GTN650 Jumpy VOR

[BobTurner]

Originally Posted by Avanza

If VOR signal passes through prop, arc modulation can be changed.
CDI needle may fluctuate as much as plus or minus 6°.

Good luck

Yes, this happens when the prop ‘cuts’ the signal at something close to its 30 Hz modulation frequency, sort of a strobe effect. Changing rpm usually makes it go away.

[Carl Froehlich]

Originally Posted by BobTurner

Yes, this happens when the prop ‘cuts’ the signal at something close to its 30 Hz modulation frequency, sort of a strobe effect. Changing rpm usually makes it go away.

Interesting theory - but.....

We have been chasing this problem with the 650 in my old RV-10. I was an early adopter of the 650 (2011 timeframe) and paid the price chasing what Garmin told me were "known software problems). But after that pain, it worked as advertised - with no VOR drift as best I can remember (I did my IFR training and check ride in this plane).

The check of the current problem included parking the plane at a VOR test site. Everything turned off (both masters off) with power applied only to the 650 and an EFIS - engine shut down. Same problem. At this point the new owner took the 650 to Garmin when at Oshkosh to check it out. They found no problems. He did get talk into trading it in (along with a pile of cash) to get the new 650 version. Same problem.

So - what do I conclude? Same plane, same install using two different 650s, same problem. The only change that is common is the updated 650 software from the original install. As other RFI type causes have been explored and ruled out, this is the only remaining unknown.

Carl

[bjdecker]

Originally Posted by Carl Froehlich

The only change that is common is the updated 650 software from the original install. As other RFI type causes have been explored and ruled out, this is the only remaining unknown.

What EFIS? What interfaces for NAV(CDI/HSI) are employed - RS-232, ARINC, or Analog?

Painful exercise, but you could wire in another display head to the 650, in place of the EFIS, and see what it does (Dynon D10A, Garmin G5, GI-275).

Asked previously, what does the 650 power up self test show on the EFIS -- a stable CDI/To/From or does it wobble?

[Cumulo]

Isn't this an almost identical problem as thread https://vansairforce.net/community/s...d.php?t=213175 ?

OP there seems to be quite astute - tried all the common things, down to snake oil guesses like floating an antenna feed-through fitting.

Beat freq interference and multi-path VOR needle waving is not usually described as "jumpy".

Was in the biz forever, looks like a box problem to me. And maybe a future Garmin service letter.

Ron

[BobTurner]

Originally Posted by Cumulo

Was in the biz forever, looks like a box problem to me. And maybe a future Garmin service letter.

Ron

Unfortunately I agree. But try inexpensive fixes first!

[jwyatt]

Originally Posted by Cumulo

Isn't this an almost identical problem as thread https://vansairforce.net/community/s...d.php?t=213175 ?

Beat freq interference and multi-path VOR needle waving is not usually described as "jumpy".

The linked thread is my saga ... I wouldn't call mine "jumpy", waving is indeed an apt description. Here's an inflight video. I'm hoping to be able to spend some more time test-flying this weekend.
https://www.youtube.com/watch?v=08DkPHj88Hg

[Walt]

To eliminate some kind of RF signal feed problem I would suggest having avionics shop plug a tester directly into the antenna port, basically a ‘bench’ check in the aircraft.
First thing I would do if you were here.

[cdeggz]

Originally Posted by bjdecker

You didn't mention this, but I thought I should ask.

During the power on of the 650 (and Xi), you will get a prompt for the Database page, followed by the Instrument Self Test Page. On this page, you will see that the 650 is driving its electrical outputs to the following:

CDI - Half Left deviation, TO indication, no Flags
GS - Half Up deviation, no Flags
Annunciators - ON
Selected Course/OBS - 150 (or 149.5)
DTK - 150 (149.5)

In this condition, what is displayed on the AFS5600?

Yes the instrument self test page all checks out. Ironically that test is sending out magenta (GPS) info which I'm not having trouble with. I'm told by AFS that the GPS and VOR/LOC come over separate data streams so that check does not really help check the VOR output!

[cdeggz]

Originally Posted by BobTurner

Unfortunately I agree. But try inexpensive fixes first!

Today I found I can't receive a VOR on the ground at my home airport (KRHV) so I flew down to the VOR checkpoint at KSNS for testing.

I was surprised today to see there was no wagging of the CDI, however the VOR error was around 30 degrees.

I powered on everything I could find in the plane and there was no change.

I then 1 by 1 powered everything off, lastly cutting the engine, and the 30-degree shift was still present.

The OBS reading on the EFIS matches the OBS on the GTN, but I ran through that recalibration step anyhow. Still no help.

Looks like I need to find someone to radio swap with me or get this thing on a test bench. Garmin wants $1350 to fix it of course. Probably not worth it to me. Which brings me to another question:

Are the INOP labeling and deactivation requirements applicable to experimentals? If it were applicable, how would one "disable" the VOR radio only? Looks like in the settings I can disable VOR/LOC but I'd like to keep my ILS functionality.

[cdeggz]

Originally Posted by bjdecker

What EFIS? What interfaces for NAV(CDI/HSI) are employed - RS-232, ARINC, or Analog?

Painful exercise, but you could wire in another display head to the 650, in place of the EFIS, and see what it does (Dynon D10A, Garmin G5, GI-275).

Asked previously, what does the 650 power up self test show on the EFIS -- a stable CDI/To/From or does it wobble?

BJdecker this is an interesting idea. Data is ARINC. I do have a G5 in the plane so I could try seeing what it will display. I don't have high hopes though. Magenta self-check is stable and accurate.