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Old 09-09-2019, 03:26 PM
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Hitec servo twitching

I have a new 7955TG servo that randomly twitches. It twitches while idle with no load, it twitches while rotating under a load. It does it on every channel and on my servo programmer.

I've sent it to Hitec who told me they replaced the circuit board but it came back doing the same thing. Anyone have a similar experience?
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Old 09-09-2019, 03:49 PM
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Some of those servos were prone to do that. Several old threads address this. You can reprogram the deadband to be slightly longer and problem should improve. The newer 7955 replacement, the D955TW has none of these problems, at least that I’ve observed.
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Old 09-09-2019, 03:50 PM
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Totally normal for those servos....

These are from my tests on the 7955:
HITEC HS7955tg (2 min 37 sec)

Check out 1:26 onwards...
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Old 09-09-2019, 04:53 PM
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I added a click to the deadband and it did help a bit. I guess I'll just live with it or throw it in the garbage and waste another $120+ on a new version. I will also have to replace my SWB offset arm to a 25T version. I should have done more research. It really sucks Hitec doesn't step-up and make this right. I just found out the servo is discontinued.
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Old 09-09-2019, 06:41 PM
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Originally Posted by Threepete View Post
I added a click to the deadband and it did help a bit. I guess I'll just live with it or throw it in the garbage and waste another $120+ on a new version. I will also have to replace my SWB offset arm to a 25T version. I should have done more research. It really sucks Hitec doesn't step-up and make this right. I just found out the servo is discontinued.
Call them or send it in again. They are very good about taking care of problems.

Yes it hurts to replace perfectly good arms but I was glad when Hitec made the switch. I have many other servos that use the 25 tooth spline and Hitec was the odd man out in my shop. If youre in the usa, you can often find good sales for the D955 or D954. I paid $89 for the last D955s I bought. The steel gear D954 works just like the 955 but weighs a bit more. Usually $10 less though.
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Old 09-09-2019, 07:19 PM
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I sent them another email, we'll see what happens. Thanks all for the input.
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Old 09-09-2019, 07:31 PM
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O55

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Originally Posted by Threepete View Post
I sent them another email, we'll see what happens. Thanks all for the input.
Something you might keep in mind too is you can replace the 25 tooth spline gear on the new D955 with the 24 tooth gear and keep using the same arms if you want. I use a ton of Hitec servos and havent seen that with the 7955s yet but have seen it in the 7950s once in a while.
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Old 09-09-2019, 07:40 PM
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Something you might keep in mind too is you can replace the 25 tooth spline gear on the new D955 with the 24 tooth gear and keep using the same arms if you want. I use a ton of Hitec servos and havent seen that with the 7955s yet but have seen it in the 7950s once in a while.
My older 7955 servos were rock solid. Later production ones often had the twitch. All of that is history with the D955. Just smooth, silent performance and they have slow start. The typical digital buzz is also gone. A very major improvement in my book and at the same price as before.

Good point on using a 24 tooth output gear but kinda spendy. A new arm would be cheaper.
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Old 09-10-2019, 11:51 AM
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Does it randomly jump about 1/4 of an inch in each direction at the end of the surface or more, and at a completely random interval?

If so, then this is from signal degradation going down to the servo. Long extensions, poor extension quality, poor pin contact, certain receivers, and even interference can cause this to happen. Hitec makes a signal booster that is specifically designed to address this issue. Please note that this is not an issue with the servo itself, this is an issue with signal quality getting to the servo.

The Hitec part number is 58496, but looks like it's currently backordered through the standard channels. Horizon Hobby also has a similar product, and I believe ServoCity as well.
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Old 09-10-2019, 11:54 AM
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Originally Posted by JoeT View Post
Totally normal for those servos....

These are from my tests on the 7955: https://youtu.be/Wc6MiX2S-Eg
Check out 1:26 onwards...
How long is the lever you are testing? More than 3" it looks like, in which case you are bound to see such movement at the end of such a long shaft. I can slap a 10" long needle on any brand of servo and see several mm of movement when the servos circuit is self correcting. This is a byproduct of high resolution servos ensuring they stay locked on center, and not an issue with servo quality or centering.

Some other things to be noted about the video, accuracy via switches is far lower than the accuracy from your gimbals. Hearing the servo try to hunt for center with zero load upon it is telling me that it's seeing a signal and is trying to correct accordingly. Having first hand experience with literally thousands of HS-7955TG's on HV, I can tell you that this servo just does not hunt around center with no load on it. In testing, switches often had a tolerance of 10s in any of their positions, whereas gimbals were often 1s or less.

By the way, just to point it out, Hitec servos are not rated based on a pull test. Hitec uses a highly precise piece of lab equipment that measures torque, not linear force.
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Old 09-10-2019, 12:13 PM
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Xpress, he said he had the same problem on any channel and on his servo tester. Agree 100% with you about cables, etc causing the problem but the above seems to rule this out. The last 7955 servos I bought had this twitch problem regardless of what they were connected to so I answered his question the way I did. Hitec solved my problem when they came out with the D955 and other D series servos.
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Old 09-10-2019, 12:18 PM
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Originally Posted by Xpress.. View Post
How long is the lever you are testing? More than 3" it looks like, in which case you are bound to see such movement at the end of such a long shaft. I can slap a 10" long needle on any brand of servo and see several mm of movement when the servos circuit is self correcting. This is a byproduct of high resolution servos ensuring they stay locked on center, and not an issue with servo quality or centering.

Some other things to be noted about the video, accuracy via switches is far lower than the accuracy from your gimbals. Hearing the servo try to hunt for center with zero load upon it is telling me that it's seeing a signal and is trying to correct accordingly. Having first hand experience with literally thousands of HS-7955TG's on HV, I can tell you that this servo just does not hunt around center with no load on it. In testing, switches often had a tolerance of 10s in any of their positions, whereas gimbals were often 1s or less.

By the way, just to point it out, Hitec servos are not rated based on a pull test. Hitec uses a highly precise piece of lab equipment that measures torque, not linear force.
Nice try tough.. hahha

Same day I tested 9 other servos one at a time, the entire test is on my youtube channel. This servo along with 1 other had the hunting issues at center. BTW all servos were plugged directly into the receiver, no extensions for this test.

Best servo of the test was actually the Futaba BLS172 in accuracy and strength as far as their indicated in/oz rating. Futaba actually understated the performance of their servo.

Using a 3 position switch, is more accurate than a pot. I programmed pre determined sweeps, the center of which was 1500us, pots drift slightly, does't matter if they are carbon or hall effect.

All tests were done using the same 9" skewer for all servos.

Totally unbiased, on the same equipment, on the same day at the same temperature, the only variable was the battery, I kept them topped up as for all of the tests.
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Old 09-10-2019, 12:52 PM
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I have a lot of 7955's some twitch others don't. the ones that do I use on the rudder and put a piece of fuzzy Velcro between the stab and counter balance to solve the problem.I just ordered five D955's for the next plane,problem solved,I hope.
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Old 09-10-2019, 01:00 PM
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I have a lot of 7955's some twitch others don't. the ones that do I use on the rudder and put a piece of fuzzy Velcro between the stab and counter balance to solve the problem.I just ordered five D955's for the next plane,problem solved,I hope.
Please let us know what you think of the D955.
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Old 09-10-2019, 01:37 PM
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all my hitec 7954 and 7955 servos twitched after about 50 flights on lipos or reg didn't matter
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