Night Basic Common Sense for Detecting PCB Board

Guidance

The detection of PCB boards is time to pay attention to details, so as to ensure product quality better. When we detect PCB boards, we should pay attention to the following night common sense.

 Night Basic Common Sense for  Detecting  PCB Board

Night common sense for the Basic Testing of PCB Board

• 1.It is strictly prohibited to detect the PCB board with a grounded test equipment to contact the live TV, audio, video and other equipment in the floor without isolation transformer.

It is strictly forbidden to directly test the TV, audio, video and other equipment of the transformer without power isolation by the equipment that has been grounded. Although have the power transformer recorder in general, when exposed to a special especially larger output power or do not understand the nature of the power of television or audio equipment, we must first understand the chassis is electrified, otherwise easily with the bottom charged television, sound and other equipment caused by short circuit of power supply, and the integrated circuit, to further expand the trouble.

• 2. The insulation performance of the electric soldering iron should be paid attention to in the detection of PCB board

It is not allowed to use live iron soldering. It is necessary to confirm that the iron is not charged. It is better to connect the iron case to the ground. It is more careful for the MOS circuit. It is safer to use 6~8V's low voltage circuit.

• 3. To understand the working principle of integrated circuit and its related circuit before detecting PCB board

Before checking and repairing integrated circuits, we should first be familiar with the functions, internal circuits, main electrical parameters, the functions of each pin, the normal voltage of the pin, the working principle of the waveforms and the components of the peripheral components. If the above conditions are available, the analysis and inspection will be much easier.

• 4, test PCB board do not cause pin short circuit

Voltage measurement or test waveform with an oscilloscope probe, probe or probe not caused by integrated circuit pin between the short circuit due to sliding, the best measure in the periphery of the printed circuit is directly connected with the pin. Short circuits at any moment can easily damage the integrated circuit, and be more careful when testing the CMOS integrated circuit of a flat type package.

• 5. Test the internal resistance of PCB board test instrument

When measuring the DC voltage of the integrated circuit pin, the multimeter with the internal resistance of the head is greater than 20K Omega /V, otherwise the voltage of some pins will have a larger measurement error.

• 6. The heat dissipation of the power integrated circuit should be paid attention to in the detection of PCB board

The power integrated circuit should have good heat dissipation and do not allow it to work in a state of high power without the radiator.

• 7. It is reasonable to check the lead line of PCB board

If we need to add peripheral components to replace the damaged part of the integrated circuit, we should use small components, and the wiring should be reasonable so as not to cause unnecessary parasitic coupling, especially to deal with the earthing end between the audio power amplifier IC and the preamplifier circuit.

• 8. Inspection of PCB board to ensure the quality of welding

Indeed when welding weld, solder accumulation, easy to cause the weld porosity. The welding time is generally not more than 3 seconds, and the power of the soldering iron is about 25W of the internal heat type. The welded integrated circuit should be carefully examined. It is best to use the ohm table to measure the short circuit between the pins, and to confirm the no solder adhesion and reconnect the power.

• 9, check the PCB board not to easily determine the damage of the integrated circuit

Do not easily judge that the integrated circuit is damaged. Because the vast majority of integrated circuits for direct coupling, once a circuit is not normal, may lead to multiple voltage changes, and these changes are not necessarily the integrated circuit damage, also measured the pin voltage and normal values close to or in some cases, also do not have that integrated circuit is good the. Because some soft faults do not cause the change of DC voltage.

Debugging method of PCB board

For the new PCB board that we just brought back, we first need to take a look at whether there are any problems on the board, such as whether there are obvious cracks, whether there is short circuit or open circuit. If necessary, check whether the resistance between the power supply and the ground wire is large enough.

For a new design of the circuit board, debugging often encountered some difficulties, especially when the plate is relatively large, the component is more often than the hand. But if a reasonable set of debugging methods is well mastered, the debugging will be twice the result.

PCB board debugging steps

• 1, for the new PCB board that we just brought back, we first need to take a look at whether there are any problems on the board, such as whether there are obvious cracks, whether there is short circuit or open circuit. If necessary, check whether the resistance between the power supply and the ground wire is large enough.
• 2, then the installation element. Independent modules, if you are not sure that they work properly, not all the best of all equipment, but part of the mount (for the circuit can be a relatively small all installed), so easy to determine the scope of the fault and problem free by the time, do not know how to start.Generally speaking, the power supply can be loaded first, and then the power output voltage is checked to see if the output voltage is normal. If you are not too sure about the power on (even if there is a great deal of assurance, I suggest you add a fuse, just in case), you can consider the use of an adjustable regulated power supply with current limiter.First, the current is better than the current protection, then the voltage of the power supply is slowly up-regulated, and the input current, the input voltage and the output voltage are monitored. If there is no over current protection and other problems in the process of up-regulation, and the output voltage is also normal, the power supply is OK. On the other hand, it is necessary to disconnect the power supply, find the fault point, and repeat the above steps until the power is normal.
• 3, then gradually install other modules. Each module is installed, and the power is tested. The power is also in accordance with the above steps to avoid burning the components due to design errors or / or installation errors.

Method of finding fault PCB board

• 1. Finding the fault PCB plate by measuring the voltage method

The first thing to confirm is whether the voltage of each chip's power pin is normal, then check whether all kinds of reference voltages are normal, and the working voltage of each point is normal. For example, the BE junction voltage is about 0.7V while the general silicon triode is on, while the CE junction voltage is about 0.3V or less. If the BE junction voltage of a triode is greater than that of 0.7V (except for special triode, such as Darlington tube, etc.), it may be that the BE junction opens.

• 2. Finding fault PCB board by signal injection

The signal source is added to the input end, and then the waveforms of each point are measured in turn to see if it is normal to find the fault point.  Sometimes we'll do it in a simpler way.For example, holding a tweezers, input to touch at all levels, to see whether the output response in the audio and video amplification circuit used in (but note that the bottom of the heat circuit or high voltage circuit, not the use of this method, otherwise it may cause electric shock). If there is no response at the first level, and there is a reaction at the first level after the collision, it shows that the problem is at the first level and should be inspected.

• 3, other methods for finding fault PCB board

There are many other ways to find fault points, such as watching, listening, smelling, touching and so on.

"See" is to see if the components have obvious mechanical damage, such as rupture, burning black, deformation and so on.

"Listen" is to listen to the work sound is normal, such as some things that should not sound, the sound of the sound of the place or abnormal sound, and so on;

"Smell" is to check whether there are any peculiar smell, such as burnt smell, the smell of capacitance electrolyte, etc., which is very sensitive to these odors for an experienced electronic maintenance worker.

"Touch" is to use the hand to test the temperature of the device is normal, such as too hot, or too cold.

Some power devices will be hot when they work. If they feel cool, they can basically judge that they do not work. But if the hot place is hot or the hot place is too hot, it is not possible. The general power triode, voltage stabilizer chip, and so on, working under 70 degrees is completely no problem. What is a concept about 70 degrees? If you press your hand up, you can hold it for more than three seconds. It means that the temperature is below 70 degrees.