The inspection found that pressing the call button could not turn on the phone, and re-checking the code was also invalid. During the test, it was found that the mobile phone could emit a "beep" alarm sound when there is no contact, indicating that the mobile phone microprocessor is normal. Use an oscilloscope to observe that there is no waveform at both ends of the 10.240MHz crystal external to the GM6532 (LGS8820) programmable PLL chip. After replacing the 10.240MHz crystal, use an oscilloscope to observe that the waveform is normal, but when the code is good or bad, it is suspected that there are problems in other parts, but the inspection is normal. When checking the 10.240MHz phase-locked loop clock signal with a frequency meter, it is found that the frequency is 10.229MHz, indicating that the frequency has a deviation. The fine-tuning clock circuit is connected with an external adjustable capacitor to restore the frequency to 10.240MHz, and then the code is all normal, and the fault is eliminated. It is worth mentioning that the new crystal parameters cannot be consistent with the original crystal parameters. After replacement, the frequency should be checked for deviations. If there is a deviation, the adjustable capacitor should be adjusted appropriately so that the frequency error is controlled within the allowable range. Normal phone presses the phone to emit a "beep" sound to the code button "CODE". Press again and it will emit two beeps, indicating that the code is successfully matched. The machine emits a beep sound when pressing the code-checking key of the mobile phone, but when pressing the code-checking key, the mobile phone cannot emit a "beep" sound, but you can see that the indicator light of the mobile phone flashes, and a few continuous "beeps" After the beep sounds, the code matching state is exited, indicating that the code matching has failed. Cross-code the machine with a normal machine of the same model and find that the fault is in the host machine. Q12 is the reset tube. Press the code key "CODE" of the host when normal. The 19th pin of the LSC89661 micro-processing chip drops from 4.28V high level to 0V low level; the 41st pin drops from 4.87V high level to 1.85V low Level, check the host code, and find that pressing the host code key, the voltage of the 19th pin of the LSC89661 micro-processing chip changes normally, and the voltage of the 41st pin is always 4.87V. Check the reset circuit related components, find that the resistance is open, and replace the test machine , The code returns to normal.
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