When you plug in the charger with the 770 off, it seems to boot up to a point. It is running, it needs to be to display the charging status. On mine, the display doesn't blank in that state but if you let it boot all the way, it will blank.

If you have a voltmeter, you can measure the voltage while it's charging. If the 770 needs more current than your supply can give, the voltage will be pulled down.

Voltage (in Volts) times current (in Amps) = Power (in Watts) so
Power divided by Voltage = current. (1000 mA = 1 Amp) ie 230 mA = 0.23 Amps.