To this problem, first check whether the power input circuit components are normal, such as: power supply insurance, varistor, and so on.
(2) If the above checks are normal, then measure whether the voltage after the rectification filter is normal.
3, measure the switching power supply stable oscillation thick chip, the pin to the ground voltage is normal, if there is an abnormality, check the chip abnormal foot around the components, such as: feedback circuit components, switching tubes and so on.
(4) If the components around the abnormal foot of the above measurement chip are normal, the problem may be abnormal or damaged the internal circuit of the thick die chip, and so on.