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-   -   DS8511 oscilating (http://www.rcuniverse.com/forum/rc-jets-120/11216966-ds8511-oscilating.html)

Propellars 09-04-2012 08:06 AM

DS8511 oscilating
Hi there.
I have had an issues with 8511 servo in the wing of my new (second hand) Flash. During power-up the servo starts to oscillate from endstop to endstop. But it only happens if the servo is out of center during startup. As sonn as I move it to neutral, the servo is good again, and I cant seem to repeat the issue. Mean while all other servos behaves fine. The previous owner had the same issue with another of the four 8511 wing servos, after 200 flights and he changed the servo to another brand.

Power supply is 2xLiPo2S and Emcotec BIC with 6V regulated supply. I can't make the servo do it again, allthough I could repeat it ten times the other day at the field. Any ideas of what goes wrong?? I'll be happy to change the servo, but I dont feel comfortable with 8511 just there, cause when two different servoes falls, I imagine something else is wrong but the servoes...

Dave Wilshere 09-04-2012 08:50 AM

RE: DS8511 oscilating
Extension leads? What type? How old?



mick15 09-04-2012 09:43 AM

RE: DS8511 oscilating
Agree Dave.


Propellars 09-04-2012 09:59 AM

RE: DS8511 oscilating
Hi Dave
Leads are 0,35 or 0,5mm2 and 1 meter long. Only one comon connector for all four servoes (green MPX gold). All other junctions are soldered.
But even if the leads might be wrong, could that ever lead to this behaviour?

Propellars 09-04-2012 10:46 AM

RE: DS8511 oscilating
Receiver is by the way FASST R6014HS and the 8511 is not on the HS chanals.

Dave Wilshere 09-04-2012 12:40 PM

RE: DS8511 oscilating
Not a Futaba guy, but I thought the HS channels were for Digital servos-like the 8511? Have you tried the same servo on the HS channels?

Is it on Flap or aileron?


Springbok Flyer 09-04-2012 01:03 PM

RE: DS8511 oscilating
Hi there,

There has been other reports on RCU about the same problem. I have had exactly the same thing happen to me with an 8511.



Propellars 09-04-2012 09:49 PM

RE: DS8511 oscilating
My servo is on flap and the last owner lost his aileron servo.
I newer found such issues on 8511 - do you have a link?

dbsonic 09-05-2012 11:11 AM

RE: DS8511 oscilating
I've seen stuff like this and fixed it. It usually comes down to low voltage/poor signal situation and I have not seen the servo itself be the problem. The culprit often lies in the extension as mentioned by Dave and connectors! The connectors can become oxidized a bit and you get a little capacitance there. And if you are doing any fanout each connector adds its parasitcs as well. This can lead to the signal you send from the rx to deteriorate a bit by the time it gets to the servo. I would not fly your model again till you get it hashed out. I also like to use twisted extensions to get a little bit of extra shielding geometry(the presence of the ground already provides shielding but only in the plane of the ribbon cable).

dubd 09-05-2012 01:35 PM

RE: DS8511 oscilating
I agree with Dave and dbsonic. Check your extensions. I've had this happen with 8711 servos and it ended up being bad extensions.

Propellars 09-05-2012 08:19 PM

RE: DS8511 oscilating
Thanks for your replies!! I'll let you know how it goes :D

darryltarr 09-05-2012 08:20 PM

RE: DS8511 oscilating
I ditto what Dave, dbsonic, and dubd have said.

I had a similar issue with one of my Aileron Servos (DS8411) on my Ultra Bandit. After a lengthy investigation I struggled to pinpoint the exact source, so before changing the servo (which was the easiest and quickest thing to do), I decided to replace the extension lead with Multiplex 3 x 0.5 PVC twisted cable and have had NO issues since.

Since then I generally do all my installs with twisted extensions. The quality and preciseness of the servos is second to none.


Dave Wilshere 09-05-2012 09:44 PM

RE: DS8511 oscilating
There was a report in JetPower magazine a few years back that said twisted leads added nothing, no worse, but no better...

I use gold plated connector (a must) and heavy duty cables throughout and never have a issue)


Springbok Flyer 09-06-2012 02:57 AM

RE: DS8511 oscilating
Hi there,

Here you go, this is where I first noticed the 8511 problem


When I had the problem, it was on a new installation, including new twisted extension leads. In fact I removed the servo fom the plane, put it on a servo tester. At first nothing happened, then after a while it simply started oscillating all by itself.

Rather worrying, I have about 20 of these servo's installed in various jets and just pray they don't start the same thing some time in future.



Propellars 09-06-2012 05:06 AM

RE: DS8511 oscilating
1 Attachment(s)
Thanks all!! What a nice forum :D

I think I can narrow it down to one point: The single MPX gold connector for all 4 servoes. Even though it has worked and worked for hundreds of flights, it might be tired now. The location is actually very close to the fuel-junction (Flash wingtank), and the MPX might have got some fuel through time.
Four leads of 0,5 mm2 is soldered to one MPX pin who is rather small, so there might be a bottleneck of some kind. And perhaps the DS 8511 are more sensitive to this than other servos.

Just for the record, the historic is:
• After 200 flights: DS8511 on Aileron 1 starts to oscillate at powerup, and both ailerons are changed to Graupner DES 707
• After plenty of more flights, and at the time where I bought it: DS8511 on Flap 1 starts to oscillate at powerup, and we concluded that his transmitter was the true issue.
• After 10 more flights, and changed transmitter + receiver brand: Flap1 starts to oscillate again at powerup.

So same DS8511 servo oscillates with two different radio brands, but only during startup. Not possible to provoke another day, but when it happened, the control surface was out of center before powerup, and it keeps on doing it on each powerup, until the control surface is manually positioned to neutral. Newer on a full charged pack. Always had more flight before on the same day.

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