Needing Help With Cobra 29 Keeps Burning Up D8

[blake25537]

I have a Cobra 29 and when I put power to it, it'll burn up D-8. I've checked a few things around it but I have no idea where to look now. I'd very grateful for any help finding the problem.

Thanks Again.

 

[Dmans]

Maybe this??

73
David

 

[Eldorado828]

Drop a slice of bread in that bad boy, hit the switch and you've got toast.

 

[blake25537]

I replaced the one that was clipped and D8 but it still burns it up when power is on

Maybe this??
View attachment 41294
73
David

 

[Dmans]

blake25537,
If you have a schematic for your particular version of 29 (I.e. 29LTD, 29 LX LE, 29NW ST etc) you could check all items tied in with D8. The audio transformer (T1) and a shorted PA jack are the first things that come to my mind.
Any modifications to the unit? Talkback added or anything else audio related?

73
David

 

[nomadradio]

That's because the main power is feeding into one end of D8 where it comes out of the modulation transformer. D8 feeds out into the final and driver transistors. Something in that section has a short to ground.

A multimeter will reveal a short to ground, using the lowest ohms range. Black lead to the negative pin of the power socket, red probe to the banded end of D8, or to the foil trace where it connects.

Unsolder and separate the center leg of the final transistor and see if the short goes away.

If not, unsolder and separate the center leg of the driver transistor and check.

If you still have a short-circuit reading to ground, one of the parts still connected to the foil trace at the banded end of D8 is what's shorted. Follow the foil trace to each remaining part. Unsolder and isolate each one by one. Eventually the short reading will clear when the failed part is disconnected.

73

 

[blake25537]

Dmans,

LTD Classic no mods or talkback looks stock to me. Sorry no schematic to the radio also.

Thanks

blake25537,
If you have a schematic for your particular version of 29 (I.e. 29LTD, 29 LX LE, 29NW ST etc) you could check all items tied in with D8. The audio transformer (T1) and a shorted PA jack are the first things that come to my mind.
Any modifications to the unit? Talkback added or anything else audio related?

73
David

 

[blake25537]

nomadradio,

Thanks I'll start check and see what I can find.

That's because the main power is feeding into one end of D8 where it comes out of the modulation transformer. D8 feeds out into the final and driver transistors. Something in that section has a short to ground.

A multimeter will reveal a short to ground, using the lowest ohms range. Black lead to the negative pin of the power socket, red probe to the banded end of D8, or to the foil trace where it connects.

Unsolder and separate the center leg of the final transistor and see if the short goes away.

If not, unsolder and separate the center leg of the driver transistor and check.

If you still have a short-circuit reading to ground, one of the parts still connected to the foil trace at the banded end of D8 is what's shorted. Follow the foil trace to each remaining part. Unsolder and isolate each one by one. Eventually the short reading will clear when the failed part is disconnected.

73

 

[Dmans]

blake25537,
The schematic in the link below should match your unit. Diode D8 is in grid square C-2. C73 is a possible culprit. Although missing in your 3rd picture, could be a solder bridge on the solder side of the board?

http://www.cbtricks.com/radios/cobra/29ltd_chr/graphics/cobra_29ltd_chr_sch.pdf

73
David

 

[blake25537]

Thanks to You both Dmans and nomadradio the Cobra 29 lives again. With the info you all gave me I tracked the problem down. The final was shorted out, I tested as explained and sure enough thats what it was. I installed a new final and she's like new again. Many thanks to you both......

 

[blasphemy000]

What's the date of manufacture sticker on the back of that's 29 say? It looks like they tidied up the PCB layout and respun it. Looks much neater inside than the normal design I'm used to looking at.

 

[blake25537]

Feb 2010 yes it looks a lot better this way lol

 

[blasphemy000]

Weird. I was figuring that was a newer board. I have a few from 2013 through 2017 and they all have the standard old style looking board. Very strange.

 

[nomadradio]

Congratulations for sticking with it and getting the radio back on the air!

Gotta love it when a plan comes together.

73

 

[Shadetree Mechanic]

Thanks to You both Dmans and nomadradio the Cobra 29 lives again. With the info you all gave me I tracked the problem down. The final was shorted out, I tested as explained and sure enough thats what it was. I installed a new final and she's like new again. Many thanks to you both......

That's some good stuff right there. Glad you got it going.