How I fixed a cheap servo

For my first attempt to work with servos, I got a pair of cheap TowerPro SG-90s from ebay. I went through all the sample code and one thing just wouldn't work - one of the servos worked fine but the other didn't go through its entire range of motion and made a clicking sound on side.

Luckily there are a bunch of videos out there showing what's inside a servo.

Imagine a clock; when the servo is first powered up and has no input, think of the horn as pointing to 12 o'clock. The good servo pointed to 12 and turned left to 9 and right to 3 no problem. The bad one would always start off pointing to 10. Then it would go to 3 OK but bind when it tried to turn past 9. (There is a small peg on the gear to stop it; the peg should be pointing to 12 at power-up.)

This wasn't a code problem, so I opened them both up to see what was going on. In the bottom of the bad servo, there were three small wires soldered to tiny terminals. And there was a solder bridge between two of them, which wasn't there in the good one.

A quick snip with the diagonal cutters and the problem was solved.

Good shooting. It was probably the feedback pot.