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Burn spot Cobra 29 LTD Chrome

Bombardier

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Dec 19, 2021
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16416876959355127480322606158540.jpg Build Aug 1
Board number KEPC-1106
APRIL 3 2008
Good afternoon, upon figuring out still why the Plastic screw melted on the TDA2003, I noticed a burnt spot on the board near the first protection diode near C123. Is there a way to repair this so I can complete the alignment, or us this board and radio scrap? I am not familiar with repairing a board, this is the only concern I have at this time.Thank you, a couple of cold beers will be bought for
 

Eldorado828

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Feb 21, 2016
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8-2-8 in the Lonestar State
The picture is pretty blurry so it's hard to tell. If it's a blown trace it appears to be a very short trace so a wire jumper soldered in place should be fine. Remove the solder, components and clean it up to assess it. Repair it but now it would be what the cause of the affect is before powering it back up.

A better picture would really help.
 
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Bombardier

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16417342778673956561627036201624.jpg 16417342778673956561627036201624.jpg Sorry, hope this may be better. I noticed the voltage regulator burned up R103. I changed both the Regulator and Resistor, only signs of any type of issue. Hope this picture is better, I checked it before posting. I will repair and check the radio over for any other spots gone bad. I will re-post after repairs.
 
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Handy Andy

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I changed both the Regulator and Resistor, only signs of any type of issue. Hope this picture is better, I checked it before posting. I will repair and check the radio over for any other spots gone bad. I will re-post after repairs.


Sad to see this...

Wanted to post a little help here, for the input "ground Lead" is the minus on the power cord.
upload_2022-1-10_9-31-11.png

StIll appears as this is a common ground chassis, so you won't have any problems finding ground to reattach the NEGATIVE (Black) wire back to foil ground. But it's in how you well this can be redone is the problem.

I'm only here to point out that what I've done before was to solder the Black wire to the Power Choke laminate to fix not just a ground - but improve (decrease) ripple noise from a bad alternator - soldering directly to the choke seems to help with this versus trying to fix the popped trace and all the extra re-work.

Now if the customer is fussy, then well, you're back up against the wall in having to fix it, but if they don't care - the soldering the Negative wire onto the choke laminate works - and in some setups - works better in reducing noise.
 

Bombardier

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Dec 19, 2021
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16418282909323050500097697629567.jpg OK sir, I repaired the trace, only issue I have now is the dreaded Motorboat noise? From reading, this is a cause of mis allignment am I correct? This is the main reason for working on this Cobra radio.
Thank you , almost done for my conclusion of post.
 

Bombardier

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Dec 19, 2021
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Sad to see this...

Wanted to post a little help here, for the input "ground Lead" is the minus on the power cord.

StIll appears as this is a common ground chassis, so you won't have any problems finding ground to reattach the NEGATIVE (Black) wire back to foil ground. But it's in how you well this can be redone is the problem.

I'm only here to point out that what I've done before was to solder the Black wire to the Power Choke laminate to fix not just a ground - but improve (decrease) ripple noise from a bad alternator - soldering directly to the choke seems to help with this versus trying to fix the popped trace and all the extra re-work.

Now if the customer is fussy, then well, you're back up against the wall in having to fix it, but if they don't care - the soldering the Negative wire onto the choke laminate works - and in some setups - works better in reducing noise.
Thank you, I shall check with my neighbor, this is his radio, I really appreciate the help from both of you guys, but I now have a Motorboat issue after fixing the trace? Reading I see a alignment may fix that problem, but a little more input may solve my problem??? Going to back track and double check the board. Thanks again, it is one thing after another.
Sad to see this...

Wanted to post a little help here, for the input "ground Lead" is the minus on the power cord.

StIll appears as this is a common ground chassis, so you won't have any problems finding ground to reattach the NEGATIVE (Black) wire back to foil ground. But it's in how you well this can be redone is the problem.

I'm only here to point out that what I've done before was to solder the Black wire to the Power Choke laminate to fix not just a ground - but improve (decrease) ripple noise from a bad alternator - soldering directly to the choke seems to help with this versus trying to fix the popped trace and all the extra re-work.

Now if the customer is fussy, then well, you're back up against the wall in having to fix it, but if they don't care - the soldering the Negative wire onto the choke laminate works - and in some setups - works better in reducing noise.
 

Handy Andy

Do Your Research First, Then Decide...
Apr 23, 2018
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OK sir, I repaired the trace, only issue I have now is the dreaded Motorboat noise?

Now, seeing what you did
Two thumbs up!
(y)(y)

But... I also see / read / interpret - Motorboat?

Could be from a Blown diode - several places where this can happen - and even affects RX "PIN" Protection diodes too...

This requires you to look at and double check - the TX diode that keeps the Pin 3 from blowing up Pin 4

upload_2022-1-10_16-45-25.png

You say it Blew R103 - not good...

It also uses a Zener - based Regulation (D22) PNP transistor TR23 - which both the 9.1V Zener (1W) and the TR23 2SD880 variant - any PNP rated 3~5A with a BCE Pinout would work, note the FACE of the Transistor to towards the Side-Panel. - and uses TO-220 case style.

It is my fear they you have a lot more caps that have to be replaced. As well as several Semiconductors.

So, if it's not giving you 8 ~ 9 volts on the LEG to R105 - Check TR23 and D22 Zener - one or both can be blown.

