An external FX unit I've connected to the MULTI I/O section won't work.

The operation varies depending on the [MODE] set for the MULTI I/O section.
Check the [MODE] selector indicator in the MULTI I/O section.

■ When [INSERT] is selected
1. Make sure the [ON/OFF] button in the MULTI I/O section is blinking.
2. Check the [CH SELECT] switch to see if the channel you want to apply the effect to is selected.
3. Check if the [LEVEL] knob is in the proper position.

■ When [CH 2/CH 5] is selected
1. Make sure the [ON/OFF] button in the MULTI I/O section is blinking.
2. Check the [CH SELECT] switches to see if the channel you want to apply the effect to is selected.
3. Check if the [LEVEL] knob is in the proper position.
4. For the channel with its channel select switch set to [MULTI I/O] (CH2 or CH5), turn the [TRIM] knob and slide the channel fader away from you.