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  #1  
Old 05-06-2020, 10:42 PM
Scott Hersha Scott Hersha is offline
 
Join Date: Jan 2005
Location: Cincinnati, OH
Posts: 2,015
Default PTT causing problems

Before I call someone at GRT, maybe someone here can help, in fact I?m pretty sure of it. On my RV4 with two GRT screens and a Garmin GTR200B com radio, I often get spurious warnings, like ?EGT too High, or Fuel Pressure too High?. These are visual warnings on my EFIS with no oral warning given, and no exceedences noted on my engine indication system. Funny thing is, this only happens when I key my mic to transmit on VHF COM. My PTT switch is on my Ray Allen stick grip. One wire to the PTT on the stick goes to the -200B, and the other one goes to the ground buss, located on the firewall, where all my grounds are located, except for a couple out in the wingtips for lights. Also, the CHT and EGT time graphs show a dive to zero when I key the mic. Fuel pressure indication sometimes goes crazy too when I key the mic. There is no noise in my headset associated with this. Sounds like some kind of grounding issue to me, but what should I look for? I?m thinking initially of grounding my PTT to an airframe bolt near the point of origin.

Thanks
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RV6/2001 built/sold 2005
RV8 Fastback/2008 built/sold 2015
RV4/bought 2016/sold/2017
RV8/2018 built/Sold(sadly)
RV4/bought 2019 Flying
RV6/Used kit purchased 2021 building
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JAN2021
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  #2  
Old 05-06-2020, 11:49 PM
BobTurner BobTurner is offline
 
Join Date: Dec 2011
Location: Livermore, CA
Posts: 7,854
Default

This likely is not the PTT, but rather too much RF in the cockpit.
1. Carefully check your coax from com to antenna. Pay particular attention to the coax shield connections at both ends.
2. Check the antenna ground connections. Make sure there?s no corrosion.
3. If not already done, use shielded cable for the RS232 line from the EIS to EFIS.
4. Wrap aluminum foil around all the wires carrying signals into the EIS.
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  #3  
Old 05-07-2020, 07:30 AM
Scott Hersha Scott Hersha is offline
 
Join Date: Jan 2005
Location: Cincinnati, OH
Posts: 2,015
Default

Thanks Bob. I?ll check those items in the order you listed them. I used the GRT supplied harness. None of it is shielded, so I?ll start with the EIS to EFIS wire.
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RV6/2001 built/sold 2005
RV8 Fastback/2008 built/sold 2015
RV4/bought 2016/sold/2017
RV8/2018 built/Sold(sadly)
RV4/bought 2019 Flying
RV6/Used kit purchased 2021 building
Cincinnati, OH/KHAO
JAN2021
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  #4  
Old 05-07-2020, 01:24 PM
pa38112 pa38112 is offline
 
Join Date: Mar 2008
Location: Clarksboro, NJ
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Default

Quote:
Originally Posted by BobTurner View Post
This likely is not the PTT, but rather too much RF in the cockpit.
Wouldn't FR always be an issue and not just when he keys the PTT ? RF causes noise. This is presenting like a grounding problem.

I would suspect a potential difference between where various things are grounded or a loose ground. You say everything is grounded to the Buss, but are there also local grounds? Maybe the trays have grounds to the air frame? You could be creating a ground loop every time you hit the PTT.

I think grounding the PTT to the air frame local to the switch VS the buss is a good starting point.
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  #5  
Old 05-07-2020, 01:44 PM
BobTurner BobTurner is offline
 
Join Date: Dec 2011
Location: Livermore, CA
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Quote:
Originally Posted by pa38112 View Post
Wouldn't FR always be an issue and not just when he keys the PTT ? RF causes noise. This is presenting like a grounding problem.

I would suspect a potential difference between where various things are grounded or a loose ground. You say everything is grounded to the Buss, but are there also local grounds? Maybe the trays have grounds to the air frame? You could be creating a ground loop every time you hit the PTT.

I think grounding the PTT to the air frame local to the switch VS the buss is a good starting point.
When I said RF I meant the actual transmission from the transmitter. Here?s a not real easy test: make up a long (like 50? or more) coax with a connector that mates to your radio on one end. Attach the other end to a 50 ohm dummy load (for short transmissions 3 150 ohm, 1 W resistors in parallel will work (not wire wound!). Connect to radio, place dummy load as far away as possible (outside closed metal hangar door preferable). Repeat test (briefly push PTT, look for egt drop out, false warnings, etc). If they went away then you know the source of your problem. BTW an open braid to antenna ground plane connection will cause all sorts of havoc, like this. I would not ground the PTT at multiple locations. This is the very definition of a ground loop, although I think unlikely to cause problems here.
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  #6  
Old 05-07-2020, 01:50 PM
bjdecker's Avatar
bjdecker bjdecker is offline
 
Join Date: Jun 2005
Location: Georgetown, TX
Posts: 952
Default Had something similar...

