Ruddermayhemcom

My Feedback: (23)

http://www.youtube.com/watch?v=9JAbl...layer_embedded

http://www.youtube.com/watch?v=f94CcpnUB-4

http://www.youtube.com/watch?v=yMWJuLfQFAE

http://www.youtube.com/watch?v=oANQHMaLX2s

Ruddermayhemcom

My Feedback: (23)

The known issue of slight jitter of the 79XX series servos causes a cascading domino effect of internal self destruction. When the pin pockets round out causing the jitter to get worse, this puts an enormous strain on the amps causing them to burn out.

I think the service bulletin is off base. http://www.hitecrcd.com/support/service-bulletins/index.html
I don't think Hitec really knows how big the problem is or how to fix it . I am merely offering my "Consumer feed back" as accurately as I can for them to better analyze the issue.

In reading the context of the service bulletin on "jitter" It appears Hitecs current position on this is the jittering is nothing to worry about ....but this is the first statement on the bulletin .

"This information pertains to known or observed issues with Hitec products."
Well we cant disagree with that. Jittering has been a known issue since the 7955 was released.And the 7950 has been out for long while now too. Much time has passed and the issues still exist. The service bulletin suggests that we just settle for it and accept it.

"Customers have noticed that these high resolution servos have a tendency to jitter when using 7.4 volt power packs and a minimal load is applied."
Others and myself know this not to be a correct statement. They jitter on 6.5v a123s also just not as bad.


"This is due to the high resolution circuit adjusting for slight position variances in the gear teeth"
We can discredit that statement after seeing evidence of pin pockets rounding out as a result of it ...on a properly set up plane.
And if that is the case , why don't they jitter when new? Its a creeper that gradually gets worse (only at the programed centers) if the jittering is "due to the high resolution circuit adjusting for slight position variances in the gear teeth" it would jitter across the entire range of motion at any given trim point.


"This usually only occurs with minimal loads, for example, when the plane is on the ground in a static position. "
Thats when the real damage is occurring! Also, myself and others know this not to be a correct statement as we can see the jitter in slow high alpha maneuvers after the pin pockets get sloppy enough from the jitter intern exacerbating the issue.

.


"Once airborne the servo experiences flying loads and the issue resolves itself and should not impact on the flying experience. "
Once airborne the servo experiences flying loads that dampen the jittering . The issue does not resolve it self. It is still present as it causes a gradual cascading domino effect of internal self destruction. The real damage is occurring when the plane is on the ground not experiencing flying loads. Especially when flipping the propeller.

Ruddermayhemcom

My Feedback: (23)

I am 99% sure the Pots are the root cause after the testing I did. I say this because there still exists the possibility that a component farther down the chain ahead of the pot could be causing the pot to prematurely fail.

To recap:



1) Pots prematurely wear out at center causing a gradual progression of jitter.


2) Next the gear pin pockets oval out intensifying the jitter.


3) And finally stress the amps to the point of failure.


The 7950TH pots have X502TD stamped on them....thats all. They are sealed. Plastic tabs are melted at the corners as a means of sealing it shut. You would have to break it to get it open.
I may search for an aftermarket pot that is the same shape and size for further testing. I am not sure the processor in the servo will recognize after market tho.

This is a pic of the 7950TH pot.

Ruddermayhemcom

My Feedback: (23)

I opened up one of the pots before I send them in . The pot has an outer track and inner track and looks to be carbon.
Outer track runs around the shaft hole and connects the green and red wires .
The inner track connects to the yellow wire at center.
The shaft has metal prongs on the bottom that contact the carbon tracks.

The outer track had visible irregularities and slight depressions almost 1mm wide in the location where the prong would have been to hold the ailerons at center. (this is where the jitter was occurring.)
The inner track had small regularities where the prongs contacted in the corresponding location for center also. About 1/4mm wide.
The spots almost looked like they had been sanded with 1000 grit sand paper. I could ever so slightly see what looks like a metallic track showing through the carbon. Like its a carbon coated metal track.


