Single channel permanently on

[sandy123]

If it is moisture, it will have got in through the glands (where the cables and leads enter the box). It doesn't take much moisture to short across tracks and components on the pcb. The FET for that channel will then 'turn on' fully when the board is powered up. Open the cover and check inside, especially underside of the controller. A Hair Dryer will do the trick, BUT be careful!!!!!!! Low heat and low speed, keep the nozzle away from the circuit board and move the nozzle around so that you don't cause 'localised' heating on any part of the board which may damage the components.
Hope that helps
Sandy