TL;DR : The output voltage from my 5V DC to 3.3V DC stepdown seems to output 4.5V DC - why?
Background: I'm trying to power an Airport Express from USB rather than a wall outlet. I have discovered that the PSU inside the APE outputs 3.3V DC which powers the device. I figured I could rip the PSU out and replace it with a little board that steps the DC from 5 to 3.3, allowing my to use USB to power the APE.
So far I have cut the end off the USB cable and tested the red/black wires with my multimeter and can confirm it is outputting 5V DC. I then connected the red wire (5V) from the USB cable onto the VIN pin of the step down, and then tried testing the output voltage from the VIN by connecting the black lead of the multimeter to the ground wire (out of the USB lead), and the red lead of the multimeter to the OUT pin of the step down, its currently reporting 4.5V.
The step down device is one of these: http://www.ebay.com.au/itm/281058278731?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649
Am I testing the output voltage incorrectly or is my step down faulty?
Where should I be placing the black lead of the multimeter to close the circuit in order to test the voltage correctly?
I can provide pictures if anything needs clarification.