Schematic isn't too clear but assuming the Regulation Loop isn't putting an excessive load on the Rectifiers output it sounds like a filter capacitor is shorted or installed in reverse. I'd remove the Regulator from the equation by lifting the supply leads to the Transformer. If the Voltage is still low then its definitly a partial short somewhere in the Rectifier/filter section and you should be able to find it with an Ohm-meter.
