====== Re-zeroing a Servo ======

<note>
Only do this if Robotical has asked you to, or if you are familiar with this repair.
</note>

If Marty receives a large knock, the servo horn can slip around the servo spline. This can prevent the joint from moving through its full range of movement. The photos below show the left knee as an example.

===== Remove the link from the affected servo =====

Using a screwdriver, unscrew and remove the blue link. Keep the screw and nut safe for reassembly.

{{ :martyv2:support_faqs:hardware:martyleglinkunscrewed.jpg?direct&600 |Blue leg link unscrewed from the left knee servo}}

===== Remove the servo horn =====

Remove the small screw from the black servo horn, then pull the horn off the servo. It may be stiff; if necessary, carefully use the end of the screwdriver to help lift it off.

{{ :martyv2:support_faqs:hardware:martylegservohornremoved.jpg?direct&600 |Left knee servo with the black servo horn removed}}

===== Zero the servo =====

Open the [[https://updates.robotical.io/zero-servo/|Robotical Servo Zero-ing Utility]], connect Marty to your computer with a USB cable, and click **Connect**. Select Marty's USB serial device in the browser's device chooser.

{{ :martyv2:support_faqs:hardware:servozeroapp.png?nolink&600 |Robotical Servo Zero-ing Utility with a button for each joint}}

Once connected, click the **Zero** button for the affected joint. For the left knee shown in these photos, choose **Zero LeftKnee**.

The servo will rotate to its true zero position. Once it has stopped moving, reassemble the joint.

===== Reassemble the robot =====

{{ :martyv2:support_faqs:hardware:martylegservohornreplacement.jpg?direct&600 |Servo horn aligned for Marty to stand straight}}

Refit the servo horn so that it is as close as possible to the position it would be in when Marty is standing straight. It may need to sit a few degrees off this position to fit onto the spline; the calibration step below corrects this small offset.

{{ :martyv2:support_faqs:hardware:martylegassembled.jpg?direct&600 |Reassembled left knee servo horn and blue leg link}}

Replace the servo horn screw and reattach the blue link using its screw and nut.

===== Calibrate the legs =====

After reassembling the leg, calibrate Marty's legs in the current app:

  - Open [[https://robotical.app/|robotical.app]] and connect to your Marty. Keep Marty powered by USB while calibrating, as recommended by the app.
  - Open **Robot settings** using the robot-and-gear icon at the bottom left. Select the Marty you are repairing if prompted.
  - Choose **Calibrate**, then **Legs** (labelled “Balance Marty's stance”).
  - On **Straighten the legs**, manually position Marty with both legs straight and both feet facing forwards, as shown below. Then click **Save leg positions** to store the calibration.

{{ :martyv2:support_faqs:hardware:servo-rezero-leg-calibration-2026-10.jpg?nolink&900 |Current Marty app leg-calibration screen showing Straighten the legs and Save leg positions}}

On the **Check the movement** screen, place Marty on a flat, firm surface and click **Test a step**. If the movement needs adjustment, choose **Adjust again**, reposition the legs and save them again. If the step looks correct, choose **Looks good** to return to the calibration menu.

===== Any issues? Contact Robotical =====

If you have any issues, or the servo horn is damaged and needs replacing, contact <support@robotical.io>.