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Old Jul 09, 2011, 07:37 PM
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RCMikeofIndiana's Avatar
United States, IN, Griffith
Joined Sep 2010
80 Posts
Help!
3rd throttle servo fried... Any suggestions

I just fried my third throttle servo in 5 weeks and I can't figure out why. In my U-Can-Do 46 I have Hitec HS-485HB for all servo's, a Spektrum AR8000 receiver and a MPI Gold Peak HR52400AS 6V battery pack.

3 weeks ago in the middle of my third flight the throttle froze fully open. After a couple of minutes it idled down so I could land. Once on the ground it was determined that the electronics of the servo was fried (it moved freely by hand but does not work plugged in). All other servo's were okay. I replaced the servo and sent the original one in for warranty repair/replacement. A week later on my forth flight the same thing happened except the servo never returned to idle. Again the servo's electronics are fired. At this point I found out that my transmitter, DX8, was on the recall list. I purchased a new DX8 (it already had the firmware upgrade). So to be smart (I thought) I replaced the servo, the AR8000 and bound it all to the new transmitter.

Well guess?? First flight today the servo stuck fully open within minutes of take off. Upon landing it was once again determined that the servo was bad. In all cases there was no binding of the throttle linkages at all. From dead center the servo arm never got past 45 in either direction. I have the same servo's in a half a dozen planes without one hiccup (at least from the servos). The battery pack never dropped below 6.2v in all cases.

Any, ANY suggestions would be appreciated.

Thanks.

Mike
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Old Jul 09, 2011, 08:27 PM
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Western KY
Joined Sep 2003
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So you're saying that at full throttle, your servo is not stalled and that the throttle linkage is not pegged in the carb arm. A HS-485HB is a lot of servo for a throttle.

Three sets of servo boards is a lot --- it really sounds like you're driving it too far, stalling it and cooking the amp, especially when running 6V.
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Old Jul 09, 2011, 10:10 PM
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earlwb's Avatar
USA, TX, Grapevine
Joined Dec 2008
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You have the servo stalling out at full throttle. Thus it burns out on you.
Check your linkage and servo throw, adjust so the servo isn't being jammed up on you.
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Old Jul 09, 2011, 10:22 PM
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Quote:
Originally Posted by RCMikeofIndiana View Post
I just fried my third throttle servo in 5 weeks and I can't figure out why. In my U-Can-Do 46 I have Hitec HS-485HB for all servo's, a Spektrum AR8000 receiver and a MPI Gold Peak HR52400AS 6V battery pack.

3 weeks ago in the middle of my third flight the throttle froze fully open. After a couple of minutes it idled down so I could land. Once on the ground it was determined that the electronics of the servo was fried (it moved freely by hand but does not work plugged in). All other servo's were okay. I replaced the servo and sent the original one in for warranty repair/replacement. A week later on my forth flight the same thing happened except the servo never returned to idle. Again the servo's electronics are fired. At this point I found out that my transmitter, DX8, was on the recall list. I purchased a new DX8 (it already had the firmware upgrade). So to be smart (I thought) I replaced the servo, the AR8000 and bound it all to the new transmitter.

Well guess?? First flight today the servo stuck fully open within minutes of take off. Upon landing it was once again determined that the servo was bad. In all cases there was no binding of the throttle linkages at all. From dead center the servo arm never got past 45 in either direction. I have the same servo's in a half a dozen planes without one hiccup (at least from the servos). The battery pack never dropped below 6.2v in all cases.

Any, ANY suggestions would be appreciated.

Thanks.

