Garmin 750XI Com Issues

[Speed Merchant]

I had a 750XI installed in my 201 about 15 months ago. I have been having difficulty hearing center occasionally. The shop that did the install used the original comm antennas and coax. They claim the connections are good. Could this be a coax or antenna issue? Have any MooneySpacers had similar problems with their 750XIs? All advice will be appreciated.

Thanks!

[jetdriven]

There is a thread on Beechtalk about this, and it boils down to lowering the squelch values in the main menu

[PT20J]

Garmin sets the default Rx squelch to 57 presumably to avoid spurious squelch breaks. I found that I could decrease it all the way to 0 which greatly improved the range without getting spurious breaks. How low you can set it probably depends on how electrically "quiet" your installation is.

Skip

[Marauder]

I had a 750XI installed in my 201 about 15 months ago. I have been having difficulty hearing center occasionally. The shop that did the install used the original comm antennas and coax. They claim the connections are good. Could this be a coax or antenna issue? Have any MooneySpacers had similar problems with their 750XIs? All advice will be appreciated.
Thanks!

Are you having any issues with being heard at distance? Also, when the avionics shop said the existing wiring and connectors were fine, did they confirm this through a SWR test or the famous “eyeball” inspection?

I ended up replacing all of my RG-58 with RG-400 because I was dealing with multiple problems with my radios. I really think the newer radios are more susceptible to interference.


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[Speed Merchant]

9 hours ago, PT20J said:

Garmin sets the default Rx squelch to 57 presumably to avoid spurious squelch breaks. I found that I could decrease it all the way to 0 which greatly improved the range without getting spurious breaks. How low you can set it probably depends on how electrically "quiet" your installation is.

Skip

I haven't tried the squelch but will give it a try. I have no trouble transmitting it is the receiving that doesn't work well. Thanks for your suggestion.

[Speed Merchant]

13 hours ago, jetdriven said:

There is a thread on Beechtalk about this, and it boils down to lowering the squelch values in the main menu

I will review the posting in Beech talk. Thanks

[Speed Merchant]

7 hours ago, Marauder said:

Are you having any issues with being heard at distance? Also, when the avionics shop said the existing wiring and connectors were fine, did they confirm this through a SWR test or the famous “eyeball” inspection?

I ended up replacing all of my RG-58 with RG-400 because I was dealing with multiple problems with my radios. I really think the newer radios are more susceptible to interference.


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I am pretty sure the shop did an eyeball test. What is a swr TEST? I may end up changing the coax. Thanks.

[Fly Boomer]

7 hours ago, Marauder said:

the newer radios are more susceptible to interference.

One of the many casualties of progress!

[Marauder]

I am pretty sure the shop did an eyeball test. What is a swr TEST? I may end up changing the coax. Thanks.

SWR tests determine the health of the wiring and connectors. That’s the issue with some of these shops. They don’t take the time to check out the wiring. Putting new with old isn’t always a good idea. Especially when they don’t see stuff like this:




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[Fly Boomer]

2 minutes ago, Marauder said:

Especially when they don’t see stuff like this:

Is that abrasion enough to cause signal problems?

[Marauder]

I haven't tried the squelch but will give it a try. I have no trouble transmitting it is the receiving that doesn't work well. Thanks for your suggestion.

When the GTN series came out I know I got a couple of firmware updates that dealt with squelch settings. And the plane was back a couple of times to adjust it.

In my case, I did notice that my old Narco at the time could transmit further than my fancy and really expensive GTN. Things improved in my case when I changed out the wiring - but not before I replaced that Narco with a GNC 255B with 16 watts. Just didn’t have the confidence with the GTN. The new wiring helped a lot.


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[Marauder]

Is that abrasion enough to cause signal problems?

The sheathing in that area wasn’t completely worn through. I couldn’t find the picture where it was compromised. And yeah, if the sheathing is exposed and compromised, you can pick up some issues. On my vintage Mooney they used metal clips to adhere the wiring to the roll cage. There were no standoffs. It was just pinned to the roll cage. After 40+ years, compression and vibration definitely had an effect on the wiring. My point is unless they do a SWR test and a physical exam of the wiring, you really never know the health of the wiring.

I cut a deal with the avionics shop that I would run all new RG-400 for both the Com and Navs, if they would make the final connections. I was the grunt and they did all the glory work.


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[Fly Boomer]

1 hour ago, Marauder said:

I would run all new RG-400

Is RG-400 expensive?  I'm tempted to replace all my coax "just because".

[Marauder]

Is RG-400 expensive?  I'm tempted to replace all my coax "just because".

$3.30 per foot at today’s price. I’ll look to see how many feet I bought. I replaced both coms and Nav wiring all the way back to the tail.


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[Marauder]

Is RG-400 expensive?  I'm tempted to replace all my coax "just because".

I can’t find the receipt for the amount I bought. The math isn’t too hard. You will need to know where your VOR splitter is. The distances you need to measure are:

1) Distance from the VOR antenna to the splitter (that will be one wire).
2) Distance from the splitter to the Nav radios. My splitter was in the avionics bay and we moved it under the panel.
3) Distance to the Com 1 radio’s antenna.
4) Distance to the Com 2 radio’s antenna.
5) A fudge factor.

