I recently found a Yamaha RM1-X on Facebook Marketplace, the guy selling it said that the display was gone, everything was working but it was almost impossible to see anything on the screen, as you can see in the above picture. It was only barely visible if viewed at an angle. I took it home for 90 euros thinking "maybe it is just a quick fix, like I have seen in this youtube video".

And yes it was!

The problem is with Capacitor C41 which is in the contrast section. The capacitor was probably gone and the display was always set to its maximum contrast, and the small potentiometer on the back didn't have any effect.

All I needed to to was unscrew the metal plate that closes the unit (lots of screws) and just desolder C41.
This is a common 10uF 16V SMD electrolytic capacitor. I didn' want to lose time searching in my SMD box and took a standard thru-hole 10uF 25V.

Desolder, resolder the new one in place, and adjust the (now working) contrast.

After the replacement. Back to normal.