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Cobra 29 RX/TX Fade. Bad PLL? Where can I get PLL?

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MightyMo
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Cobra 29 RX/TX Fade. Bad PLL? Where can I get PLL?

Post by MightyMo » Saturday, 11 June 2016, 11:05 AM

I've had this problem for a while. I think I've finally got it nailed down. One minute the radio is fine. Normal static and radio chatter. I can talk and get radio checks as I please. The next, the static goes quiet, (still there, but MUCH quieter). RX/TX light still changes, but I can't get anyone to talk to me. After extensive google research, I came across a page on cbtricks.com that suggest it may be a bad Phase Locked Loop. (PLL) IF THE FINAL AND DRIVER ARE FINE. How can I check the final and driver? The only markings on my PLL are "COBRA 29 LTD 0246 11600-00" I googled those and can't find anything. I found one page that suggests the PLL I need is a uPD2816. Does anyone know if that's the correct PLL? I found a uPD2816 for $5.00, where another sight has a COBRA 29 LTD 0246 11600-00 fro $20. Any thoughts? Does it sound like its a PLL problem? I noticed this problem around the same time my meter stopped working on the radio.

Also where do you get your electrical components. Many of the RS around me are out of business. The closest is an hour away. I've found resistors and diodes online, but I'm looking for a website and/or store that is much more inclusive. In upcoming projects, I'm going to need radio pots and knobs, LEDs, 2 digit channel displays, this PLL, diodes, etc.

Thanks in advanced.

(edited by MDY)


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sonoma
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Re: Cobra 29 RX/TX Fade. Bad PLL? Where can I get PLL?

Post by sonoma » Saturday, 11 June 2016, 12:12 PM

sounds more like you have a bad solder joint around the crystal and PLL area. check the solder joints good and resolder them to make sure they do not get a open circuit when the radio warms up or gets moved around.

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MightyMo
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Re: Cobra 29 RX/TX Fade. Bad PLL? Where can I get PLL?

Post by MightyMo » Saturday, 11 June 2016, 13:18 PM

I've done a visual inspection. To my untrained eye, I can't see any cold/broken solder joints. It doesn't just happen when it's warmed up. It seems to be random. I'll take a double look just to be sure and re solder the joints when I get a chance.

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Re: Cobra 29 RX/TX Fade. Bad PLL? Where can I get PLL?

Post by wjb11583 » Thursday, 16 June 2016, 16:20 PM

Is it a newer 29? I had one do this and it was IC4 TDA2003 it's a 5 pin 10W audio amplifier, had a broken pin and would sit just right sometimes and others just quiet noise. As far as places to get components try Jameco . com, Mouser . com, and Digikey . com very resourceful sites IMO
P.S. the TDA2003 is obsolete so expect to pay between $1.50-$5.00 depending on where you find it.

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MightyMo
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Re: Cobra 29 RX/TX Fade. Bad PLL? Where can I get PLL?

Post by MightyMo » Thursday, 23 June 2016, 22:10 PM

Define "newer". It's a front mic, made in China. MFG date is Aug 03. Was yours newer or older? Thanks for the site list.


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Re: Cobra 29 RX/TX Fade. Bad PLL? Where can I get PLL?

Post by pabxone » Friday, 24 June 2016, 5:04 AM

sonoma wrote:sounds more like you have a bad solder joint around the crystal and PLL area. check the solder joints good and resolder them to make sure they do not get a open circuit when the radio warms up or gets moved around.
That will most likely be the cause of your problem and not a PLL fault.

Tap all around on the solder side of the circuit board whilst the fault is there with the PLASTIC end of a medium sized screwdriver.

The tapping tends to make cold / dry solder joints and cracks in the PCB come and go.

Not all dry solder joints look poor on a visual inspection - often you need to localize the area where the faulty joint(s) are and then rework all the joints in the area to fix the problem.

Area's to pay particular attention to are often covered in a wax pouring on the component side (VCO and OSC) - This wax if it migrates thru to the solder side due to a prior rework or repair creates dry / cold solder joints as it melts and mixes in with the solder.

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Re: Cobra 29 RX/TX Fade. Bad PLL? Where can I get PLL?

Post by 47bw022 » Friday, 15 July 2016, 12:28 PM

Hi MightyMo

If what you suggests that it's a bad pll loop, then it might just be an adjustment of the pll loop coil. I don't know the radio, and haven't looked at any schematics for it.
Often it just requires a trimming pin and a digital voltmeter and a soldering iron for the allignment. The soldering iron for melting the wax in the coil so you can get the trimming pin into the coil.

Best regards
47bw022
Dennis

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wjb11583
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Re: Cobra 29 RX/TX Fade. Bad PLL? Where can I get PLL?

Post by wjb11583 » Sunday, 24 July 2016, 15:30 PM

MightyMo wrote:Define "newer". It's a front mic, made in China. MFG date is Aug 03. Was yours newer or older? Thanks for the site list.

Mine was a 2013 front mic china model, if its the china model it probably has it, looking from the component side it's about an inch to inch and a half towards the front of the radio mounted on the chassis.

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MightyMo
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Re: Cobra 29 RX/TX Fade. Bad PLL? Where can I get PLL?