This is the best pick I could get. My camera wont focus down that small. If I had a large magnifying glass I might be able to pick it up

I am sending the pots in to Hitec in the morning for testing.

Meanwhile I got a few other things up my sleave....

Ruddermayhemcom

My Feedback: (23)

The bottom line is this gittering issue with the 7950TH is a result of the worn spots in the tracks. The pots are PREMATURELY wearing at programed center and on just 6.5v . Of coarse this was confirmed in troubleshoot vid part 2.



Jittering has been observed as a wide spread issue. The service bulletin even confirms "jittering as a known and observed issue". Tho it incorrectly describes the cause totally oblivious to the severity of how bad this issue really is. It also suggests the consumer accept the issue as if action will never be taken to resolve it.



The evidence is solid .



Further more jittering leads to ovaled out gear pin pockets and burnt amps from the stress. Thats why when you guys send them in for service they need center cases and more than often new amps also depending how early you catch it.



Now the looming question.... WHY?



Is something farther down the line in the servo causing this? Is Hitec using pots not up to specks for its counterparts ? Or are the pots just poor quality ?

MetallicaJunkie

keep us posted...i have a 7950 that runs sweet...but a couple of 7955s that jitter.... one on my rudder looks like its having a seizure.... i sent it off twice and hitec said there is nothing wrong with it. with no control surface attached it's fine, thats probably how they test them.


interesting videos... hitec needs to take a look at them. looks like i might have to switch brands


let us know if you find an alternative pot.... i might be able to revive a couple of bad servos that hitec couldnt fix, because they said they were fine


post this on FG there is way more giant scale traffic there

jonkoppisch

My Feedback: (162)

If you give up 3d and start pattern flying you wouldn't have that problem

Capt Cash

My Feedback: (1)

Wish I had known this before I bought 10 Hitec 7954SH's earlier this year. One is starting to jitter now on the rudder after about 10-15 flights on a 25% MX2. I fly it hard with lots of knife edge but Iwould think a plane this size shouldn't even phase ahigh torque servo like this. It does it only around center while sitting on the ground and with the engine off.Whata crapper knowing it's only gonna get worse. I'm using A123's.

Thanks for sharing your findings Ruddermayhemcom. I'm sure you've explained this in detail to Hitec, hopefully they will make something of it.

Can anybody comment on comparabe Futaba servos?

Jezmo

I have a number of HS7985MG and HS7955TG servos that are doing this. They only do it on 6+ volts. My nephew has a JR8711 on his rudder that also does it. If the pots fix it let us know as I would certainly be willing to change them out. I have looked at mine and the shaft holes are still good and tight in the case. Maybe I can catch it before any major damage is done.

Edited to change 97 to 87, my dumb thumbs at it again...LOL

Ruddermayhemcom

My Feedback: (23)

hahaha pattern shmattern! Hey I finally got the 1'' arm lecture.[:@]

Ruddermayhemcom

My Feedback: (23)

Ruddermayhemcom

My Feedback: (23)

I found this just now .

http://www.f l y i n g g i a n t s.c...1&postcount=11 (take the spaces out of f l y i n g g i a n t s for the link to work)

Heck even his plane has a very early case jitter setting in and he is sponsored by Hitec.
Judging by the jitter it looks to only be on its 5th flight at that point.

http://www.youtube.com/embed/VfGvOriHuU4

Ruddermayhemcom

My Feedback: (23)

I found this out too. Just look. The JR8711 had jittering issues too.

The serial no. is different but other than that the pots are identical on the outside ( same dimensions). Made from the same mold. Same stamps and all on the top and bottom....too small for the camera to focus on. One stamp is a lightning bolt and the others are very small circles with a no. 5 or 6 .

