My radio just recently incurred this problem. It receives WX band, uses PA correctly (mic transmits over PA). It will not receive or broadcast over any standard CB channel (1-40). The needle is just dead. It doesn't even receive anytthing with squelch all the way open. Don't know if it's a bad antenna (checked wiring looks okay) or something wrong with CB.

It sounds like you are using the wrong mic for the radio or the mic has been wired incorrectly or the mic has become faulty or the radio has become faulty. Unplug the mic and see if the radio receives and works correctly. You should be using a Yaesu model MH-36b or a MH-42B microphone.

Since the radio cannot transmit on Weather bands, there is no need for the radio to detect if it is transmitting when receiving weather.
In CB mode (on CB channels) the radio will mute when transmitting. So... a guess is that it is muting, or transmitting all the time in CB mode. Check for a mic problem by removing the Mic from the radio ... if it receives CB with no mic connected, then there is an issue with the mic. Otherwise, unless the unit is in PA mode (public address mode) it is likely something internal and beyond the scope of this service.

Antenna good? Receive meter move at all? radio show transmit when key mic? more you tell me the better I can help---I will assume you know enough to make sure RF gain and squelch are set correctly.

and cb/pa in right position........

That's usually a defective amc stage that produces RX audio and TX modulation. It's an integrated circuit that will require a tech fixing it. You will have a transmitted carrier with no modulation, and no sound in your speaker. However, make sure you have a good mike plugged in first, and substitute a known good mike to verify it's not the mike. With the WX band working, the above may not apply.

See if you have a carrier...signal going out, but no modulation. Get your Tech to check the mike wiring/circuitry inside the radio, also.

You have a bad microphone. There is most likely a broken wire in the microphone jack.

Without a microphone the radio will not go into receive mode, at all. Same thing happens when there is a disconnected wire in the mic cord. Because the radio you have, has Talkback in it, you can always hear a little audio coming through, but not a lot.

Try a new mic or one you know is good.

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....

It could be..
Final blown : worst case
bad mic : try another one
CB/PA switch on PA instead of CB
CB/WX swithc on WX instead of CB :you cannot transmit on weather.
Broken wire on mic jack.

Try the mic and switches first, and if that doesn't help you may need to take to tech. You can find one at a cb shop usually at truck stops.

If try another mic and see if problem is still
there then you have half of the problem,the
TX an RX all go through the mic if mic good
you will have to go inside of the unit.
Good Luck

IF YOU CAN RECIEVE THAT IS A VERY GOOD SIGN THAT YOUR SWR MATCH IS OFF....I DONT KNOW IF YOU MATCHED UP YOUR RADIO WITH YOUR ANT.IF NOT YOU CAN BUY A CHEAP SWR METER @ RADIO SHACK...CHECK THAT YOUR COAX CABLE IS OK TRY TO REPLACE THE COAX......GOOD LUCK

the mics on these cobras have a receive wire in them so the transmit will still work even if the receive wire is broken,if this wire is broken the unit will not make any sound...........remove the plug from the radio end of the mic and check the connections on the 5 pin connector.....(could be a cold solder or broken wire) or try a new mic or one that is proven work on another radio
hope this helps.................good luck!