Make sure your meter is working correctly. With no load, there should be no voltage drop. What's the voltage right at the transformer? What's the line voltage going into the transformer? ...And just to confirm, this is an actual TRANSFORMER, right? A heavy thing with lots of iron in it and no electronics? Because if it's actually an electronic power supply, then your meter will probably not read the voltage correctly and it probably won't produce the correct voltage without a load applied anyway.