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Cobra 29 LTD Cl,assic R58 keeps burning? ( Replaced L7 ) 3 Times

Mark A Bradley

Active Member
Mar 22, 2019
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R58 keeps burning up, after replacing L7,LA111, 3 different LA111. Lifted legs on Driver Final, and Amp, replaced Voltage Regulator, no good. All I did was replace this for a noise, and would not adjust, was over tightened. Any ideas where to start? Thank you, enjoy your day,,,,,
 
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You got me. In the schematic for the Cobra 29LTD Chrome, R53 is in the SWR meter circuit. Can't recall seeing one of those overheat.

L7 is in the receiver's second IF 455 kHz.

Which one of a dozen (or more) versions of the Cobra "29" is this radio? The schematic callout numbers will change from one revision to the next.

73
 
You got me. In the schematic for the Cobra 29LTD Chrome, R53 is in the SWR meter circuit. Can't recall seeing one of those overheat.CL,Classic 29,LTD Classic,Thanks.

L7 is in the receiver's second IF 455 kHz.

Which one of a dozen (or more) versions of the Cobra "29" is this radio? The schematic callout numbers will change from one revision to the next.

73
 
With a problem like this where things just don't seem to line up you have to look for the seemingly unrelated things.

There is a good chance that it's not about L7 and has more to do with something that happened while replacing L7.

Time to get out the magnifying glass and a pc board layout showing the solder side.
Look for solder bridges and splashes.

Who knows what you might discover.
LC
 
Since were shooting in the dark, can you spare a pic of that area?

R53 is part of the SWR circuit - so it is getting fried may mean that the "sensor" (ferrite and coax feedthru) may be damaged.

upload_2021-12-3_8-3-19.png

LA111? YIKES that is by FET2 - that FET may have shorted - it can be replaced with any SK192 - 2nd IF Amp part of the IF strip.

C18 sits atop a POWER rail to the TX side towards the rear panel - if the SOLDERING job shorts out C18 - there goes the ballgame!

upload_2021-12-3_8-8-56.png

But before you start just dumping parts in, take a magnifying glass and go over those areas; SWR and it's coax section on the board - then over by L7 - locate the 100Ω ohm resistor R22 and also C18 - a tiny cap that feeds the 10.240 from the Xtal (main crystal X1) to make the IF generate it's 2nd stage of 455kHz IF after L7 into L8 and L9

Follow the power traces - make sure C18 is not a dead short which can kill the FET at FET2 and affect L7.
 
20211202_224300.jpg 20211203_081932.jpg 20211203_081932.jpg 20211203_082023.jpg 20211202_224300.jpg
Since were shooting in the dark, can you spare a pic of that area?

R53 is part of the SWR circuit - so it is getting fried may mean that the "sensor" (ferrite and coax feedthru) may be damaged.


LA111? YIKES that is by FET2 - that FET may have shorted - it can be replaced with any SK192 - 2nd IF Amp part of the IF strip.

C18 sits atop a POWER rail to the TX side towards the rear panel - if the SOLDERING job shorts out C18 - there goes the ballgame!

But before you start just dumping parts in, take a magnifying glass and go over those areas; SWR and it's coax section on the board - then over by L7 - locate the 100Ω ohm resistor R22 and also C18 - a tiny cap that feeds the 10.240 from the Xtal (main crystal X1) to make the IF generate it's 2nd stage of 455kHz IF after L7 into L8 and L9

Follow the power traces - make sure C18 is not a dead sh
With a problem like this where things just don't seem to line up you have to look for the seemingly unrelated things.

There is a good chance that it's not about L7 and has more to do with something that happened while replacing L7.

Time to get out the magnifying glass and a pc board layout showing the solder side.
Look for solder bridges and splashes.

Who knows what you might discover.
LC
Thanks for the tip, will look over the board and my work, I appreciate your input, enjoy your Wed.

ort which can kill the FET at FET2 and affect L7.
Nomad, this is a Resistor sitting by the Driver ,it is a 8.2 Ohm Resistor, as it calls for in my Cobra 29 LTD Classic parts. I keep getting some buzzing in TR8,and from my Signal tracer, it may be coming from L7,as I went back one step to this part. The noise it coming out of the Right side leg, so I assume it is L7,anable to adjust the Voltage when peaking out the receive side. I will go ahead and go with your fix, and look around, if this changes anything, let me know, and a good day to you. Top two pics,R58.
 
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This is a Cobra 29 LTD Classic, just a reminder there is a mistake of the radio,a Clasic,not Chrome. Thank you.
 
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With a problem like this where things just don't seem to line up you have to look for the seemingly unrelated things.

There is a good chance that it's not about L7 and has more to do with something that happened while replacing L7.

Time to get out the magnifying glass and a pc board layout showing the solder side.
Look for solder bridges and splashes.

Who knows what you might discover.
LC
This all happened when replacing L7, but I think L7 was not producing any voltage, just making a noise, traveling to Tr8 this is why I changed the L7,as it did not make a change on my oscilloscope.
 
20211203_083528.jpg
Since were shooting in the dark, can you spare a pic of that area?

R53 is part of the SWR circuit - so it is getting fried may mean that the "sensor" (ferrite and coax feedthru) may be damaged.


LA111? YIKES that is by FET2 - that FET may have shorted - it can be replaced with any SK192 - 2nd IF Amp part of the IF strip.

C18 sits atop a POWER rail to the TX side towards the rear panel - if the SOLDERING job shorts out C18 - there goes the ballgame!

