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