Of course, the first thing to check is the fuse. It should be in a holder at the bottom of the control box, accessible from the outside. The fuse holder that they used in my box was not a high quality type, and was eventually damaged by corrosion. I replaced it with an internal automotive type fuse holder that sits inside the box.
I had my control box fail last year but was able to fix it very inexpensively. I'd be happy to share what was wrong, but proceed at your own risk. If you have some (or know someone who has) experience soldering and working with electricity then you should be fine.
Keep in mind that it could be any one of a hundred different things causing the failure. This just happens to be what was wrong with mine.
If you take the front cover off the unit (turn it off first) you will see the main circuit board. On the board is a component that looks like a black disk, about 1" across. It is mounted vertically by two wires. Have a close look at it. If it looks cracked and burned it is likely your problem.
This component is a "inrush current limiter". When it is cold, its resistance is fairly high. When power turns on it allows a small amount of current in. This small current causes it to heat up, which then lowers the resistance, allowing more current in. This continues until the resistance is near zero. This happens fairly quickly, maybe over 10 seconds or so, but it prevents large initial currents from damaging the cell.
If this is the problem, you can order the part from Digikey (
www.digikey.com). It is only a few dollars. Replace it and you are back in business. If you need help or more details, please let me know. I don't know much about pools but I am pretty good at fixing electronics :-D
Good luck
Mike M.