...on my 1980 Mooney 231; Keying up either KX-155's to transmit would cause all of the instrument gauges to indicate offscale. This was traced to a ****-poor ground contact on the gauge cluster - Mooney used a single point contact "finger" that just oxidized overtime. Fresh ground lead with proper termination resolved the issue.

Look carefully at the EIS grounding - are they using fast-on terminations or PIDG ring terminals?

B
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  #7  
Old 05-07-2020, 01:59 PM
BobTurner BobTurner is offline
 
Join Date: Dec 2011
Location: Livermore, CA
Posts: 7,854
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Quote:
Originally Posted by bjdecker View Post
...on my 1980 Mooney 231; Keying up either KX-155's to transmit would cause all of the instrument gauges to indicate offscale. This was traced to a ****-poor ground contact on the gauge cluster - Mooney used a single point contact "finger" that just oxidized overtime. Fresh ground lead with proper termination resolved the issue.

Look carefully at the EIS grounding - are they using fast-on terminations or PIDG ring terminals?

B
Yes, a definite possibility. You?ve heard of MOS (metal oxide semiconductors). What happens is an oxide layer between contacts acts like a diode, and, in the presence of RF from the transmitter, generate all kinds of harmonics that in turn cause trouble.
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  #8  
Old 05-07-2020, 02:31 PM
jselvatici jselvatici is offline
 
Join Date: Mar 2019
Location: FL
Posts: 188
Default Similar problem with E.I. FL-2C

This post just came right in time.

I have an EI FL-2CA installed on my new (old) build Vans RV-9A and during recent tests, I was able to notice something different on the unit. I power up the plane, the fuel quantity comes up normally and stays that way. If I key the PTT (Push To Talk) to transmit on the radio, the fuel quantity slowly goes down until reach zero. Once I release the PTT, the quantity indication start to slowly goes back to were it was.
Have you guys ever seen something like this? My antenna cables runs far from the fuel probe wires.
I have recorded a video in case you want to have a better understanding of the reported behavior:

https://youtu.be/czW2fE2RYms

I will look for potential ground issues as reported for some other problems here.
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  #9  
Old 05-07-2020, 03:05 PM
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N941WR N941WR is offline
 
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Location: SC
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For me, the problem usually comes after I press the PTT and say something (stupid).
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  #10  
Old 05-07-2020, 08:46 PM
Scott Hersha Scott Hersha is offline
 
Join Date: Jan 2005
Location: Cincinnati, OH
Posts: 2,015
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Guess I’ll have to check all my grounds tomorrow. I checked the COM antenna today and the ground plane and braid was grounded solidly. I’ll need to remove my boot cowl to access my ‘forest of tabs’ ground block, but I can check the security of all grounds right there. I don’t suspect a corrosion problem, because I did a panel upgrade just two months ago where I replaced all wiring in the plane, including all new RG400 coax. The wingtip mounted LED position lights, wing LED landing lights, and tail mounted LED position/strobe lights are all locally grounded, but the lights were all off today when I tested my PTT - in my hangar with the engine not running. Pressing the PTT switch resulted in my fuel pressure indication starting to indicate pressure, oil pressure indicating a value, EGT and CHT jumping around. I’m thinking I may need to check the security of my engine to airframe ground(s). I have two fat wires going from engine bolts to the firewall, but not directly attached to the brass bolt that is part of the forest of tabs. The firewall is common to those two fat wires. The fat wires are attached to engine mount bolts that hold the engine mount to the firewall. The inside end of one of those bolts is where the battery negative terminal is connected. Battery is on the cockpit side of the firewall in an enclosure. I’m thinking as I’m typing. Maybe I shouldn’t allow any distance on the firewall between these big grounds, and maybe not use engine mount bolts as a ground path? What do you think? I didn’t have a problem before my panel upgrade, but I didn’t have any electronic gauges, except for the original GRT EIS, and it worked fine. Now I have two new GRT EFIS screens, which is where the problem is showing up.

Opinions about my grounds please.....
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SH
RV6/2001 built/sold 2005
RV8 Fastback/2008 built/sold 2015
RV4/bought 2016/sold/2017
RV8/2018 built/Sold(sadly)
RV4/bought 2019 Flying
RV6/Used kit purchased 2021 building
Cincinnati, OH/KHAO
JAN2021
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