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02-21-2010, 08:56 AM
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Join Date: Aug 2009
Location: Gadsden, Alabama
Posts: 41
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Audio system noise caused by Trutrak servos
Has anyone ever had any issues with a Trutrak autopilot causing a high pitched whine in the audio system whenever the autopilot is engaged? I just put a Sorcerer in an L-39 and also a Garmin GMA-340 audio panel and whenever I engage the autopilot I get a high pitched whine in the audio system. It's not just a noise; it sounds like an inverter whine only this one is at about 4.4khz. I am positive that it comes from the servos somehow because whenever the pilot is engaged, you can actualy hear this same sound from the servos like there is some sort of high frequency inverter in them. The real kicker of this whole thing is that the sound gets into the audio system through the Garmin GDL-69 audio output to the 340. If that line is not connected, you barely hear the noise and only during radio or intercom transmission. I have relocated audio and data lines, moved this, grounded that, ungrounded the other thing and nothing helps. I have heard this noise one time before when I installed a Sorcerer in another jet along with a 340. That ended up being one of the headset audio jacks touching ground and was easily remedied. This one though, has me pulling my hair out. Even the Trutrak guys were stumped. They had no idea where this noise would even be generated. All the signals going to the servos are either straight DC or ground. This noise is somehow generated in the servos and is feeding back into the audio system, through the GDL-69. If the GDL-69 is not in the circuit, in particular the audio output, no noise. Any input on this would be appreciated. Thanks in advance.
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02-21-2010, 11:30 AM
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Join Date: Sep 2005
Location: Dothan, Alabama
Posts: 1,487
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Stick one of these on the avionics bus. I had the same noise issue with the electronic ignition. This made it all go away.
http://www.bandc.biz/electrolyticfiltercapacitor.aspx
+ to bus, - to ground. Keep the leads as short as possible.
__________________
Alton DeWeese
N526RV RV7A Tip Up, IO360 180 W/Hartzel BA prop.
Flying ~950 hours since Aug 2010
N4IDH
Construction Log
?The secret of getting ahead is getting started. The secret of getting started is breaking your complex overwhelming tasks into small manageable tasks, and then starting on the first one.?
?Mark Twain
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02-21-2010, 12:04 PM
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Join Date: Feb 2005
Location: 20km outside of Zurich, Switzerland
Posts: 468
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Ask Bernie!
Bernie has mounted a filter on his installation, maybe he has an aswer why and where you can get it.
http://www.flyvans.com/log/2009_11_18_front_skin.html
You will find him here or contact via his page.
Regards, Dominik
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02-21-2010, 05:06 PM
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Join Date: Feb 2005
Location: Zurich, Switzerland
Posts: 466
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we put in a DB-25 filter at the trutrak control-head connector.
however, our problem wasn't audio noise but rather a noticeable autopilot pitch change while transmitting on com1.
in any case, i would certainly try the db-25 filter as it's cheap and the trutraks appear to have a history of some emi/rfi issues.
check http://trutrakap.com/forum/showthread.php?t=43
rgds, bernie
__________________
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Bernie Daenzer, Alex Lichtensteiger
www.flyvans.com
RV-7A
S/N 72072, Flying!
HB-YMT (Switzerland)
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02-22-2010, 01:32 AM
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Join Date: Feb 2005
Location: 20km outside of Zurich, Switzerland
Posts: 468
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Thanks Bernie ;-)
Dominik
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02-23-2010, 07:50 AM
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Join Date: Aug 2009
Location: Gadsden, Alabama
Posts: 41
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Quote:
Originally Posted by flyvans.com
we put in a DB-25 filter at the trutrak control-head connector.
however, our problem wasn't audio noise but rather a noticeable autopilot pitch change while transmitting on com1.
in any case, i would certainly try the db-25 filter as it's cheap and the trutraks appear to have a history of some emi/rfi issues.
check http://trutrakap.com/forum/showthread.php?t=43
rgds, bernie
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Well I have tried so many things, including an 8400uf cap across the servo DC line. That made a noticable difference but did not cure it. As I said, this isn't noise getting into the Trutrak, it's noise getting into my audio system from the Trutrak servos. It's a 4.4 khz whine. Yesterday I even went so far as to replace all the servo lines including the DC and ground with double shielded multiconductor cable which was rerouted away from everything. The noise was definitely diminished but still there, especially in the radio sidetone. I just got off the phone with Trutrak and they are sending me some new servos. I am not 100% convinced the servos are the problem but I'm not convinced they aren't the problem either. Bernie, you said Trutraks have a history of emi/rmi issues? Like what kind of issues?
