Finding a fault and repairing an electronic board is a science. If the troubleshooting is a science, it requires the perfect knowledge of general electricity and the usual electronic functions which constitute, by the assembly, the electronic boards. It is necessary to know how to interpret the role of each function and the reaction in the event of a malfunction by analyzing its block diagram.
1 - To find a fault you will essentially need, in addition to some of your usual senses such as vision, smell and hearing, a digital multimeter equipped with an ohmmeter and a diode test function.
The diode test function, symbolized by a diode pattern on the multimeter's function selector, is used to measure the threshold voltage in the on and off direction (tending towards infinity in this case) of a junction measured between its terminals.
It can be used to evaluate the state of the diodes, but also that of the transistors, which appear to the eyes of the instrument, like two diodes mounted head to tail (between the base and the collector and the base and the emitter).
The second very useful function (apart from the voltmeter, ohmmeter and all that follows) is the continuity tester function. It is quite simply an audible signal which is emitted by the multimeter if the electrical resistance of a circuit is less than tens of ohms in general. It is therefore a very practical function for checking the presence or absence of a short circuit, bad contacts, etc.
2 - Fault finding methodology:
Observe the symptoms of the failure. Try to see them for yourself rather than having them described to you. Because the person entrusting you with the broken device may very well have omitted some detail which is very important to begin the investigation correctly.
Once the fault has been noted, work if possible with the power off (unplug the plug, etc.) to locate the fault. This will save you the risk of electrocution. In fact, most failures are sought with the ohmmeter and / or the continuity tester, or simply "by eye", so there is no need to work under voltage and take unnecessary risks. In the case of low voltage devices, do the same, this will prevent you from inadvertently shorting and burning out the rest of the circuit. Working with the power off also avoids a possible aggravation of the breakdown.
Check with the naked eye first if some components, welds or printed circuit tracks have not blackened or are not chipped. Beware of dust deposits that could make you think that certain components have burnt when it is not. Try to smell the suspect component for a burnt, melted plastic like smell.
In the absence of a diagram, get the Datasheets of the components (mainly integrated circuits) on the Internet likely to be involved in the failure. This will allow you to check, for example, that the switching power supply manager must be supplied with 15 volts on its leg 8, and therefore to go back to the "source" if these 15 volts are absent (eg: resistor blown). In addition, most manufacturers add a few application diagrams of their components, often taken over by device manufacturers.
If you have an inverter (not connected to the mains) or an isolation transformer (230V / 230V) use it when working under (high) voltage, you will be isolated from the phase and the low power will limit the damage if you leave drop a metal tweezer or a screwdriver into the chassis of the device.
Figure 1.3: low power isolation transformer protection.
When recovering a device, check, before any intervention, that there are no missing cards or electronic components, that they are not physically damaged and that the components are dry (In case of rain, let dry device for several days).
For visual inspection, some specifications may call for the use of magnifying instruments for examining electronic boards. The tolerance of magnifying instruments is ± 15% of the selected magnification ratio.
Magnifying instruments used for inspection must be suitable for the item being treated. Lighting must be suitable for the magnification instruments used. Unless the required magnification is specified in the contract documentation, the magnification in Table 1.1 is determined by the items under inspection. Arbitration conditions are used to verify rejected products using inspection magnification.
3- Location of the faulty board or circuit
All automated machines such as PCB-controlled, PLC-controlled machines, one of the first steps in system troubleshooting.
e consists of checking external devices such as display cards, keyboards, indicator lights, sensors, etc., these external devices make it possible to locate the faulty circuit; the system power supply represents a second source of clues.
One of the advantages of looking for clues from outside sources is that you don't have to take the whole system apart to fix it – it's much easier to start from the outside.
Before going into the circuits of the system, you should know that it is divided into sections, figure 1.4 shows the different elements. The system shown has three sections but could have more. Each of the sections fulfills a specific function or role.
Each section contains electronic circuits often a section consists of a simple card on which the circuits are mounted, which facilitates their location and use. When one component fails, the entire system can fail.
To troubleshoot a circuit, you must start from the outside by finding the way to the inside. This is a step by step process and each step leads you a little further inside the circuits, it indeed leads you to the next step.
