Hi! This is my first mail on this site, so be gentle!

I have a problem with one of the servos on my glider: it works fine, then suddenly pitches up or down and stays there while juddering away about that new point. I can still apply up and down inputs, but the trimmed position is a good way up or down and the whole elevator vibrates.[&o] I've tried swapping receiver channels and it continues with that servo only so it's not a transmitter or receiver problem.

Makes flying a bit tricky!

Any ideas what is going on here?

There could be several problems. Was the servo ever damaged in any manner? You could have a gear damaged (one or more teeth sheered off a plastic gear), an intermittent electrical connection internal to the servo or an intermittent open in the signal lead. From your description, it does seem that the problem is with the servo and not the other components. When the servo is acting up, does flexing the wires at the base of the servo do anything? Does a gentle shaking or rapping on the servo case make it settle down? It is not to difficult to open the servo up and look at the gear train.

Thanks Rodney. I've opened up the servo and it still happens with the servo motor disconnected from the gearing - it sits on the table and jumps about like mad! I tried flexing all the cables while this was happening, but with no change. However, when I turn the transmitter off it stops immediately, then starts when I switch back on. A mystery.

Thanks for finding the time to reply!

What servo is it? If it is a cheap one, I would just throw it away. No use spending time trying to fix a $15 servo that could cost you a ton more than that if it goes nuts in the plane. The S-148 or equivelent servos are really cheap. I don't even open them any more. Any problems, they go in the garbage.

I agree with Flyboy. If you can't find the problem and it's not worth getting repaired, then take out the motor and gears and put them in your stuff box and toss out the rest. You shouldn't even be flying this plane if it has a known problem like this.

You can't disconnect the servo motor from the gear train and expect anything but bizarre behavior.

The whole point of a servo is to run the output gear to a specific position based on your control input. That position is sensed by a potentiometer, usually connected directly to the output gear. The motor runs the gear train until the potentiometer reaches the desired position. If you disconnect the motor from the gears it can't move the output potentiometer and will just run at full speed. It will probably change direction (CW or CCW) as you move the control stick.

Look at those gears more closely! I bet you have missing or damaged teeth somewhere in the gear train. You can buy replacement gears for most servos. Check on the Tower Hobbies web site by searching for "gear set". You might be surprised at how many sets are available!

dear friend in all flying machine the saving is the base of the misery, so change that servo please!!!

Sounding to me like the position sensing pot has gone bad... or is flopping inside the servo case.

Definitely an unsafe servo to use whatever is wrong wth it. Replace before it puts your airplane in the dirt. (or worse... it crashes the plane ito you or someone else.)

Get a Tower TS-53. Peel the label and you raise its value $5 (its a Futaba 3003 with a label stuck on. TX-53 = $9... S-3003 = $14 Tower pays you to let them label the servo...)