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Twitching Servos

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Old 09-02-2018, 03:09 PM
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Mini Pistons
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Default Twitching Servos

I have a nitro P51 Mustang with an Evolution Motor (idk what size it just says Evolution & has an: a stamped in casing.) Iíve got a JR R700 7ch reciever (Batt., Aux1, Aux 2, Gear, Rudd, Elev, Aile, Thro) running off a JR Propo 4N600 4.8V 600mah battery. 5 Servos are a JR NES-537 and 1 servo is a Command dual B.B. servo CS-67 (that runs my 2 flaps on my receivers gear port) My transmitter is a JR XF421 EX running a XF-421EX 8N-600 9.6V battery. I got the plane used. Plane probably sat for a bit. Engine runs great. I replaced all fuel lines etc.
MY ISSUE:
When outside & I turn the transmitter on first then the receiver with fully charged batteries & semi drained batteries, I get a lot of servo twitching & chatter. Mainly my rudder, ailerons, flaps. I have tried unplugging 1 at a time but it still exists until all 3 are unplugged. I have tried seperating my transmitter from my receiver (various distances & it still happens.) I have tried separating my power wire & antenna. I have even disconnected the servo arms but the servo still twitches & chatters. This drains my receiver battery in approx 10-15 minutes. Crystals seem ok since only prob. W a few servos. Are the servos bad, receiver bad?
Im new to RC planes so please help!
Also anyone know the size my motor is?
I canít fly her till I get the servos to calm so this is the hold up!

Last edited by Mini Pistons; 09-03-2018 at 06:05 AM.
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Old 09-02-2018, 04:29 PM
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Common occurrence due to a variety of things. Only a couple of which are due to bad equipment. First of all, where are you experiencing this? In your work room? Basement, spare room or garage? Or at the flying field?

I'm guessing you bought the radio used since it's an older 72 MHz system. Might want to send it in to be serviced and checked out. How old are the batteries? Chattering servos will drain the Rx battery but 10-15 minutes suggests that battery has issues.

One thing that will cause this is being in an enclosed room. Basically the Tx signal bounces off the walls and dozens of signal reflections hit the Rx all at once. It's called swamping the Rx. The same thing can happen out doors if the batteries are fresh and the Tx is too close to the Rx. And by too close I've seen this happen out to 6-7 feet separation.

First test. Disconnect the CS-67 servo and see what happens. Sometimes different servos simply will not play nice together.
Second test. With fresh batteries take it outside and turn everything on. If it twitches, start walking away with the Tx and see how far away you are when the twitching stops. Shouldn't be too far.

Last test. Again outside, discharge the batteries a little. Turn things on and just let them sit for a while. Then see how things are. You should notice you can have the Tx and Rx closer without the twitching.

I did some research on the servos. Here's some specs:
JR - NES-537 Torque:4.8V: 40.00 oz-in (2.88 kg-cm) 6.0V: 49.00 oz-in (3.53 kg-cm)

Hobbico CS-67 Torque:4.8V: 43.00 oz-in (3.10 kg-cm) 6.0V: 52.00 oz-in (3.74 kg-cm)

These are old school analog servos. Nothing wrong with that but they could simply be past their time and need replacing.
My next question is how big is the model? Wing span and approx weight. And did these servos come with it? My concern is at the very least these servos may need some new gears and a pot cleaning. Both can be done by you or you can pay someone to do it. Worse case they may simply be worn out.

Alternative would be to simply replace the servos. Here's a compatible one of similar design and a bit more power. New Servo At $13 each they'd be a good price. And by the time you purchased new gears and messed with learning how to clean the pots and such, buying new would be far easier and probably better. But I'd like to know more about the plane itself before sending you off to buy new servos. Just in case a different or more powerful servo is called for.

Last edited by Appowner; 09-02-2018 at 04:32 PM.
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Old 09-03-2018, 07:01 AM
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Common occurrence due to a variety of things. Only a couple of which are due to bad equipment. First of all, where are you experiencing this? In your work room? Basement, spare room or garage? Or at the flying field?

I'm guessing you bought the radio used since it's an older 72 MHz system. Might want to send it in to be serviced and checked out. How old are the batteries? Chattering servos will drain the Rx battery but 10-15 minutes suggests that battery has issues.

One thing that will cause this is being in an enclosed room. Basically the Tx signal bounces off the walls and dozens of signal reflections hit the Rx all at once. It's called swamping the Rx. The same thing can happen out doors if the batteries are fresh and the Tx is too close to the Rx. And by too close I've seen this happen out to 6-7 feet separation.

First test. Disconnect the CS-67 servo and see what happens. Sometimes different servos simply will not play nice together.
Second test. With fresh batteries take it outside and turn everything on. If it twitches, start walking away with the Tx and see how far away you are when the twitching stops. Shouldn't be too far.

Last test. Again outside, discharge the batteries a little. Turn things on and just let them sit for a while. Then see how things are. You should notice you can have the Tx and Rx closer without the twitching.

I did some research on the servos. Here's some specs:
JR - NES-537 Torque:4.8V: 40.00 oz-in (2.88 kg-cm) 6.0V: 49.00 oz-in (3.53 kg-cm)

Hobbico CS-67 Torque:4.8V: 43.00 oz-in (3.10 kg-cm) 6.0V: 52.00 oz-in (3.74 kg-cm)

These are old school analog servos. Nothing wrong with that but they could simply be past their time and need replacing.
My next question is how big is the model? Wing span and approx weight. And did these servos come with it? My concern is at the very least these servos may need some new gears and a pot cleaning. Both can be done by you or you can pay someone to do it. Worse case they may simply be worn out.

