How to diagnose PCB circuit board fault?

First, 9 Common Sense Tips for Testing PCB

1. In the case of no isolation transformer, it is strictly prohibited to use the grounded test equipment to contact the live TV, audio, video and other equipment on the bottom plate to test the PCB board

It is strictly prohibited to directly test TV, audio, video and other equipment without power isolation transformer with the equipment whose shell has been grounded.

Although generally the recorder has power transformer, when exposed to a special especially or larger output power of the nature of the power supply do not know much about TV or stereo equipment, first make clear the machine chassis is charged, otherwise extremely easy and floor live TV, audio and other equipment power supply circuit, spread to the integrated circuit, further breakdowns.

2, the test PCB board should pay attention to the insulation performance of the electric iron

It is not allowed to use the soldering iron for welding with electricity. To confirm that the soldering iron is not charged, it is best to ground the shell of the soldering iron. More care should be taken for the MOS circuit, and it is safer to use the low-voltage electric iron with 6~8V.

3. Understand the working principle of integrated circuit and its related circuits before testing PCB board

Before the inspection and repair of integrated circuit, we must be familiar with the function of the integrated circuit, the internal circuit, the main electrical parameters, the role of the pins and the normal voltage of the pins, the waveform and the working principle of the circuit composed of peripheral components.

If the above conditions are present, the analysis and inspection will be much easier.

4, test PCB board do not cause short circuit between pins

When measuring voltage or testing waveform with oscilloscope probe, the meter pen or probe should not cause short circuit between integrated circuit pins due to sliding. It is best to measure in the peripheral printed circuit directly connected with pins.

Any momentary short circuit can easily damage integrated circuits, and extra care should be taken when testing flat CMOS integrated circuits.

5, test PCB board test instrument internal resistance to be large

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

6. Detecting PCB board should pay attention to the heat dissipation of power integrated circuit

Power integrated circuit should be good heat dissipation, not allowed to work without a radiator in the state of high power.

7, PCB board lead detection should be reasonable

If you need 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 causing unnecessary parasitic coupling, especially to handle the grounding terminal between the audio power amplifier integrated circuit and the preamplifier circuit.

8. Test PCB board to ensure welding quality

When welding is indeed welded firmly, the accumulation of solder and porosity are easy to cause false welding.

The welding time is generally not more than 3 seconds, and the power of the soldering iron is about 25W.

The integrated circuit that has been welded should be carefully checked. It is best to measure whether there is a short circuit between pins with an ohmmeter, and then connect the power supply to confirm that there is no solder adhesion phenomenon.

9, the detection of PCB board do not easily determine the damage of integrated circuit

Do not judge the integrated circuit to be damaged easily.

Because the majority of integrated circuits are directly coupled, once a circuit is not normal, it may lead to multiple voltage changes, and these changes are not necessarily caused by integrated circuit damage. In addition, in some cases, when the measured pin voltage is consistent with or close to the normal value, it may not always indicate that the integrated circuit is good.

Because some soft faults will not cause a change in the DC voltage.

Second, PCB board debugging method

For the new PCB board just back, we should first probably observe whether there is a problem on the board, such as whether there is an obvious crack, whether there is a short circuit, open circuit and other phenomena.

If necessary, check that the resistance between the power supply and the ground is high enough.

For a newly designed circuit board, debugging will often encounter some difficulties, especially when the board is relatively large, more components, often do not know how to start.

But if you master a set of reasonable debugging methods, debugging will be twice the result with half the effort.

Third, PCB board debugging steps:

1, for the new PCB board just back, we should first probably observe whether there is a problem on the board, such as whether there is an obvious crack, whether there is a short circuit, open circuit and other phenomena.

If necessary, check that the resistance between the power supply and the ground is high enough.

2. Then it's time to install the components.

Independent modules, if you are not sure to ensure that they work normally, it is best not to install all of them, but a part of the installation (for a relatively small circuit, can be installed at a time), so that it is easy to determine the scope of the fault, so as not to encounter problems, do not know how to start.

In general, you can put the power supply part installed, and then on the power supply output voltage test is normal.

If you don't have a lot of confidence (and even if you do, it is recommended that you add a fuse, just in case), you can consider using an adjustable regulated power supply with current limiting function.

Preset the better current protection current first, and then slowly increase the voltage value of the stabilized power supply, and monitor the input current, input voltage and output voltage.

If in the process of upward adjustment, there is no overcurrent protection and other problems, and the output voltage also reaches normal, it means that the power supply is OK.

Otherwise, disconnect the power supply, look for the fault point, and repeat the above steps until the power supply is normal.

3, then gradually install other modules, each installed a module, on the power test, the power is also following the above steps, in order to avoid due to design errors and/or installation errors caused by the overcurrent and burn out components.

Fourth, find the method of PCB board failure

1, measuring voltage method to find fault PCB board

The first thing to confirm is whether the voltage of the chip power pin is normal, and then check whether the reference voltage is normal, and whether the working voltage of each point is normal, etc.

For example, when a typical silicon triode is switched 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.), the BE junction may BE open.

2, signal injection method to find fault PCB board

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

Sometimes we will use a simpler way, such as holding a tweezer, to touch the input end at all levels, to see if the output end has a response, which is often used in audio, video and other amplifier circuits (but note that hot bottom circuit or high voltage circuit, can not use this method, otherwise it may lead to electric shock).

If there is no reaction before the touch, and after the touch of a reaction, it shows that the problem is in the previous level, should focus on inspection.

3, other methods to find fault PCB board

There are many other ways to find trouble spots, such as seeing, hearing, smelling, touching and so on.

"See" is to see whether the components have obvious mechanical damage, such as rupture, burning black, deformation, etc.;

"Listening" is to listen to whether the working sound is normal, such as some things that should not be ringing in the ring, the ring of the place is not ringing or the sound is not normal, etc.

"Smell" is to check whether there is a smell, such as the smell of burning, capacitor electrolyte taste, etc., for an experienced electronic maintenance personnel, these smells are very sensitive;

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

Some power devices, when working, will be hot, if it feels cold, it can basically judge that it does not work.

But it doesn't work if it's too hot where it shouldn't be or too hot where it should be.

General power triodes, voltage regulator chips, etc., work below 70 degrees is completely no problem.

What does 70 degrees mean?

If you press your hand up, you can hold it for more than three seconds, indicating that the temperature is about 70 degrees below (pay attention to the first tentative to touch, do not burn the hand).