G1000 COM1 and COM2 constant RX

TangoWhiskey

TangoWhiskey

Touchdown! Greaser!
Joined
Feb 23, 2005
Messages
14,210
Location
Midlothian, TX
Display Name
Display name:
3Green
OK, that was weird. Went to fly yesterday, and found the COM units on the G1000 in the 172 were in constant receive mode, on both COM1 and COM2, any frequency.

This wasn't "broken squelch"... the COM radios were actually annunciating "RX". Stuck mike? Nope. Unplugged my headset, hand mike, worked the transmit button on both yokes (FBO could hear us transmit and we could hear them on UNICOM, and heard all traffic on CTAF or other frequencies we listened to). We checked the MAN SQ settings (put audio to the cabin speaker since headsets were unplugged) and it was still doing the same, could not set squelch high enough to break stop the constant background noise.

It SOUNDED like when squelch is broken and you're hearing the white noise, but for the "RX" between the frequencies at ALL times, not just when somebody was transmitting.

Mechanic and CFI couldn't find out what was wrong either... we shut down both avionics buses, and it was doing it with the line units offline, too, just running off the standby battery with only the PFD up.

Will see if they get it fixed by Thursday--going to try again, 5am, and reset night currency.
 
Did you bring any personal electronic devices on board? Sounds like RF interference.
 
Does the G1000 work like the 430 in that you can essentially disable the "squelch" by pushing in on the volume button? For instance, if you're too far out for the ASOS to be strong enough to break the radio into "rx" mode, you can push the button and it does exactly what you're describing. It kicks the radio into 100% rx.
 
jason said:
Does the G1000 work like the 430 in that you can essentially disable the "squelch" by pushing in on the volume button? For instance, if you're too far out for the ASOS to be strong enough to break the radio into "rx" mode, you can push the button and it does exactly what you're describing. It kicks the radio into 100% rx.
Click to expand...

Yup. If you press the volume knob it will open the squelch and display "RX" next to the respective frequency tuned in that COM radio. As far as the radio is concerned, it believes it is receiving a transmission (even though you are essentially inducing the noise) and displays "RX."
 
Last edited:
denverpilot said:
Did you bring any personal electronic devices on board? Sounds like RF interference.
Click to expand...

My cell phone and my personal radio, but both were off--and it continued to exhibit this behavior when I (and my gear) left the plane and went into the FBO to hear if the CFI's broadcasts were making it through to the base radio.

jason said:
Does the G1000 work like the 430 in that you can essentially disable the "squelch" by pushing in on the volume button? For instance, if you're too far out for the ASOS to be strong enough to break the radio into "rx" mode, you can push the button and it does exactly what you're describing. It kicks the radio into 100% rx.
Click to expand...

Yes, it does... and we checked that. In fact, I ALWAYS check it, as one of the first things I do after I turn on the ANR on my headset is push the button and set the volume level of the radios by breaking squelch. That became a habit early in my flying career when I discovered somebody turned the volume all the way down... it's embarrassing to transmit the same thing four or five times to Ground Control, thinking they're not hearing you, when in fact they ARE, but you're not hearing THEM! ;-)

HPNPilot1200 said:
Yup. If you press the volume knob it will open the squelch and display "RX" next to the respective frequency tuned in that COM radio. As far as the radio is concerned, it believes it is receiving a transmission (even though you are essentially inducing the noise) and displays "RX."
Click to expand...

Does it say "RX", Jason? I don't think it does--I think it goes from the dual-headed-arrows to showing the percentage of volume level. The PC simulator doesn't emulate this, so I'll have to look more closely next time I'm in the plane.
 
TangoWhiskey said:
Does it say "RX", Jason? I don't think it does--I think it goes from the dual-headed-arrows to showing the percentage of volume level. The PC simulator doesn't emulate this, so I'll have to look more closely next time I'm in the plane.
Click to expand...

It does in the Cirrus and the Phenom. It will only display % volume if you turn the knob left or right. I would imagine it would behave the same in the 172 but can't say with certainty.
 
On my first xc, a 6 hour trip with one stop at 3 hours, that happened to me in a 182 G1000, and I couldn't figure it out until the first stop. 3 hours of noise! Yuk! Anyway, it turned out to be some button pushed or selected that should not have been. It was Sooo quiet for that second 3 hour leg. Sorry, I don't remember which button...
 
HPNPilot1200 said:
It does in the Cirrus and the Phenom. It will only display % volume if you turn the knob left or right. I would imagine it would behave the same in the 172 but can't say with certainty.
Click to expand...

