Beyond just plugging it in and seeing if it moves, how can I check whether a servo is in good condition. Superficially, the servos on my Legionnaire look and move fine. Ditto with several other servos I have stilling around from prior models. But are there some clever tests I can do check them out more thoroughly?

Of course, I could just buy new servos every time I have some doubt, but that seems a bit wasteful.

As far as I can tell, everything is OK inside the servo. No missing teeth and everything engaging. The only weird thing, which means nothing since I know nothing about servo mechanics, is that when the main gear (the one that rotates the servo arm) is lifted out, the other gears actually all spin with the stick at neutral. This stops as soon as the main gear is replaced. Is this normal? I'm tested the arm "under load" so to speak and am not getting any slippage.

As I said everything looks and feels normal to me. And yet, I can't explain how the servo arm ended up in a completely different position after the crash landing. Could the crash have effected the servo's circuit board? Honestly, it wasn't that much of crash. The servo arm would have been "stressed" during the crash and that stress transferred to the gears but I don't see how that could have causes a glitch in the servo position. I certainly haven't been able to duplicate the glitch.

Hi: You can take servo testing to an extreme, but requires test equipment.
What you are experiencing is normal because the servo electronics is sensing a small error when you pull the main gear out. Doing this disconnects the physical connection between the servo potentiometer that revolves with the main gear. Normally a small error would result in the motor rotating the main gear/potentiometer so that the error is "nulled out".

In my experience, the main failure point in a servo is the the gears and secondly the servo potentiometer. Replacing the gears is obvious if you detect any noise or roughness in the servo response. The surface of the potentiometer (pot) will wear with time from the wiper arm contacting the surface. The result is "hunting" and a servo that does not return to the same point every time. Replacing the pot requires soldering technique and a source for the spare parts.

I hope this answers your question?
Best regards Charles Johnson. N7JWH

Thanks! That's very helpful. Obrigado!

The best way to test a questionable servo is by placing it in a rectangular storage device and if it hops from there into your airplane, use it. If not, replace it. :-)

Excellent advice! Would a circular filing cabinet work as well?