used, you must first find out whether the chassis of the machine is charged, otherwise, it is very easy to connect with the TV and audio equipment with the bottom plate charged. Other equipment causes a short circuit of the power supply, which affects the integrated circuit, causing further expansion of the fault.

d) If it is necessary to add external components to replace the damaged parts inside the integrated circuit, small components should be selected, and the wiring should be reasonable to avoid unnecessary parasitic coupling, especially the connection between the audio power amplifier integrated circuit and the preamplifier circuit. the ground terminal.

e) When soldering with a soldering iron, make sure that the soldering iron is not charged. It is best to ground the shell of the soldering iron and be more careful with the MOS circuit. It is safer to use a low-voltage electric soldering iron of 6~8V.

f) The power integrated circuit should have good heat dissipation, and it is not allowed to work in a high-power state without a radiator.

g) Before checking and repairing the integrated circuit, one must first be familiar with the function of the integrated circuit used, the internal circuit, the main electrical parameters, the function of each pin, the normal voltage of the pin, the waveform and the working principle of the circuit composed of peripheral components. If the above conditions are met, analysis and inspection will be much easier.

h) When measuring voltage or testing waveforms with oscilloscope probes, the test leads or probes should not cause short circuits between the pins of the integrated circuit due to sliding. It is best to measure on the peripheral printed circuit that is directly connected to the pins. Any momentary short circuit can easily damage the integrated circuit, and extra care should be taken when testing flat-packaged CMOS integrated circuits.

i) Make sure that the soldering is firm during soldering, and the accumulation of solder and pores are likely to cause false soldering. The welding time generally does not exceed 3 seconds, and the power of the soldering iron should be about 25W with internal heating. The integrated circuit that has been soldered should be carefully checked. It is best to use an ohmmeter to measure whether there is a short circuit between the pins, and then turn on the power after confirming that there is no solder adhesion.