Voicetone Create (original) - Volume problem / very quiet

Hello,

My friend purchased a second hand 'Voicetone Create' (original version) from ebay in May.

It's been working fine until the other night...

He set it up the same as usual, all the patches we're still stored.
You could hear the effects still but everything was just really quiet.
Turning the input knob didn't alter the volume.

He's done a factory reset and tried different mic cables and tested the PA.

Just wondering if anyone could give us an idea as to what might have gone wrong?

Thanks in advance.