Killing a 7955

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Tired Old Man · 09-28-2009, 05:21 PM

Though I've known about what happens when a charged servo is bumped for a long time, in the last month I've had two HS 7955TG servos killed on one of my planes. Be careful with people walking around your plane if the electrical system is powered up. Someone bumping into a flight surface will quickly wipe out the electricals in a 7955 servo.

It doesn't seem to take much of an impact.

mbacosa · 09-28-2009, 05:46 PM

Just a momentary bump? Why does it cause the servo to fail? That is really odd.

Tired Old Man · 09-28-2009, 06:06 PM

Just guessing, but the servo is under the impression it's supposed to be at neutral and something comes along and knocks it out of the commanded position causing some type of electrical spike. Don't know if it's an amp or a pot that takes the hit but they just stop functioning after that.

JoeAirPort · 09-28-2009, 06:17 PM

Hmmm, another guy posted a reply a while back about his theory of why these servos burn up. He said if the surface was over extended and the push rod was in a weird position...and you powered up that way, it can burn up the servo (locks up). Maybe not right away but in short order of that happening. Maybe this is a similar thing. This servo doesn't like power spikes. Thanks for the heads up. Maybe we are finally figuring out the failure mode of these things.

Mike Mayberry · 09-29-2009, 12:34 PM

The servo is just going to try and hold it's position if bumped but unless the load is sustained for a long period of time it should not burn up.

It is possible to set overload protection in the 79XX series servos which would protect the servo from burning up if it's loaded for too long.

Mike.

Kevin B · 09-29-2009, 12:46 PM

Mike, might be unrelated, but I have a question about the same servos.

I am running 7955's all the way around on my AW 300QB 100cc. I have not adjusted the bandwidth, but have programmed them with the HFP-20. I am running A123 batteries into an EQ Sport Power expander. Started off fine no problems. Then the left wing servos started shimying and jumping all by themselves. Now the right Elevator (single servo) is doing the same thing. This happens at all different charge states between freshly charged or 3rd flight of the day. Any suggestions????

Mike Mayberry · 09-29-2009, 01:49 PM

If you apply a load it should go away. This has been discussed here... it's most likely normal.

Mike.

09-28-2009, 05:21 PM	#1
Tired Old Man Bad-ass Super Contributer! Join Date: Jul 2006 Location: California Posts: 4,323	Killing a 7955 Though I've known about what happens when a charged servo is bumped for a long time, in the last month I've had two HS 7955TG servos killed on one of my planes. Be careful with people walking around your plane if the electrical system is powered up. Someone bumping into a flight surface will quickly wipe out the electricals in a 7955 servo. It doesn't seem to take much of an impact.

09-28-2009, 05:46 PM	#2
mbacosa Bad-ass Super Contributer! Join Date: May 2008 Location: Mesa, Arizona Posts: 939	Re: Killing a 7955 Just a momentary bump? Why does it cause the servo to fail? That is really odd.

09-28-2009, 06:06 PM	#3
Tired Old Man Bad-ass Super Contributer! Join Date: Jul 2006 Location: California Posts: 4,323	Re: Killing a 7955 Just guessing, but the servo is under the impression it's supposed to be at neutral and something comes along and knocks it out of the commanded position causing some type of electrical spike. Don't know if it's an amp or a pot that takes the hit but they just stop functioning after that.

09-28-2009, 06:17 PM	#4
JoeAirPort Obsessed Join Date: Jan 2006 Location: Milwaukee, WI Age: 47 Posts: 5,501	Re: Killing a 7955 Hmmm, another guy posted a reply a while back about his theory of why these servos burn up. He said if the surface was over extended and the push rod was in a weird position...and you powered up that way, it can burn up the servo (locks up). Maybe not right away but in short order of that happening. Maybe this is a similar thing. This servo doesn't like power spikes. Thanks for the heads up. Maybe we are finally figuring out the failure mode of these things. __________________ Videos: 88" EF Yak http://www.youtube.com/watch?v=ynkFiYZOS7k 88" EF Yak http://www.youtube.com/watch?v=z1SD21qAJoA 110" EF Yak http://www.youtube.com/user/Vortran?.../5/UyaC9xGFKTk

09-29-2009, 12:34 PM	#5
Mike Mayberry Bad-ass Super Contributer! Join Date: Mar 2006 Location: Poway, CA Posts: 704	Re: Killing a 7955 The servo is just going to try and hold it's position if bumped but unless the load is sustained for a long period of time it should not burn up. It is possible to set overload protection in the 79XX series servos which would protect the servo from burning up if it's loaded for too long. Mike.

09-29-2009, 12:46 PM	#6
Kevin B Bad-ass Super Contributer! Join Date: Aug 2006 Location: Oklahoma Posts: 621	Re: Killing a 7955 Mike, might be unrelated, but I have a question about the same servos. I am running 7955's all the way around on my AW 300QB 100cc. I have not adjusted the bandwidth, but have programmed them with the HFP-20. I am running A123 batteries into an EQ Sport Power expander. Started off fine no problems. Then the left wing servos started shimying and jumping all by themselves. Now the right Elevator (single servo) is doing the same thing. This happens at all different charge states between freshly charged or 3rd flight of the day. Any suggestions????

09-29-2009, 01:49 PM	#7
Mike Mayberry Bad-ass Super Contributer! Join Date: Mar 2006 Location: Poway, CA Posts: 704	Re: Killing a 7955 If you apply a load it should go away. This has been discussed here... it's most likely normal. Mike.

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