1. Observation Method
When we receive a circuit board for repair, the first step is to carefully observe its appearance. If the circuit board has been burned, it is crucial to check whether the power circuit is functioning correctly before powering it up, ensuring that no further damage will occur. The observation method is a type of static inspection, and generally follows these steps.
The first step is to check if the circuit board has been damaged by human actions, focusing on the following aspects:
① Check if the circuit board has been dropped, which may cause deformation at the corners or damage to the chips.
② Inspect the chip sockets to see if they have been forcibly pried open due to a lack of proper tools.
③ Observe the chips on the circuit board; if they are socketed, first check if the chips are inserted incorrectly. This is to prevent the operator from mistakenly inserting the chips in the wrong position or direction during repairs. If the error is not corrected in time, powering the circuit board could burn out the chips, leading to unnecessary losses.
④ If the circuit board has shorting terminals, check whether they are inserted incorrectly.
Repairing a circuit board requires a solid theoretical foundation and meticulous attention to detail. Sometimes, with careful observation, one can identify the cause of the problem at this stage.
The second step is to check whether the components on the circuit board have been burned out. For example, check if resistors, capacitors, or diodes show signs of blackening or charring. Normally, even if a resistor is burned, its resistance value remains unchanged, and its performance does not affect normal usage; a multimeter is needed for measurement. However, if capacitors or diodes are burned, their performance will change, preventing them from functioning properly in the circuit, which affects the entire circuit’s operation, requiring replacement with new components.
The third step is to observe integrated circuits on the circuit board, such as 74 series, CPU, co-processors, AD chips, etc., for signs of bulging, cracks, burning, or blackening. If such conditions are found, it can be determined that the chip is damaged and must be replaced.
The fourth step is to check if the traces on the circuit board are peeling, burned, or broken. Check if the plated holes have detached from the pads.
The fifth step is to check the fuses on the circuit board (including fuse tubes and thermistors) to see if the fuses are blown. Sometimes, if the fuse is too thin to see clearly, a multimeter can be used to determine if the fuse is damaged.
The occurrence of the above four conditions is mostly caused by excessive current in the circuit. However, the specific reason for the excessive current must be analyzed on a case-by-case basis. The general approach to troubleshooting is to carefully analyze the circuit diagram of the circuit board, then check the upper circuit based on the burned components, deducing step by step, and using accumulated experience to analyze the most likely problem areas and identify the cause of the failure.
2. Static Measurement Method
For most circuit boards, the previous observation method may not reveal any issues. Only a small number of circuit boards may experience physical deformation due to special reasons, making it easy to identify the fault. However, most faulty circuit boards still require the use of a multimeter to systematically measure some key components and points on the circuit board to identify and resolve issues.
Before measuring, first determine whether the circuit primarily uses analog or digital signals. For circuit boards with a schematic, this can be determined by reviewing the schematic. For circuit boards without a schematic, the following two methods are generally used:
① Observe the components on the circuit board to see if there is a microprocessor. Whether it is an early 80 or 51 series or the widely used DSP today, if such a chip appears on the circuit board, it indicates that there is a bus structure, and digital signals will occupy a significant part, allowing it to be treated as a digital board.
② For circuit boards without microprocessors, observe the components to see how many 5V power supply chips are present. If there are many 5V supply chips, it can also be treated as a digital circuit for repair. The repair methods for digital and analog circuits differ; generally, analog circuits are simpler to repair as one can deduce step by step to find the problem. However, for digital circuits, since all circuits are connected to a bus without a clear hierarchical relationship, repairs can be more challenging. The following focuses on the static measurement method for digital circuits, which generally follows these steps.
The first step is to use a multimeter to check for a short circuit between the power supply and ground.
The method of checking is: find a chip powered by 5V and measure the two diagonal points (for example, for a 14-pin chip, measure between pin 7 and pin 14; for a 16-pin chip, measure between pin 8 and pin 16). If there is no short circuit between the two points, the power supply is functioning normally. If a short circuit occurs, the cause must be identified through troubleshooting.
These steps are the basic thought process for power supply repairs; for particularly complex circuit boards, specific issues must be analyzed individually.
The power supply is the foundation of the circuit; only when the power supply is functioning normally can subsequent circuit applications be discussed. Therefore, measuring the power supply is very important and is often overlooked by repair technicians.
The second step is to use a multimeter to measure the diodes to check if they are functioning properly. Normally, when measuring with the resistance range, the forward voltage should be tens to hundreds of ohms, and the reverse voltage should be one thousand to several thousand ohms. Generally, diode damage occurs due to excessive current in the circuit, resulting in the diode being burned out.
The third step is to use the multimeter’s resistance range to measure the capacitors to check for short or open circuits; if present, it indicates that there is a problem with this part of the circuit. The next step is to determine whether the component itself is faulty or if the issue lies with the connected circuit. The method is to desolder one leg of the suspicious component and check if there are any open or short circuits. This allows for pinpointing the problem.
The fourth step is to similarly use a multimeter to measure the integrated circuits, transistors, resistors, etc., on the circuit board to see if they meet their logical performance. If the circuit board includes a bus structure, there will generally be pull-up resistor arrays on the bus. Measuring the resistor arrays is a very important step, as their condition can preliminarily test the status of the chips hanging on the bus.
After checking with the observation method and static measurement method, most problems encountered in circuit board repair can be resolved. It is important to ensure that the power supply is functioning normally to avoid causing secondary damage to the circuit board in the next steps.
3. Online Measurement Method
The online measurement method is typically used by manufacturers that produce circuit boards in bulk. To facilitate repairs, manufacturers usually set up a common debugging and maintenance platform that can conveniently provide the necessary power supply and some essential initial signals for the circuit boards. The online measurement method primarily addresses two aspects: one is to subdivide the problems identified in the previous two steps and ultimately pinpoint the faulty components. The second is to find the cause of the fault when the issues have not been resolved through the previous two steps. The online measurement method generally follows these steps.
The first step is to power the circuit board. It is important to note that some circuit boards do not have a single power supply; they may require 5V, plus and minus 12V, 24V, etc., so ensure that all necessary power supplies are provided. Once the circuit board is powered, touch the components on the circuit board to check for any hot or overheated parts, focusing on the 74 series chips. If any component feels hot, it may indicate that it is damaged. After replacing the component, check whether the circuit board fault has been resolved.
The second step is to use an oscilloscope to measure the logic gates on the circuit board to see if they conform to logical relationships. If the output does not conform to logic, two scenarios need to be considered: one where the expected output should be low but is measured as high, indicating chip damage; the other where the expected output should be high but is measured as low, which does not necessarily mean the chip is damaged. In this case, the chip should be disconnected from the subsequent circuit and measured again to observe whether the logic is reasonable, determining the chip’s status.
The third step is to use an oscilloscope to measure the crystal oscillator in the digital circuit to check for output. If there is no output, all chips connected to the crystal oscillator should be removed as much as possible before measuring again. If there is still no output, it can be preliminarily determined that the crystal oscillator is damaged; if there is output, the removed chips should be reattached one by one to find the fault.
The fourth step is to measure the three buses of the digital circuit, which generally include digital, address, and control buses, using an oscilloscope to compare with the schematic to check if the signals are normal and identify the problem.
The online measurement method is mainly used for comparing good and faulty circuit boards, discovering and solving problems through comparison, thus completing the circuit board repair.
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