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meatmonkey · 07-06-2011, 07:54 PM

So short story, I've been running a 7954SH in my SCX for a few weeks now without issue, until today when it started to jitter whenever it was under water. Like it can't find center. It still steers strongly back and forth, but whenever you hold the wheel still it jitters a couple degrees back and forth. It has been run submerged for several hours of use without issue, until now. Works fine on dry land, but as soon as you dunk it it jitters. Never gotten hot either.

I did pull the gear cover off when I first got it and put dielectric around the case seams and around the output shaft bearing. I then pulled the circuit board cover off and covered that with dielectric as well. Now here is where I am perplexed.

I normally would dielectric grease both sides of the circuit board, but I did some research and found that supposedly Hitec Coreless servos have the motor directly soldered to the circuit board, meaning your not supposed to be able to easily service either. I tried to gently separate the aluminum heatsink part of the case from the plastic portion with the mounting ears on it, but I could only pull them a mm apart without overstressing anything. Relunctantly, I put dielectric around the aluminum/plastic seam and put it all together hoping for the best against water intrusion.

I currently have pulled the gear cover off and the rear circuit board cover, and not found any sign of water intrusion, still nice and clean and covered in dielectric grease.

So my questions are: Where is the sensor that tells the servo where to hold still? Is it in the section that I can't seem to seperate (meaning the aluminum heatsink section and the plastic portion with the mounting tabs)? What are the odds I got water in there and not anywhere else?

Has anyone else had this or a similar problem with a Hitec servo?

Has anyone pulled one of these servos apart and reasembled successfully? Any pointers on making it easier? Anyone have a link to an exploded diagram for this servo? I searched but couldn't find anything.

I haven't pulled any gears off of the shafts to look underneath, but if I do, must I mark the relationship of the gears with each other? Meaning, if any of them are a tooth or two off from factory will it cause negative impact on the servo?

I've heard reports that Hitec advises hobbyist not to mess with their coreless motors because they are delicately directly soldered to the circuit board. Can anyone confirm or deny this? I know that my 645MG had short wires from the non-coreless motor to the circuit board, which made it easy to pull the board out and dielectric both sides, but on my 7954SH the circuit board is a very tight fit in the case and doesn't seem to want to come out, which is why I was wondering it if was soldered directly to the motor.

Sorry for the long post, and thank you for any assistance in making this servo work properly again. Yah, I know some of you are thinking "stay out of the water", but I can't. It is just too much of a guilty pleasure. I have already assumed that Hitec won't warrentee anything that's been "waterproofed" and then tested as such. I'm no noob, but I don't have much experience inside servos, as I am sure is shown by this post.

Charlie-III · 07-06-2011, 08:55 PM

I can only state that, "A gear is a frikkin gear.....". Unless it has a stop or lever or sensor, the servo can give a rats butt where the gear is.
Just saying.
The centering pot is likely attached to one end or the other of the main motor so it knows where it is (rotationally).

SuperRoo · 07-06-2011, 11:09 PM

That was a long short story....

From what you've described it sounds like the rx could be the problem not servo.

Also worn gears can cause a servo to hunt around at center, this is the down side to the cheaper 7954 with metal gears and tons of torque.

meatmonkey · 07-07-2011, 10:03 AM

I can dunk the front axle in water and it jitters, while the RX is high and dry in the reciever box.

The servo only has a few weeks of use, so I'd hope the gears are still good. I run another 7954SH in my AX-10 with Rovers and it is still precise, quick, and strong after a year of horrible abuse.

Yeah, sorry about the length. Just trying to be specific. And I incorrectly stated the section I am trying to seperate. The mounting ears are on the gear cover side, and there is another plastic case section under it that abuts the aluminum portion, and that is where I can't seem to seperate the servo to inspect for water intrusion.

