Hi Michael,
Could be that the filament supply required to illuminate the tube is
low due to a dry electrolytic capacitor or a resistor value gone high.
Trace back from the outer points of the tube (filament points) to the
power supply. If you see an electrolytic capacitor in series with one
of the lines, try blowing some hot air on it. If that makes your
display brighter, replace the capacitor.
Jango