rotation, the potentiometer could come completely out
without affecting the operation of the gearbox. Other
servos might require further modification, like drilling out
the spline. Fortunately, that wasn’t something we had to
worry about with the Savox, Hitec, or Futaba servos.
We turned the Savox servo right side up and carefully
removed the top casing. Be careful not to accidentally take
apart the gearbox as you take off the casing — you’ll want
to see how everything fits together so you can put it all
back together once you’re done. Taking a picture is a good
way to give yourself a guide for reassembly.
Take apart the gearbox, taking care to keep the gears
clean, and you’ll find one gear with a stop on it. Many
servos have plastic gearboxes, so the stop can be easily
removed with some flush cuts. The Savox servo, however,
was equipped with titanium gears, which are not exactly
susceptible to hand cutters. To shave down the metal stop,
we busted out our trusty rotary cutter. We secured
the gear in a vice and shaved down the stop until it
was flush with the gear.
With the potentiometer and gear stop both as
gone as Alderaan, we were ready to put the servo
back together. Our last modification would be to cut
the casing to allow the second PWM cable an escape
route next to the original cable. Our rotary cutter
made quick work of the mod. We reassembled the
gearbox with reference to our picture, put the top
casing back on, flipped the servo around, resoldered
the motor leads, and reattached the bottom casing.
The Hitec was easier to modify. The pot was held
in with only one screw, and it was soldered to the
PCB via some wires that we connected to a PWM
cable. The gears were plastic, and we were able to
snip the stop by hand. The Futaba was even easier to
modify because we had previously modified it for
continuous rotation. The gearbox was ready to go,
but we did need to make one change to the PCB.
When modifying a servo for full rotation, you remove
the potentiometer like we had been doing here, but instead
of replacing it with a device like the Force servo arm, all you
need is a simple voltage divider comprised of two resistors.
By replacing the pot with a voltage divider, the servo
sends back the same position no matter how far the horn
has rotated. Since it will never reach that position because
the resistance of the voltage divider won’t change, the
servo will rotate continuously. It’s a good idea to set the
voltage divider to the middle of the range of the pot
because the resistance of the divider will set your neutral
position. A common value for the pots found in servos is 5
kΩ, so divide that by two to get your ideal neutral position
of 2. 5 kΩ.
To make the voltage divider as easy as possible to
construct and fit on the PCB, you’ll want to use as few
SERVO 06.2015 73
REMEMBER THIS ASSEMBLY!
THE MANUAL DUAL SERVO DRIVER FROM SERVOCITY.