In the meanwhile I got CD4050 and VKVPA020.
Many thanks to the customer support.
However when replacing CD4050 - nothing changed - exact the same behaviour.
The I got back to my own CD4050 and replaced the processor (ATMEGA328P) with the new one (dsPIC33FJ32GP) the behaviour is different. When pressing the button the backlight goes on, resuming the button the backlight goes off. I have no knowledge about the controller.
Changing both CD4050 and processor with the two new items I have the same behaviour.
So it seems the replacing CD4050 makes no change in behaviour.
Replacing the processor doesn’t make it better.
The display still doesn’t work (except the backlight).
How can I check the LCD board?
What type of LCD is used - is there any datasheet?
I found in a different forum this hint
https://forumtest.whadda.com/t/edu-08-spareparts/13827
part No: ATPFF12864WP67
and another in
https://forumtest.whadda.com/t/another-lcd-display-for-audio-analysator-k8098/11970
Display used is the ATPFF12864WP67 from ArrowTech. Maybe you can find a compatible device.
However nothing else that could help me to test the LCD board.
Any ideas?