As Jofemodo said above:-
You have to change the framebuffer in the next files:
/etc/systemd/system/zynthian.service
/zynthian/zynthian-sys/sbin/splash-screen.sh
/zynthian/zynthian-ui/zynthian.sh
Look for the references to /dev/fb1 and change them to /dev/fb0
The screen resolution is available at the begining of zynhian_gui.py
#-------------------------------------------------------------------------------
Define some Constants and Parameters for the GUI
#-------------------------------------------------------------------------------
width=320
height=240
Just adjust these value to match your screen resolution & the GUI will scale itself.
I’m building a Zynthian with a 800 x 480 HDMI display. This is a non standard resolution so I needed to add some lines to config.txt to force this. I followed the instructions on the manufacturer’s website.
Let us know how you get on!