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Electrical system issue during Comm Transmission

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Not that it matters for this question, but I have an RV-8A that I bought a little more than a year ago, constructed by a reputable builder and with avionics installed professionally.

About two weeks ago, keying the radio for transmission with the switch on the grip caused the AP to disengage and reboot. Further, I noticed that the manifold pressure increase 10# during transmission, dropping back down to the correct pressure when I released the mic key.

There is no static in the radio at any time, including during transmission.

I was on a trip and determined to get back to home base and did so, but an additional symptom occurred - now when I key the mic the ammeter goes to -20.

I do not know the make of the grip, but it has switches for the mic, AP disengage and trim (although no problems when I trim the aircraft). The logical place to start the search seems to me to be in the grip - looking for a frayed wire. My bet is that I have something going to ground when I trip the mic hence the ammeter reading.

Anyone have any similar experience or advice?
 
I believe you're on the right track - but look at the wiring exit at the bottom of your stick first..... and track to the first tie-down of the bundle.
 
What you are experiencing is leakage of the rf from the antenna coax. When you are transmitting you are putting out about 10 watts of RF. If the is any issue with the coax it will reflect back into the aircraft and show symptoms of what you report. Start by check the antenna coax and both ends. Most likely place for it fail. Might also be a bad antenna but that is rare. Hope this helps.

Jay
 
What you are experiencing is leakage of the rf from the antenna coax. When you are transmitting you are putting out about 10 watts of RF. If the is any issue with the coax it will reflect back into the aircraft and show symptoms of what you report. Start by check the antenna coax and both ends. Most likely place for it fail. Might also be a bad antenna but that is rare. Hope this helps.

Jay

+1

This is where I would start.

Larry
 
Also check the shielded wire grounds on your PTT connection to the audio panel. I had a similar issue and that is where I found a problem. Fixed it and the issue went away.
 
What kind of radio and what brand of Autopilot?

The fact that it seems to be a new effect on a previously working installation is very suspicious...

i would also go look into the shielding / wiring of control sticks and PTT wiring first.

We had a similar effect, but during installation/build phase with the combination of a trutrak autopilot and an SL30. The resolution was installing a cheap DB25 filter dongle between the Autopilot head and the harness plug. The effect was a noticeable pitchdown everytime the PTT was pressed.
 
My RV-9A had the avionics installed (for a previous owner) by a "professional" avionics shop, with atrocious workmanship. Much of that was torn out when I got the plane and updated the avionics.

"Professional" sometimes onlly means that somebody got paid for the work.
 
Voltage Drop maybe

I would check the voltage drop from the Battery; Master Sol and Bus bar and then to the AP unit and other maybe shared connections, Use a Multi meter that’s reads Min and Max hold option, If for example the Voltage at the Master solenoid has little or no drop but at the AP unit power input has significant drop say to 9 volt the problem is between the Master Sol and AP and Manifold and maybe other connections. Do this test with all power on in running config and key the mic at each point of the test. Hope this helps
Ian. Rv 9 Australia
 
Seems strange that the PTT would be the problem as it is just a switch to connect a pin to ground, no real signal.
I agree that it is strange, and I was very skeptical, but I was also amazed at the difference it made. Somebody more versed in RF than I would have to explain it :rolleyes:
 
When I put in my GTN635, my help incorrectly wired the bnc connections. They stripped the shielding to far (didn't butt up against the connection) and when I keyed the mic I had similar issues (afs4500 engine indications went crazy) also lost GPS reception. It was an RF issue. I would bet you have some leakage somewhere anything unshielded is getting blasted. I did some Google searches and figured out how to test coax and made new connections, no more issues.
 
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