I have a MASSFIDELITY relay, wireless Bluetooth receiver for which the power supply is missing. I found a manual for the device that states it requires DC 5V 1A power, but says nothing about polarity. I ordered a so-called replacement power supply, but the one I received was 12V so I no longer trust power supply retailers to know what this device needs. Is there a way to determine what the polarity of a power supply should be by looking inside the device? The device can be opened easily so as to see the back of the power input lug etc..