It doesn't seem all that sophisticated. It is an adjustable power supply and a (presumably) separate volt/ammeter in a single box.
To get a circuit diagram you could open it up and reverse engineer it.
Also, to test the meter portion, you could try various voltages and currents with the unit unplugged. Most likely the meter does not require power to work.
Be careful not to overload the meter with more than 50 mA though, or you could damage it.