72 SERVO 06.2015
the target. In the meantime, the gearbox would be
struggling against the stop, and you would risk burning out
The difficulty of the continuous rotation modification
depends on the servo you’re working with. ServoCity
provides a very helpful “Rotation Modification Difficulty
List” that ranks servos on a scale from 1 to 10, with 1
being the easiest and 10 being the hardest. Modifications
are deemed more difficult where more soldering is
required, where the stop in the gearbox is metal instead of
plastic, and whether the spline needs to be drilled out.
To give ourselves a few options as far as what servos to
use with our servo arm, we set out to modify a few servos
for continuous rotation. We selected a servo from Hitec
(the HS 322), one from Futaba (the S3003), and a servo
from Savox (the SC-1258-TG). We started with the Savox.
Savox servos are high-end, with an all-metal construction
and serious torque. We previously worked with them back
in the July 2011 issue, where we equipped an off-roading
bot with a Savox servo for its steering mechanism.
The first step for a continuous rotation modification is to
remove the bottom plate of the servo. Generally, servo
casings are held together with four long screws that can be
removed with a Phillips head screwdriver. Be careful when
you remove the screws because they are also what hold the
top park of the casing and gearbox in place.
Once the bottom plate comes off, the servo PCB will
be exposed. The potentiometer is hidden under the PCB, so
it has to some out. The circuit board is usually fixed in
place, mainly by the leads from the motor. The motor leads
are easily identified as they are
normally the only connections
on the PCB slathered in so
much solder they looked like
Han Solo’s hands poking out
from his carbonite storage.
Our trusty solder sucker
made the desoldering process
quick, but the PCB still
wouldn’t budge much.
After desoldering the motor leads, on most servos you
can extract the PCB — at least for a certain distance. The
potentiometer is often screwed into the bottom of the
gearbox, but the pot is usually connected to the PCB via
some wires that at least give you enough leeway to pull out
the board a good distance. The Savox PCB, however, was as
immobile as Jabba the Hutt nestled comfortably on his dais.
We were able to peek under the PCB enough to find the
culprit. Instead of connecting to the PCB with flexible wires,
the pot was soldered directly into the PCB and, in turn, the
pot was screwed into the bottom of the gearbox with not
one, but two screws.
The pot leads were flexible enough to give us room to
slide in a small screwdriver and remove the two screws
holding the potentiometer in place. Once you get the PCB
free, the next step is to desolder the potentiometer. For
other servos like the Hitec, we were simply able to desolder
the wires from the pot, leaving the connections to the PCB
unscathed. With the Savox, we would have to perform
some precise solder surgery.
The three leads were very securely soldered into the
PCB and a challenge to remove. It was a situation almost as
sticky as being trapped in a garbage compactor in the
underbelly of the Death Star. With some helping hands and
our trusty solder sucker, we soon liberated the
The manual for the Force servo arm suggests wiring
the device directly into the servo PCB. We preferred to keep
our options open, so instead of soldering the arm directly
into the PCB, we soldered a PWM cable into the PCB that
would, in turn, connect to the device. With the cable
soldered in place, we were
ready to move onto the
Now, This is
In all of the servos we’ve
modified for continuous
READY FOR REASSEMBLY.
REALLY GRINDING MY GEARS.