After going through piles of used servos to see what needs fix'n, I figured I'd write a little Servo Survival Guide for anybody new to servo repair like I was and am. Most of this info is based on the Hitec HS81 servos, but for the most part when you've seen one servo you've seen them all. You'll also find a few Tower Pro SG90 (Also called HXT900...a Hitec HS55 clone) tips as well. By the way, this servo is a great little servo for the price (around $4). I've read nothing but good things about it, some people even liking it more than the HS55. It is said the United Hobbies HXT900 version is of better quality, though I haven't compared it to the SG90 to see. For the most part, I use Hitec HS81 servos in my larger builds or ones with more speed and/or larger control surfaces, while the HS55 or SG90 is what I use in slower, smaller, or lighter builds. These are probably the two most popular servos in the Park Flyer universe.

WARNING: I'm no expert. Following my advice could destroy your servo or damage some other component. Use at your own risk and don't cry to me if you burn the house down or lose a finger. Always remember to unplug the battery that is powering the RX, unplug the servo, and turn off the radio before doing any work on the servo.

PRE-TESTING SET-UP: You want to check that your trims and sub-trims are at neutral, the radio is set up for full throws and full rates (125% on JR radios). This will insure that the servo is cycling through it's full gear travel and that any bad pots or stripped gears will show themselves under these conditions.

TROUBLESHOOTING STAGE ONE:

Step 1: Plug the servo into an RX, turn on your TX, and hitch the battery up to the speed control or whatever you are powering the RX with.

Step 2: Move the stick on the TX fully in each direction to see if the servo is moving like it should.

Step 3: If you don't hear any gears stripping and the servo is moving properly and smoothly, grab the servo arm lightly but firmly and put some resistance on it in both directions as you move the TX stick fully in both directions. Many times a stripped gear won't show it's self until a load is put on it or the servo is moved completely in one direction. If any gears are stripped move on to REPLACING STRIPPED GEARS.

Step 4: Set the servo onto a table and look down through the top hole on the horn. Mark the table directly below it so that you can see the mark through the hole's line of sight. Now quickly move the TX stick all the way one direction and release the stick. The horn's hole should return to the marked spot. Move the stick quickly in the other direction all the way and once again release the stick. Repeat this process several times to confirm the horn (which shouldn't be held or touching anything) always returns to the same neutral point. If it doesn't then the POT is more than likely dirty. Move on to TROUBLESHOOTING STAGE TWO.

Step 5: Slowly move the stick from neutral all the way to full throw in one direction and watch the control horn on the servo. It should smoothly and slowly move in that direction with no jumping or jitters. Slowly return the TX stick to neutral and check to see if the horn (again, shouldn't be touching anything) is still returning to neutral properly. Repeat the test in the other direction. If the servo doesn't work smoothly or return to neutral this more than likely is a dirty POT. Move on to TROUBLESHOOTING STAGE TWO.

Step 6: Slowly move the servo all the way in both directions with the TX stick and make sure it looks like the servo is traveling an equal distance from neutral both ways. If it isn't then make sure your throws, sub trims, and trims on the radio are set right. This condition is more than likely a bad or dirty POT but can be a stripped gear. Go down to TROUBLESHOOTING STAGE TWO.

Step 7: Hold the servo in your hand (without touching the control horn) and move the TX stick around, holding it still in random and various positions for a few seconds at a time. Also try holding it still at full throws in both directions. The servo shouldn't buzz, vibrate, or make noise in these various spots when the stick is being held still. If it does then more than likely the POT is dirty. Move on to TROUBLESHOOTING STAGE TWO.

Step 8: If the servo buzzes, doesn't always move, or especially gets hot then this could indicate a seized motor rather than a dirty pot. Jump down to FIXING A TIGHT OR FROZEN MOTOR.

REPLACING STRIPPED GEARS:

Gear sets are fairly cheap at around $3 to $5 for nylon sets. Still, if you have an HS81 and want to install metal gears in it to prevent further stripping, pick up a pack of HS81MG gears. You can also save money by saving all the good gears out of a servo that has stripped. Look at them under a magnifying glass. More often than not you'll find that the top or second from the top gear is the one that has went bad. Throw the good ones in a bag and hold onto them for replacing a bad gear in the future.

While HS81MG servos have a notched shaft that the top gear must mate to properly, some servos such as the SG90 Tower Pros have a cogged top that the top gear can fit onto in various positions. If this gear is stripped you can break away the two stop tabs on it's bottom and position the gear so that the stripped portion is pointing directly the other direction from the other gears. When the servo is moved to full throws chances are this bad portion of the gear will not engage the others. The same can be said for the other gears on both servo types, but only putting it back together and checking movement is going to tell you if it's going to be OK. This should only be done in a pinch when no other replacement gears are present, but involves high risk of having the servo fail so be aware.

Before taking the servo apart there is one last thing to check to make sure it is really stripped. Often the control horn is slipping on the top gear due to the horn being bad or it being the wrong horn. Horns are usualy very specific to the model and brand of servo they are used for, featuring different teeth patterns than others. Try replacing the horn with one that you know is meant for that servo and tighten the screw holding it snug.

