RCCrawler Forums - How to diagnose servo failures and (hopefully) repair them.

If your servo dies you're usually not going to repair it, about the only thing people are comfortable repairing is the gears. It's a shame because with a little know-how and the right equipment it is pretty easy to repair servos. I've been repairing servos for years and here are some pointers to get you going.
If you have 2 or more broken servos of the same type you can quite often get one of them working using parts from the other ones. One could have dead motor and the other broken gears, then you just move the unbroken gears to the servo with the working motor. Sometimes you can use parts from other types of servos, even other manufacturers servos, to get a working servo. You have to realize that if you mix and match components from different servos, the servo specs will change. The servo might lose some of its speed, torque or accuracy. But you'll have a working servo, which is better than broken one.

Over half the failures in servos can be diagnosed by looking closely at the servo, listening to it or smelling it. Simple Multimeter and 4-7 volt batterypack can help you to diagnose the rest.
Always start repairs by testing the servo to confirm that the servo is broken. I use old servo tester made by Multiplex (now owned by Hitec) but radio and receiver with battery-pack is very good solution. Start by testing the test equipment with servo that is known to be working. It's very annoying to spend hours trying to find faults in a servo that is working fine but the batteries were flat.

The servo can be broken in the following ways, starting with the most common ones:
A. You can't fasten it to the chassis: Broken “ears” on the case.
B. It's dead: Electronic failure.
C. It makes sounds but the servohorn doesn't move, it moves erratically or the servohorn rotates without resistance from the servo: Gears are broken.
D. It jitters or glitches: Usually electronic failure but can be worn gears or other problem with the gears. Sometimes the servo is “not broken”. Some servos always jitter or glitch with one receiver, ESC or even batterypacks but not with others. To make it even more fun, sometimes they only behave like this on the track. You could say that some servos don’t like some receivers, ESCs or batteries. This is more common than most people realize, which is why I recommend that people buy servos from the same manufacturer as their radios.
E. It doesn't center properly: Could be electronic failure (position sensing resistor, usually called potentiometer or pot for short) or worn/bad gears.
F. It only moves to one side but doesn’t center: Usually electronic failure, broken transistor on the PCB.

The diagnoses. First you have to take the servo apart. Almost all servos are held together with 4 screws at the bottom of the case. The bottom is a lid on the electronics and the top is a cover for the gears. The center is the case with the motor and the position sensing resistor. Remove the lid, the cover and the gears, noting how the gears fit together, and set them aside.

