GNS 430 reception problems

[Kevin Clarke]

Hi,

I'm looking for folks that might have a similar experience. Recently
upgraded the panel to include a GNS 430 and replace one of those old
NARCO all in one VOR instruments plus an old NARCO radio.

Anyway great upgrade. The GPS function works wonderfully. Filing /g is
the best. The NAV function works great. Dual glide slope indicators(the
other is slaved to a KX-155) really makes IFR manageable.

The problem is the COM function of the GNS 430. We're only getting about
40 miles of range on the receive side of the COM. I'll call for flight
following on COM1(the GNS) and never get an RX indication. When I flip
over to COM2 (the KX-155) it works just fine. The KX has a range of at
least 80 miles. The GNS seems uneven. We can xmit and be heard over
distances but the receive side is really only useful in and about the
terminal area. This is less than optimal.

When we did the upgrade we kept the original antennas that were on the
plane(PA28-180).

Our avionics guy is baffled, maybe useless at this point. He swapped out
the GNS plus ran a myriad of other tests. He says everything is checking
out just fine. But that is surely not our experiences. So I turn it over
to the collective wisdom of the net. Has anyone else ever experienced
anything similar?

Thanks.

[Clay]

Sounds like the antenna or the coax.

[[email protected]]

The avionics shop should be able to test COM receive sensitivity through the
plane's antenna with the unit installed in the plane, very simply using a
suitable signal generator (that every shop should have.) The signal
generator is set up to transmit a very low level signal, modulated with a
tone, through a whip antenna located some distance (maybe 100 feet or so)
from the plane, in line-of-sight to its COM antenna. Signal level is
adjusted to the point that the tone is just heard through the COM receiver.
Calculating free-space loss for the distance (a standard calculation), one
can determine within a few dB the signal strength incident to the COM
antenna. This is not as precise as doing it on the bench with the signal
generator connected directly to the receiver, but it is sufficient to find a
gross problem and to troubleshoot it. This method CAN accurately measure
the difference in sensitivity between a plane's two COM receivers.

If your avionics guy doesn't understand this test you need a new avionics
guy.


-Elliott Drucker

[The Visitor]

Clay wrote:
> Sounds like the antenna or the coax.
>
Good point. An overy bent or kinked and pinched coax mould be a problem.

[ktbr]

The Visitor wrote:
>
>
> Clay wrote:
>
>> Sounds like the antenna or the coax.
>>
> Good point. An overy bent or kinked and pinched coax mould be a problem.
>
An easy way to test this would be to swap the coax between
the two radios and see if the 430 improves and/or the 155
degrades.

[Dave Butler[_1_]]

ktbr wrote:

>>> Sounds like the antenna or the coax.
>>>
>> Good point. An overy bent or kinked and pinched coax mould be a problem.
>>
> An easy way to test this would be to swap the coax between
> the two radios and see if the 430 improves and/or the 155
> degrades.

Just as another data point, we had the same problem with our GNS480. The comm
section may be the same as the 430, I'm not sure about that. Anyway, we swapped
the antenna cables between the 480 and the KX155 to see whether the problem
stayed with the radio, or moved with the antenna. Oddly, the problem just went
away, or at least became less pronounced. Maybe something got unkinked, I don't
know. The KX155 is still the more sensitive receiver.

DGB

[Mike Noel]

You may find the coax cables are joined by bnc connectors under the panel
near the pilot's left knee. This may be an easy(er) place to swap com
antennas before trying to access the back of the radio trays to swap the
coax. The hardest swap is laying on your back in the tail cone checking the
connectors at the base of the antennas.

I have heard that com antennas come with a special coating to suppress
static buildup. Static charges can interfere with the radios. This coating
will eventually deteriorate and could also degrade radio performance. I
suspect such a buildup probably produces 'static' or popping in the
receiver.

