My Cobra 29LX is Fading In And Out

[GigaFish]

Hello all,

I am having an intermittent issue with my Cobra 29 LX. I recently gave up my Wilson fiberglass antenna mounted on the toolbox of my truck because I never could get a solid ground and good SWR readings. I purchased a Little Wil, and have since achieved much better results!

Now for the issue. My radio will intermittently (3-4 times per hour) fade in and out on both receive and transmit. I know it is not antenna related, as I have hooked it up to another vehicle/coax/antenna and it does the same thing. When the radio has faded out, no static can be heard even with squelch all the way counterclockwise. Sometimes, I will turn the radio on and there will be no static, and other times it will just gradually (over a minute or so) fade out until there is none. In addition, if I key the mic when the radio is "out", the radio goes into transmit mode but shows no power. I have also bought a new mic thinking that could be related, but it wasn't. When the radio is working correctly, I have confirmation that it can be heard over a distance, and the talkback works regardless.

I have had the radio peaked and tuned and brought up the issue, but (naturally) the radio was on its best behavior on the bench (It acted this way before the P&T so they are also not related). I've hooked up a multimeter to the power in to measure current draw, and when the radio is "working" the amps spike up when keying, but when it is not, amps do not move. I am a little confused as to whether this is happening due to something being wrong, or the radio is sensing something else wrong and is pulling power as a result.

Wiggling the coax connector appears to have no effect, but I have removed the covers and poked around the components with an eraser, and can get the issue to replicate by pushing in the center of the board. I am pretty (very) new to the world of how all of this works, but if it is something I can fix, I am all for it! I have read tons about cold solders being common, does this seem like a case for one of them? The radio was manufactured in May of 2011, so it does have a bit of age.

Sorry for the long post, but I figured it would be most helpful to eliminate everything I have tried! In addition, (and this is fully showing my inexperience), is it normal for there to be some points on the board where there is absolutely no solder? As in there is a "pin" of something on the other side sticking through, but not a bit of solder. Thanks in advance!

[redlght]

A cold solder joint is most likely or a crack in the trace, there are some used pins best to do the poke test using a eraser or plastic stick on the solder side till you home in the trouble spot then reflow the joint.

[pabxone]

^^^ Ditto on a cold / dry solder joint.

A bit of patience and you should find it (or them if there are a few) after a little bit of time.

Here are some examples of poor solder joints :-
https://learn.adafruit.com/ad ... n-problems

[pabxone]

"whoops" Double posted (sorry)

[Scipio Kid]

First off, having had old and new radios and not being much of a tech, I'll say the LX, even if a few years old, is one of the most advanced radios out there. The antenna warning light, in particular is far better at sensing a problem then the typical SWR warning light. I found this out experimenting with several antennas while monitoring internal SWR meters as well as external units. At first I was annoyed that I could get good matches with other radios but not on the LX. But with some experience and testing I realized the LX wasn't only looking at SWR's, it was working more like an antenna analyzer. Using it as a tester, I was able to find and fix problems with coax, damaged ends, antennas etc. I'm really impressed with what these radios can do.

Now that doesn't mean they don't or can't break and what Pax and Red said about a cold or broken pathway sounds very likely. I had a nearly identical problem with a Uniden and had a good tech troubleshoot it while it was acting up. It only took him 5 minutes to find the problem which was an adjustable pot near the finals (can't be more specific because I'm very lacking in electronics but getting educated the hard way sometimes). While it was an easy fix, the radio wasn't worth it. Doing it myself might have been but I didn't know where to get the part nor am I very good with a soldering iron. so It's on a shelf, waiting.

I had a similar problem however in my pickup after installing a brand new Cobra. Every thing about the installation was great. Got a perfect SWR match right off the bat (well took an hour to fine tune the antenna) and the thing worked flawlessly but about a week after the install I was listening to a long-winded driver and noticed him fading in and out and instantly concluded it was his radio, not mine. The next day I noticed there was almost no static and had the squelch all the way down. Then while driving it suddenly got loud. I called for a radio check and was told I was breaking up. I got to checking things out and thought I'd figured it out with my coax connection being loose. Once I snugged it up, everything was back to normal. But then other things started going wrong. My channel display flickered and I'd notice the background noise fading in and out from time to time. So I looked real close at everything, coax, antenna etc. I put three other radios in and found the same problems but they came and went. This went on for some time until I finally decided to check my original wiring. I'm an electrician and am often shocked at the shoddy work and really stupid ways others do electrical work. So I'm always confident in my work, which was my problem ... In re-checking everything I discovered my ground connection was junk. It looked good but it wasn't. So my coax shield was carrying the load off-and-on instead of the ground wire, duhhhhh! A simple mistake and a simple fix (once I finally figured it out). So I'd advise you check your connections, both positive and negative including your power cord and plug on the back of the radio. These radios can't work with partial or fluctuating voltage and a faulty connection anywhere can cause both.

One more observation from just a couple days ago. Like I said, I love my LX and think they're great radios. My partner got a new (slightly used) tractor and we were moving some trailers Saturday with it. I jumped in and noticed he had an LX which came with the truck new, 3 years ago. As he left, I jumped on the base to see how well his radio was getting out. He only got a mile away and I lost him. I could hear him fine but he couldn't hear me at all. We got together later and figured out his RF gain seems to be stuck in low no matter how he adjusts it. The radio works fine as long as he's talking to someone close but with a little bit of separation, he can't hear anyone. I'm hoping it's just a little dust in the pot and we can clean it out but who knows. With a lot more of the functions being integrated, maybe down the road the wonderful LX won't be as easy to fix as the old Classics.

Let us know what you find with yours. I'm sure I'm not the only one who'd like to know.

Also, welcome to the forum! Good to have you along.

[sonoma]

you might make sure the screws that hold the board to the radio frame could be loose and causing a problem. here is a bulletin on this issue.

http://downloads.cobra.com/CB ... B10025.pdf

[GigaFish]

Thank you all very much for the responses! I think I will take the radio out and give it a real good look over this weekend. Hopefully it is just a cold joint or path, as suggested! They really are complex radios! My other radio is a Uniden 510, and is crazy the number of features that the LX has! I figure that if I can't find/fix it, then I will put the 510 in the truck for now until I can get it looked over!
I will definitely post anything I find out!

[sonoma]

touch up the solder around the VCO, audio chip and the crystal. just touch up all the joints in those areas and make sure you do not slop any solder across the joints while doing this. you need a good magnifying glass to make sure you can see all the joints real good. do not rely on the strength of your eyes to do this. when you touch the solder gun tip to the joint and if you nsee rosin flow from under it is a good sign it may not be making a good connection. just touch them all up in the locations I mentioned. those are the most common problem areas.. hope you find your problem. just make sure you have good glasses to do this. I even have a small scope I use when I am searching for bad joints. it hooks to my computer so it blows them up nice and large. you can find them on the auction site rather cheap if you search for them.

[GigaFish]

Wanted to post an update!
I was able to locate the area on the radio that made it cut off when touched. I took it back to the shop so trained eyes could work on it, and it turns out it was a bad solder by the crystal, just as sonoma suggested! He fixed it up and cut me a break since he had already looked at it once. Thanks for the help narrowing it down!

[sonoma]

glad to hear you now have it working.

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