Odd servo centrepoint

[Groomi]

Hello all, please help determine if I'm just being thick here...

I use Savox 1251mg servos in all my buggies. However, one has a wildly different neutral point to the others. All the others I can plug in, turn on the transmitter and car and they sit motionless at their centrepoint, but this one will jam to one side, well beyond the transmitter trim settings.

I've opened up the casing and removed the gears so I can see the servo is just endlessly spinning unless I move the stick almost fully to the right.

How can I set the servo to the same neutral point as the others?