I can hear it key up on my scanner but it says rf output fail and when i put an SWR meter inline it doesnt move. It has a brand new antenna. I tried replacing the final transistor with a 2sc1969 and it didnt change anything but the original final did look a bit dark like it got really hot. Im not sure where to go from here.
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How to diagnose Cobra 29lx with RF output fail. SWR meter doesnt move, new transistor didn't help
- Thread starter Sevabeva
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K4EET
Chaplain
The only Service Manual that I could find appears to be for a European model but it may help you troubleshoot the radio.
My understanding is, you can't just replace the finals, there's also some capacitors that need changing and you should realign it (using a scope)... That's my understanding and I could be way off the mark as I have never have done it (or burned up my finals)...
tvengr
Well Known Member
If you hear a weak signal on your scanner, the oscillator is working. It is usually the higher power components that fail. Since the final has been replaced, I would look at the driver. I would also suspect a transmit/receive relay if your radio has one. Also, check the connection to the antenna connector. To be safe, check your antenna for a short or high SWR.
K4EET
Chaplain
Sevabeva, can you tell if the "EU" designation impacts the Service Manual accuracy for your rig? Try to match the foil traces on Page 16 of 16 with your rig's PCB to see if they are the same. Let me know if they are different and I'll try to do some more Googling for additional information that may help you if this Service Manual does not pan out.
Best wishes with your troubleshooting endeavors! Dave
I havent checked the values yet but the steps should be similar. Ill update when i test it
Use a scope good to at least 30MHz and high frequency probe to check the base and collector of the final, driver, pre-driver transistor, etc. You should see some voltage gain from the base to collector, although it's not a big gain in some cases due to the impedance of the circuit. A bad transistor will show a definite loss of RF going from the base to collector.
When an RF transistor fails in a CB it should not affect any capacitors in the circuit and none should need replacing. Sometimes a resistor can be affected and they usually look cooked if bad. After changing a transistor the radio should work nearly as good as new without alignment but the radio should be aligned because of all the Chinese counterfeit transistors that are nowhere near original spec.
When an RF transistor fails in a CB it should not affect any capacitors in the circuit and none should need replacing. Sometimes a resistor can be affected and they usually look cooked if bad. After changing a transistor the radio should work nearly as good as new without alignment but the radio should be aligned because of all the Chinese counterfeit transistors that are nowhere near original spec.
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