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KST Servo trouble


NickS
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After faultless service for 18 months 2 of the KST 08 servos in my DLG  (running on 1s Lipo) have decided to play up.  They are both on the tail surfaces and I have two others in the wings for the ailerons/flaps.  The model is a Flitz 2 that was beautifully made by Mike Stern.  I have removed the output discs to check that there is no unusual friction in the line runs and all is fine.  It's just that both servos have very little power despite battery being at full charge.  Just light pressure on the output cog with a finger stalls them. 

The receiver is a Lemon variety and has been faultless too.  However, I don't use a switch and just plug the battery lead direct.  It is possible that I have inadvertently touched the contacts in reverse but I don't understand why the Aileron/Flap servos are fine.  I never remove the wing so all four servos stay connected at all times, hence if I fried the tail servos why not the ailerons?  The issue arose just a few days ago with both tail servos adopting the issue at the same time.

Any suggestions would be most welcome.  Someone has suggested taking the tops of the servos and spraying with electrical contact fluid but I really can't see that helping.

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Paul Gleeson

If you did plug the battery in the wrong way round, even for a moment it can damage any electronics. Let me guess you have red JST connectors. Easy enough to do which is why I never use them. You have longer cables and additional connectors on the ailerons servos so that “might” have saved them.


Try connecting the servos to another receiver and see if that makes a difference. If not consider the servos scrap and buy new ones. 
Not worth risking a £500 plane for a £50 servo.  The elevator servo is the critical one. Other single servo failures do not make a crash inevitable. 

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yes to be absolutely sure just try a process of elimination.

  • change out battery
  • change out RX
  • reset TX programming to something basic
  • replace servos.
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Yes JST plug 😬 so my fault entirely If that is what happened.  

Good point about the longer wires to the wing servos.

I will have to dig the receiver out of my big biplane to test with, though I did try hooking the elevator servo to the wing servo output and nothing better there, so pretty sure the receiver is ok and servos faulty.

Have three fully charged batteries so not on the list for concern.

 

Looks like a new pair of KST's and a switch to prevent a similar expensive error.

 

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Paul Gleeson

If you do use red JST connectors I highly recommend you take a black permanent marker and mark the negative side all over both the plug and socket.

I use the micro heli MCPX connectors. Much shorter than the red JSTs

Be careful with a switch, you can still plug a red JST the wrong way round. And I switch is another point of failure 

s-l500.jpg

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Nick,

I'm guesing from the age of the model that they are the v3 of the X08 rather than the v5. I've had at least 2 out of 4 of my v3 X08s fail - not like you describe, but fail in other "interesting" ways. Could have been more than 2 but hard to tell after a crash.

I had on aileron break causing major damage as I think it was stuck on launch and then a couple of months later the rudder in the same plane started a weird glitch. Went full deflection on launch which was interesting. Landed safely that time though.

https://www.superfly.online/product-page/gdw-ds1906

These are £10 per servo cheaper than the X08 v5 and I've got two of these and two Bluebird A10S in the model now.

If an obvious fix to battery or RX doesn't solve it then I'd just change them rather than wish you did when you loose elevator control at a vital moment.

 

We must meet up again this summer for some more flying and hopefully I can drag you along to a comp - all assuming we're ever allowed to leave the house again!

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I use a pig tail wire always plugged into Rx.  That way I just plug battery into pigtail.  Makes things much easier although with JST I guess you could still plug in wrong way if you tried.

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Thanks Paul for advice Paul and  useful servo link. You wouldn’t happen to know if the top output cogs match the KST ones so I can simply switch the pulleys over?

Yes would be good to meet up again when allowable. As for a comp I could be persuaded.

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Just a thought Nick, The wing servos aren’t under any load when plugged in (unless the ailerons have moved in storage ) whereas the tail servos work against the springs so will need to drive all the time. This could explain why the tail ones failed and not the wing ones on momentary contact.

Keep safe 

Neil P

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So I use servo leads as connectors, I remove the white wire (for futaba) if I accidentally connect it the wrong way round nothing happens as there’s no circuit . Just thought as an option it’s cheap and works

 

i also had issues with early KST’s but mine just wouldn’t centre and would only move in one direction and then not move back. I now use bb equivalent

dave

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2 hours ago, NickS said:

You wouldn’t happen to know if the top output cogs match the KST ones so I can simply switch the pulleys over?

The servos I linked are a direct match in output splines and case size for the X08. Not as tried and tested as the X08 v5 but Neil H @ Superfly seems happy with them and it saves a few quid...

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  • 4 weeks later...
StraightEdge

I had KST v3 in my old Blaster 3, the rudder servo suddenly stopped working for no apparent reason.  I wondered whether I had inadvertently knocked the rudder in transit and stressed the servo, but had no recollection of this and there was no visible damage to the rudder.  Later its replacement also suddenly stopped working!

On lead-polarity I always use a permanent jumper-lead:  Futaba plug into RX at one end (then all plugs sealed over pins with light tape) and JST socket to battery at other - that then being the 'switch' - and never managed to force the plug in the wrong way round.

 

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