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  #1  
Old 02-21-2010, 08:56 AM
brettt777 brettt777 is offline
 
Join Date: Aug 2009
Location: Gadsden, Alabama
Posts: 41
Default 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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  #2  
Old 02-21-2010, 11:30 AM
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AltonD AltonD is offline
 
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Location: Dothan, Alabama
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Default

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.
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  #3  
Old 02-21-2010, 12:04 PM
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swisseagle swisseagle is offline
 
Join Date: Feb 2005
Location: 20km outside of Zurich, Switzerland
Posts: 468
Default 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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  #4  
Old 02-21-2010, 05:06 PM
flyvans.com flyvans.com is offline
 
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Location: Zurich, Switzerland
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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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  #5  
Old 02-22-2010, 01:32 AM
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swisseagle swisseagle is offline
 
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Thanks Bernie ;-)

Dominik
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  #6  
Old 02-23-2010, 07:50 AM
brettt777 brettt777 is offline
 
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Location: Gadsden, Alabama
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Quote:
Originally Posted by flyvans.com View Post
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
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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  #7  
Old 02-23-2010, 01:35 PM
TrutrakTech TrutrakTech is offline
 
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Location: Springdale, AR
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Quote:
Originally Posted by brettt777 View Post
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?
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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  #8  
Old 02-24-2010, 12:59 PM
brettt777 brettt777 is offline
 
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Location: Gadsden, Alabama
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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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  #9  
Old 02-24-2010, 01:26 PM
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L.Adamson L.Adamson is offline
 
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Quote:
Originally Posted by brettt777 View Post
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?
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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  #10  
Old 02-24-2010, 04:07 PM
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647jc 647jc is offline
 
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Location: Ankeny, Iowa
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Quote:
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?
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.
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