Casio CZ/ VZ/ FZ - Pro Series

I've recently picked up a bargain Hohner HS-2/E (Casio VZ10m) and am 
unsure if the backlight is on its way out or not as it's a little 
dim. I know backlights can fail but do they just die suddenly or do 
they get dimmer over time? Can anyone advise please?

Thanks in advance.