I looked online this morning for a video or schematic to help you and was astounded at the lack of info on this...
...So i ended up taking a 320+ apart to find it myself.
It appears to be located by the usb port and is marked R-14.
I can't seem to find ANY schematics right now (and don't have any more time today) so don't take this as fact.
Is there anybody out there that can help with this?
Help us out if you can...
If you try this be very gentle and mark the original location prior to moving the screw.
Here's a quick video of a voltage adjustment (different counter), should help you to understand the process...
https://youtu.be/joIuux8gUqwNotice the blue block? this is the voltage adjustment pot for that particular unit. The 300's seems to be VERY small and metallic...
The manufacture is willing to fix it for free but you'll be without it while they perform the service. Maybe something to think about...
Good luck and let me know what happens.