UNPLUG THE BATTERY FROM THE RX AND UNPLUG SERVO! Turn off radio.

Once you replace the gears you should still go back to TROUBLESHOOTING STAGE ONE and do all the steps again to look for further servo problems.

Step 1: Remove the screw holding the servo horn and the horn it's self. Always hold the servo horn and apply equal force in the other direction as you tighten or loosen this screw to avoid putting pressure on the gears inside. Next, remove the four screws on the bottom of the servo. It's better to loosen (or tighten) these four screws in a zig-zag fashion, much like tightening lugnuts on a car. This helps to insure that the servo case is being closed (or opened) with equal pressure. The bottom and top cover plates should now easily pop off. Be careful that the top cover doesn't come off with the gears and/or a metal rod with it. You want them to stay in place to help guide gear placement.

Step 2: Pull the gears off one at a time from top to bottom and place them on a table, staggered from top to bottom and left to right a few inches from each other to indicate the sequence and direction they were on the servo. Make sure the middle metal pin stays in place and you don't loose it.

The metal gear on top of the servo is the motor's output shaft. If you spin it with your finger you'll find it to be very loose. Don't let this make you think the metal gear is loose. It's just very easy to spin this tiny motor. The middle metal pin isn't attached to anything inside the servo, it just seats into a hole to hold it. On the inside of the top cover is also a hole for it to seat in. Nine times out of ten if the servo doesn't want to snap back together the pin isn't fitting into this hole for some reason. The other, slightly larger, metal shaft on top of the servo is the POT shaft. When the motor moves it this POT also moves, and via feedback it tells the servo what it's position is. Notice a flat spot on top of this shaft. This is where the top gear that the control horn screws into has to seat properly on in order for that gear to not slip on the shaft. If you haven't noticed, all the other gears are not secured to the two metal shafts in any fashion. They simply spin on the shafts when moved.

Step 3: Take your new gear set and install the gears, using the old ones position on the table as your guide, starting from bottom to top. When you get to the top gear make sure it lines up with the flat spot on the POT shaft and press it down firmly. There shouldn't be any large gap between this gear and the next one down or you've probably not seated it properly. Grease the servo gears with the right greese if you feel the need to do so.

Step 4: Place the top cover back onto the servo and make sure it is completely flat with the servo case. If it isn't then more than likely the top middle pin's top portion isn't seating into the top hole, or the top gear isn't installed properly (it can bump the top of the cover if it isn't sitting right. Normaly the "fat" portion of this gear will be pointing towards the other gears.

Step 5: Place the bottom cover back onto the servo and re-install the four screws. Do not overtighten the screws or they will break that bottom cover's four mounting holes. Again, tighten them in a zig-zag fashion (top left, then bottom right, then top right, then bottom left). If one or two of the bottom plate's mounting holes are broke still use all your screws. These will hold the top cover on, which is where things need to be snug to avoid gear play and stripping. You can also get by with only having two or three screws available to use so long as the top cover is tight. If not, apply a little CA around it's mounting surface to keep the top cover snug. Replace the servo horn and it's screw. Again, apply force in the other direction on the horn to the screw's direction as it is being tightened to avoid stress being put on the servo gears.

Step 6: Go Back To TROUBLESHOOTING STAGE ONE and start at the beginning of the test steps again.

TROUBLESHOOTING STAGE TWO:

Before going through the trouble of cleaning the POT there are still a few possible easy fixes to the problem.

Before following any of the steps below UNPLUG THE BATTERY TO THE RX, unplug the servo, and turn off radio.

If you find that any of the below checks fix the problem you should still go back to TROUBLESHOOTING STAGE ONE and go through all the steps again to look for further problems that might be present.

On one SG90 servo I thought I had a dirty POT on it since the unit wouldn't move fully in one direction compared to the other and would "hum" as is tried to reach it. It turned out that there was a tooth missing on one of the gears very near full throws and the servo couldn't advance the last bit of travel it needed. It wasn't producing any stripped gear noises so be aware to check for bad gears when cleaning the POT.

Step 1: First, clean the contacts on the RX and servo plug to insure they aren't dirty and causing a random short. You might also try another port (IE: switch from elevator to rudder on the RX) to insure that it isn't that RX port that is causing the problem. This might also indicate a bad TX POT (read below).

Step 2: If the problem is still present, next try another RX with the servo to rule that component out. Also try a different port on the RX (IE: Rudder port instead of Aileron). If this fixes the problem then it suggests the POT controlling the Aileron on your TX is dirty or bad and not the servo's POT.

Step 3: If the problem is still happening, make sure the battery pack you are powering the servos with is fully charged. A low voltage level could be causing the servo to act up.

Step 4: If the problem is still yet present, make sure the TX battery is fully charged, the antenna is fully extended, and that you aren't experiencing any interference. If the problem still persists then move on to CLEANING THE POT.

