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How to repair circuit boards without drawings?
2020-11-12
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1. To have a solid picture

It is necessary to thoroughly understand the principles of some typical circuits and be familiar with them. The drawings are dead, and the thoughts in the head are alive. They can be analogized, reasoned, and inferred. For example, the switching power supply is always inseparable from the oscillating circuit, switching tube, and switching transformer. During the inspection, check whether the circuit is vibrated, whether the capacitor is damaged, whether the transistors and diodes are damaged, and no matter what switching power supply is touched, it can be operated. They are almost the same, do not have to force a circuit diagram; for example, a single-chip microcomputer system, including crystal oscillator, three buses (address line, data line, control line), input and output interface chips, etc., are also inseparable from these ranges for maintenance; another example is various operational amplifiers The composed analog circuit, despite its numerous changes, is inferred on the basis of "virtual shortness" and "virtual disconnection", and it can also have a clue, analyze each one, and make it clear. After practicing good analysis and reasoning skills, even if you encounter a device you have never seen before, you only need to understand the principle.

2. Pay attention to the order of maintenance

Pay attention to the maintenance sequence to find the shortest path to solve the problem, avoid chaotic dismantling, failure to repair, and cause the failure to expand. Maintenance is like a doctor's treatment, but also pay attention to "seeing, hearing and asking." "Hope" is to check the appearance of the faulty board to see if there are any obvious signs of damage on it, if there are any components burning black or bursting, whether the circuit board is broken or leaking caused by corrosion, whether there is leakage of capacitors, and whether there is a drum on the top. Wait; "sniff" with your nose to see if there is a burning smell, where did the smell come from; "ask" is very important, you must ask the person in detail about the situation at the time when the equipment fails, and it is possible to reason from the situation The fault location or component; "cut" means using certain testing instruments and methods, divided into two cases, power-on and non-power-on, check the resistance, voltage, waveform, etc. of the circuit location or component, compare the good and bad circuit boards, and observe Differences in parameters, etc.

        In fact, there are many faults that you can solve without using the multimeter, and the circuit diagram is naturally avoided.

       3. The same circuit board has great value for fault repair:

It is better for the maintenance personnel to request a good circuit board that is exactly the same as the faulty circuit board or a bad circuit board that is exactly the same as the faulty circuit board.

     In the current component-level maintenance, many test instruments have a relatively strong function of comparing good and bad circuit board components. The value of a good circuit board to successful repair is sometimes far greater than the circuit diagram, which can greatly increase the speed of repair and the rate of one-time repair.

        A bad circuit board that is exactly the same as the faulty circuit board is also of great reference value for maintenance. Because the failure points of two bad circuit boards are not necessarily the same, even if the failure points are the same, the degree of damage is not necessarily the same. Therefore, it is often more convenient and easier to repair several identical bad circuit boards at the same time than repairing a single bad circuit board.