My inclination would be to think that your unit was re-wired at some point with a mains power lead and a plug that does not have an earth ground.
In the U.S. it is common for certain appliances like lamps to have only two wires, both the same color, because the lamp doesn't care which side is hot and which side is return.
And, in fact, most devices that run on AC don't care either, if they are not connected to any other electrical devices. But when you have two devices hooked up to opposite AC polarities and you come between them, you can be in for an unpleasant and potentially lethal shock. (As has happened to me, apparently, with a PA system in one circuit and my guitar amp in another, wired opposite, and nearly getting knocked on my ass when my mouth touched the microphone...)
The only solution that comes to mind before you take it to a qualified repair shop is to open the thing up and see if there is any indication inside where the main power lead connects. There may be some kind of legend inside that indicates hot and return. Earth ground would be connected to the case body.
Good luck!
