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  #1  
Old 03-11-2019, 08:56 PM
dbegeman's Avatar
dbegeman dbegeman is offline
 
Join Date: Mar 2018
Location: Conifer, CO
Posts: 38
Default Electrical Gremlins

I have an issue that has been bugging me since I bought my airplane a year ago. Unfortunately I didn't realize it was an issue until after the pre-purchase inspection was done and the paperwork signed.

What happens is, when ever I push the PTT button (on either stick) my manifold pressure gauge starts decreasing and the ammeter deflects 180 degrees from zero amps. It does it on either radio (ICOM 200 and a Bendix/king nav/comm) and with the engine running or just on battery power. The ICOM makes the ammeter deflect one way and the Bendix/King the other way but both deflect 180 degrees from zero amps. In addition I have a switch labeled AMP that on the right selection says ALT and the left selection is unlabeled and I have no idea what function it serves. I included pics (and hopefully I can lable them). Any help would be appreciated!!


MAP before pushing PTT


MAP with PTT pushed


Ammeter before pressing PTT


Ammeter with PTT pressed


Ammeter with PTT held down longer and the "AMP" switch in the upper right hand corner.



Thanks in advance,
Dan
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Last edited by dbegeman : 03-11-2019 at 08:59 PM.
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  #2  
Old 03-11-2019, 09:27 PM
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N804RV N804RV is offline
 
Join Date: May 2013
Location: Mount Vernon, Wa
Posts: 827
Default

What kind of intercom/audio panel set up do you have? The push to talk switch should just connect the transmit key-line to audio common. There should never be any 12vdc on that circuit.
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  #3  
Old 03-11-2019, 09:49 PM
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dbegeman dbegeman is offline
 
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Location: Conifer, CO
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Default

Ken,

The audio panel is a ICOM200 and a Bendix/King Nav com (I'll get the model tomorrow along with the intercom model). There is a switch that toggles between com1 (bendix) and com2 (Icom) for transmission only.

Dan
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  #4  
Old 03-12-2019, 08:53 AM
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Walt Walt is offline
 
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Location: Dallas/Ft Worth, TX
Posts: 6,342
Default

This is 'normal' operation for the unshielded Van's gauges.
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  #5  
Old 03-12-2019, 09:49 AM
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airguy airguy is offline
 
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Location: Garden City, Tx
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Default

Quote:
Originally Posted by Walt View Post
This is 'normal' operation for the unshielded Van's gauges.


A little tongue-in-cheek perhaps, but unfortunately Walt is right. The standard analog gauges are not good at rejecting RFI and this is a common complaint.
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  #6  
Old 03-12-2019, 10:07 AM
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N804RV N804RV is offline
 
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Location: Mount Vernon, Wa
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Default

How well do the radios work? Do people complain that your transmit is garbled, noisy or weak? How does the sidetone sound?
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  #7  
Old 03-12-2019, 10:17 AM
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olyolson olyolson is offline
 
Join Date: Dec 2009
Location: St Louis, MO
Posts: 1,082
Default Electrical Gremlins

Ok I admit I am an electrical neophyte. For the brain trust here what would happen if he re-wired the manifold pressure and ammeter gauges with shielded wire?
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  #8  
Old 03-12-2019, 10:25 AM
rv7charlie rv7charlie is offline
 
Join Date: May 2006
Location: Pocahontas MS
Posts: 3,888
Default One word: Grounds

Check ground locations for both antagonists and victims.

Make sure that your antenna coax has solid shielding termination on both ends. Make sure that the antenna(s) are properly and securely grounded to the airframe. Make sure that the ground paths for the radio(s) and the instruments are grounded to a 'single point' on the airframe, preferably near the battery negative terminal (near where the battery is grounded to the airframe).

No doubt others have had issues with those gauges, *but* if not everyone has the issues, then either mfg of the instruments are highly inconsistent, or installation practices are highly inconsistent. Which is more likely?
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  #9  
Old 03-12-2019, 10:28 AM
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skylor skylor is offline
 
Join Date: Aug 2009
Location: Southern California
Posts: 1,022
Default Antenna Grounding

Quote:
Originally Posted by dbegeman View Post
I have an issue that has been bugging me since I bought my airplane a year ago. Unfortunately I didn't realize it was an issue until after the pre-purchase inspection was done and the paperwork signed.

What happens is, when ever I push the PTT button (on either stick) my manifold pressure gauge starts decreasing and the ammeter deflects 180 degrees from zero amps. It does it on either radio (ICOM 200 and a Bendix/king nav/comm) and with the engine running or just on battery power. The ICOM makes the ammeter deflect one way and the Bendix/King the other way but both deflect 180 degrees from zero amps. In addition I have a switch labeled AMP that on the right selection says ALT and the left selection is unlabeled and I have no idea what function it serves. I included pics (and hopefully I can lable them). Any help would be appreciated!!


MAP before pushing PTT


MAP with PTT pushed


Ammeter before pressing PTT


Ammeter with PTT pressed


Ammeter with PTT held down longer and the "AMP" switch in the upper right hand corner.



Thanks in advance,
Dan
I previously had my #2 COM radio (SL-30) connected to an Archer wingtip antenna and I experienced similar issues on a GRT avionics Engine Instrumentation System. Pushing the push-to-talk to transmit from this radio would cause large spikes in the readings of various engine parameters. When I switched from the wingtip antenna to a belly mounted bent-whip, the problems went away completely. I suspect that the problem was actually due to poor grounding of the wingtip antenna that caused RF energy to radiate into the aircraft and be picked up by the EIS wiring.

Other things that may cause this is poor quality antenna cable connector installation. The moral here is to check the quality of installation of your antenna and antenna wiring to be sure that the antenna and cable shields are well grounded.

Skylor
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  #10  
Old 03-12-2019, 10:56 AM
rocketman1988 rocketman1988 is offline
 
Join Date: Jun 2010
Location: Sunman, IN
Posts: 2,724
Default Also...

There is a section in the Aeroelectric Connection book that may help.

All that happens when you push a PTT is the mic key wire goes to ground. I would start by looking at how it and the affected instruments are grounded. Are you setting up a ground loop when you key the mic?
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