With the display unplugged use a 9v battery and aligator clips put them on the two outside pins on the display glass and a wire should glow let it glow for about 5-10 second and see how it goes
I have tried this and it didn't make any difference whereas it usually brightens them right up.
There isn't wrong with the machine. I bought the displays on the off chance I might be able to get them working again. Some are missing segments which I am hoping new chips may fix but there are a few displays that are very very dull. You win some you lose some 
Did the filiment lines across glass glow white when u did this? Are the filiments straight or wonky? If wonky the glass is stuffed