Alternative would be to simply replace the servos. Here's a compatible one of similar design and a bit more power. At $13 each they'd be a good price. And by the time you purchased new gears and messed with learning how to clean the pots and such, buying new would be far easier and probably better. But I'd like to know more about the plane itself before sending you off to buy new servos. Just in case a different or more powerful servo is called for.



Wingspan: 58Ē
Weight: ?? Approx. 5 + lbs.
I am experiencing the issue outside. I have tried seperating the transmitter & receiver at various distances with issue still existing just goes from twitching to jitter. My ailerons do full movement back & forth at times. I would like to get by without having to replace anything till I get to fly the plane a time or 2 to try it out. I figured on a new battery already and found it online for about $15. Iím not sure if this is how the plane came setup (Iím thinking so because just about everything says hangar 9). I donít think itís the Hobico CS-67 servo causing it because the issue is still there when that servo is unplugged it just settles down a little. Iím trying to get to 10 posts so I can post pictures (dislike that rule by the way)

any idea
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Old 09-03-2018, 07:30 AM
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Isolate the switch harness. Try plugging the receiver battery directly into the receiver. Good luck.
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Old 09-03-2018, 07:38 AM
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I'm thinking that your batteries are in need of being replaced. With a good RX battery you should be able to operate the system without flight loads for a couple of hours. The glitching could be caused by " dirty voltage " that the battery is providing especially if the internal resistance of the battery is high enough to intermittently short. Either way you are going to have to replace the batteries and there is a good chance it may fix the glitching issue. I personally stopped using 4.8V RX batteries 25 years ago and suggest 6V. That however with 72 MHz is just a personal preference but IMO a must with 2.4 ghz.

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Old 09-03-2018, 07:54 AM
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Originally Posted by GREG DOE View Post
Isolate the switch harness. Try plugging the receiver battery directly into the receiver. Good luck.
Agreed! I'd forgotten about suggesting that. There's an issue that occurs when batteries are left connected during long term storage. It's called black wire disease. Basically the black or negative wire corrodes badly and spreads to everything the battery is connected to. The switch being the first victim. The corrosion will cause your twitchiness.
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Old 09-03-2018, 07:56 AM
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Another thought. If you have a friend with a radio, try plugging your servos into his Rx and see if they still jitter. If not, then we can probably eliminate the servos as a source of the problem. And the ones you have should be fine for the plane you have. Assuming they're in good condition.
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Old 09-03-2018, 08:46 AM
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Originally Posted by Mini Pistons View Post
I get a lot of servo twitching & chatter.
Yep. Remember this sound very well. Been 35yrs since I heard it last.
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Old 09-03-2018, 06:52 PM
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Sounds like batteries. Maybe the charger doesn't work or like some others mentioned, dirty voltage. Sometimes bad batteries are left in the plane or transmitter, and you get the deadly green negative wire. The old servos sometimes have dirty pots, and some contact cleaner sprayed inside the servo arm screw hole gets through. I would put my money on the battery though. Check with a load test/discharge cycling...
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Old 09-03-2018, 08:40 PM
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My question on the corroded battery wire, if that was the issue would it only affect 4 of the 6 servos instead of all of them?
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Old 09-04-2018, 07:15 AM
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It seems logical but it could just be that the jittery ones just can not handle the low voltage. I have seen some servos that jitter when the voltage is low, and some that just slow way down. I would still do a battery cycle and see how many mah are available. If you want to get more posts, to get your ten posts, just send them quickly like ttt, Submit. Then you can send your pics. I would guess the motor is the .40 or .46. There are lots of them around. You can measure the piston diameter and stroke. Ti R squared x stroke. You would need a vernier and have to take the head off if you are comfortable doing that.
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Old 09-04-2018, 09:25 PM
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First, if the battery packs are the originals they are over 10 years old and need to be replaced. Second if your transmitter has loose parts (diodes) like you said in the other thread you need to replace your radio.
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Old 09-05-2018, 05:04 AM
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Originally Posted by Mini Pistons View Post
My question on the corroded battery wire, if that was the issue would it only affect 4 of the 6 servos instead of all of them?
Possibly! In 43 years as a radio tech, I've seen some weird things which eventually we were able to explain. But there are a few that I still shake my head over.
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Old 09-05-2018, 07:52 AM
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Start by changing the batteries in BOTH the transmitter and receiver. You may be getting a weak signal from the transmitter to the receiver that it's trying to resolve. As already stated, if the receiver pack is dying in 15 minutes or less, that pack is shot and won't hold up to even one flight since the servos will suck a lot more current when being used than when sitting on the deck. If you still get twitching with all new batteries, then it's time to do a bit more troubleshooting to find the issue
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Old 09-08-2018, 05:59 AM
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I'm with the others that the most likely cause is a bad battery or switch. Before you spend your money though...

If you are new to the hobby, a P51 is a very poor choice for learning to fly. The H9 Mustang is a sweet plane and will be a lot of fun once you have some skills. But your chances of flying it successfully as a beginner are practically zero. So I'll suggest taking the battery and switch out of the plane and tossing them, then hang up the plane for a while and get a trainer plane. While you are at it, get an instructor if you don't already have one. With a little luck, you will find a trainer with a more current 2.4ghz radio included, or you can budget for one and get it when you can.
On figuring out which engine you have, you don't need to take it apart and measure the bore. Look at Evolution's website and get the external dimensions of their various engines. Then measure the head and crankcase length of yours to see what you have. That said, if the plane has been sitting for a few years, the engine could be gummed up with old castor oil. If it is, it will need to be disassembled and cleaned.
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