I had a similar problem with my Sprite. In my case it was my fault because I used the wrong power supply: I used 24V DC instead of 24V AC.
It's interesting to see that the Sprite itself also works connecting it to 24V DC but the valves fail to do a proper job on DC (since my valves require AC. But according to Wikipedia there are also DC valves). It left me with a feeling of having a broken device until getting clarification from the support.
It was hard to find a 24VAC power supply because it's very uncommon to use AC on low voltage devices, but now that I have a 24VAC everything works as expected.
Oh, and if you have to buy a new power supply make sure it has at least 800mA (0.8A) if you have a Sprite. (Or look it up in the user manual if you have another device)
Hope this helps