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Cobra 29 LX LE Dead Receive

Unit 75

"TRAINMAN"
Jul 29, 2014
527
1,014
153
Central Alabama
I have a Cobra 29 LX LE that transmits fine. It has great modulation. PA works just fine. I have NO signal on the digital S meter and no audio coming thru the internal or external speaker. Not a mic issue. I can hook a signal generator to the antenna jack and hear no audio or signal. I have tried tuning the rx cans to see if I have a signal or audio. I have checked TR7, FET1, FET2, C8,D4, D6 etc. I DO have my 10.240MHz signal from TR20. I have removed TR13 and no audio. TR8,TR9,TR10 are ok. Any suggestions?
 

NODENS

Member
Aug 2, 2018
45
7
18
Have you checked if Rx voltage is present? you can check those RX transistors to see if they are feeded.
 

Unit 75

"TRAINMAN"
Jul 29, 2014
527
1,014
153
Central Alabama
Nomad......TR8,9 and 10 are ok.
TR8
E=.778v
C=7.11v
B=1.5v
TR9
E=(ground 1.44mV)
C=1.65v
B=.758v
TR10
E=.801V
C=.818v
B=1.6v
TR23 Regulator is good supplying voltage rail for RX supply.
D4 detector is tested good as well.
Volume control pot is good. I'm going to check out TR10 again. Just looked at the 29 voltage chart and it is saying the base voltage should be 13.2v in both tx and rx. Don't know if that is correct because the base voltage of TR10 should be the same as the collector voltage of TR9.
 

nomadradio

Analog Retentive
Apr 3, 2005
5,323
7,650
573
Louisville, KY
www.nomadradio.com
TR10 collector has 8/10 of a Volt on it. Should be closer to 12 or 13 Volts DC.

Makes it sound as if the primary winding of L9 (the side with only 2 pins) has become an open circuit. One of those two pins leads to TR10 collector. If the other pin on that side of has 12 Volts or more on it, L9 has failed open circuit,

73
 

Handy Andy

Do Your Research First, Then Decide...
Apr 23, 2018
5,583
7,693
573
www.cbtricks.com
Just to help...

upload_2021-2-8_21-40-33.png
Your LX model has Weather, - look above, there is a Transistor from the main Front panel - that has a wire from it, to the Junction of Tr9's Collector and TR10's Base...

upload_2021-2-8_21-54-10.png
IT can kill your receive because when WX alert hits, or the front panel commits a fart, the programming of the RSW switch - it can blow shorted from triggering...too many times or a failure in the Switch circuit itself - being TR25...

Remove the Wire from the RSW hole on the main PCB - if Receive NOW works , your have a bad TR25 being fed from the the front panel - and it's controlled by a transistor on the front panel PCB - may not be worth the effort to try and fix it yourself.

Just remove TR25 if you don't care for WX to interrupt your day...

You'll have a Receive though.

The Radio will also have WX - just not the ability to "inject" a tone and blare out the speaker when you're listening to the CB side when an alert comes on the WX channels

WX works like a Monitor function - the RSW wire - being removed, stops the Monitor - but you will have WX if you switch to the WX to listen for the forecasts and any alerts.

There is also a "feeder" resistor - being that your voltages are low, this same TR25 can blow the Resistor that biases these two parts (TR9 and TR10) and that is R32...1.5K although it shouldn't be the problem child, this Resistor can blow or cause a load condition on TR25 - and make it blow.
 
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Unit 75

"TRAINMAN"
Jul 29, 2014
527
1,014
153
Central Alabama
TR10 collector has 8/10 of a Volt on it. Should be closer to 12 or 13 Volts DC.

Makes it sound as if the primary winding of L9 (the side with only 2 pins) has become an open circuit. One of those two pins leads to TR10 collector. If the other pin on that side of has 12 Volts or more on it, L9 has failed open circuit,

73
Hit the nail right on the head, Nomad. The primary of L9 had a discontinuity. Replaced L9 and I have receive. Re-tuned the rx chain and I can hear a mouse fart in Mexico.Thank you for your help.

Handy Andy.....Thank you for the info. I'm going to add that in my notebook for future reference. TR25 is a TO-220 package regulator on my main board that supplies the rx chain with power. It isn't a surface mount in this radio. Cheers rascals!
 
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Unit 75

"TRAINMAN"
Jul 29, 2014
527
1,014
153
Central Alabama
It's all good, Handy Andy. My unit is the 29 LX LE non Bluetooth. It has been discontinued for awhile. Not sure of the board number as it's all buttoned up now. Looking at your schematic.....If you follow where the RSW (weather audio) connection is from the front smd board to the main PC you will see where TR25 (TO-220 casing) is mounted and TR25 (LC9450) collector connects to TR9 collector / TR10 base junction. Removing TR25 will kill your rx as this is what biases your whole rx chain. Info is never old and always useful my friend.
 
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