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PTT Disengages AP74

T

Tcheairs

Well Known Member
I have a legacy D-180 with the AP-74 and 34 installed in my RV7A. Autopilot works flawlessly in all modes when engaged, but lately when I key the mic button on the stick, it (most of the time) disengages all functions of the AP 74 autopilot. all lights go off on the main switch and the servos and it is completely disconnected..I'm usually able to get everything re connected quickly, but it makes for a dicey situation when being vectored for an instrument approach in the soup..

anyone experienced this or anything similar?
 
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Tcheairs said:
When I key the mic button on the stick, it (most of the time) disengages all functions of the AP 74 autopilot...anyone experienced this or anything similar?
Click to expand...
Assuming you have "PTT" and "Autopilot Disconnect" buttons on your stick grip, both are actuated by grounding, so perhaps these two wires in the grip have become connected? See if your Autopilot Disconnect button also activates the PTT.
 
I do not have an A/P disconnect switch on my stick grip, or anywhere on the control stick. Same thing happens when I key the copilot PTT button..Will check and see if the A/P disconnect on the dash keys the mic.
 
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Some things to check:

1) Does the battery voltage drop when PTT is pressed?
2) Does the AP share any power or ground connections with the radio? If so, a weak power or ground connection would be exacerbated with the increased load of the radio when transmitting.
3) Are the radio antenna connections corroded or loose? Stray RF from an antenna problem can cause other electrical devices like the AP to go haywire.
4) Try turning off the radio before pushing the PTT and see if the problem still occurs.

-Jerald
 
More things

Does the radio antenna coax run anywhere close to the autopilot or Dynon wires under the panel? Check the condition of the coax connectors ... not unusual to have a bad connection there which will make all kind of RFI (radio frequency interference).

Do you have two radios? If so does the AP disconnect regardless of which radio transmits?
 
Lots of diagnosis material here. I'll start checking and report back, Thanks for the suggestions. The RV7a is not my build. But I do have all of the electrical schematics (which someone will be able to decipher)..
 
Got around to some troubleshooting yesterday. Here are my results.

1. When keying the mic button on the primary radio (Garmin 430w), the DC voltage only drops from 12.2v to 12.1v. This seems to me to be normal. If the AP-74 is on, it cuts off

2. Turning on the AP-74 does not cause the 430 radio to transmit (no tx symbol in the 430 frequency window)

3. When I switch to the #2 radio and PTT, the AP-74 does not turn off (disconnect) but the Garmin 430w unit SHUTS DOWN!! and reboots from scratch.

4. I had a GDL-82 installed a couple of months ago and obviously there is coax cable running from it to the GTX 327 which is stacked with the 430 and 340 audio controller in the center of my panel.

gremlins beyond my RF capability and I guess the plane is going to have to go back into the avionics shop
 
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