Problem with burned out servos in my RPG - RC Groups
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Jun 10, 2017, 09:17 AM
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Ricardo1500's Avatar
Discussion

Problem with burned out servos in my RPG


Hi All

Yesterday I went to fly my RPG.
Plugged in my 4s Battery as normal and the usual beeps were emitted, but on checking control i found one roll/pitch servo didnt work.
I de and re energised the system and then found that both pitch and roll servos had stopped working, although throttle and rudder continued to function fine.

Today Ive had a look and found both servos (TowardPro 996Rs) are burned out. See pic .

Tested the BEC and its putting out 5v as advertised and 2 new servos put into the model work fine.

I know servos will fail occasionally and yes both of these had had over 200 flights of hard use in my RPG, but for both to fail on the same day seems more than coincidence to me.

Any ideas anyone
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Jun 10, 2017, 03:42 PM
Mike's Ma Maaaaaaaaan
Two servos on one day, probably not worth buying a Lottery ticket with luck like that....

Seriously, I would be strongly doubting my BEC, or was the head somehow blocked stalling the servos???

Paul
Jun 10, 2017, 03:49 PM
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Ricardo1500's Avatar
Hi Paul
No the servos weren't being stalled in any way, the model was sitting on its wheels as normal. Graham also suggested the bec may have been the culprit, but the rudder servo was unaffected which puzzled me.
Jun 10, 2017, 04:36 PM
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106flyer's Avatar
Nope sorry , no idea other than stalling or power supply overload

Steve
Jun 11, 2017, 12:37 AM
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britinoz's Avatar
That is a strange one....not that I'm a great fan of that particular servo.
I do remember Bruce Simpson from RC Model Revue telling us that there was a problem with the circuit board and it could go at any time....Possibly the second one went in sympathy ....sorry bout that.
Chris...
Jun 11, 2017, 01:18 AM
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106flyer's Avatar
I have some blue bird ones Chris recommended and you can have them...

Steve
Jun 12, 2017, 02:12 AM
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britinoz's Avatar
Well Steve..... it would appear that you don't have much luck with some servos.........You have to talk to them nicely. you did know that ?
Chris
Jun 12, 2017, 07:20 AM
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Ricardo1500's Avatar
Chris
Didn't try talking nicely that maybe where I went wrong!
Steve thanks for the kind offer but I've already swapped them.
Last edited by Ricardo1500; Jun 12, 2017 at 11:11 AM.
Jun 12, 2017, 11:16 AM
Registered User

If You Really Want to Know Why


To learn why the servos failed you have to dig deeper. In my experience modern servo electronics are very hardy and not often the cause of servo failure.

Pop the servo motor out from its mount disconnecting it from the gears.

Now plug the servo into your receiver. Using the transmitter stick for that channel move it back and forth to the extremes. Does your servo motor run fast in each direction? If it does, the electronics are doing their job and the motor is OK. Look for a mechanical jam in the gear train for the source of the problem. If the motor doesn't run, do this:

Unplug the servo from your receiver.
Unsolder the wires from the motor where they attach to the circuit board. Now briefly connect the wires from your servo motor to a DC power source like the 5 Volt supply from your ESC, or any other handy source such as a flashlight battery. Any supply from about 1.5 to 5 Volts will work. Now, does the motor run?

If the motor runs, the electronics are the fault. If it doesn't, the motor has failed. Of course it is possible that both motor and electronics failed. If you want to learn if the electronics are OK, simply connect a similar small known good motor to them and repeat the test.

Now, take the failed motor's housing apart and inspect the armature and commutator. Look for burned spots, melted wires, or burned off insulation (causing internal shorting). If you find any of this you can be sure the motor was working so hard, or stalled that it drew excessive current and burned out.

Again...this is only if you really want to know why your servos failed.

Cheers!
Russ

OOPs! didn't see the obvious failure on the circuit board until I enlarged it. If both circuit boards have exactly the same obvious fault, you have your answer. This type of fault is caused by excessive current, so you may still want to look at the motor's internal parts on that servo. Does the other circuit board have the same appearance?
Last edited by rus1; Jun 12, 2017 at 11:23 AM. Reason: Saw larger pic
Jun 12, 2017, 01:11 PM
Registered User
Hi Rich

Chas had a similar problem a while back. It is fair to say if one servo fails the current surge is enough to take out another. I seem to remember that one of Chas' failed and while we were looking at the problem the second which was working fine went a few seconds later. In his case he replaced both servos and all was. Ok again.

Maybe worth checking with him to confirm this.

Cheers

Graham
Jun 12, 2017, 03:47 PM
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Ricardo1500's Avatar
Graham
That describes what happened in my case exactly.
Russ
Both circuit boards failed in exactly the same manner..........blistering to the same component in both servos
Jun 13, 2017, 11:57 AM
Registered User
Ricardo,
I would say those servos have a design flaw and to avoid them in the future.

It would be instructive to examine the servo motor internally unless you have already binned them.


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