Modifying an RC servo for continuous rotation

Graupner C677 servo For an upcoming hobby project I need an RC servo that can rotate continuously through 360°. Most (cheap) RC servos are limited to a rotation angle of 180° degrees or less, including the Graupner C677 that I got. But by modifying the internal hardware and electronics it can be made to rotate continuously.

Because this procedure removes some material from one of the sprockets, it is a permanent hack (unless you are prepared to mess with tiny plastic parts and superglue to reverse it).

Note that this procedure is meant for a Graupner C677. It may not work on all servos: some are mechanically unsuited for modification. Please examine your servo's innards carefully before doing anything permanent to it.

What you need

You will need the following tools and consumables for this procedure:

small phillips screwdriver
ohm meter
soldering iron with a fine tip, solder
2 resistors of 2.2 kΩ (value may depend on servo type, see below)
some heat-shrink tubing
tiny saw or hobby knife
small file
scalpel or other thin and small knife

The procedure

Remove the four screws that fit the top cover to the servo body: two on top and two on the bottom.
Carefully pull the top upwards, exposing the outgoing gears and the feedback gear.
The white stuff is grease, it caused two of the sprockets to stick to the top cover. I just left them there because they don't need modified.
Make a note (or take a picture) of how the gears are installed.
Lift the outgoing sprocket of its axis and put it aside.
Note the protrusion on its underside: that is the mechanical stop that prevents rotation through more than approximately 180º. It will be removed in a later step.
Remove the two small phillips screws that fit the cover plate to the servo body (just visible to the left and right of the brass sprocket of the servo motor itself).
Lift the top cover up and move it free from the servo body, keeping it vertical.
On the left we now see the feedback potentiometer, its wiper controlled by the gearing.
Turn the cover over. The feedback potentiometer is secured by two plastic latches and a little bit of glue.
Make a note of the way the wires are attached to the pot.
Carefully insert the scalpel between the pot and its plastic retaining bracket to cut through the glue.
Those retaining brackets are fragile: I broke one. This is not a problem.
Push the two retaining clips sideways and pull the potentiometer out so it comes loose of the cover plate.
Put the cover plate aside.
Unsolder the pot.
Measure the resistance between the two outside leads of the pot.
Divide the value obtained in the previous step by two.
You now have the value of the two resistors you need. This value will likely not be one of the standard values for resistors, but that's not a problem: choose a value that is as close as you can get. In the case of this C677 servo, the pot has a value of 5 kΩ so I went with two resistors of 2.2 kΩ.
Solder the two resistors together.
Put some heat-shrink tubing over the wires that you unsoldered from the pot.
Solder the wire that was attached to the center lead of the pot (the 'wiper') to both resistors (right, photo below).
Solder the wires that were attached to the outside leads of the pot to the other resistors (one on each).
Because the resistors are identical in value, this setup simulates that the potentiometer is in its center position.
Warm the heat-shrink tubing so it covers the wires and resistor leads nicely.
Carefully place the resistors into the servo.
Using a small saw (or Dremel tool), carefully remove the protrusion from the outgoing sprocket.
If necessary, smooth the surface with a file, so that the sprocket is smooth and does not catch on the end stops.
Put the servo back together and test it.