The Motorboat can be from several other locations with one being by the PIN Diode near the rear - centerline by L3/L4 and the Rear Heat sink panel for Final and Driver. Caused by a blown Cap or PIN Diode - to get by you can replace the PIN with a 1N4148 - just don't use a Schottky-type - use standard PN junction or one that gives you 0.7V drop - not those 0.3V drop ones.
 
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Bombardier

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OK? This is what I have done. Went around with my meter on diodes, found D316, D1, D2, D14 no good. Replaced T23 with a D880 Voltage Regulator with the face looking at R105, and after that , yes R105 I'd producing 9 volts from T23. But the Metal Oxide 1 Watt 5% 15 Ohm Resistor done burnt up again, I should possibly go over solder joints again? I know I did the Capacitors about 1 year ago, radio has been no problems until now. I shall pull schematics and use also what was sent in this forum also.
I will trace back from T23 and see what may be causing this Resitor to burn up. Thanks for reminding me that yes, there are Diodes in the path of all the circuits, as I was overwhelmed to see R103 burnt. Upon pluging and initiating current, right away I see smoke and Resistor starting to discolor. Right now I am scratching my head but? I shall go over what you sent to me and try to diagnose the problem. Every bit put into my head may help me,.is there a chance the power supply may be overvoltage? I shall check that ASAP when I get out if this packed grocery store. Here for 5 things, and guess what, the cart us full, oh my wife drags me along when I have work to do. I am sorry no more information than what I gave you, trying to get you caught up with my prodject. I will re post my fi ding after 9:30 PM Central time. Thank you.
 
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Handy Andy

Do Your Research First, Then Decide...
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When the polarity has been reversed, and you still burning up resistors, then yes, a recap is needed.

This time though - you'll have to work the power supply input from the switch back to the board - you'll have to make a break point for the power - so you can then look for potential shorts and blown caps without having to replace or refix a section every time it's powered up.

If you know the part, and it becomes the fuse, then what is going on AFTER the part is where the problem is.

So, remove R103 again, this time keep it out - do a continuity check (no power applied) - one side should be open while the other pad towards the side panel goes to the front panel thru that trace to a wire leading up - locate that pad check continuity from it to ground and follow the trace. R103 gets power from this area and it uses the CB PA switch to do it - from the main power the resistor sourced from - towards the front panel - might be something there. Do a continuity check on the trace - from it to ground - then locate its next "stop" or part soldered across - one to the trace the other say - to ground - remove - lift one leg, of the part, see if continuity check doesn't change - or if it does - if the short (or low resistance reading) goes away, you've found the culprit.

While you're at it, check IC 4 - the Audio amp - make sure it's not shorting itself to the case thru the tab.
 

Bombardier

New Member
Dec 19, 2021
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When the polarity has been reversed, and you still burning up resistors, then yes, a recap is needed.

This time though - you'll have to work the power supply input from the switch back to the board - you'll have to make a break point for the power - so you can then look for potential shorts and blown caps without having to replace or refix a section every time it's powered up.

If you know the part, and it becomes the fuse, then what is going on AFTER the part is where the problem is.

So, remove R103 again, this time keep it out - do a continuity check (no power applied) - one side should be open while the other pad towards the side panel goes to the front panel thru that trace to a wire leading up - locate that pad check continuity from it to ground and follow the trace. R103 gets power from this area and it uses the CB PA switch to do it - from the main power the resistor sourced from - towards the front panel - might be something there. Do a continuity check on the trace - from it to ground - then locate its next "stop" or part soldered across - one to the trace the other say - to ground - remove - lift one leg, of the part, see if continuity check doesn't change - or if it does - if the short (or low resistance reading) goes away, you've found the culprit.

While you're at it, check IC 4 - the Audio amp - make sure it's not shorting itself to the case thru the tab.
Hello backO repaired the burning of the resistor, I had 2 bad Voltage Regulators burning up the R104. Now I have 13.8 at R103 and R104 and still have the motorboat. I found that C156 had a bad or corroded leg, and it broke off into the board, 470uf 16v Capacitor in the audio section. Now I will start pulling Capacitors C119 C36 C79 C38 and C67 , this is the area if concern. Report when done. This is all I had time for, but I have time to pull all of these now.
 

Bombardier

New Member
Dec 19, 2021
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Hello backO repaired the burning of the resistor, I had 2 bad Voltage Regulators burning up the R104. Now I have 13.8 at R103 and R104 and still have the motorboat. I found that C156 had a bad or corroded leg, and it broke off into the board, 470uf 16v Capacitor in the audio section. Now I will start pulling Capacitors C119 C36 C79 C38 and C67 , this is the area if concern. Report when done. This is all I had time for, but I have time to pull all of these now.
Now. Noticed when you turn the squelch up or down, the Motorboat go fast or slow. Might be 0n to something?

Caps
1uf 50v = 1.0
1000uf 25v =96.7 Replaced
3.3up 25v =35.2
470uf 16v =470
 

Handy Andy

Do Your Research First, Then Decide...
Apr 23, 2018
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Are you able to get 8 ~ 9 volts off of TR23 so the radio is getting power from the main regulator?

From the MAIN regulator - the PLL taps it's 5V regulated output from there.

Locate PLL, by it is D19 and R124 - a 10Ω ohm (or thereabouts) are you seeing 5V there at R124?

The Squelch is (should be) TR13 with C36 it's "thunk cap"
 

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