cdp-X33es dim screen

Three possibilities... and dim only at the corners actually suggests it or the display window needs cleaning...

1/ Have you removed the front panel and cleaned the glass of the actual display tube itself? Be extremely careful not to break the glass 'pip' on the tube.

The display should be bright all over when viewed without the front panel in place.

2/ The display tube is worn out (low emission).

3/ Problem with the supplies to the tube such as dried out electrolytic caps in the negative bias supply and heater supply.

Note... 2 and 3 above don't fit the 'dim at the corners' scenario 🙂
 
In that picture the words continue and repeat on the display looks like low emission to me.

If you leave the characters that are dim illuminated then they may recover somewhat after a few hours of illumination... maybe... it can work that way.