07-06-2011, 07:54 PM	#1
meatmonkey I wanna be Dave Join Date: Aug 2004 Location: central VT Posts: 2,300	Hitec 7954SH jitters in water? So short story, I've been running a 7954SH in my SCX for a few weeks now without issue, until today when it started to jitter whenever it was under water. Like it can't find center. It still steers strongly back and forth, but whenever you hold the wheel still it jitters a couple degrees back and forth. It has been run submerged for several hours of use without issue, until now. Works fine on dry land, but as soon as you dunk it it jitters. Never gotten hot either. I did pull the gear cover off when I first got it and put dielectric around the case seams and around the output shaft bearing. I then pulled the circuit board cover off and covered that with dielectric as well. Now here is where I am perplexed. I normally would dielectric grease both sides of the circuit board, but I did some research and found that supposedly Hitec Coreless servos have the motor directly soldered to the circuit board, meaning your not supposed to be able to easily service either. I tried to gently separate the aluminum heatsink part of the case from the plastic portion with the mounting ears on it, but I could only pull them a mm apart without overstressing anything. Relunctantly, I put dielectric around the aluminum/plastic seam and put it all together hoping for the best against water intrusion. I currently have pulled the gear cover off and the rear circuit board cover, and not found any sign of water intrusion, still nice and clean and covered in dielectric grease. So my questions are: Where is the sensor that tells the servo where to hold still? Is it in the section that I can't seem to seperate (meaning the aluminum heatsink section and the plastic portion with the mounting tabs)? What are the odds I got water in there and not anywhere else? Has anyone else had this or a similar problem with a Hitec servo? Has anyone pulled one of these servos apart and reasembled successfully? Any pointers on making it easier? Anyone have a link to an exploded diagram for this servo? I searched but couldn't find anything. I haven't pulled any gears off of the shafts to look underneath, but if I do, must I mark the relationship of the gears with each other? Meaning, if any of them are a tooth or two off from factory will it cause negative impact on the servo? I've heard reports that Hitec advises hobbyist not to mess with their coreless motors because they are delicately directly soldered to the circuit board. Can anyone confirm or deny this? I know that my 645MG had short wires from the non-coreless motor to the circuit board, which made it easy to pull the board out and dielectric both sides, but on my 7954SH the circuit board is a very tight fit in the case and doesn't seem to want to come out, which is why I was wondering it if was soldered directly to the motor. Sorry for the long post, and thank you for any assistance in making this servo work properly again. Yah, I know some of you are thinking "stay out of the water", but I can't. It is just too much of a guilty pleasure. I have already assumed that Hitec won't warrentee anything that's been "waterproofed" and then tested as such. I'm no noob, but I don't have much experience inside servos, as I am sure is shown by this post.

07-06-2011, 08:55 PM	#2
Charlie-III I wanna be Dave Join Date: Nov 2010 Location: 07456 N. NJ USofA Posts: 8,314	I can only state that, "A gear is a frikkin gear.....". Unless it has a stop or lever or sensor, the servo can give a rats butt where the gear is. Just saying. The centering pot is likely attached to one end or the other of the main motor so it knows where it is (rotationally).

07-06-2011, 11:09 PM	#3
SuperRoo Quarry Creeper Join Date: Dec 2008 Location: Australia Posts: 269	That was a long short story.... From what you've described it sounds like the rx could be the problem not servo. Also worn gears can cause a servo to hunt around at center, this is the down side to the cheaper 7954 with metal gears and tons of torque.

07-07-2011, 10:03 AM	#4
meatmonkey I wanna be Dave Join Date: Aug 2004 Location: central VT Posts: 2,300	I can dunk the front axle in water and it jitters, while the RX is high and dry in the reciever box. The servo only has a few weeks of use, so I'd hope the gears are still good. I run another 7954SH in my AX-10 with Rovers and it is still precise, quick, and strong after a year of horrible abuse. Yeah, sorry about the length. Just trying to be specific. And I incorrectly stated the section I am trying to seperate. The mounting ears are on the gear cover side, and there is another plastic case section under it that abuts the aluminum portion, and that is where I can't seem to seperate the servo to inspect for water intrusion.

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