Cobra 29LX driver failure

[Larry-b]

"First Post" Not sure if this is the correct place to post this one.

Radio stopped transmitting. Found R58 and driver burned. Replaced R58 and driver. Powered up and had .5 watts DK and some swing. Had it on about a half hour and something around VR5?? smoked, now driver is out again. The only other problem I found was a broken trace where JP36 ties into L16, repaired that. Replaced driver and powered on, driver out again. Could that broken trace have caused the driver failure the first 2 times? and possibly knocked something else out.

 

[unit_399]

Check C126 and C143. One or the other is shorted. 73s.

- 399

 

[Larry-b]

Check C126 and C143. One or the other is shorted. 73s.

- 399

Thanks. I will replace both. I do not have a capacitor tester.
I will reply with results when parts come in.

 

[Radio Tech]

I agree with unit 399. Something is making the base of the driver pull a lot of current. Like if L16 was shorted. C143 could cause that. If upstream the final would had smoked.

Now I guess that you found only a burnt trace close to VR5 area? Possible a power supply rail over current and smoked. This would have been caused by a short around the driver so the trace was the weakest link.

 

[Larry-b]

I agree with unit 399. Something is making the base of the driver pull a lot of current. Like if L16 was shorted. C143 could cause that. If upstream the final would had smoked.

Now I guess that you found only a burnt trace close to VR5 area? Possible a power supply rail over current and smoked. This would have been caused by a short around the driver so the trace was the weakest link.

Actually the trace problem I found was a broken trace where JP36 ties into L16 which left one leg of L16 and C143 not connected. I have been unable to find any type fault in the area of R5? Ive checked the area with a DMM and found nothing out of tolerance. I used a remote webcam at 5X zoom on a 21" monitor to check all components and traces and found no sign of anything burned near R5.
I will be replacing C126, C143 and C62 along with the driver and as to the smoke from around R5? I will see what does not work after cap and driver replacement??
Thanks
Larry-B

 

[Larry-b]

Problems Fixed

I replaced c126, c143 and c62 and that solved the driver problem. Dead Key was still only 1/2 watt and barely swinging to 3 1/2. I figured that had to be a problem with either feed to the final or the final itself. Voltages checked out so I replaced the final with a new IRF520 and still only 1/2 watt DK. So I tried another "NEW" IRF520 and still no change. I pulled a final out of another radio (IRF520) and it worked fine. I had purchased 10 IRF520's and it appears they are ALL bad. Thats what I get for buying "good deals" from ebay.
Lesson learned
Thanks for all the help
Larry-B