CLEANING THE POT:

Step 1: Follow all the above REPLACING STRIPPED GEARS steps for taking the servo apart, and don't forget to UNGPLUG THE RX FROM THE BATTERY, unplug the servo, and turn off the radio.

Step 2: Flip out the little circuit board on the bottom of the servo and look inside. There are two components inside. One is the motor (it has two wires going to it). The other is the POT (it has three wires going to it). Notice the location of the three pot wires to indicate how it was sitting. Normaly the three wires are pointing toward the motor side of the servo.

Step 3: Remove the screw that is holding the POT in place. Press on the POT's shaft from the top of the servo and it should slide out towards the bottom of the case. Wiggle the wires a bit to help it come out.

Step 4: Mark the POT top and bottom in various spots to indicate how it lined up if you feel the need to.

Step 5: There is a small "U" shaped clip attached to the middle wire which goes around the base of the POT shaft. Bend this clip's ends back and away from the POT shaft. Pull on the other end of the shaft and the POT should come apart. Notice the position of it's insides as you do this to help you put it back together in roughly the proper spot.

Step 6: Take brake cleaner, alchohol, electrical contact cleaner, or any other product that leaves no residue and place some on a Q-Tip. Scrub lightly the inside of the POT's black area along with the three metal contact points. Also clean the metal "feelers" on the other half of the POT and the metal plates below it. I then like to slightly bend these "feelers" up for better contact with the other surface. Blow on the two parts and inspect them closely for any debri or Q-Tip fuzz.

Step 7: Re-install the two halves back together in the proper orientation and bend the "U" clip back around the bottom of the shaft's groove.

Step 8: Re-insert the POT in the proper position in the servo case and replace the screw. Don't tighten too much.

Step 9: Refer to REPLACING STRIPPED GEARS above for the rest of the steps in replacing the gears and closing up the case.

Step 10: Go back to TROUBLESHOOTING STAGE ONE and follow all those steps. If the servo is still responding like a dirty POT then one or two more cleanings may be required, scrubbing a little harder. One HS81 took me two cleanings before it started working properly. You may also have a tight or frozen motor. Refer to FIXING A TIGHT OR FROZEN MOTOR.

FIXING A TIGHT OR FROZEN MOTOR:

Some times the servo won't move or move very well and seems to get very hot. This could indicate the motor is having trouble moving.

STEP 1: Follow all the steps in the REPLACING STRIPPED GEARS section above to take the servo apart and remove the gears.

STEP 2: Take a drop of bushing oil and place it on the motor at it's rear shaft and front shaft.

STEP 3: Plug the servo back into the RX and turn on the TX and RX. The motor will spin all the time, trying to figure out where it's at. Since there are no gears in it to move the POT it will keep spinning. Allow it to run for ten to twenty seconds or so. This may fix the problem.

STEP 4: Follow the above steps under REPLACING STRIPPED GEARS to put the servo back together, then go back to TROUBLESHOOTING STAGE ONE and do all the steps to look for further problems.

FINAL NOTES: If you can't fix the servo after following the above troubleshooting guide then one final last ditch effort is to closely inspect all the servo wires, including the servo lead that goes to the RX. Look for any broken or cut wire, or any loose solder connections to the motor, pot, circuit board, etc. If the servo is still beyond figuring out it's problem then don't pitch it. You can salvage parts of it for other good servos, or combine the parts from two bad ones to build a working servo. IE: The pot may be good on one servo that has a bad motor, while this might be vise versa on another. A little solder work and you can build one good unit out of it. The little metal gear pins always seem to get lost as well, so it's good to have a few spares around. Keep them on a magnet or you're going to loose them. I know from experience...

Hope this helps anybody who was lost like I was...

thanks =)

Very good guide. Thanks for taking your time writing this to help us out!

MX57

I bought one of the "YTV Servo Cycler & Driver" units from [link=http://www.york-electronics.com/]York Electronics[/link] and that little jewel has been a real timesaver for testing servos. It can automatically cycle the servos for you or set them on center, and only needs a 4.8V battery to do it, so you don't even need to OWN a receiver to setup your plane.
They are one of the Retail Advertisers here on RCU, and they have a great product with this one!

Added more tips, information, and troubleshooting steps to the guide.

A link to some cheap-vs-brand servo comparisons I made up, along with a great servo comparison chart somebody did...

http://www.rcuniverse.com/forum/m_62...tm.htm#6221204

Good article, but would like to add some experience I have earned:

When I have cleaned pot-meters I add a SMALL amount of contact-lubricator to a Q-tips and wipe the "black area" on the pots. Have worked great.

In addition to swapping RX-channels to make sure the problem is not in the RX be aware that also the transmitters potensiometers can be worn out! Causing "glitches" and other odd behavior. The problem will seem to disappear when you check the servo in an other RX-output/channel... People often forget this possible source of failure I had to swap one complete stick on one of my older TX`s some years ago. First I did suspect bad RX, but the problem followed all my planes. [:@]
A good servotester is great.

Also would like to remind people NOT to use Loctite when they mount the servohorn-screw at servoes with nylon or karbonite gear! Only at metal-gear. The karbonite will fail if Loctite is used.