But before you start just dumping parts in, take a magnifying glass and go over those areas; SWR and it's coax section on the board - then over by L7 - locate the 100Ω ohm resistor R22 and also C18 - a tiny cap that feeds the 10.240 from the Xtal (main crystal X1) to make the IF generate it's 2nd stage of 455kHz IF after L7 into L8 and L9

Follow the power traces - make sure C18 is not a dead short which can kill the FET at FET2 and affect L7.
Repaled R22,burnt out,forgot to mention me bad. Will look into C18, and FET2,first on my list this morning.
 
Since were shooting in the dark, can you spare a pic of that area?

R53 is part of the SWR circuit - so it is getting fried may mean that the "sensor" (ferrite and coax feedthru) may be damaged.


LA111? YIKES that is by FET2 - that FET may have shorted - it can be replaced with any SK192 - 2nd IF Amp part of the IF strip.

C18 sits atop a POWER rail to the TX side towards the rear panel - if the SOLDERING job shorts out C18 - there goes the ballgame!

But before you start just dumping parts in, take a magnifying glass and go over those areas; SWR and it's coax section on the board - then over by L7 - locate the 100Ω ohm resistor R22 and also C18 - a tiny cap that feeds the 10.240 from the Xtal (main crystal X1) to make the IF generate it's 2nd stage of 455kHz IF after L7 into L8 and L9

Follow the power traces - make sure C18 is not a dead short which can kill the FET at FET2 and affect L7.
Hey,it was my fault, I put down R53,it is R58,the number 8 looked like a 3, all the dirt and whatever was covering half of the number. I will fix the title, again Thank upu for the help.
 
With a problem like this where things just don't seem to line up you have to look for the seemingly unrelated things.

There is a good chance that it's not about L7 and has more to do with something that happened while replacing L7.

Time to get out the magnifying glass and a pc board layout showing the solder side.
Look for solder bridges and splashes.

Who knows what you might discover.
LC
Hello,I am going to start pulling legs, one at a time in the 2 circuits and see what I find. And also I will check all solder joints. I am going to spend some time on this radio, as this is the radio that,s goes in my truck, And weekend, busy catching up to sit, it will be a few days. Enjoy Thanks you.
 
so, im only going off of that one pic you posted of the driver transistor, but it looks as though that insulator is possibly crooked enough for the back of the transistor to make contact with the heatsink?

also, that thermal paste looks grey.
if it is, there is a chance that you bought conductive thermal paste which is not what you want for doing work on CB radios.
you want the white stuff that is non-conductive.
i could be wrong, but if it's grey, i would switch it out.

you might at least check for a short from the middle leg of the transistor to the heatsink.
there should be no connection.

seeing a burnt resistor in front of a large transistor is a pretty good bet that the transistor itself has issues.

if you replaced this resistor and the new one burnt up as well, that's even a stronger indication that this transistor might be shorted.

seems worth checking.
you can google how to test a transistor with a DMM if you're not already familiar with the process.
LC
 
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so, im only going off of that one pic you posted of the driver transistor, but it looks as though that insulator is possibly crooked enough for the back of the transistor to make contact with the heatsink?

also, that thermal paste looks grey.
if it is, there is a chance that you bought conductive thermal paste which is not what you want for doing work on CB radios.
you want the white stuff that is non-conductive.
i could be wrong, but if it's grey, i would switch it out.

you might at least check for a short from the middle leg of the transistor to the heatsink.
there should be no connection.

seeing a burnt resistor in front of a large transistor is a pretty good bet that the transistor itself has issues.

if you replaced this resistor and the new one burnt up as well, that's even a stronger indication that this transistor might be shorted.

seems worth checking.
you can google how to test a transistor with a DMM if you're not already familiar with the process.
LC
Thank you,will check my Trandistors before installing them.And I shall look into the paste, I purchased this second hand. I did not notice, thanks, great observation. Ok, will give it my best, I will buy you guys a beer, when done, have a great weekend, stay safe.
 
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I check the pack or resistor they came up 5.2 on my tester, and meter. I believe they call for a 560ohm, and apparently a miss marked bag. I have to check the bag and see the ring colors.Grey,Red,Gold Gold,these are just
In my resistors.
Checking 8.2, this is what my app is telling g me. Going to try a real 560ohm,keep you posted
 
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Reactions: Handy Andy
20211206_114708.jpg I check the pack or resistor they came up n my tester, and meter. I believe they call for a 560ohm, and apparently a miss marked g. I have

I think I may have found another issue,take a look at the driver. What do I see, a Crack going down the side. Now will this also play a part in burning up the Resistor even if it the Resistor was at the correct value? So far, the Resitor has not burned,I also installed a new Final, and Driver and Voltage Regulator, just to cover everything I can think of, as it already had a replacement TA7310P.
Now, the re eive is out, but I can work on that. When all is said and working, I will explain that it seems I had bad value or mismanaged Resistors,and a cracked Driver, so going to fix the RX,and give it a test runView attachment 48874 View attachment 48874

so, im only going off of that one pic you posted of the driver transistor, but it looks as though that insulator is possibly crooked enough for the back of the transistor to make contact with the heatsink?

also, that thermal paste looks grey.
if it is, there is a chance that you bought conductive thermal paste which is not what you want for doing work on CB radios.
you want the white stuff that is non-conductive.
i could be wrong, but if it's grey, i would switch it out.

you might at least check for a short from the middle leg of the transistor to the heatsink.
there should be no connection.

seeing a burnt resistor in front of a large transistor is a pretty good bet that the transistor itself has issues.

if you replaced this resistor and the new one burnt up as well, that's even a stronger indication that this transistor might be shorted.

seems worth checking.
you can google how to test a transistor with a DMM if you're not already familiar with the process.
LC
 

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