I think you need to make sure the USB plug that goes into the 770 does not have +5V? I base this on experience trying to use i-Blue charger (USB connector) with 770... all what happes is that 770 thinks it is connected to a PC. (The charger has absolutely no USB functions at all!) It looks like 770 senses the PC by monitoring the +5V supply line... Try to disconnect the +5V and see if it works? (You need the +5V for the Keyboard, though...)