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Radio Repair/Cobra 29LTD/ dead radio

2FB327

2FB327

Active Member
Oct 31, 2012
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As I am working my way thru a stack of donated radios, I have a 29 LTD with the KEPC 060G board.
Here is a link to some help I found from The Radio Shop, I'm referring to post #10. The whole post is great for someone that has an interest in repair.
http://gokarters.com/smf/index.php?topic=2330.0

I have a dead radio
The radio lights up, the channel selector lights, there is power in and out of the on/off switch.
There is no hiss from the speaker or the external speaker, no carrier, no mod, when I key the bench radio there is nothing, Nothing when I inject the 1kHz tone.
IC1 with a FC I have:
pin10 -5.1240
pin19 -10.240
pin 20-10.240
pin 21 -10.240
IC 1 voltages are dead, where as I should have reading according to the pin charts.
The radio looks to be virgin, D-11 is intact and the coils have never been spread and I can't find any burnt traces or radio hackery.

The pa switch is in cb and I have 5 volts at pin 11. There is 10,240 at both pins of the crystal.

On the audio side the voltages at the audio chip are:-
1-13.4
2-7.0
5-1.2
9-6.7
10-13.4
If you plug in the external speaker into the pa jack you can hear yourself.

Any help you could lend to get it up and running would be appreciated.
 

if you have not done this yet I would touch up all the solder joints at the vco, crystal and audio IC and see if you find any thing. even push around on the board to make sure if any thing comes up. you say there is no voltage at places it should be. so touch up the solder around key areas 1st off.
 
do you have a good magnifing glass so you can see the solder joints? looking with your bare eye's is a waste of time. just at the IC and vco area check all the solder joints for a dull solder joint or resin showing under the solder. you can hit a joint with a solder iron and resin will ooz out from under them when they are not soldered good. make sure all the board screws are tight also. there is so many things that can cause this and looking for ic voltage traces is good but a waste of time if you have a bunch of bad solder joints. it does not take that long to hit the solder joints in those area's.
check the vco voltage.
 
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sounds like TR23 is bad. sometimes when this goes it will take D22 with it.
check TR23 (2SC1096 or any equivalent) and D22 (9.1 volt 1/2watt zener diode) using your DMM diode function. if D22 tests good with this function then you can assume its good.
don't replace TR23 without at least checking D22.

if it were on my bench, i would first re-heat the solder connections around TR23, and check the radio. then i would remove TR23 and test it.
this can eliminate the possibility of removing TR23, finding out it's good, and then sitting there scratching your head instead of putting it back.

good luck,
LC
 
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Results: inspect and reflow solder around TR23 and D22, nothing.
Remove and test D22, tested good, compared it with new one and installed a new one.
Remove and test TR23 with a Peak DCA 53, tested good, compared findings with two other ones I had, reinstall.
Checked voltage at regulator:
Base rx/5V--tx/ drops to zero.
Collector rx 13.8--tx same
Emitter rx 13.8--tx same.
I'm trying to make sense of the current flow but am still stumped.
 
you do seem to have a problem here.

here is what you should be seeing at TR23:

RX
E=7.8vdc
C=11.4vdc
B=8.4vdc

TX
E=7.8vdc
C=12.8vdc
B=8.4vdc

there is a shorted or open component somewhere associated with TR23 as you are seeing the full supply voltage at the collector.

im afraid this could be tough to troubleshoot without being able to trace and interpret the voltage readings in this area.

the biggest deal i see is that you have the full supply voltage at the emitter, which should be your regulated 8 volt line. this could have caused more damage down the line.

ill have to look more and get back to you.
LC
 
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2FB327, i'm afraid im not really getting anywhere in trying to help you, as the readings you have taken have me a little stumped.

i hate to say it, but at this point it seems like some of the readings you have taken were either incorrect or misinterpreted. please don't take this as me saying you don't know what you are doing, but i don't have anything to go off of besides your tests.

i wish i could help more, but i don't feel comfortable putting out multiple guesses based on measurements that i can't interpret.

im not saying no one can help you, as there are probably guys out here that can tell you exactly what the problem is based on your post, but i am not one of them.
If i had the radio in front of me that would be a different story.

i wish you luck in your endeavor.
i do believe your problem lies in the TR23 area.
LC
 
None taken and thanks again, it's a donated radio and it will be a good learning experience down the road.
 

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