Continous Rotation Servo Mod - Adjustable Method

I know there are a lot of methods for doing this but I was unhappy with the other methods.  Servos have 4 basic parts motor, gear train, controller board, and poteniometer(pot). The two things we need to modify are the gear train and the pot.  What I will do different is by using a different pot that allows for adjustment in case your servo doesn't spin the same speed in either direction or needs to match speeds with another servo.  The pot im using is a 5k Ohm, 10% tolerance, 25 Turn trough hole trim pot from Digikey.  I'm also assuming that you know the basics on how a servo motor work and if not see the pdf below.  

5k 25Turn Pot - Digikey
http://www.digikey.com/product-search/en?x=12&y=16&lang=en&site=us&KeyWords=A105772-ND

My Specific Servo
http://www.ebay.com/itm/MG996R-Torque-Digita-RC-Metal-Gear-Servo-FOR-RC-Helicopter-CAR-Boat-/390528100232?pt=US_Radio_Control_Control_Line&hash=item5aed49e788
Futaba compatibile arms/spline shaft

Tools/Materials Used:
#1 Phillips Screwdriver
Grease - General Purpose
Needle File Set
Heatshrink
Soldering Iron
Hot Glue Gun
Small Gauge wire
Wire Strippers
Drill 
etc.

Step 1: Remove Existing Pot

A) Remove the four screws on the rear cover.

B) The circuit board and motor should easily slide out, the motor may require some jiggling.

C) With my servo there is a screw holding in the pot that needs to be removed.  Your pot may be directly soldered to the board as in the fifth picture.

D) Now unsolder the pot's leads from the circuit board or from the pot itself.

Step 2: Removing Gear Stop

A)  Now remove the front cover showing the gear train.

B)  Remove the output shaft/gear combo, it is most likely a snug fit and may require a flat screwdriver to be used.

C)  The gear will have a "nub" that needs to be filed off or cut of with a pair of flush cut pliers if you have nylon gears.  Mine was a steel pin pressed into the bronze gear that only took 5 minutes to file down but make sure you don't file any of the brass gear away on accident.

D)  Wipe the gear clean of debris and install back into its home location.

E)  Install the remaining gears and before you install the front cover add a little grease to replace what was removed from handling.  I have a wealth of 40 weight motor oil and no car that takes it so I used that instead of wheel bearing grease that is just to thick.  I would recommend all purpose grease but light machine oil will suffice also if you don't use a lot to prevent leaking into the motor causing a short.

F)  Install the front cover.  Try not to force it as you may damage the cover, instead try wiggling it until it closes.  We have a little bit of work to do still so use some electrical tape to keep the cover on.

Step 3: Installing the New Pot

A)  Plug in hot glue gun to start warming up, I'm using a low temp glue gun.

B)  Grab your heatshrink and cut pieces to cover the connections that will be made to the new pot.

C)  Slide your pieces of heatshrink on to the wires before soldering to the pot.

D)  Solder the wires to the circuit board, if you left the existing wires on the board then skip this step.

E)  Drill a hole in the case to access the trim pot's screw.

F)  Install the motor back into the case.

G)  Place a bead of glue that will go under the new pot.

H)  Quickly install the new pot before the glue cools.

I)  Apply more hot glue around the sides and top to ensure it wont move out of place.  Try not to get any glue by the wires as you will need the extra space to fold them in when placing the circuit board back.  

J)  Once the glue has cooled place the circuit board to where it belongs, folding in extra wire neatly.

K)  Install the rear cover and tighten the screws.  Don't over tighten the screw as they are easy to strip out or split the case.  I usually snug them up and will periodically check them when in use.  

L)  You may remove the electrical tape holding the front cover unless you like the statement.  

Sorry if I was a little vague on placing the access hole or pot inside the cavity.  As this will vary with every case you will have to use your best judgement or invite that smart friend over to pick his brain.  First make sure that the pot will fit inside the cavity before you worry about drilling a hole.   

Step 4: Adjust the Servo

I will be using my TH9X & R8B radio system to plug in the servo and adjust the pot to the center position.  You may also use an Arduino board if you give it a command for 90 degrees.  Where this mod will shine is if one servo is faster or slower when working with another given the same commands.  We have to set up the center position, plug the servo into a channel that has a spring return back to center.  The servo will start spinning in one direction and using a small screwdriver trim the pot until it stops moving.  A good method is to turn it one full turn in one direction and if nothing changes or speeds up, turn it two turns in the other direction.  Once you get it to stop explore how much you can turn the pot in either direction before it moves and then try to set it in the middle of the "dead band".  You are done!  

Where this mod will shine is if one servo is faster or slower when working with another given the same commands.  Lets say we are using the vertical axis of both joysticks to control two servo motors in a skid/tank steer configuration.  You will have to reverse or flip one channel so that when both sticks are pressed up, both servos move forward.  Now with both sticks maxed out one side moves faster than the other and needs to be adjusted.  There is two ways to go about this, you can make the faster servo slow down or make the slow servo speed up.  To slow down the faster servo you will want to trim the channel in the positive direction and then using a small screwdriver adjust the pot to use the new center position.  To speed up the slow servo, trim the channel in the opposite or negative direction and adjust the pot to the new center position.  

I have loved using my servos since doing the mod because when I turn on my RC receiver and Arduino Uno it seems to have a new home position every time.  After two seconds of turning the trim-pots The robot works great and I surprisingly no longer have an issue with one servo being slower in reverse than before!  

If you have any questions feel free to ask!  I will also be releasing another instuctable where I discuss RC control using an Arduino Microcontroller in great depth and will be showcasing my tank steering code as part of the final demonstration.