My 857 receives fine, but when I key the mic it turns off. The 857 works fine on HF both receiving and transmitting.

The static tells me that your audio chip is working, but the stages ahead of that are not feeding a signal to the audio amplifier. The intermittent receive sounds like a bad antenna connection, or bad contacts on your transmit - receive circuits. I don't know the Cobra 29 LXBT circuitry, but likely a relay is switching between transmit and receive when you press the mic key. You could try connecting your antenna to the pins on the relay that carry the rf signal to the receiver circuitry. If you suddenly starting picking up signals, you know the switch is the problem, as you have by passed the switch momentarily. Don't try to transmit in this condition.

Your RF gain control needs to be turned all the way up to be able to receive signals and if you are on one of the HF bands like 40 meters or any HF band the squelch control needs to be all the way off! You do not use squelch on HF! When on VHF or UHF then you would rotate the squelch control just to the point that the background noise stops and go no further. Higher squelch settings will only block weak signals and your radio will be deaf! Squelch control rotated just to the point the background noise stops! NO SQUELCH ON HF. Turn the squelch all the way off on HF!

Put SWR meter between your antenna and output, test wave length receive/transmit, might need a ground plane or center loaded stick.
Could be transceiver relay, try clicking mic fast, it should click out as fast as you key the mic.

this is strange as most times if a proper transmit key is depressed , then the radio switches from receive to transmit , the mfg know this would cause some great squeal from receiver /transmitting so close to a receiver so the part that is the receiver is muted to keep this from happening , id say there is some fault in the part that does this muting of receiver

Tough call. could be the antenna but if someone transmitted with a bad antenna it might have ruined the output circuit. the receiver will still work if the output circuit is damaged. Verify the antenna and its wiring first.

Another possibility is the microphone switch. the push to talk switch might have a loose wire and its not activating the transmitter when you push the side in.

Have you ruled out a receiver PL setting problem?

If you have talk back in this radio.( you hear your self through the radio when you transmit) then its a good bet that the RX wire in the mic plug has broken off.

The mic switch is responsible for putting the radio into receive mode when you release the key. If the RX wire is broken, the radio will not go into receive.

Normally when this happens you have no receive at all no matter what, but you will have very low sound if you have talk back installed in the radio.

Because of this, if you have talk back in your radio, and the volume is faint even at full blast, its a good bet that its a broken RX wire in the mic cord or plug.

You can attempt to see if the break is at the mic plug. if it is, you can re solder it and it will solve your problem. If you have no clue, then you can just go buy new 4 pin mic.

One of 2 reasons. First is common. Your mic or mic jack may be bad. The way the radio works, is the mic switch switched from receive and transmit. When the mic is not keyed then the switch is actually on receive, and when you key it, you switch it to transmit. So if there is something wrong with your mic, or it is not plugged in, the radio will not be in receive mode.

The next problem if a new mic doesn't solve the problem is...

there is an issue with your antenna or feed system. Make sure coax is good. Use an ohm tester. Make sure its not shorted.

Next make sure the antenna is good and tuned properly.

You may need to replace one or both.

Sound like either an antenna problem, or a mic problem.

The mic on most cb's also serve as transmit/receive switching. A broken wire or bad switch in the mic or cord can cause either or both to fail.

If you have a bad antenna (or cable), you should still hear squelch noise on receive (maybe lower in volume) and be able key the mic and get a transmit light on the unit. Visually inspect the connector, cable and antenna. If need be use an swr meter to prove the antenna system is bad.

Hope that helped...

Geno

Moving the volume/on/off knob to make the radio work again is interesting and a good clue. On most CB's, the pot is probably soldered directly to the printed ciruit board.

It's possible that there is a bad solder joint to the power connections on the pot that turn the radio on & off. If the solder joint is bad, it can still make a connection but the connection will have added resistance in line. The on/off switch can be bad too.
Switches arc each time they are used and over time, the contacts become pitted and make poor connection.

I had a stereo receiver where the tuner lights flickered and there was electrical noise in the receiver. It turned out to be a bad on/off switch.

This resistance may be low enough not to affect the radio when in the receive mode but when you transmit and draw more current that resistance will be noticable during transmit.

The fact that you rotate or move the volume/on/off knob and the radio starts working again, tells me that there may be a bad connection in the radio.

A cheap thing you can try is spraying some electrical contact cleaner into the pot and rotate the pot back & forth several times and turn the switch on & off several times to clean it.

Please let me know if I can be of further assistance to you.

Good luck!

- Jim