PCB board detection 9 common sense, PCB debugging and fault detection analysis

- Jul 13, 2018-

The inspection of PCB board should pay attention to some details, so as to ensure the quality of products more prepared. In the inspection of PCB board, we should pay attention to the following 9 common sense.

1. It is strictly prohibited to use grounded test equipment to touch the charged TV, audio and video equipment on the bottom panel to detect PCB board without isolating transformer

It is strictly prohibited to directly test TV, audio and video equipment without power isolating transformer with grounded equipment.
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 insulation performance of electric soldering iron should be paid attention to when testing PCB board

It is not allowed to use soldering iron for electric soldering. To ensure that soldering iron is not charged, it is better to ground the soldering iron's shell.

3. Understand the working principle of integrated circuits and related circuits before testing PCB boards

Inspection and repair of integrated circuit, first to be familiar with the function of the integrated circuit used, internal circuit, main electrical parameters, and function of each pin pin the normal voltage and waveform and peripheral components of the working principle of the circuit.
If the above conditions are met, the analysis and inspection will be much easier.

4. Test PCB board to avoid short circuit between pins

Voltage measurements or use the oscilloscope probe test waveforms, pens and not caused by sliding or probe integrated circuit short circuit between pins, best in the periphery of the connected directly with pins printing circuit measurements.
Any momentary short circuit is liable to damage the integrated circuit, especially when testing a flat encapsulated CMOS integrated circuit.

5. The internal resistance of the PCB test instrument should be large

Dc voltage measurement IC pin, should choose header Ω resistance is greater than 20 k/V multimeter, or for some pin voltage will have larger error of measurement.

6. Pay attention to the heat dissipation of power integrated circuit in PCB board detection

The power integrated circuit shall have good heat dissipation and shall not work at high power without a radiator.

7. Reasonable testing of PCB plate leads

If need add components outside instead of the integrated circuit inside damaged part, should choose small components, and reasonable wiring to so as not to cause unnecessary intercoupling, especially to handle audio power amplifier IC, and between the preamplifier circuit earthing terminal.

8. Ensure the welding quality of PCB board

It is true that the welding is firm, and the accumulation of soldering tin and the porosity are easy to cause virtual welding.
Generally, the welding time is no more than 3 seconds, and the soldering iron's power is about 25W.
The welded integrated circuit should be carefully checked. It is better to use ohm meter to measure whether there is short circuit between each pin, and confirm that there is no soldering tin bonding before switching on the power.

9. It is not easy to determine the damage of the integrated circuit to detect PCB

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 cause many voltage changes, and these changes are not necessarily IC damage caused by, and in some cases measure each pin voltage consistent with normal or close to, also not necessarily can explain integrated circuit is good.
Because some soft failures do not cause changes in the dc voltage.

PCB board debugging steps

1. For the new PCB board we just got back, we should first roughly observe whether there are any problems on the board, such as whether there are obvious cracks, whether there is short circuit, whether there is open road, etc.
If necessary, check to see if the resistance between the power supply and the ground wire is large enough.

2. Then there is the installation element.
Independent modules, not sure if you keep them working properly, it's best not to install, all but part of part of the mount (for smaller circuit, can be a whole mount), so easy to determine the scope of the fault, lest meet with problems, do not know how to start.

In general, the power supply can be installed first, and then power on to check whether the power output voltage is normal.
If you don't have a good handle on the power (even if you have a good handle, you are advised to add a fuse, just in case), consider using an adjustable voltage source with a current limiting function.

The current is preset to be better than the current protection, then the voltage of the voltage-stabilizing power supply is slowly raised, and the input current, input voltage and output voltage are monitored.
If there is no problem in the process of upward adjustment, such as current protection, and the output voltage is normal, then the power supply part is OK.
Otherwise, disconnect the power, find the fault point, and repeat the above steps until the power is normal.

3, then gradually to install other modules, each module is installed, test on electricity, electricity is according to the above steps, in order to avoid because of design errors and/or installation errors caused by the flow and burn out elements.

Look for ways to troubleshoot PCB boards

1. Find fault PCB board by measuring voltage method

First of all, it is necessary to confirm whether the voltage of each chip power pin is normal. Second, it is necessary to check whether the various reference voltage is normal.
For example, a typical silicon transistor has a junction voltage of about 0.7V when it is on, and a CE junction voltage of about 0.3V or less.
If the BE junction voltage of a triode is greater than 0.7V (except for special triode, such as darlington tube, etc.), it may BE the BE junction and open the circuit.

2. Signal injection method to find fault PCB board

Add the signal source to the input and measure the waveform of each point in turn to see if it is normal to find the fault point.
Sometimes we will also use more simple way, with a forceps in hand, for example, to touch at all levels of the input, the output side reaction, which used in audio, video and other amplifier circuit (but note that hot plate or high voltage circuit, can't use this method, otherwise it may lead to electric shock).
If there is no reaction at the first level and there is a reaction at the second level, it indicates that the problem is at the first level and should be carefully examined.

3. Other ways to find fault PCB board

There are many other ways to find fault points, such as looking, listening, smelling, touching, etc.

"See" is to see whether the component has obvious mechanical damage, such as cracking, blackening, deformation, etc.

"Listening" is to hear whether the working sound is normal, for example, some things that should not be heard are ringing, the place where the sound should not be heard or the sound is abnormal, etc.

"Smell" is to check for odors, such as the smell of burning, the smell of capacitor electrolyte, etc.

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

Some power devices, when they work, get hot, and if they feel cold, it's basically a sign that they're not working.
But if it's too hot where it's not supposed to be or if it's too hot, it's not going to work.
General power triode, voltage regulator chip, etc., working below 70 degrees is completely ok.
What is the concept of 70 degrees?
If you press your hand up, you can hold it for more than three seconds, which means the temperature is below 70 degrees.