In the case of unclear chip circuit and working principle, you can use the ground impedance method to measure the circuit in the full discharge state, with a multimeter 200K ohm file (preferably with 4 and a half), measure the corresponding impedance of each pin, and compare it with the good.
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If it is convenient, cut off its positive and negative power supply from the main line and measure its impedance to the ground, which can measure most of the problems!