Common sense and methods of PCB inspection: look, listen, smell, touch…
1. It is strictly forbidden to use grounded test equipment to touch the live TV, audio, video and other equipment of the bottom plate to test the PCB board without an isolation transformer
It is strictly forbidden to directly test TV, audio, video and other equipment without a power isolation transformer with instruments and equipment with grounded shells. Although the general radio and cassette recorder has a power transformer, when you come into contact with more special TV or audio equipment, especially the output power or 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 The TV, audio and other equipment that are charged with the bottom plate cause a short circuit of the power supply, which affects the integrated circuit, causing further expansion of the fault.
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 for soldering with power. Make sure that the soldering iron is not charged. It is best to ground the shell of the soldering iron. Be more careful with the MOS circuit. It is safer to use a low voltage soldering iron of 6~8V.
3. Know the working principle of integrated circuits and related circuits before testing PCB boards
Before inspecting and repairing the integrated circuit, you must first be familiar with the function of the integrated circuit used, the internal circuit, the main electrical parameters, the role of each pin, and 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.
4. Do not cause short circuits between pins when testing the PCB
When measuring voltage or testing the waveform with an oscilloscope probe, do not cause a short circuit between the pins of the integrated circuit due to sliding of the test leads or probes. It is best to measure on the peripheral printed circuit directly connected to the pins. Any momentary short circuit can easily damage the integrated circuit, so be more careful when testing the flat-package CMOS integrated circuit.
5. The internal resistance of the PCB board test instrument should be large
When measuring the DC voltage of the IC pins, a multimeter with the internal resistance of the meter head greater than 20KΩ/V should be used, otherwise there will be a large measurement error for the voltage of some pins.
6. Pay attention to the heat dissipation of power integrated circuits when testing PCB boards
The power integrated circuit should dissipate heat well, and it is not allowed to work under high power without a heat sink.
7. The lead wire of the PCB board should be reasonable
If you need to add external components to replace the damaged part of the integrated circuit, small components should be used, 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 end.
8. Check the PCB board to ensure the welding quality
When soldering, the solder is firm, and the accumulation of solder and pores can easily cause false soldering. The soldering time is generally no more than 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 checked carefully. It is best to use an ohmmeter to measure whether there is a short circuit between the pins, confirm that there is no solder adhesion, and then turn on the power.
9. Do not easily determine the damage of the integrated circuit when testing the PCB board
Do not judge that the integrated circuit is damaged easily. Because most integrated circuits are directly coupled, once a circuit is abnormal, it may cause multiple voltage changes, and these changes are not necessarily caused by the damage of the integrated circuit. In addition, in some cases, the measured voltage of each pin is different from the normal When the values match or are close, it may not always mean that the integrated circuit is good. Because some soft faults will not cause changes in DC voltage.
02
PCB board debugging method
For the new PCB board that has just been taken back, we must first roughly observe whether there are any problems on the board, such as whether there are obvious cracks, short circuits, open circuits, etc. If necessary, check whether the resistance between the power supply and the ground 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 set of reasonable debugging methods, debugging will get twice the result with half the effort.
PCB board debugging steps:
1. For the new PCB board that has just been taken back, we must first roughly observe whether there are any problems on the board, such as whether there are obvious cracks, short circuits, open circuits, etc. If necessary, you can check whether the resistance between the power supply and the ground is large enough.
2. Then the components are installed. Independent modules, if you are not sure that they work properly, it is best not to install all of them, but install 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, you can’t start.
Generally speaking, you can install the power supply first, and then power on to check whether the output voltage of the power supply is normal. If you don’t have much confidence when 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 function.
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 is no overcurrent protection and other problems during the upward adjustment, and the output voltage has reached 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.
3. Next, install other modules gradually. Each time a module is installed, power on and test it. When powering on, follow the above steps to avoid over-current caused by design errors and/or installation errors and burn out components.