I needed to modify a number of Parallax standard servos to continuous rotation for a kids' robot workshop project. I came up with a method that requires no soldering and allows the servo to be changed back to position operation if desired. The following procedure is specific to the Parallax servo although the concept may work for other brands if their construction allows it.
What I did was to adjust the position feedback potentiometer (pot) to its electrical center and reposition the pot so it was mechanically decoupled from the gear train. I also cut the stop nub off one of the gears. After modification, any commanded position ahead or behind center causes continuous rotation. You can return the servo to standard service by restoring the pot to its original orientation. A small plastic pin or screw stop can be added if desired.
Step 1: Disassembling the top and bottom
• Unscrew and remove the servo horn if attached. Set it aside.
• Remove the four long screws that hold the servo assembly together.
• Pull off the bottom and top covers. Be careful. Some of the gears may come out when the top cover is removed. They are often stuck together with the lubricating grease.
• Remove the gears. The large gear at the bottom may be left in place. Preserve as much of the grease as possible.
• Remove the two short screws that were exposed when the gears were removed. This detaches the board and motor assembly from the case.