might have a bad connection to the receive wire in the mic plug.
sometimes if you turn the volume all the way up, you can hear some faint noise through the speaker, with no mic connected.
you might be mistaking that phenomenon for this symptom.
try resoldering the connections on the astatic's plug, and also make sure its wired right.
here is a link to a page with all kinds of info for your cobra 29.
including the mic wiring for whichever type of astatic mic you have.
http://www.cbtricks.com/radios/cobra/29ltd_29ltd_st_29wx_st/index.htm
the pin numbers are stamped on the little black plug, you just have to look real close.
want an easy method to make sure its the mic and not the radio?
have a voltmeter handy?
set the voltmeter to read continuity. (so that when you touch the leads together the meter shows a direct short)
now unplug the mic from the radio, remove the power cord from the back, and any other connections such as antenna, or an external speaker.
take off the bottom cover of the radio, (the one with the speaker on it)
and set the radio down on the edge of it so that you can leave the speaker connected. (have the radio upside down so you can see the components. the bottom cover should be on its side with the weight of the radio holding it up)
can you picture it?
now, put one lead of your voltmeter on any one of the metal tuning cans that you see in the radio, (you know, the ones we're not supposed to touch).
the metal chassis of the radio will not work.
now, while holding one lead on a tuning can, touch the other lead to each mic pin on the connector until you see the meter move or however the voltmeter reads a direct short.
you have found your ground pin.
draw a picture of the connector on the radio and label this pin.
now hook the radio back up, but leave the mic off.
turn it on, and using a short length of wire as a jumper; touch one end of the jumper to the ground pin, and the other end to each of the other pins.
one pin will cause the radio to go into transmit mode.
that is your TX pin.
one pin will cause the receiver audio to be heard.
that is your RX pin.
and one pin might scare the crap out of you by making a loud BOOOP! sound when you touch it.
that is your audio pin.
there ya go.
sorry if you already know all this stuff, i like to type it out so that in the future, someone might find this thread helpful.
later,
LC