Servos can be divided into 3 major components:
1. The case: Always look very closely at the servo, before and during disassembly. Cracks or deformed, maybe even melted plastic can give clues where to look for faults. Jewelers loupe or magnifying glass and good light are essential.
The most common case failure in servos is when the mounting "ears" brake off the case or the gear cover. The best solution is to replace the broken part. But, depending on the breakeage you can sometimes use washers under or doubletape to fasten the servo to the chassis and keep on using the servo. Look ma, no screws .
There are few other faults that can occur with the servo-cases: Sometimes the "seats" for the axles, in the gears, become oval instead of round so that the axle, and gears, can move sideways. I've found this to be quite common in certain cheap high-torque Metal Gear servos, it can cause the servo to jitter and premature gear failure. The best solution is new case.
2. The gears: This used to be the most common cause of failures in servos but with the increasing popularity of metal gear servos, the case and the electronics have become the weak parts. If you can move the servohorn easily from side to side or the servo makes noise when being tested but the servohorn doesn't move, then you most likely have broken gears. Clean the lubrication from the gears. I usually clean the gears using motorspray or white spirit but chemicals can sometimes damage the gears so the correct method is to use clean soft tissue. Look closely at all the tooths on the gears. I use jewellers loupe with 12 times magnification but simple magnifying glass works too. Look for broken or deformed teeth on the gears. The one closest to the motor (the 1st gear) is the one most likely to fail. Even in metal gear servos the first gear is often made of plastic. If the servo has the option of metal or plastic gears it's common to have the same 1st gear in the sets so you can save money by buying the plastic replacement gear set. Futaba even sells 1st gears seperately for some of it's servos. The splines on the output gear can sometimes "go bad" but that is almost always because the user used incorrect servohorn and forced it on the output gear. I have on few occations been able to "fix" servos with gears from other types of servos but you usually need new gear set. Always use proper lubrication (grease) when assembling servogears and for those that don’t know, WD40 should never be used as a lubricant.
3. The electronics: The electronics can be divided into 4 parts:
3a. The Servo-wire. If the plug is broken or the isolation on the wires is damaged it's usually best to replace the whole wire (wireloom?). A servo-wire is cheap from you local hobby shop but I usually cut one off a servo where I've diagnosed the PCB as dead. Three solderjoints are all that hold the wire to the PCB. Note the position of each wire and with three quick jabs with finetipped soldering iron you have the old wire out. Pretin the ends of the new wire and with three quick jabs you've attached the new wire. I've said three quick jabs like it's something simple, and it is, but you need practice. It's like learning to ride a bike, you'll fall of it quite often before you get the hang of it. Use good fine tipped soldering iron and quality solder. Do not use soldering gun or wide tipped soldering iron and the junk solder that comes with most soldering irons is only fit to pollute our enviroment. Go to local electronic repair shop and ask them where to buy good soldering iron and solder for beginners in electronic repairs. Then practice on a broken servo.
3b. The PCB (Printed Circuit Board) most commonly known as the green electric thing with all the little parts on. This is usually not repairable. If any parts on it are burned or if the unmistakeable smell of cooked electronics infuses the air it's most likely dead. Use this servo for parts.
3c. The position sensing resistor (potentiometer or pot). The pot is connected to the output gear and has three wires going to the PCB. If it fails it can cause all kinds of strange behaviour in the servo. Bad centering, jittering and glitching being common. I almost never replace it and if I do I always replace it with new one from electronic parts shop. It's nearly impossible to test properly but if you want to try, it's best to use analog ohm-meter. Connect one test lead to the middle output and the other test lead to one of the end outputs. Move the resistor from end to end and watch the needle on the meter. It should go smoothly from end to end. Any jerkiness of the needle on the meter while moving the resistor smoothly means that the resistor is bad. But even if the needle moves smootly the resistor may still be bad. Remember to test both end outputs. If you're not sure if the resistor is good it's best to replace it. Some people say that contact cleaner can make the potentiometer like new. In my experience contact cleaner can only fix a problem for short while. In my opinion it would be more accurate to say that it can mask the problem for short while.
3d. The motor. To test the motor find the outputs from the PCB to the motor. (It’s important to make sure that you’ve removed the gears from the servo.) Plug the servo into the test equipment and set it so that the servo is supposed to move from side to side (a friend or girlfriend can help here. The Multiplex servotester can do this automatically). While the servo is supposed to move from side to side, measure the DC voltage over the motor terminals. If you get alternating plus 4-7 volts to minus 4-7 volt, on the meter, but the motor doesn't move, the motor is dead. The good news is that all the other parts of the electronics are working and if you can't get new motor you can use the PCB as a switch. For example, connect diodes to the motor wires and you have brake lights. Confirm that the motor is dead by removing it from the servo and connecting 4-7 volts directly to the motor terminals, old 4 cell pack is perfect for this. If the motor spins you did something wrong, doublecheck everything.
If you don’t get any readings on the meter, the PCB or test equipment are not working. Doublecheck that the test equipment is working and the servo is correctly plugged in. If you still don’t get any readings on the meter the PCB is dead, use this servo for spares. Confirm that the motor is OK by removing it from the servo and connecting 4-7 volts directly to the motor terminals. If the motor is dead it may have caused the PCB to fail. If it’s working perfectly you can use if for spares.

I hope someone has found this useful. If you have any questions I'll try to answer them here.

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Servo parts: Cover, Gears, Case, Motor, Potentiometer, PCB, Servowire.

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My Multiplex servotester. I'd be lost without it.

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Cheap servocenter unit. Can be used as servotester.

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Radio used as servotester.

January 2009 update. Slight rewrite and pictures added.