When a mobile device is connected to the USB-A terminal, the SEND/RETURN ON/OFF button is not lit blue.

When the [RETURN TYPE] selector switch is set to [AUX] and the FX CH selector switch is set to [MASTER] or　[AUX], you cannot turn on/off the SEND/RETURN option in order to prevent loop oscillation. (The SEND/RETURN ON/OFF button is not lit blue.)