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Apr 14, 2019, 12:42 PM
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Defective Servo Mystery.

Disclaimer : I’m not trashing any company or vendor. Just trying to solve a mystery.

I’m building my second DLG. It’s a 4 in the Pod design. I have four brand new KST X08 V5s installed. They were bench tested before and after installing them in the servo tray. Last night, I tested them before getting ready to install the pushrods. They all worked. I finalize the pushrod assembly and was about to start installing them and discovered one aileron servo had stopped working. I plugged in a cheapo servo in its place and it worked, so the RX is fine (Spektrum, fyi). Just to remove all doubt, I bound another Spektrum RX and plugged everything into it and the non working X08 servo still wasn’t working. Annoyed, I decided to call it a night.

This morning, I discovered that now a second X08 has stopped working. So, I did the tests again. Swapped out different servos, RX, and batteries and concluded it is indeed the X08 servos.

This isn’t the first time this has happened. My previous DLG I built a year ago had three KST servos die within minutes of each other. I thought it was a defective RX, who’s brand I stopped using as a result, but maybe that wasn’t the issue after all.

So what the heck is going on? I know KST makes good servos. Is there someone making a knock-off imitation that’s infested the market? I’ve never had this happen with any other servo brand. Not even a $5 Emax.

The one constant in all of this is that, while I’ve bought KSTs from at least three different vendors over the years, the ones that have been defective came from one particular vendor (though not all were defective - just to clarify).

Any theories?
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Apr 14, 2019, 01:28 PM
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What are you using for a battery?
Apr 14, 2019, 01:35 PM
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eitanro's Avatar
This sounds really weird... One servo can be faulty, may be even two, but 4 servo's? That's really weird... If it was a broad malfunction we would hear more about it.
Perhaps an issue with the power supply? Continues over load at some point?
Apr 14, 2019, 02:17 PM
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It’s always been a 1S battery between 180 and 700mah. JST connector soldered to a servo end with the signal wire removed. I’m very paranoid about reversing polarity, so I always double check every time I connect when I’m on the work bench.
Apr 14, 2019, 02:28 PM
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Iím not the best at soldering, but I know enough to prevent a short from occurring. Can a Ďdirtyí solder cause voltage issues that could damage a servo? Iím thinking no, but Iím willing to consider anything right now.
Apr 14, 2019, 02:31 PM
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Sensoar's Avatar
When connecting the battery... if the power plug has intermittent connection by fumbling with the connectors (where there are multiple on / off / on power events) it will cause a failure to boot the servo occasionally.
Apr 14, 2019, 02:32 PM
Lift is where you find it.
Do you have a manual servo tester you can use? That would eliminate all of the other possible points of failure- get the Tx, Rx and battery out of the loop. I usually power mine with the throttle lead from a brushed motor ESC and a 2S or 3S battery. You can also use a BEC that puts out 6v.
Apr 14, 2019, 03:09 PM
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KST - enough said. I'll never use them again
Apr 14, 2019, 03:46 PM
iVoid Warrenties
flyinghedgehogs's Avatar
I only had one defective out of 44, so maybe your luck is spectacularly awful?
Apr 14, 2019, 04:47 PM
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Originally Posted by flyinghedgehogs
I only had one defective out of 44, so maybe your luck is spectacularly awful?
Iím a double amputee with male pattern baldness, so thatís certainly a possibility. =)
Apr 14, 2019, 06:03 PM
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eitanro's Avatar
I would suggest you to contact KST for this issue and perhaps send them for inspection, their customer service is very good.

Apr 14, 2019, 06:13 PM
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I'd like to know how the servos were damaged. I've killed two MKS servo during a build up of two gliders. Have no idea of how they were killed. Maybe my servo tester wiped out the servos. I'll get a new servo tester before my next build. The servos were good when I tested them and bad once installed and connected to a receiver.

All I know is I did something to wipe the servos out. Could be bad servos. I may send the servo back to see if they can help me as to what went wrong.
Apr 14, 2019, 07:25 PM
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I’ve never used a servo tester. I had one a long time ago, but when I saw how easy it was to plug something in wrong and damage things, I refused to use it. I always use the RX, TX, wire harness and battery that I’m going to use in the glider to test and center the servos. That way I can encounter and troubleshoot problems as early as possible (case in point).

Is there any type of an RX defect that could damage a servo? The three I lost last year and these two were all plugged into an RX that had been de-cased and covered with a few pieces of electrical tape. Grasping at straws but worth asking.
Apr 14, 2019, 09:09 PM
Aurora Builder
Strange. Unless you are reverse polarizing the voltage to the servos I find it hard to believe a receiver is to blame, and we all know the smell of reverse polarity...

The only real thought I have is the 11ms frame rate on Spektrum DSMX might be too much for the X08’s?

I would try a higher voltage pack (2S), fresh receiver at 22ms and all factory wiring. My gut is the frame rate issue or a bad batch at the vendor but anything is possible!
Apr 14, 2019, 09:25 PM
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pmackenzie's Avatar
I doubt it is frame rate, supposed to be good to 333hz , which would be 3 msec.

I would guess inadvertent reverse polarity, very easy to do with de-cased receivers. Plugging a battery or servo connector in backwards is usully fine, but if you plug it in offset by one pin ( which the case would prevent) then things get burnt out.

Having said that, not a fan of the X08. Too expensive and they are not up to V5 because V1 through V4 were so good

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