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sportav8r(at)gmail.com
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 Posted: Sun Apr 11, 2010 6:17 pm Post subject: intermittent transponder mystery solved |
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I finally ran to ground my transponder intermittent that has been plaguing me for years! My set-up is a Garmin GTX 320 in a tray that formerly housed a Narco AT-150, using Garmin's plug-and-play adapter to fit between the two. The first time it crapped-out, I sent the transponder back to Garmin: $250 bench fee to be told the box is fine. Worked fine for awhile after that. Then it started becoming intermittent - the interrogation light would flicker normally at the start of a cross-country, and ATC could give me VFR advisory service, but often by the time I got to the destination I was only showing as a primary target on radar.
I limped along like this for awhile, but the prospect of having to enter the Orlando mode C veil next week with "known issues" motivated me to try again. I started with what has worked before - removing the transponder and adapter tray, spraying every connection with contact cleaner and reassembling it all. Well, that worked for a few seconds and then it quit again, so I got out the continuity checker and started checking the RF path for opens and shorts. The meter showed an open on the coax center conductor when I wiggled it in the back of thel Narco tray. Put the innards back in, and sure enough, if I held the coax a certain way, I saw interrogation flashes; if I let go, it stopped. Now all I need is for the pax to reach under the panel and hold the coax the whole trip. Not happening.
The BNC jack on the transponder itself looked good; the male mating plug on the front side of the adapter looked good - no bent pin. The female jack in the back of the Garmin-to-Narco adapter looked good. There is a small length of RG-184-type teflon coax inside the adapter, but it is totally protected inside the chassis and is under no mechanical strain, and it's new Garmin manufacture, so the problem can't be there <hold that thought> so the fault must be in that ancient RG-58 pigtail that came with the Narco tray and was probably soldered in the 1950's. But the continuity checker showed no opens or shorts no matter how I wiggled it on the bench, and its coax shield is well-crimped and fully strain-relieved. Maybe the male pin on the end of that pigtail, the one that's captive in the back of the tray, is worn/undersize. So I added a thin layer of solder to the gold pin and pressed it back into the fitting on the Garmin adapter tray - still showed an intermittent open when I wiggled it! (at)#$%!
At this point, I noticed that as I twisted the jack into the plug, the shell of the Garmin female (the barrel) would move with it. That's not supposed to happen. In fact, the entire guts of the Garmin female fitting is threaded into its mounting flange, and since it is not secured or safetied in any way (and was doubtless under-torqued at assembly), movement of the male coax pigtail on the outside of the tray eventually broke the BNC solder connection inside the Garmin adapter. I re-threaded the outer shell of the Garmin adapter's female connector with red LocTite and re-soldered the mini-coax to it. Problem fixed - hopefully for good!
Moral of the story: never assume that a factory avionics assembly is properly done, or that a wire that "can't possibly" move/break hasn't done just that. I only wish that there was some lifetime warranty where the Garmin would comp me for the time and aggravation this has caused.
I'm attaching pictures that show the adapter tray, the fitting as it comes apart but isn't supposed to, and how it broke the solder joint internally. The last one shows the guts of the BNC fitting completely unscrewed from its mounting flange and the coax just hanging there. May this never happen to you!  If it does, remember to question everything you're assuming.
I'm gonna feel bad if the pictures don't upload - never done that on Matronics list before.
-Bill B. / "Stormy" RV-6A
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skywagon
Joined: 11 Feb 2006
Posts: 184
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| Posted: Sun Apr 11, 2010 9:51 pm Post subject: intermittent transponder mystery solved |
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Bill,
If you reach the right service engineer at Garmin, repeat the story, I am sure that they would be happy to compensate you and also return the $250 left on their table. I have had issues with Garmin several times in the past, and when the story was told, they were gracious about working with me. Unless things have changed at Garmin, I would take your service problem back to them. They may have an assembly problem or Q/C problem and would want to know about it...... Shouldn't matter if the unit is out of warranty....
If you don't have luck with your first try at the refund, review their management team via the web and write the key guy a detailed letter.... he will make it happen....
David
[quote] ---
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nuckolls.bob(at)aeroelect
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| Posted: Tue Apr 13, 2010 7:39 am Post subject: intermittent transponder mystery solved |
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| Quote: | I'm attaching pictures that show the adapter tray, the fitting as it
comes apart but isn't supposed to, and how it broke the solder joint
internally. The last one shows the guts of the BNC fitting
completely unscrewed from its mounting flange and the coax just
hanging there. May this never happen to you! If it does,
remember to question everything you're assuming.
I'm gonna feel bad if the pictures don't upload - never done that
on Matronics list before.
|
Good work! That itty-bitty coax has been problematic
when it comes to durability of soldered joints.
As you've discovered, the center-conductor is
pretty skinny.
Bob . . .
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sportav8r(at)gmail.com
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| Posted: Tue Apr 13, 2010 12:04 pm Post subject: intermittent transponder mystery solved |
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Actually, Bob, under 10x loupe examination of the coax and the center terminal of the fitting, there was no visual indication that there had been an actual joint there to fracture - just solder on each, and no impression of the conductor in the little puddle on the coax fitting. But you can bet there is, now! There was no need to re-trim the coax to prepare it for re-soldering - all the wire was still there.
-Bill
On Tue, Apr 13, 2010 at 11:39 AM, Robert L. Nuckolls, III <nuckolls.bob(at)aeroelectric.com (nuckolls.bob(at)aeroelectric.com)> wrote:
[quote]--> AeroElectric-List message posted by: "Robert L. Nuckolls, III" <nuckolls.bob(at)aeroelectric.com (nuckolls.bob(at)aeroelectric.com)>
| Quote: | I'm attaching pictures that show the adapter tray, the fitting as it comes apart but isn't supposed to, and how it broke the solder joint internally. The last one shows the guts of the BNC fitting completely unscrewed from its mounting flange and the coax just hanging there. May this never happen to you! If it does, remember to question everything you're assuming.
I'm gonna feel bad if the pictures don't upload - never done that on Matronics list before.
|
Good work! That itty-bitty coax has been problematic
when it comes to durability of soldered joints.
As you've discovered, the center-conductor is
pretty skinny.
Bob . . .
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