Cobra model 18 ultra, no RX or TX

[big mike 8113]

I have a Cobra Model 18 Ultra, Weatherband CB here on the bench. The radio has no receive audio, and when mic. keyed no RF output but the TX indicator light dose come on. I did check with a known good mic. still same results. I thought maybe the audio IC might be shot , but figured check with the forum first to see if any one else has run across theese symptoms on this type radio. I havent been able to find a schematic or voltage pinout on this model on line. The PLL IC is a LC7185. And the numbers I found on the PC board are as follows., 411-032-D, 4A1030-D.
One other thing I forgot to mention, is that all three weather band positions work fine.
Any help will be apreciated.
73's Big Mike 8113

[ramblingman]

hello;.does your s meter indicate received signal..like on a know busy channel say 19....also if you have a external speaker,plug it into the pa jack in back of radio,,put radio in pa mode,and see if you get any audio through the speaker.this will let you know if your audio chip is shot...but it sounds more like maybe a loose connection..have you checked for that..on this models they are notirious for that...those small boards that stand up along the rear and side of the main pc board.sometimes you find loose connections there.power it up,and take a plastic scratch tool..and wiggle around on the boards and see if audio returns.......

[absorbentgnome]

Sounds like a missing internal supply to me, but unfortunately I am not familiar with the radio chassis. The audio chip supply and RF output transistor supply are often linked, so check for an open resistor or fried track. Even if the audio chip is dead, there still should be some kind of TX if the supply is good. If a resistor is open, it could be because of another fault that caused it to burn out.

Download the datasheet for the audio chip and check there is 12V on the appropriate pin, if not follow the circuit back until you find the supply line and the faulty component.

[big mike 8113]

Thank's for the input, I found Q104 sitting a little crocked and solder joints didn't look so good, same with the solder joints at the rear of the radio in the vicinety of the ant. connector. After I got done cleaning up all the factorys soldering efforts, I powered the radio back up, and to my suprise I had a room full of receive. Next I keyed the mic. took a look at my Hygain meter, and man what a swing.
73's Big Mike 8113