Servo Reversing with a Soldering Iron

This will show how to reverse a servo mechanically by swapping wires inside the servo.   It is for very specific instances and not a first or second choice to reverse servo operation.

Why?

Why indeed.  I confess to a problem.  I can’t seem to ‘just build it’.  If it’s simple I complicate it.  If easy, I find a way to make it more difficult.  It does lead to leading stuff.

I’ve been looking for a slow flyer and decided to scratch build an FT Old Speedster.  Do I download the plans and just get to it? No. Not me. I choose to do an 80% build... fiddle fiddle…  OK, plans done, start building.  Am I satisfied with a 3 channel?  No, I want ailerons AND flaps...  fiddle fiddle…  Looks good!  But wait…

Mounting the flap servos presents a problem.  No matter how I mount them in the area it makes sense to do so, on the edge of the fuselage under the wing, one always goes forward while the other moves to the back.  What to do? Mounting the servos so they work correctly puts them into the wind under the wing.  I briefly toy with the idea of connecting both to one servo but the geometry results in a slot that weakens the fuse.

So I did what any other, reasonably intelligent and mechanically inclined individual would do: I google it.  I find a post talking about the possibility of reversing the wires inside a servo.  Why not?  Why not indeed…  Do not try this unless your Solder-Fu is good.  The pictures show a HXT900 9 gram servo. Others are probably pretty close to the same.  Time to fiddle some more…

How

Note: the example servo, while an HTX900, has seen better days. I’ve used it in 3D builds and trimmed the tabs and broken gears and it’s done. Looks like heck, but for taking pics in the re-creation of what I did it will work.

First, remove or peal back the stickers. They cover the bottom plate. I put tape on the side of the servo case to keep the top on and the gears in place. 

Remove the screws.

Take note of what it looks like before you start. 

Start by moving the two wires on the motor. Swap them across, left on right, right on left.

Then carefully move the control board aside and remove the plastic insulator to gain access to the potentiometer. Don’t lose the insulator; don’t forget to put it back afterwards.

 Just like the motor, a simple swap. This time it’s the two outside wires. It’s real tight in there. I used a bit of tape to hold things out of the way. 

Take your time and maybe use a smaller soldering iron than the one I have.

 Put it all back in reverse. Note there is a slot for the servo lead in one end of the servo bottom plate. 

There you go. Not something you have to do all the time (thank goodness) but nice to know it can be done.

It works!

Thanks for looking! As I was preping this I googled some more and found videos of the same process. If that is better for you than pics, go for it. Going to put this up anyway. ;)

JD