Mike
Hi Mike ,

Just curious , , what kind of engine is on the plane ? If its one of the newer small Gasoline engines , , is there a return spring on the Carb ? Ive seen the Walbro style carb's factory supplied return spring fry a couple of servos in the manner you describe , so thats the basis for my question .......
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Old Jul 09, 2011, 10:35 PM
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Sounds like your servo is stalling at full throttle. Try setting the throttle servo using either mechanical settings or the limiter on your transmitter so that the servo moves JUST less than full throttle (on the carb) when you move the throttle to full on the transmitter. Do the same for idle cutoff so that you have to use full down trim (or the idle cutoff switch) to get full off on the carb. Those settings should eliminate any stalling at either end of the throttle.
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Old Jul 10, 2011, 02:00 AM
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United States, KS, Overland Park
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Ohh, I've gone through this. Made me mad as heck too. Vibration will do strange things. I don't know your setup, but I will tell you my little story.

I lost two servos on the throttle in a weeks time last year, same way. the electronics fried. I did everything I could to figure out the problem until someone at the field made a suggestion. Sometimes we have linkages that are perfect, no bind or drag at all, so perfect in fact that it lets the connecting rod itself develop a resonance between the throttle servo and the carb. The vibration causes the servo to try to move that minute amount back and forth at incredible speeds. That causes the electronics to fry. Oddly enough it seems the higher quality the servo on the throttle the more likely it is to fry. The solution is fairly simple. you need a little resistance. For me it involved adding a piece of foam rubber at the midpoint of the pushrod. I glued it to the frame of the plane and it provided just a touch of resistance. This keeps the pushrod from transfering vibration to the servo, at the same time it doesn't resist enough to cause any concern with longevity. A year later and about 5 gallons of nitro and still running strong. SO if it sounds like what you are going through give it a shot. All you need is a bit of resistance and it should isolate that servo a bit.
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Old Jul 10, 2011, 05:41 AM
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Jacksonville Fla.
Joined Mar 2007
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You don't say whether or not you using an extension...I'll assume you're not....but if you are check it.....after you have checked everything above you have to suspect the receiver.....
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Old Jul 10, 2011, 06:24 AM
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United States, IN, Griffith
Joined Sep 2010
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Thanks to all for your responses.

I have a Saito 82 installed. In all but one case the throttle arm and rod were no where near the top of the of the curve. In the one case it was a little farther then the normal 45. I was able to hand twist it back to neutral like it was butter. No binding. No extensions on the servo for throttle. I am wondering if Wookster has something. The Saito does put out a bit resonating vibration while full throttle. If the vibration is making the servo fight the control rod to stay at a certain place then I can see it burning out. It would also explain why I can not repeat it on the ground. I ran 3 - 10 minutes tests at home duplicating everything (except running the engine) with a fresh servo in place and did not fail.

I would think that if it was a voltage problem more then one servo would be effected.
I am planning on purchasing a standard servo, perhaps a different brand, to try out.
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Old Jul 10, 2011, 07:13 AM
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United States, MO, Springfield
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what frame rate are you using in the DX8 and is the throttle servo analog or digital
11ms frame rate + analog servo will = dead servo
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Old Jul 10, 2011, 09:25 AM
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USA, TX, Grapevine
Joined Dec 2008
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I have been using a Airtronics standard 102 servo for many years now on my Saito .80 engine with a straight wire for the pushrod. But I think the wire is rubbing a little, inside its plastic tube, on bulkheads so maybe that dampened it out.
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Old Jul 11, 2011, 07:54 AM
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United States, IN, Griffith
Joined Sep 2010
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Thanks to all for your suggestions.

I replaced the Hitec servo with a Futaba S3003. After centering the servo arm I dialed in the top and bottom to 75%. With the throttle linkage re-attached I started working the top and bottom back out. Bottom was still about 100% when set right. The top was at 95% when the carb was 100% open. I am hoping that this will eliminate any binding or straining the servo at full throttle. I guess I'll find out Friday or Saturday when I fly
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Old Jul 12, 2011, 11:53 PM
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Birmingham, Alabama
Joined Jun 2002
2,970 Posts
as said, need to adjust the travel of the servo. you might also want to check the being vibration translated to the servo.

had a 4-stroke one time that was stripping gears. travel was fine & wound up being vibrations taking their toll on a micro servo.
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