I had the exact same problem. I found the problem was the power supply, which was weird because it still output voltage when unplugged from the control box. When I would plug it in, the voltage would drop to next to nothing. I purchased a new 12VAC 3500ma power supply for $5 from Amazon. I had to splice the connector from the old power supply on to the new one, but it fixed it. It is working right now.
The unit does work really well for cleaning problems with algae bloom.
I highly recommend UV filters, but I wonder if another manufacturer may be more reliable.