Adapting an AM transmitter  for a Hitec 725 winch

The Hitec HS725BB is a good reliable sail winch of surprising power. A number of builders though, find that it gives too many turns (usually 4.5) on the drum for the amount of sheeting movement required on the model. However it is possible to make a simple modification in the transmitter to control the number of turns the winch makes with end to end throw of the Sheeting Stick. Note that with a new unit this modification would probably cancel the warranty of the transmitter and even maybe the winch. By adding a resistor to one end of the Sheeting Stick potentiometer the number of turns made by the winch can be reduced. However if the servo-reversing switch on the transmitter were to be thrown, great confusion would result. So we add a resistor at both ends of the sheeting pot and by using a variable, ganged, dual-potentiometer to do so, an effective means of trimming the winch can be achieved. Click here to view a schematic circuit. On most units the value of the unit's potentiometer is 5 Kohms and the required trimming dual-pot should be of the same value. Make sure it is a linear and not logarithmic version. Space to fit it will be at a premium so get a miniature size. Carefully saw off the pot's shaft but leave enough to saw a screwdriver slot across its end. Remove the rear case from the Tx and find some space within the unit to position the new trimming potentiometer. Fix it in place, either with an aluminium flange or with hot glue in such a way so that later its shaft can be reached by a small screwdriver through a hole in the case for adjustment. On my ACOMS transmitter there was room across the back of the antennae to mount the trimming potentiometer on an aluminium flange. The brown triangular "thingy" is hardened brass sheet shaped to provide a friction engagement with the serrated arc of the sheeting stick to hold it in position. Locate the three wires going to the Sheeting Stick potentiometer. De-solder the two outer wires and re-affix them again, one to each of the center contacts of the new trimming potentiometer. Then solder a pair of new wires from the end terminals of the trimming potentiometer back to where the wires were removed from the unit's sheeting potentiometer. If coloured wire is used it is easier to keep the logic of the arrangement under control. When everything is back together the system should be capable of reducing over the full range of the trimming potentiometer rotation, from 4.5 to about 0.5 turns of the winch drum. When setting up on the model, bear in mind that the reduction occurs at both ends of the winch's travel. So if it is required to reduce the length of the sail sheet travel by say 10 cm then adjust things so that the sheet moves back 5 cm (the other 5 cm will be taken up at the other end!). It is possible to make a similar modification on the winch itself but I have never been brave enough to take the Hitec apart and try it! Back to "Building a J"