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Crash diagnosis. 2nd servo failure?

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Old 04-20-2006, 10:13 PM
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hdsoar
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Default Crash diagnosis. 2nd servo failure?

This is my first post on RCU. I hope this is the right forum. My Mini Ultra Stick went in today for the second time due to what looks like on the surface to be a failed servo.

The plane is set-up with a tower-pro brushless motor, melody/dream-fly 25amp esc, a Dimension engineering SmartBEC, a GWS d/c 8 channel reciever, and a combination of 4 e-flite and 2 GWS servos. Battery is the Milehigh RC 2200 Mah 3 cell li-po pack. Several weeks ago during straight and level flight the elevator servo stopped responding and the plane went in. This was an E-flite servo that stopped working. After that crash I hooked everything back up and the offending servo would work for a little while and then just quit. But when it would quit sometimes some of the other servos would go nuts. Anyway I replaced the servo thinking it was just a bad servo that caused the crash. I also put in a different GWS d/c receiver just in case. I probably flew the plane 1-2 hours of trouble free flight time after that crash. Today during mid-flight the plane stopped responding and went in. This time it seems the right aileron servo failed. This was a GWS servo. When it failed it not only stopped responding but it caused full deflection and twitching of most of the other servos and no response to controls. As soon as I un-plug the right aileron servo all control surfaces go back where they belong and control is regained. I have plugged this servo into other channels of the receiver and it has the same effect on the rest of the system. If I plug one of the other servos into the right aileron channel it works fine as long as I don't plug in the one that failed. So again on the surface it seems I have a defective servo.

My question is what is causing this? I have a hard time believing that I have just had 2 bad servos. I'm scared to just replace the servo for fear of another one going out. Anyone have any Ideas of what else to check?
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Old 04-21-2006, 02:50 PM
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jooNorway
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Default RE: Crash diagnosis. 2nd servo failure?

I have experienced failure on 9 out of 11 HS-55 servoes which I bought at once. Sure it can be a bad batch! All my servoes failed on the first day they were used...
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Old 04-25-2006, 03:18 PM
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Default RE: Crash diagnosis. 2nd servo failure?

That sounds crazy. Can you tell us more about the history of the servos? Are they very old? Do you fly in a harsh environment, like near saltwater? Had they ever been submerged or damaged in any way? The occasional bad servo can happen, but with two failures of different brands, I would be concerned, too.

Could your BEC be supplying too much voltage?
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Old 04-25-2006, 09:14 PM
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hdsoar
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Default RE: Crash diagnosis. 2nd servo failure?

The first servo, the eflite, was maybe 4 flights old. The second one had been in several crashes but had many flights on it. The plane has never been in what I would call a harsh environment. Don't live near the ocean and have'nt flown in the rain or in freezing weather.

Since the first post I tested the voltage from the BEC and it was at 5.08 volts. When I plug in the latest failed servo the voltage drops and fluctuates to about 2-3 volts. Some sort of short in the servo I suppose.

I'm thinking that maybe 6 servos are just too much draw for the BEC? The BEC is supposed to be able to power 6 servos, but I don't know what else it could be, other than just 2 failed servos. Maybe the GWS servos draw more amps than the e-flite's. I guess I'll do an amperage draw this weekend. I'm away from the plane until then.
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Old 04-26-2006, 09:12 AM
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Default RE: Crash diagnosis. 2nd servo failure?

If there are too many servos for the BEC, I would not expect that to result in a damaged servo. Can you take the 2 bad servos apart and try to diagnose the nature of the problem inside the servo?

Maybe the servos are undersized for their workload, or maybe you just got unlucky with 2 bad servos.
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Old 05-12-2006, 04:06 PM
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Default RE: Crash diagnosis. 2nd servo failure?

or running too many amps through the BEC is causing the servos to malfuntion in flight in certain conditions (when several of the servos are moving at the same time) and the damage to them is coming from the impending crash after loss of control?
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