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  #11  
Old 03-12-2019, 11:36 AM
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Carl Froehlich Carl Froehlich is offline
 
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This is not a ground loop problem. This is basic RFI.

Get someone with an antenna analyzer to ring out our feed lines and antennas. If you have a bad connection this will show it. What you can do in the meantime is look at wire bundles - do you have feed lines bundled with coax?

Shielding instrument wires would not hurt. You may need to experiment with small disc capacitors across the analog gauge terminals ls - this will bypass the RFI to ground. This was standard practice in ?back in the day? radio work.

Carl
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  #12  
Old 03-12-2019, 01:53 PM
rocketman1988 rocketman1988 is offline
 
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Default Funny...

Funny, Carl, that you can make that statement with absolute certainty.

I had a similar issue in my C172...and it WAS a ground loop issue. That is why I suggested it...
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  #13  
Old 03-12-2019, 02:07 PM
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airguy airguy is offline
 
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Quote:
Originally Posted by rocketman1988 View Post
Funny, Carl, that you can make that statement with absolute certainty.

I had a similar issue in my C172...and it WAS a ground loop issue. That is why I suggested it...
Was it though? Or was it just a mechanic telling you it was a ground loop issue because he didn't figure you were smart enough to know the difference?
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  #14  
Old 03-12-2019, 03:54 PM
rocketman1988 rocketman1988 is offline
 
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Default No

No it actually was a ground loop problem...
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  #15  
Old 03-12-2019, 05:40 PM
rv7charlie rv7charlie is offline
 
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Importantly for analysis, 'ground loop' is only one way (of several ways) to have a ground problem.

Check your grounds and ground paths.
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  #16  
Old 03-13-2019, 07:33 AM
Southern Pete Southern Pete is offline
 
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It is likely to be a RFI problem as many others have found in the past.
Depending on how much your time is worth it may be easier to install a different (better designed) gauge.
Pete
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  #17  
Old 03-13-2019, 02:28 PM
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dbegeman dbegeman is offline
 
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Thank you everyone for the help! The antennas are belly mount so I will check the connections. I tried running an independent ground for the ammeter and it made no difference, I did not try isolating the ground on the manifold pressure gauge but will when the weather gets better. I will look at the wire bundles and try to see if the coax runs with the electrical. I did have a previous owner reach out to me and said he had the same problem and just ignored/dealt with it. The radios did have some issues but that was when the "AMP" switch was in ALT. Also I still have no idea what the "AMP" switch does.

Thanks,
Dan
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  #18  
Old 03-14-2019, 07:40 PM
Robb Robb is offline
 
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Mine has done it since it was built. Never could figure what causes it in 380 Hours.I always thought it was a bad ground but never figured it out. I guess I am used to it
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  #19  
Old 03-14-2019, 08:14 PM
molson309 molson309 is offline
 
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The first thing to check is if there is high SWR on the antenna. if this is high it causes the coax to radiate RF which causes all kinds of problems.

If you have high SWR there's all kinds of things you can do to fix it. I had a bad connector at the antenna end of my coax and it caused **** with my instruments. Fixed that and the problems went away.

However - as others have already posted - if you have instruments that are sensitive to RF you may have to experiment with bypass options at the instruments themselves.
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  #20  
Old 03-15-2019, 08:33 AM
Flyingleap Flyingleap is offline
 
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Default Belly Mount

Everything Charlie and Mark X 2. As soon as he said "belly mount" my eyes perked up. Don't think I've ever seen better than 4 to 1 SWR on a Comant
bent whip ( the kind that is parallel to the fuse ) . A place to start. It is 400 coax, right? Dual shielding, it helps. Then start bypassing with disc caps, helps a little more. You will get it, but more than one treatment will be necessary.
John
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