--
Best Regards,
Mike

http://photoshow.comcast.net/mikenoel

"Kevin Clarke" > wrote in message
nk.net...
> Hi,
>
> I'm looking for folks that might have a similar experience. Recently
> upgraded the panel to include a GNS 430 and replace one of those old NARCO
> all in one VOR instruments plus an old NARCO radio.
>
> Anyway great upgrade. The GPS function works wonderfully. Filing /g is the
> best. The NAV function works great. Dual glide slope indicators(the other
> is slaved to a KX-155) really makes IFR manageable.
>
> The problem is the COM function of the GNS 430. We're only getting about
> 40 miles of range on the receive side of the COM. I'll call for flight
> following on COM1(the GNS) and never get an RX indication. When I flip
> over to COM2 (the KX-155) it works just fine. The KX has a range of at
> least 80 miles. The GNS seems uneven. We can xmit and be heard over
> distances but the receive side is really only useful in and about the
> terminal area. This is less than optimal.
>
> When we did the upgrade we kept the original antennas that were on the
> plane(PA28-180).
>
> Our avionics guy is baffled, maybe useless at this point. He swapped out
> the GNS plus ran a myriad of other tests. He says everything is checking
> out just fine. But that is surely not our experiences. So I turn it over
> to the collective wisdom of the net. Has anyone else ever experienced
> anything similar?
>
> Thanks.

[William Snow]

I had a similar problem with two KX-170's.
Solution, corrosion, under the antenna at the BNC connectors....we replaced
coax, and antenna's. Problem fixed, radios AOK!

[Kevin Clarke]

Dave Butler wrote:

Interesting that is the first time someone else has reported a similar
problem. We also (allegedly) did the swap the antenna test. I say
allegedly because I'm not convinced it ever occurred.

But because you had a similar issue with the KX155 could there be some
interaction there? I know that is a long shot but interference is a
funny thing.

Do other folks have a GNS4x0 and KX155? I've got to believe that is a
very very common config.

KC

> ktbr wrote:
>
>>>> Sounds like the antenna or the coax.
>>>>
>>> Good point. An overy bent or kinked and pinched coax mould be a problem.
>>>
>> An easy way to test this would be to swap the coax between
>> the two radios and see if the 430 improves and/or the 155
>> degrades.
>
> Just as another data point, we had the same problem with our GNS480. The
> comm section may be the same as the 430, I'm not sure about that.
> Anyway, we swapped the antenna cables between the 480 and the KX155 to
> see whether the problem stayed with the radio, or moved with the
> antenna. Oddly, the problem just went away, or at least became less
> pronounced. Maybe something got unkinked, I don't know. The KX155 is
> still the more sensitive receiver.
>
> DGB

[Mike Noel]

I have a Garmin SL30 nav/com and a King com. Using the 'both' button on the
audio panel, the audio output level from the King drops precipitously.
Until I discovered this it appeared the King radio was no longer working
correctly after the SL30 was installed. In order to hear the outputs from
both of the radios at the same time the volume of the King must be
increased. When the radios are listened to by themselves, the King's audio
level is normal.

--
Best Regards,
Mike

http://photoshow.comcast.net/mikenoel

"Kevin Clarke" > wrote in message
ink.net...
> Dave Butler wrote:
>
> Interesting that is the first time someone else has reported a similar
> problem. We also (allegedly) did the swap the antenna test. I say
> allegedly because I'm not convinced it ever occurred.
>
> But because you had a similar issue with the KX155 could there be some
> interaction there? I know that is a long shot but interference is a funny
> thing.
>
> Do other folks have a GNS4x0 and KX155? I've got to believe that is a very
> very common config.
>
> KC
>
>> ktbr wrote:
>>
>>>>> Sounds like the antenna or the coax.
>>>>>
>>>> Good point. An overy bent or kinked and pinched coax mould be a
>>>> problem.
>>>>
>>> An easy way to test this would be to swap the coax between
>>> the two radios and see if the 430 improves and/or the 155
>>> degrades.
>>
>> Just as another data point, we had the same problem with our GNS480. The
>> comm section may be the same as the 430, I'm not sure about that. Anyway,
>> we swapped the antenna cables between the 480 and the KX155 to see
>> whether the problem stayed with the radio, or moved with the antenna.
>> Oddly, the problem just went away, or at least became less pronounced.
>> Maybe something got unkinked, I don't know. The KX155 is still the more
>> sensitive receiver.
>>
>> DGB

[Kevin Clarke]

Thanks everyone for their feedback and opinion. I am now convinced what
is going on is a simple case of demonic possession.