Ruddermayhemcom

My Feedback: (23)

I checked the pots in the trusty old 5955s as a potential candidate for swapping and of coarse they are the unsealed older style(big and round) ...be like jamming a round peg in a square hole ..been there done that! lol
I wonder if Hitec's POT supplier discontinued the old style pots . Could be why they discontinued the 5955 and redesigned the cases in the 79XX around the (newer version square POTS)

Hmmmm....... the JR 87XX series came out at the same time too using the newer square pots notably from the same supplier. Surely looks like the POT supplier's (pot redesign) was the driving force for a servo redesign (Hitec7955 and JR8711).

Hitecs got to know the square pots are not up to par. ...could be why they are throwing that 7990 out there with the magnetic encoder for there consumers to test.
When your pot supplier switches up on ya it just changes everything....
Just like a pot supplier, they give ya the good stuff at first then they give ya the junk ...

Ruddermayhemcom

My Feedback: (23)

I finally got on the ringer and called Hitec up.... When I toldum who I was ...he says "Wait a minute your....."

I says "Yep."

"Oooh theres been some talk around here ." he says


Then we had an interesting conversation...

Ruddermayhemcom

My Feedback: (23)

I gave him the whole summery of the trouble shoot in a nutshell.

I explained the reoccurring jitter problem.

I told him that when I replaced the pots with new pots from a new 7950 it totally eliminated the jitter even on 7.4v .
He says " Well it probably won't fix it for long"

I says I KNOW! Then there was silence ...I'm thinking what does he know??? When I began to pry him he told me to deal with Mike Mayberry.


With precedence he informed me that they want me to send the pots in .

"They want to get them in there hands to run some tests." he says.

He told me for now he would send me out some more pots to get me by for a little while longer.I proceeded to tell him this IS a wide spread issue and not just isolated. He didn't deny it one bit and again urged me to send the pots in for testing.

I think we are finally getting some where guys.

Before I let him off the hook I got him to confirm the pots are purchased from a wholesale manufacturer in Asia.
I told him I already suspected this because the JR 8711 uses the same ones from the same manufacturer. He chucked about that.
Then in all seriousness . I blurted out " They had jittering problems too ya know"

Ruddermayhemcom

My Feedback: (23)

The Hitec service bulletin is written in such a way, it suggests that the consumer accept the known and observed issues with the 79XX servos.

They need to know the consumer does not except there excuses. We want the known and observed issues resolved. Thats why we pay for premium servos.

Ruddermayhemcom

My Feedback: (23)

I already sent all 8 of the servos in last winter for Hitec to take 7 weeks to fix. They put new center cases and amps in them. I was leery of keeping them after this because I knew something just wasn't right with this servo. They assured me I would never have a problem again with them. They started to jitter again a couple flights later until the jitter progressively got to the point it was in the fist place. [:@] Just as I suspected they would! I serviced them my self and again the jitter came back. Hitec will tell you jitter is normal tho. Clearly we see what it can do to a servo. who are they fooling?! [:@]

jonkoppisch

My Feedback: (162)

I'm tellin ya, you're over driving the servos! If you have a maximum of 1/4 inch throw it wont matter how much they jitter. You'll never see it

Capt Cash

My Feedback: (1)

There is definitely some jittering going on in that video of Bonedoc's plane. Being that he's a Hitec sponsor I'd be interested in his viewpoint on the subject. Hemight be able to pull a little moreweight withthem on this issue too. I think there might be a few fellasrunning 7950's at my field, I'm gonna try and get their thoughts on the subject.

BBBlue

Subcribed.

mike31

My Feedback: (67)

Why just not use that type of servo?

Ruddermayhemcom

My Feedback: (23)

Were not letting Hitec off the hook that easy [sm=tired.gif] Besides I am sitting on 18 defective 7950s!

Ruddermayhemcom

My Feedback: (23)

Oooh, So the 7950TH is an IMAC servo?
TROLL ! LOL

jonkoppisch

My Feedback: (162)

When you decide to get rid of those shaky suckers I'll give ya .25 a piece for them