The 5th foot of the UC3840 is ground.
First test the fifth and eighth pins, the eighth pin output reference voltage 5V, testing this foot will soon be able to judge the UC3840 good or bad
3, if the 8th pin voltage is normal, under normal circumstances, there is no problem with UC3840, and the others do not need to be measured.
4, if the 8th pin voltage is not, test the 7th pin to see whether the power supply voltage is correct. However, the 7-pin voltage value depends on the specific circuit design.