Electrolytic capacitor at negative voltage regulator input
We'd need to see your schematic but I suspect that you may be relying on the 47 uF to do all your smoothing on a full-wave rectified supply. You need a large capacitance to do the primary smoothing - typically 1000 - 4700 uF. The local capacitor at the regulator is to avoid any high-frequency instability. The output capacitor is not optional either.