Are you testing your voltages going in and out with the unit under test? Often the input voltage will be loaded down by the transformer so you need to hook up the test and THEN test the voltage going in to see if that 78V is being loaded down. Conversely, try the test in reverse by putting a low voltage into the secondary and measuring at the primary to see if the results change. Again, test the input voltage with the unit under testing conditions. Source impedance for the test signal will almost always allow the test signal to be loaded down some.