Which rf power transistor is used in the cobra 18 wx st cb radio

start unsoldering you final , check it for proper voltage. then do the same for the driver. Check the voltage regulator. make sure you have the proper out put voltage in the vco section before doing anything. Also make sure you are on frequency.you more then likely have a short somewhere in the receive section and you are just going to have to break it down area by area , transistor to ICs .Its going to be painstaking but you will find it.

The cobra 18 wx st is the worst radio Cobra ever made. I don't know why they still make it. It's not gonna do much no matter how much you tune it up. Its a bad radio.

Get a cobra 29, and turn VR4 all the way counter clockwise.
(this will not work on the 18)

VR2 is modulation in the 18 wx st. adjust it while checking on a watt meter to see when its all the way up.

Take the cover off, flip it over solder side up, check all solder joints for cracks, cold solder joints, check the safety diode where it comes in the radio, before I go further, can you pick up the weather channel?...Does the PA work?...need to check the pre-driver and finals,audio chip, hook up the power meter using a coax three foot jumper from the output from radio to the input of the watt meter, then the antenna, first check the SWR, on the scale on the meter to cal/swr mode,adjust it to spec, then while still keyed down turn the dial to SWR,if it is the safe range your ok,then turn the Dial to 10 or 20 watt scale and key the mic. Does it have any wattage? If not suspect the finals, they are located on the back rail, if it does have RF then its the audio chip, its located on the right side rail facing you,closest to you.remove it and replace it with same ....hope this helps....

You probably don't want to hear this, but the cobra 18 wx st is just about the worst, and most hated radio ever made. Its a very poor radio.

to be very honest with you, you should get rid of that radio, or return it. Then buy a cobra 29 instead,

for proper mic wiring, please reference this page.

get an swr meter and check your antenna first. if good problem may be in the circuit board.. they are famous for loose IF, TX mixer boards. or call us

Your power output has blown, they are called your "finals" they can be replaced quite cheaply.

There are a few common reasons the fuse keeps blowing:

1. The transmitter final transistor is bad, it usually is directly connected to the 12v source and not switched by the power switch.
2. The power wires were accidentally reversed I.E. positive to ground, neg to 12v. There is a protection diode on the circuit board that did its job and is now shorted. it will keep blowing fuses until its replaced.

to be sure that u dont burn yr radio try to check yr swr with an external meter to be mounted between the radio and the antenna and check if it works! good luck.
rgds stefano

Sounds like the radio is working meter I
would think is bad you could replace it or
run as is.
Maybe this helped

Sounds to me like your finals in the rig have blown stopping you using the CB mode, Either that or you have a loose connection on the power output inside the radio.