Post by MightyMo » Monday, 24 October 2016, 16:34 PM

Update as an FYI, I know this is an old post. A lot has happened since I've been on last. Couldn't find the bad solder joint despite trying. Finally found a CB tech close-ish to me. Took the radio to him and it took him all of 5 min to find the bad joint. He charged me $45 to re-solder and align the radio. I was just glad to have it done. But about a week later, the same thing happening. I was planning on taking it back to him because he says he'd warranty the job for a year. In the mean time I took it apart to change the channel indicator from stock to blue (combined with the new red filter glass gives a purple indicator). I realized the ribbon connecting the channel indicator to the main board had literally half of the joints broke on the non-solder side of the board. With time and patience I un-soldered the rest of the ribbon and broken contacts, stripped the ribbon exposing fresh wire, and got it all put back together. It works great. For what it's worth, thanks for responding to the post.

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Re: Cobra 29 RX/TX Fade. Bad PLL? Where can I get PLL?

Post by jessejamesdallas » Monday, 24 October 2016, 19:02 PM

Try Fry's Electronics for parts...Fry's is like a Radio Shack on steroids...
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MightyMo
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Re: Cobra 29 RX/TX Fade. Bad PLL? Where can I get PLL?

Post by MightyMo » Monday, 24 October 2016, 19:49 PM

Sweet. Thanks.

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Re: Cobra 29 RX/TX Fade. Bad PLL? Where can I get PLL?

Post by Scipio Kid » Tuesday, 25 October 2016, 11:37 AM

How do you figure you got that kind of damage? I've had Cobra 29's in off road trucks and dune buggies and not seen breakage like that. Did the radio get dropped?

At any rate, all of this is excellent information. I have a real nice tweaked 29 that does the same thing from time to time. Now, I can't wait to get into it to see if it's the same as any of the stories above.

A problem with getting CB's fixed; any tech worth his salt will charge 40 bucks minimum to fix your radio. The better (more expensive) your radio is, the easier it is to pay the bill when it needs fixing. I took a Uniden 76 and a Cobra 29 to the local guru for fixing. He took literally 4 minutes to diagnose both radios and then told me to junk them. He said it'd cost 40 bucks per radio to fix plus parts, 12 for the Uniden and 29 for the Cobra. He said he could sell me good used units for less with a warranty. Now if they'd been Galaxy or Connex radios, 50 to 60 bucks wouldn't have been an issue. When the FCC first went to 40 channels, we bought a bunch of Sears Roadtalkers for our trucks and were thrilled to get them for 100 bucks each. 5 years later I took one of them in for repairs and paid 40 bucks to get it back on the road and felt pretty good about it. I was at Sears the next day and they had the same radio on sale for 35 dollars.

Now, if you can DIY the thing, then great but if you're like me, DIY almost always ends in DOA. But when it works ... It's GREAT!
If I could only learn to solder ...

-- Tuesday, 25 October 2016, 12:08 PM --

Sort of along those same lines; my Cousin stored his Freightliner tractor at a friends yard out in the sticks for a couple of months when things were slow and when he went to get it, it'd been vandalized terribly, mostly with shotgun blasts. One blast through the windshield hit the side and back of the Uniden pc78ltw that hung on the ceiling peppering the case with shot and tearing off the antenna and power cables and even wrecking the mic cable. It was lying on the floor of the sleeper when we found it. The blast actually tore the screws out of the ceiling holding it in place. It looked like junk but I took it apart a while later, straightened out the case halves some, cleaned it out and couldn't see any real damage to the internals so I fixed the mic cord, put it back together and fired it up ... it worked great! I put it in my pickup and love the comments I get when folks climb in and see it all blasted up like that. (I just say I had a little 12 gauge malfunction.) Now if it survived that, again I wonder what happened to your Cobra to tear those pins loose? Maybe it came out of an M1 Abrams or something!
Happy Trails



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MightyMo
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Re: Cobra 29 RX/TX Fade. Bad PLL? Where can I get PLL?

Post by MightyMo » Wednesday, 26 October 2016, 8:34 AM

Ha ha. I really enjoyed your post. Those "Shotgun malfunction" type radios are the best to have. You can't by character, history, and stories like that.

I sure was regretting trying to put the new channel indicator in my 29. Not only did I find the ribbon damaged like I mentioned, after I desoldered the contacts on the indicator, I tried to gently get it free of the circuit board by pushing it in. Instead, the remaining solder (that I couldn't see) ripped several of the contacts off the board. In the end, most of the contacts still were intact enough to work with very careful soldering. I used one little jumper where the contact disintegrated completely. Lesson learned!

As for the ribbon. I have no idea how it happened. This 29 is my first CB. I've taken the radio apart a couple of times just to get to know the radio and to start educating myself. When put back together, the ribbon folds into an "S" shape underneath a metal cube mounted to the frame of the radio. My only thought is it must have been taken apart and put back together enough times to flex those contacts enough to break. There was a bead of hot glue across the non-solder side of the board for added strength. I think that is what kept the ribbon in place allowing for sporadic RX/TX. I've only taken it apart 2 or 3 times. But I'm pretty sure this radio has had work done to it before. I'm not familiar enough with radios yet to know what may have been done. It only has 4-6 watts of output. I did the resistor talk-back mod. It does seem to have better receive. It doesn't squeal with my power mic like my other 29 does. So who knows.


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