It is time to pay attention to some details in the detection of PCB boards, so as to be more prepared to ensure product quality. When testing PCB boards, we should pay attention to the following nine common senses.
1. Nine common senses for detecting PCB printed circuit boards
1.1. It is strictly forbidden to use grounded test equipment to touch the live TV, audio, video and other equipment on the bottom plate to test the PCB without an isolation transformer.
It is strictly forbidden to directly test TV, audio, video and other equipment without power isolation transformers with equipment with grounded shells. Although the general tape recorder has a power transformer, when you come into contact with a more special TV or audio equipment with a large output power or a lack of understanding of the power supply used, you must first find out whether the chassis of the machine is charged, otherwise it is very easy to The TV, audio and other equipment that are charged with the bottom plate cause a short circuit in the power supply, which affects the integrated circuit and further expands the failure.
1.2. Pay attention to the insulation performance of the soldering iron when testing the PCB board
It is not allowed to use a soldering iron to solder with electricity. To confirm that the soldering iron is not charged, it is best to ground the shell of the soldering iron. Be more careful with MOS circuits. It is safer to use a low-voltage electric soldering iron of 6~8V.
1.3. Before testing the PCB board, it is necessary to understand the working principle of the integrated circuit and its related circuits
Before checking and repairing an integrated circuit, you must first be familiar with the function of the integrated circuit used, the internal circuit, the main electrical parameters, the function of each pin, and the working principle of the circuit composed of the normal voltage, waveform and peripheral components of the pin. If the above conditions are met, analysis and inspection will be much easier.
1.4. Test PCB board so as not to cause short circuit between pins
When measuring voltage or testing waveforms with an oscilloscope probe, the test leads or probes should not cause a short circuit between the pins of the integrated circuit due to sliding, and 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 must be taken when testing flat-pack CMOS integrated circuits.
1.5. The internal resistance of the PCB board test instrument should be large
When measuring the DC voltage of integrated circuit pins, a multimeter with an internal resistance greater than 20KΩ/V should be used, otherwise there will be a large measurement error for some pin voltages.
1.6. Pay attention to the heat dissipation of the power integrated circuit when testing the PCB board
The power integrated circuit should have good heat dissipation, and it is not allowed to work in a state of high power without a heat sink.
1.7, the detection of PCB board leads should be reasonable
If it is necessary to add peripheral components to replace the damaged part of the integrated circuit, small components should be selected, and the wiring should be reasonable to avoid unnecessary parasitic coupling, especially the grounding between the audio power amplifier integrated circuit and the preamplifier circuit should be handled well. end.
1.8. To ensure the welding quality of PCB board detection
When welding, it is indeed welded firmly, and the accumulation of solder and pores are easy to cause virtual welding. The welding time is generally not more than 3 seconds, and the power of the soldering iron is about 25W for the internal heat type. The soldered integrated circuits 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.
1.9. Do not easily judge the damage of the integrated circuit when testing the PCB board
Do not easily judge that the integrated circuit is damaged. Because most integrated circuits are directly coupled, once a certain circuit is abnormal, it may cause multiple voltage changes, and these changes are not necessarily caused by the damage of the integrated circuit. When the values are consistent or close, it does not necessarily mean that the integrated circuit is good. Because some soft faults do not cause changes in DC voltage.
2. Debugging method of PCB printed circuit board
For the new PCB board that has just been taken back, we must first observe whether there are problems on the board, such as whether there are obvious cracks, whether there are short circuits, open circuits, etc. If necessary, check whether the resistance between the power supply and the ground wire is large enough.
For a newly designed circuit board, debugging often encounters some difficulties, especially when the board is relatively large and there are many components, it is often impossible to start. But if you master a reasonable set of debugging methods, debugging will be more effective.
PCB board debugging steps:
2.1. For the new PCB board that has just been taken back, we should first observe whether there are problems on the board, such as whether there are obvious cracks, whether there are short circuits, open circuits, etc. If necessary, check whether the resistance between the power supply and the ground wire is large enough.
2.2, and then install the components. Modules that are independent of each other, if you are not sure to ensure that they work normally, it is best not to install all of them, but to install them part by part (for relatively small circuits, you can install them all at once), so that it is easy to determine the fault range, When you encounter problems when you get it, there is no way to start.
Generally speaking, you can install the power supply part first, and then power on to check whether the output voltage of the power supply is normal. If you are not too sure about powering up (even if you are sure, it is recommended that you add a fuse, just in case), consider using an adjustable regulated power supply with current limiting.
First preset the overcurrent protection current, 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 also reaches normal, then 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.
2.3. Next, install other modules gradually. After each module is installed, power on and test it. When powering on, follow the above steps to avoid overcurrent and burn out components due to design errors or/or installation errors.
3. How to find the fault of PCB printed circuit board
3.1. Find the faulty PCB board by measuring the voltage
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.
3.2. 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, so as to find the fault point. Sometimes we also use simpler methods, such as holding a tweezer by hand to touch the input terminals of each level to see if the output terminals respond. circuit or circuit with high voltage, this method cannot be used, 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.
3.3. Other methods for finding faulty PCB boards
There are many other ways to find fault points, such as looking, listening, smelling, touching, etc.
"Looking" is to see whether the components have obvious mechanical damage, such as cracking, blackening, deformation, etc.;
"Listening" is to listen to whether the working sound is normal, for example, something that should not be sounding is ringing, the place where it should be sounding is not sounding or the sound is abnormal, etc.;
"Smell" is to check whether there are peculiar smells, 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;
"Touch" is 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 is the concept of 70 degrees? If you press your hand up and you can 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 hand.
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