If you have the time to take the plane down, I would pull the interior and see where the wires run. Then measure the lengths. My top com antenna was routed through the pilot’s side. I re-routed to the co-pilot’s side, under the door. My pilot’s side is filled with wiring for the Aspens, the Lynx and who knows what else.



The co-pilot’s side was much easier to work with.



My belly antenna was routed up through the left side.

If you take on the project, it is also a good time to check on the condition of the antenna groundings and the general health of everything behind the panels. I spent a lot of time correcting poor routing and storage of wiring from avionics shops.

If you look at this picture, you can see the Aspen’s wiring was literally just piled in behind the panel. It was a rat’s nest. I cleaned it up and made sure everything had a stand-off to prevent rubbing and chafing chances (first picture).



I’ve owned my plane for 31 years. My philosophy has always been to be proactive on updating and upgrading things over time. I don’t want to be flying over some crappy weather to find out that I should have probably done some preventative maintenance. A side benefit of being proactive, the plane is ready when you need it.

It’s not to say things will fail (like my famous alternator failure of 2019), but the likelihood is a lot less if you are staying ahead of the storm.


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[EricJ]

2 hours ago, Fly Boomer said:

Is that abrasion enough to cause signal problems?

It doesn't even have to be abraded, a kink that distorts the cross-section can change the characteristics significantly.  Dirty or corroded connectors can cause problems as well.

[Fly Boomer]

15 minutes ago, Marauder said:

My philosophy has always been to be proactive on updating and upgrading things over time.

This is incredibly helpful.  Huge head start in planning this project.  Many thanks!

[Fly Boomer]

18 minutes ago, Marauder said:

My splitter was in the avionics bay and we moved it under the panel.

Was this primarily to conserve coax?  Or perhaps weight -- I notice that RG-400 weighs about twice as much.

[Fly Boomer]

22 minutes ago, Marauder said:

My top com antenna was routed through the pilot’s side. I re-routed to the co-pilot’s side, under the door.

I can't tell from the pictures if the old coax was routed through conduit.  Did you use conduit for the RG-400?

[Marauder]

Was this primarily to conserve coax?  Or perhaps weight -- I notice that RG-400 weighs about twice as much.

Probably a little of both. I spoke to the avionics shops as I was running the new wires and asked about the splitter’s location. They said there was nothing magical about the location. So, being the former President of the Cheap Bast$#d Club, I moved it forward. I have not seen any issues with the VOR signals. Here is my last inflight VOR check per 91.171:




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[PT20J]

For new installs, RG-400 is a better cable. It is double shielded, has lower loss at high frequencies and has a FEP jacket. It is required for the low signal levels and higher frequencies used for GPS, and I think most transponders now specify it. For the lower VHF nav/com frequencies and relatively short runs in light airplanes, RG-58 is fine if it is in good condition. The most common issues are bad connector terminations and crimps along the length. At 100 MHz, depending on who's specification you use, RG-58 and RG-400 both have about the same loss (in the vicinity of 4-5dB per 100 feet).  RG-58 is also much more flexible than RG-400.

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[PT20J]

Garmin's release notes are pretty cryptic. There is a note from a service bulletin in 2013 that says:

COM Software Version 2.12 contains the following changes from Version 2.11:

• COM Software Version 2.12 provides improved squelch control over Version 2.11. It is helpful only in those installations that experience open squelch (receiver too sensitive) after utilizing the COM Monitor Mode.

This may be the change the increased the default squelch setting. 

If you have a second com with good reception, you can climb to  3-5000 feet and establish communications with a distant facility on the good radio. Then try the GTN, and if you cannot hear a reply, transmit on the GTN but listen on the good radio. If you get a response, you know that the GTN transmitter is good and you can try adjusting the squelch.

Skip

[Marauder]

I can't tell from the pictures if the old coax was routed through conduit.  Did you use conduit for the RG-400?

The old wire did not have any conduits that were used. The wire was either tie wrapped or attached to the roll cage with metal clips. When it went through a bulkhead, there was a rubber grommet.

I refreshed the grommets, and when there were questionable areas to transverse, I would wrap the wire in a plastic spiral wrap. You can get the spiral wrap at Aircraft Spruce.

Like what I used for the CiES wiring:


BTW - I almost lost an arm putting in the new blue grommet!


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[Marauder]

Garmin's release notes are pretty cryptic. There is a note from a service bulletin in 2013 that says:
COM Software Version 2.12 contains the following changes from Version 2.11:

• COM Software Version 2.12 provides improved squelch control over Version 2.11. It is helpful only in those installations that experience open squelch (receiver too sensitive) after utilizing the COM Monitor Mode.

This may be the change the increased the default squelch setting. 
If you have a second com with good reception, you can climb to  3-5000 feet and establish communications with a distant facility on the good radio. Then try the GTN, and if you cannot hear a reply, transmit on the GTN but listen on the good radio. If you get a response, you know that the GTN transmitter is good and you can try adjusting the squelch.
Skip

That was the firmware update I remember being applied in 2013. I thought there was another update later that also addressed squelch issues as well.


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[PT20J]

36 minutes ago, Marauder said:

That was the firmware update I remember being applied in 2013. I thought there was another update later that also addressed squelch issues as well.


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Quite possible. Garmin also messed around with the GNC 255 squelch settings over different revisions. Mine is set to 0 (highest sensitivity) and works fine there.