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Odd servo behavior

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Old John B20/02/2017 16:19:08
109 forum posts

I bought a 92inch span Moule from a friend complete except for the radio. Put in a Futaba 6 channel S-FHSS. There are a mixture of servo makes in the machine the wings had Hitec 625's for the ailerons. Did all the necessary checks and had the first flight. It was obvious that all was not well but got her back safely and did more checks. The port aileron was vibrating and occasionally waving to us. Put in a brand new Hitec servo, no difference.Changed the radio to a futaba 6 channel FASST. Still no difference.Now I changed ALL the wiring in the plane, tried again,and still the vibrating and waving.Had I bought a duff new servo? I didn't have another Hitec servo handy so put in an ordinary Futaba 3001. It worked!!! WHY. Both servos work out of the plane. It would appear that Hitec and Futaba don't mix but I have been modelling since 1974 and have never met this problem before. Has anyone had this happen before and is there a simple explanation that I have missed. Allthe best brains in our club have not come up with an answer.

ben goodfellow 120/02/2017 17:17:58
1069 forum posts
41 photos

i had something similar years ago with an ff8 super / spekky dm8 module using hs85 servos. the left aile twitched alot . if i remember i think it also twiched when the elev and rudd were moved to. i did solve it in the end , but for the life of me cannit remember how .....ill add that no mixing was in effect it was a glitch / problem, with the radio

Edited By ben goodfellow 1 on 20/02/2017 17:19:24

cymaz20/02/2017 17:34:06
9177 forum posts
1180 photos

It does sound like a duff servo. I bought a Hitec 645 mg ages ago from Blackburn models...duff out of the box. I returned it and they sent me another without question..excellent service.

You've tried the servo in another plane?

Edited By cymaz on 20/02/2017 17:35:08

ted hughes20/02/2017 17:37:47
466 forum posts

I have been told it is nomal for digital sevos to chatter and jitter about when the Tx is close by.

I had a Radian Pro that did it with all 4 wing servos, in the air all was fine

ben goodfellow 120/02/2017 17:52:14
1069 forum posts
41 photos

digis buzz and move a little , but should not jump much they calm down once loaded up . cant think why tx proxi would make a difference. also i think with my problem i went to a 7c that i had and no probs after that

kc20/02/2017 19:35:49
6414 forum posts
173 photos

I suggest inspecting the extension leads and the fit of the servo plugs might provide some answer. Hitec don't have the 'tabs' that Futaba have do they? That seems to suggest a reason why two servos of the same make don't work but a Futaba does.

Jon Laughton21/02/2017 04:32:43
1206 forum posts
72 photos

Had a similar problem with an in wing aileron servo on an aerobatic model...ended up changing the servo & extension lead to resolve it but could never figure out why it happened but suspect perhaps some very fine carbon dust or similar was causing an intermittent short somewhere between the Rx and servo...

Denis Watkins21/02/2017 07:19:50
4318 forum posts
104 photos

I think we are very lucky continually setting up our models, attaching the wings and thus our aileron servos on arrival at the field, and disconnecting at the end of the day. There lays a chance to stretch cables, bend connectors and generally wear out the joint. Miraculously some last for years, but the moment they slacken, then anything can happen with both power and signal.

Again, thankfully these arrangements do work usually and for a very long time. Now "stray capacitance" is another ball game, and I suspect is responsible for some " odd servo behaviour"

Capacitance can and does occur in flat plates close to one another, and also in cable?

Edited By Denis Watkins on 21/02/2017 07:20:43

Bob Cotsford21/02/2017 10:18:27
8380 forum posts
463 photos

As the 625s and 3001s are not digital servos the 'digital chatter' doesn't apply. I've found one batch of 645s to be a bit queasy if fed over 6.0 volts eg 5s NiMh or 2S LiFe unregulated, but fine on a regulated supply. I suspect the servo amps might be susceptible to electrical noise when used with long extensions, which only gets worse as they jitter and create their own noise on the supply. Try fitting ferrite beads on the longer extensions, might be worth a go.

edit - if the ailerons are fed via a Y-lead in the cabin, try changing that.

Edited By Bob Cotsford on 21/02/2017 10:19:58

Geoff S21/02/2017 12:38:16
3576 forum posts
14 photos

Further to Bob's suggestion of ferrite beads I would recommend twisted wire extension leads rather than than then usual flat ones. 92" is quite a big span and leaves servos very open to interference along the wires.

My biggest model is a Maher's Thunderbird (84" ws) and as it was also my first petrol powered model (Zenoah 26 magneto ignition) I was quite nervous about possible interference and one of the measures I took was to use twisted servo extensions. The only problems I've had with it concern the nut holding the transmitter


Edited By Geoff Sleath on 21/02/2017 12:38:58

Martin McIntosh21/02/2017 21:27:42
3301 forum posts
1169 photos

Don`t know those servos or the battery voltage you are using but I had similar problems when using JR 591`s on 6V. They worked OK but the rest of the servos were decidedly unhappy and flapped up and down.

Old John B05/03/2017 14:58:53
109 forum posts

Thanks chaps for all the advice etc.. Haven't been able to put any of it into practise as the weather has been foul. Also I'm not having much luck with electrics for although the computer is supposed to remember all the details, when I logged in , it just didn't want to know. Anyway changed the password and we 've got here and when I can finally get flying I will let you all know what happens

Trevor05/03/2017 15:28:35
421 forum posts
57 photos

Just one other remote possibility that hasn't been mentioned so far - I once spent a frustrating few days trying to sort a similar fault, only to discover that the radio was operating on a 10ms frame rate, which is not a good idea for analogue servos. The model has five identical (Hitec) servos and strangely only one of them was showing symptoms of distress. Changing back to a 20ms frame rate fixed the problem.

Good luck.

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