The earth lug is a decent ground point, just be sure that the resistance between it and terminal 3 is very low.
With the Speedball in, you need to measure the resistance of every terminal where the unstriped lead of a 220uF 250V cap lands. They should all be something high. Then you should measure the path of the black wires that connect all the striped sides of the 220uF 250V caps. You should be able to read about 0 Ohms from terminal 3 all the way back to where the unbanded ends of the UF4007's meet the first 220uF/250V cap.
With the Speedball in, you need to measure the resistance of every terminal where the unstriped lead of a 220uF 250V cap lands. They should all be something high. Then you should measure the path of the black wires that connect all the striped sides of the 220uF 250V caps. You should be able to read about 0 Ohms from terminal 3 all the way back to where the unbanded ends of the UF4007's meet the first 220uF/250V cap.
