I think that is only dust. The same dust is on the capacitor nearby.Just looked at your picture. The rectifier next to the capacitor looks a bit burnt?
You could have a dry joint on the rectifierI think that is only dust. The same dust is on the capacitor nearby.
However, that picture does show that the rectifier is raised off the board, with the four connecting pins easily accessible. That would make it very easy to do the basic power supply test that I suggested in post #5. The two diagonally-opposite pins marked with ~ ~ should have 42v AC across them, and the + and - should read 60v DC.