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Pumpkin drop event

HXT-900 servo sticks right when power on

kulfsson

Junior Member
#1
Hi,

I am having an issue with a Bixler build. The plane came with a faulty servo so I had to replace it with a HXT-900 servo that i had lying around.

The problem I am having is that the HXT servo now goes to the extreme right when I connect the battery to the receiver and I am not able to move it at all with the TX. After a while the servo becomes quite hot as it seems to be holding the extreme right position.

I have tested the servo on all channels on the receiver and the behaviour is the same. I have also tested a second HXT and it shows the same issue.

All other servos that came with the plane seem to react and work fine.

Once I disconnect the power the servo seems to move freely.

My TX is a Turnigy 9x with a FrSky DJT module and the receiver is a FrSky D8R-II PLUS.

Does anyone have any idea what can be at fault here ?

Regards,

Kristjan
 

kulfsson

Junior Member
#4
Is this happening with the servo bare or installed? Maybe the horn is not installed at center...
This happens in both instances. One servo I glued in to the Bixler without testing (i know), and another one is bare. It is not about the horn since it will always move directly to one direction and stay there until I shut off the power.
 

kulfsson

Junior Member
#5
As a test, I connected the old servo (with broken gears) on the same channel and that seems to work fine. i.e. it responds to stick movements unlike the HXT and doesn´t jam into one direction.

I suppose that both of my HXT's could be faulty, but I find it very strange. I have some other HXT's coming soon from Hobbyking. Perhaps these will work better ?
 

xuzme720

Dedicated foam bender
Mentor
#6
Maybe...it almost sounds like they were assembled backwards...if you pull the gears out and plug it in does it ever stop? If not, then they have bad pots in them
If you aren't sure about them, don't use them.
 

kulfsson

Junior Member
#7
Maybe...it almost sounds like they were assembled backwards...if you pull the gears out and plug it in does it ever stop? If not, then they have bad pots in them
If you aren't sure about them, don't use them.
Thanks for the troubleshooting tip. I pulled the gear out of the servo that was not in the plane and connected it to the receiver. Sure enough, it just spins around like crazy with no end in sight.

I assume therefore that my other servo also suffers from this same problem. I bought these a long time ago and this is the first time I have connected them. Must have been a faulty batch ? What else can explain that I got 2 bad ones ?

I have a shipment on the way with 3 new HXT-900 servos so I will test them and post my results here after testing.

As a side note, what other 9 gram servos do you recommend ?

UPDATE: So, I jiggled around with the gears a little bit and then tested it again and got it to work. I should probably wait for my shipment instead of using it on the plane, even though it is just for the rudder it probably wouldn´t be nice to have it lock in flight.

On a side note, here is an image of the servo that came with the plane. Notice the missing teeth.
servo.jpg
 
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kulfsson

Junior Member
#9
Brown/black is minus.
Servos tend to go one direktion and stick if connected the wrong way.
Thanks for your comment. I knew about this and made sure to connect them that way.

Since the servo is already questionable, and for the sake of science :), I reversed the connection and nothing happened. It did not turn in either direction, did not stick and I could move it with my fingers. Plugged it in correctly again and it seems to work fine now, except for me knowing that it was faulty and I shouldn't use it ;-)
 

stay-fun

Helicopter addict
#10
Hmm, what happens if you take your HXT900, remove the gears, plug it in (the motor will start spinning) and then use some pliers to change the position of the output shaft? If done correctly, you should be able to reverse the direction of the motor, and when put in center, the motor should come to a stop.

If not, there's something weird going on with the electronic board in the servo. Or maybe it can't handle the signal of your receiver? I'm not familiar with FrSky.
 

xuzme720

Dedicated foam bender
Mentor
#11
Frsky uses the same signal as the rest. It's something going on in the servo itself.

The reason it kept spinning to begin with is there was no connection between the motor and the potentiometer, which is how the board in the servo can tell(or is supposed to tell) where the servo is currently. I neglected to mention to remove only the gears that have the stop built in to them so the pot was still connected to the motor...It won't harm anything to pull all of them, but isn't a fair test of the servo.
If it's working like it's supposed to now, chances are the pot and the rest of the gear train were not synched the way they need to be, ie. center of pot in line with center of limit tabs on the gearing.