Thank you for your help.
As I said I tested a few settings with xrandr the other day and ran into the problems described above.
Today I wanted to proceed and started out with setting the external display settings in recalbox.conf. After reboot everything works now.
I wonder why the behavior was different when setting the same modes on my TV with xrandr.
Anyhow, works perfectly now! Now I can finally dive into some oldschool Gaming 
Thank you for your help.
As I said I tested a few settings with xrandr the other day and ran into the problems described above.
Today I wanted to proceed and started out with setting the external display settings in recalbox.conf. After reboot everything works now.
I wonder why the behavior was different when setting the same modes on my TV with xrandr.
Anyhow, works perfectly now! Now I can finally dive into some oldschool Gaming
@Zing: I just saw your post, thanks for the hint. Amazing how fast you get help in this community!
I tried some configurations using xrandr today and got it somehow to work.
I used display HDMI1 and set it to "preferred" mode. I then got an output on my TV (ES UI). However, it got only displayed in about 2 third of the screen (upper left corner, rest of screen is black).
When I start a game though, it is displayed on the whole screen.
Are there any additional configurations to "scale" ES UI?
Additional Info: Got the same result when connecting to a LCD display.
Hi community,
First of all thank you for your work. Installation on my spare Lenovo Thinkpad T420S was a breeze and everything works...except plugging it into my TV using a Display Port to HDMI Cable
Observed behavior:
When I connect via SSH xrandr --lsitmonitors only shows one monitor.
I wanted to set the screen and preferred resolution in recalbox.conf. Do you have any tips for me how to get this solved or where to investigate this further?
Regards
radi