Yeah, that makes sense... I remember the % volume showing up AS you start turning--couldn't recall what it shows before or after.
 
skidoo said:
On my first xc, a 6 hour trip with one stop at 3 hours, that happened to me in a 182 G1000, and I couldn't figure it out until the first stop. 3 hours of noise! Yuk! Anyway, it turned out to be some button pushed or selected that should not have been. It was Sooo quiet for that second 3 hour leg. Sorry, I don't remember which button...
Click to expand...

Hmmm..... I'll be interested to see what they find!
 
TangoWhiskey said:
Yeah, that makes sense... I remember the % volume showing up AS you start turning--couldn't recall what it shows before or after.
Click to expand...

Could be that Automatic Squelch was broken.

The Cessna NAV III Pilot's Guide describes Automatic Squelch behavior in two ways, depending on the version of the guide. One way (the older one) describes the symptoms I was seeing, except I know we pushed the VOL/SQ knob (and then pushed again to re-enable automatic squelch).

In the 190-00498-00 Rev. A document, it says:

190-00498-00 Rev. A said:
attachment.php
Click to expand...

The 190-00498-06 Rev. B document, also for Cessna Nav III changes that behavior to display SQ constantly, instead of RX.

190-00498-06 Rev. B said:
attachment.php
Click to expand...

I wonder if perhaps somebody pushed the VOL/SQ button on the MFD?! We didn't check that one!
 

Attachments

  • ScreenShot037.png
    ScreenShot037.png
    55.6 KB · Views: 78
  • ScreenShot038.png
    ScreenShot038.png
    60.3 KB · Views: 77
Last edited:
TangoWhiskey said:
The 190-00498-06 Rev. B document, also for Cessna Nav III changes that behavior to display SQ constantly, instead of RX.
Click to expand...

Now that you mention it, that sounds more familiar. I'll check when I meet with my student next to see what the Cirrus displays. Probably SQ.
 
Yea, I think that is the button I pushed when the noise went away...
 
denverpilot said:
Did you bring any personal electronic devices on board? Sounds like RF interference.
Click to expand...

It did end up being RF interference--from the ELT.

Avionics guy traced it to the ELT; disconnected the ELT, reconnected it, and the problem went away. He said he's seen it before--some sort of feedback loop from the closely placed antennas.
 
Sounds like a something is multiplying to make up a mix at the IF frequency of the receivers in the G1000. Icky. That or a 2A+B or 2A-B type mix. (Where A and B are frequencies.)

406 ELT or just 121.5? Just curious.

The ELT and G1000 are shielded enough that it has to get started by leaking out/in the antennas.

Interesting also that he's seen it enough that it's "common" to him. That means it's reproducible and could be designed out... which also means it's "designed in" to a bunch of airplanes... oops. :)

Fascinating little RF engineering problem for someone to solve.
 
Most likely a little oxidation on the connectors. It can show up as a semiconductive layer, which can cause rectification, which can lead to all kinds of weird RFI problems. It's not at all uncommon, and the fact that the problem went away when the connector was disconnected/re-connected tends to confirm it. Interesting that disconnecting the ELT antenna did it. I'd almost bet he disconnected and re-connected the NAV/COM antenna as well. I would expect to see that being a problem with the receiver antenna connections, rather than a transmitter.
 
Ahhh, yeah... that's likely it.

Or as a friend says, "Passive Intermod is the Devil's Snack Food."
 
denverpilot said:
Sounds like a something is multiplying to make up a mix at the IF frequency of the receivers in the G1000. Icky. That or a 2A+B or 2A-B type mix. (Where A and B are frequencies.)

406 ELT or just 121.5? Just curious.
Click to expand...

121.5

denverpilot said:
The ELT and G1000 are shielded enough that it has to get started by leaking out/in the antennas.

Interesting also that he's seen it enough that it's "common" to him. That means it's reproducible and could be designed out... which also means it's "designed in" to a bunch of airplanes... oops. :)

Fascinating little RF engineering problem for someone to solve.
Click to expand...

Yeah, he mentioned moving/relocating an antenna as the ultimate fix...
 
denverpilot said:
Ahhh, yeah... that's likely it.

Or as a friend says, "Passive Intermod is the Devil's Snack Food."
Click to expand...

More commonly called "varactor effect" as the output collector-base diode becomes the varactor and the collector is nearly directly tied to the antenna port to try and get both fundamental and first harmonic into the antenna without going through a lot of matching.

Once the sucker gets to oscillating pretty much the only answer is to shut the whole system down and reconnect things one at a time.

Known problem for a long time. Only answer is distance between antennas or 406 ELTs.

Jim
 
Last edited:
You must log in or register to reply here.
Back
Top