Finding a fault and repairing an electronic board is a science. If the troubleshooting is a science, it requires the perfect knowledge of general electricity and the usual electronic functions which constitute, by the assembly, the electronic boards. It is necessary to know how to interpret the role of each function and the reaction in the event of a malfunction by analyzing its block diagram.
1 - To find a fault you will essentially need, in addition to some of your usual senses such as vision, smell and hearing, a digital multimeter equipped with an ohmmeter and a diode test function.
The diode test function, symbolized by a diode pattern on the multimeter's function selector, is used to measure the threshold voltage in the on and off direction (tending towards infinity in this case) of a junction measured between its terminals.
It can be used to evaluate the state of the diodes, but also that of the transistors, which appear to the eyes of the instrument, like two diodes mounted head to tail (between the base and the collector and the base and the emitter).
The second very useful function (apart from the voltmeter, ohmmeter and all that follows) is the continuity tester function. It is quite simply an audible signal which is emitted by the multimeter if the electrical resistance of a circuit is less than tens of ohms in general. It is therefore a very practical function for checking the presence or absence of a short circuit, bad contacts, etc.
2 - Fault finding methodology:
Observe the symptoms of the failure. Try to see them for yourself rather than having them described to you. Because the person entrusting you with the broken device may very well have omitted some detail which is very important to begin the investigation correctly.
Once the fault has been noted, work if possible with the power off (unplug the plug, etc.) to locate the fault. This will save you the risk of electrocution. In fact, most failures are sought with the ohmmeter and / or the continuity tester, or simply "by eye", so there is no need to work under voltage and take unnecessary risks. In the case of low voltage devices, do the same, this will prevent you from inadvertently shorting and burning out the rest of the circuit. Working with the power off also avoids a possible aggravation of the breakdown.
Check with the naked eye first if some components, welds or printed circuit tracks have not blackened or are not chipped. Beware of dust deposits that could make you think that certain components have burnt when it is not. Try to smell the suspect component for a burnt, melted plastic like smell.
In the absence of a diagram, get the Datasheets of the components (mainly integrated circuits) on the Internet likely to be involved in the failure. This will allow you to check, for example, that the switching power supply manager must be supplied with 15 volts on its leg 8, and therefore to go back to the "source" if these 15 volts are absent (eg: resistor blown). In addition, most manufacturers add a few application diagrams of their components, often taken over by device manufacturers.
If you have an inverter (not connected to the mains) or an isolation transformer (230V / 230V) use it when working under (high) voltage, you will be isolated from the phase and the low power will limit the damage if you leave drop a metal tweezer or a screwdriver into the chassis of the device.
Figure 1.3: low power isolation transformer protection.
When recovering a device, check, before any intervention, that there are no missing cards or electronic components, that they are not physically damaged and that the components are dry (In case of rain, let dry device for several days).
For visual inspection, some specifications may call for the use of magnifying instruments for examining electronic boards. The tolerance of magnifying instruments is ± 15% of the selected magnification ratio.
Magnifying instruments used for inspection must be suitable for the item being treated. Lighting must be suitable for the magnification instruments used. Unless the required magnification is specified in the contract documentation, the magnification in Table 1.1 is determined by the items under inspection. Arbitration conditions are used to verify rejected products using inspection magnification.
3- Location of the faulty board or circuit
All automated machines such as PCB-controlled, PLC-controlled machines, one of the first steps in system troubleshooting.
e consists of checking external devices such as display cards, keyboards, indicator lights, sensors, etc., these external devices make it possible to locate the faulty circuit; the system power supply represents a second source of clues.
One of the advantages of looking for clues from outside sources is that you don't have to take the whole system apart to fix it – it's much easier to start from the outside.
Before going into the circuits of the system, you should know that it is divided into sections, figure 1.4 shows the different elements. The system shown has three sections but could have more. Each of the sections fulfills a specific function or role.
Each section contains electronic circuits often a section consists of a simple card on which the circuits are mounted, which facilitates their location and use. When one component fails, the entire system can fail.
To troubleshoot a circuit, you must start from the outside by finding the way to the inside. This is a step by step process and each step leads you a little further inside the circuits, it indeed leads you to the next step.
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