We swapped the antennas. No difference
Tonight we replaced the BNC connector; no difference.
We tested it with and without the KX-155 on; no difference.

When running the recv sensitivity test, the KX drops out much later than
the GNS.

So I am left with demonic possession.

I'm thinking I need a new avionics guy. Unless someone has a wild idea.

thanks,

KC

Kevin Clarke wrote:
> Hi,
>
> I'm looking for folks that might have a similar experience. Recently
> upgraded the panel to include a GNS 430 and replace one of those old
> NARCO all in one VOR instruments plus an old NARCO radio.
>
> Anyway great upgrade. The GPS function works wonderfully. Filing /g is
> the best. The NAV function works great. Dual glide slope indicators(the
> other is slaved to a KX-155) really makes IFR manageable.
>
> The problem is the COM function of the GNS 430. We're only getting about
> 40 miles of range on the receive side of the COM. I'll call for flight
> following on COM1(the GNS) and never get an RX indication. When I flip
> over to COM2 (the KX-155) it works just fine. The KX has a range of at
> least 80 miles. The GNS seems uneven. We can xmit and be heard over
> distances but the receive side is really only useful in and about the
> terminal area. This is less than optimal.
>
> When we did the upgrade we kept the original antennas that were on the
> plane(PA28-180).
>
> Our avionics guy is baffled, maybe useless at this point. He swapped out
> the GNS plus ran a myriad of other tests. He says everything is checking
> out just fine. But that is surely not our experiences. So I turn it over
> to the collective wisdom of the net. Has anyone else ever experienced
> anything similar?
>
> Thanks.

[[email protected]]

On 7-Sep-2006, Kevin Clarke > wrote:

> When running the recv sensitivity test, the KX drops out much later than
> the GNS.


Was the RX sensitivity test done on the bench or in the airplane? How "much
later" (in dB)? If it's more than a few dB then the GNS 430 is probably
defective. There should be a published specification for receive
sensitivity, which will be shown in the Garmin shop service manual if
nowhere else. If what you get, with a bench test, does not meet spec then
troubleshooting needs to move to internal circuitry. Probably best done by
a Garmin authorized repair facility if there is such a thing.

Good luck!

-Elliott Drucker

[Kevin Clarke]

wrote:
> On 7-Sep-2006, Kevin Clarke > wrote:
>
>> When running the recv sensitivity test, the KX drops out much later than
>> the GNS.
>
>
> Was the RX sensitivity test done on the bench or in the airplane? How "much
> later" (in dB)? If it's more than a few dB then the GNS 430 is probably
> defective. There should be a published specification for receive
> sensitivity, which will be shown in the Garmin shop service manual if
> nowhere else. If what you get, with a bench test, does not meet spec then
> troubleshooting needs to move to internal circuitry. Probably best done by
> a Garmin authorized repair facility if there is such a thing.

The test was done in the plane. The difference is about 20dB, 50 vs 70.
We've already replaced the GNS430. Could I have gotten 2 defective brand
new Garmins? I'll ask if that value is within spec, that's a good thought.

That was why I asked the net community for experiences. I'm either the
unluckiest SOB on the planet or there is something else going on.

thanks,
KC


>
> Good luck!
>
> -Elliott Drucker

[[email protected]]

No way the sensitivity difference between the two radios should be 20 dB --
that reflects an incident signal strength difference of 100-fold! If the
Garmin is not defective (and the unit swap seems to confirm this) then the
receive signal is being attenuated by roughly 20 dB someplace. This should
be fairly easy to find using a spectrum analyzer or similar instrument,
again something any avionics shop should have available.

However, I must wonder why your avionics shop isn't going through these
rather obvious diagnostic steps without outside help.


-Elliott Drucker