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02-23-2010, 01:35 PM
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Join Date: Mar 2008
Location: Springdale, AR
Posts: 547
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Quote:
Originally Posted by brettt777
Well I have tried so many things, including an 8400uf cap across the servo DC line. That made a noticable difference but did not cure it. As I said, this isn't noise getting into the Trutrak, it's noise getting into my audio system from the Trutrak servos. It's a 4.4 khz whine. Yesterday I even went so far as to replace all the servo lines including the DC and ground with double shielded multiconductor cable which was rerouted away from everything. The noise was definitely diminished but still there, especially in the radio sidetone. I just got off the phone with Trutrak and they are sending me some new servos. I am not 100% convinced the servos are the problem but I'm not convinced they aren't the problem either. Bernie, you said Trutraks have a history of emi/rmi issues? Like what kind of issues?
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I think what Bernie is referring to is the RF/pitch change problem. This is a symptom of RF bouncing around inside the airframe and/or getting transmitted back to the panel through wiring into the AP controller. The pitch gyro is quite sensitive to RF energy input. When the controller gets hit with RF, the energy spike skews the gyro's analog output, causing a pitch change.
If I'm off target and someone knows something I don't know, please fill me in! 
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02-24-2010, 12:59 PM
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Join Date: Aug 2009
Location: Gadsden, Alabama
Posts: 41
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One thing about this noise, you can actualy hear an audible high pitched whine coming from the servos when they are engaged. I don't remember hearing this before. Has anyone else noticed this? It is the same exact noise that I hear in my audio system. I actualy measured it on the servo DC line. It was 4.4 khz. Lucas is there some sort of inverter in the servo circuitry that could somehow be feeding back into the DC system?
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02-24-2010, 01:26 PM
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Join Date: Apr 2005
Location: KSLC
Posts: 4,021
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Quote:
Originally Posted by brettt777
One thing about this noise, you can actualy hear an audible high pitched whine coming from the servos when they are engaged. I don't remember hearing this before. Has anyone else noticed this? It is the same exact noise that I hear in my audio system. I actualy measured it on the servo DC line. It was 4.4 khz. Lucas is there some sort of inverter in the servo circuitry that could somehow be feeding back into the DC system?
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I've followed this thread for a few days. I don't get the whine, and a friends aircraft doesn't either. My Trutrak is for altitude, and he has both pitch and roll.
P.S. --- I mentioned the friend's airplane because I have a lot of time in it. Just called him to verify, and he say's it must be something else.....besides the servos.
L.Adamson --- RV6A
Last edited by L.Adamson : 02-24-2010 at 01:33 PM.
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02-24-2010, 04:07 PM
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Join Date: May 2006
Location: Ankeny, Iowa
Posts: 434
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Quote:
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One thing about this noise, you can actualy hear an audible high pitched whine coming from the servos when they are engaged. I don't remember hearing this before. Has anyone else noticed this? It is the same exact noise that I hear in my audio system. I actualy measured it on the servo DC line. It was 4.4 khz. Lucas is there some sort of inverter in the servo circuitry that could somehow be feeding back into the DC system?
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I certainly can't speak for Trutrak but I know some Servo systems use PWM (pulse width modulation) to control torque or current supplied to the Servo motor windings. Could be the 4.4 khz is the PWM frequency they are using to control Servo torque. You would be able to physically hear this coming from the Servo motor and it could also cause noise on the supply voltage feed.
__________________
Joe Condon
Ankeny, Iowa
RV9A - 647JC - 300 hrs - SOLD 6/23/15 
OneX - Under Construction - For Sale
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