I measured a bunch of open circuit voltages across a variety of power supplies using a CRO. the signal is just spurious. Adding a resistor across the output of the same power supplies gives the expected trace. I used high and low impedance option which was 10 megaOhms on the CRO leads. What is going on, in theory the open circuit voltage in this case max 12 volts should be measurable without a load resistor to draw a current?? Apologies if a dumb question.