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Electrical Interference Hunt

Rallylancer122

Well Known Member
Hey Everyone,

This one is stumping me. A couple flight hours ago the GNS530w in my RV8 started developing static on the com radio. Only when receiving a transmission (ie it's not constant, only when someone else is transmitting, I'm receiving ATIS, etc..). Apparently I'm having trouble transmitting also as when I tried to call the local TRACON 20 miles away they couldn't get me. I troubleshot the antenna, cables, audio panel, headset, etc.. and pretty much narrowed it down to the 530 itself. Garmin wants $2300 for a flat rate repair. Darn!

So I flew again tonight to double check something and now my TruTrak EFIS started acting up. Static on the screen, eventually so bad the screen was unreadable. Hmmm....

I'm starting to wonder if something in the plane is generating interference. One by one I pulled each breaker in the plane, but the problem persisted. I can duplicate the radio problem on the ground as well with the engine off. Voltage and amperage are good.

Am I missing something? I'm inclined to think the problems are coincidental, but I figured I'd throw it out there and see.
 
The common thread in all your avionics is the ground system. Make sure your battery and common ground points are clean and in good service. Avionics are more sensitive than starters and lights.
 
If the static problem is intermittent and only the two components affected, I would not think it was a system problem. You only get static during reception, so that points away from the electrical system. My guess is a bad antenna or coax cable connected (incl BNC's) to the 530. If either is bad, it is possible that during transmission you are throwing off enough RF to impact the AP screen. Also possible that it is coincidental, though the timing is suspect. Without a good antenna connection, the high energy radio transmission cannot properly radiate away from the plane, creating high energy output right around the plane. Possible that this is enough to mess up an LCD display. Without the ant connected, the 530 is spitting out 7 watts from the BNC with no polarization. This is probably only a few inches away from our AP.

If you have a second comm, turn off the 530 and confirm that the AP doesn't have display problems while it is off. That would eliminate the second variable. Also, if a buddy has a comm ant laying around, you could make up a new coax and connect this antenna to your 530 for testing.

Larry
 
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Coax grounds

Since it seems to have come on suddenly, I would suspect a coax or twisted shielded pair ground or some other harness ground connection broke.
 
Pulled the breakers for 530, usb, led strobes....no change. EFIS was fritzing even when not transmitting or recieving. Ohmed out the antenna cables and no shorts.

I forgot to mention that the tach display on my vm1000 was intermittently wonky also.

I'll check the gounds. Thanks! Any other ideas through them my way.
 
Know anyone else with a 530? It’s easy to swap radios and test, then you’ll know if it’s your 530 or something else.
 
Maybe this...

In my last RV-7, I noticed static that I couldn't attribute to the usual suspects (Ignition, Strobe, LED lights, USB power supply drivers, Cell Phone, etc.).

Static became progressively worse and ultimately would break the squelch when flying IMC.

After several flights, and quite by accident, I discovered that the isolation washer stack-up on the co-pilot headphone & microphone jacks had failed.

Closer inspection with the iPhone (click the lock button 3x) showed that the black shoulder washer had cracked through and was missing part of the "shoulder" portion, allowing the barrel of the jack to contact the bulkhead.

I'm not saying this is your issue, but it's another thing to look at...
 
It sounds like you are actually having trouble across multiple instruments. For that you need to look for a cause that would be common to all of them.

I would check your main power and ground connections. If there is some resistance in that path it will show up when you try to transmit because you are trying to draw more current during that time. I don't remember the exact number (maybe 5 or 6 amps) but I'm sure that current is higher than your EFIS steady state current for example. The other instruments can deal with lower voltages generally but your radio will have low output tx power if it is getting a lower voltage.
 
Thanks for the ideas! I did double check headsets and jacks, and does not seem to be related. I gave up and pulled the panel on Friday, and found a loose ground wire on the 530. Eureka! To boot, found a the tach wire loose also. And on the subsequent test flight the EFIS behaved itself. But.....

The static on Rx is still there! Didn't seem to change anything at all. Shoot. BTW, a nearby airport broadcasts AWOS on the VOR, so I tuned that in and no static. Came in clear. It just seems to be related to that Com.

Everything appears to be pointing towards that 530. To be honest, I'd be happy to pay Garmin the $2300 if I was sure that would fix it. Don't suppose anybody in NE Wisconsin has a 530 I can plug into my rack for a few minutes?
 
Swap the vor and com coax inputs. If the problem is now with vor reception, it’s the coax/antenna. If not, the radio itself.
 
I had "static" noise in my headset only when receiving a weak transmission. The weak transmission was enough to break the squelch on the VHF receiver, which allowed the "Static" to enter the system.

The noise turned out to be ignition noise from my Left Mag. If you have standard mags, do an inflight mag check whenever you are hearing the noise. If the noise is present only on one mag, then you are on your way to finding it.

My noise issue revealed two issues: One spark plug wire shield was not grounded at the Mag end, and one other spark plug center conductor wire had high and intermittent resistance when checked.

Your comment that you hear the noise when receiving made me think of this.

Ya never know!
 
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