Well, it is an inherent problem with older manufacturing techniques used in the production of monochrome LCDs. Essentially there is a conductive material which joins the LCD cells (one of the bars of a number) to the connections which are then soldered to on the outside of the LCD package- this conductive material fails over time and causes the LCD cell to go dim/not work. Back then this was relatively new technology.
Originally Posted by alistair.o
I also repair analogue synthesizers and this is a common problem on gear from the 1980s, especially from Japan. I believe the parts were all made by the same company (Alps)