Move the test point to the control section. First of all, the VCC of UC3843 is only about 7.5. According to its Datasheet, the VCC of UC3843 is at least above 8.5. This indicates that there is something wrong with the starting circuit and the voltage is not enough. Through the schematic analysis, the way to improve the starting voltage is to remove the resistance R4 from the auxiliary power supply, reduce the voltage difference, and increase the filter capacitor C7. After recharging, the circuit has no noise and the output voltage is normal.