1. The fifth foot of UC3842 is ground.
2, first test the fifth and eighth pin, pin 8 output reference voltage 5V, test this foot will soon be able to judge the UC3842 good or bad.
3, if the 8th pin voltage is normal, there is no problem with UC3842 under normal circumstances, 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.