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Bad Jitter On 6V (Futaba R138DP)

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Old 11-25-2010, 02:38 PM
  #1  
msiminoff
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Default Bad Jitter On 6V (Futaba R138DP)

I have a problem that suddenly appeared this week...
My receiver is glitching/jittering badly when powered with a 6V flight pack but doesn't jitter on 4.8V. I have tested this with multiple battery packs and my bench power supply. The receiver is a Futaba R138DP (Tx is T7CHP) and the servos are a combination of HiTec (rudder & elevator are 645MG's, ailerons are 475HB's) and Futaba (throttle). ALL of them are rated for 6V. The 6V battery is a 5-cell AA 1650mAh NiMh pack.
The problem is most pronounced when moving the sticks to their extremes and then quickly returning them to neutral. The snap-roll switch really exacerbates the problem. When the servos return to neutral it is as though they completely power-down for a second and then power-up again. There is also a consistent jitter about 1X per second.
The crazy thing is that I have been flying with this exact configuration for 4 years; Same airplane, same battery, same servos, same Rx! Suddenly this week I power up the plane at the field and... JITTER! Needless to say, I'm frustrated and not flying until I get the issue sorted-out.
Not that it matters but the plane is a Fliton Extra 330 freestyle powered by a Saito FA-82 .
Any help would be appreciated.
Thanks
-Mark
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Old 11-25-2010, 03:25 PM
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Default RE: Bad Jitter On 6V (Futaba R138DP)


ORIGINAL: msiminoff I have a problem that suddenly appeared this week...
My receiver is glitching/jittering badly when powered with a 6V flight pack but doesn't jitter on 4.8V. I have tested this with multiple battery packs and my bench power supply. The receiver is a Futaba R138DP (Tx is T7CHP) and the servos are a combination of HiTec (rudder & elevator are 645MG's, ailerons are 475HB's) and Futaba (throttle). ALL of them are rated for 6V. The 6V battery is a 5-cell AA 1650mAh NiMh pack.
The problem is most pronounced when moving the sticks to their extremes and then quickly returning them to neutral. The snap-roll switch really exacerbates the problem. When the servos return to neutral it is as though they completely power-down for a second and then power-up again. There is also a consistent jitter about 1X per second. The crazy thing is that I have been flying with this exact configuration for 4 years; Same airplane, same battery, same servos, same Rx! Suddenly this week I power up the plane at the field and... JITTER! Needless to say, I'm frustrated and not flying until I get the issue sorted-out.
Not that it matters but the plane is a Fliton Extra 330 freestyle powered by a Saito FA-82 . Any help would be appreciated.Thanks-Mark
If battery is fresh of charger, especially a field charger, the voltage on 5 cells is to high for those servos and frequently results in jitters, especiallyolder servos. (highervolts decreases life expectancy of servos)especially if TX is to close and swamping is also a factor. Does same occur after TX&RX batteries have been left sitting say 30 minutes off the charger for float to diminish?
One servo could have developed a dirty pot creating feedback, or lead/connctionhad developed a fault.
Test by removing one lead at a time to isolate which one if any. If a servo, open the case and remove the gears. Spray a zero-residue tuner cleaner, plastic friendly or protect plastic parts,into and around the "pot" and work it in. Once the cleaner has dried, re-install the gears and close up the case.
Any change to leads/extensions?
Jittering servos on 6V - different brand voltages on signal line
Servo - Extentions and Jitter

See also sub Section "Glitches & Jitter in Receiver, Servo & ESC - causes and cures " under
"Radio Systems, Accessories, Alterations and FAQ." at:
Alan's Hobby, Model & RC FAQ Web Links
Regards
Alan T.
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Old 11-26-2010, 02:20 AM
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msiminoff
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Default RE: Bad Jitter On 6V (Futaba R138DP)

Alan, thank you very much for your thoughtful response!

Yes, there is still jitter after the surface charge on the 5-cell pack has diminished. Using my variable power supply (& Fluke meter), I observed the following; Jitter always occurs from 6.35 down to 5.60V and then gradually diminishes as the voltage is further reduced. There is NONE at 5.10V. As expected there is no jitter with a freshly charged 4.8V NiMH pack.

The jitter occurs regardless of the distance between Tx and Rx.

The only extensions are for the (2) aileron servos and the jitter is there even when the ailerons are disconnected. I have not changed any of the extensions or servo leads since I originally assembled the airplane.

I did try disconnecting the servos one at a time to see if I could identify a single bad one and I even tried plugging in a brand new 645MG and it jittered too.
I will try cleaning the servo pots with some tuner cleaner and see if that helps. Good tip!

The strange thing is that this problem has shown up after several years (probably 100+ flights) of trouble free operation. I do understand that the 6V flight pack may have caused accelerated wear on the servos... but I am surprised that this issue appeared so suddenly.

Seems like the simple solution will be to switch to a 4.8V flight pack... though for a aerobatic plane like this one it will be a shame to lose the speed and torque of the higher voltage battery.

Happy Thanksgiving,
-Mark
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Old 11-26-2010, 08:38 AM
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Default RE: Bad Jitter On 6V (Futaba R138DP)

I had problems, years ago, with 6v batteries and servo jitters. In my case, it turned out to be cheap, 24ga Y Harness. Replacing the Y Harness or extension solved my problem. These were servos rated for 6v and NiMh batteries. Switching to 20ga or 22ga wire solved the problem.
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