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did you replace it with one that is equal to the one that burnt .The one you replaced should have been the reverse polarity diode which should have been right near the smaller transformer .If this is the radio it looks like you should replace some of those electriclytic caps also.Did you replace the diode with the arrow pointing in the direction the arrow is pointing ?
The Cobra 29, like most CB radios, contains a reverse-polarity diode whose purpose it is to protect the radio against accidental reverse-polarity hookups. Normally all you will need to do is replace the fuse, then hook the radio up correctly.
If that doesn't get you going, then possibly the polarity-protection diode is now shorted and will have to be replaced (or removed - not recommended).
Beyond that, my experience is that the next component most like to get damaged is the Audio Output circuit, either an IC or a power transistor (larger than a regular transistor). At this point you're going to have to take it to a repair shop.
reverse polarity diode.open the radio and put your nose to work and sniff out the area where the power wires come into the set. There is a little black diode in that area and it s called the rev pow diode. Check it to see if you blew it.if you did then replace itand make sure the arrow on the diode is going the same way as the number on the board shows.
If you have an inline fuse make sure to check and replace it if needed. If the radio was hooked up backwards then you need to go to radio shack and pick up a replacement diode. 1N4001 will be the diode you need. It allows current to travel one way but not the other. When you hooked the radio up backward it blew, like a fuse. Here is the diode at Radio Shack. http://www.radioshack.com/product/index.jsp?productId=2036268
When you pull the bottom cover off you will see this diode in the board right by the power cord connector for the radio. Most of the time they will be chipped or even split after blowing from reverse polarity. Pay attention to the little "arrow" on the diode because there is a negative and a passive to diode. Anode and Cathode as it were. The Anode flows to the Cathode. The "tip" of the arrow is the negative side.
Look with a flashlight to insure you did not blow a tracer too.
The radio was probably wired backwards at some point. There is a
reverse polarity diode inside that must be replaced to fix this. If you
have continued replacing fuses and plugging it back in, there may be
further damage such as finals or audio amplifier chip as well. you will
need to take the radio to a qualified repairman for service. If it is
simply the reverse polarity diode, your bill should not be more than an
hour's labor charge.
It sounds like it may have been hooked up backwards when hooked up to the battery charger. If so it probably blew the polarity diode (D23) (1N4004) and with no sound it probably blew the audio IC (TA7222AP) mounted to the case of the radio. Also look at all of the large capacitors and make sure that the tops are not swollen or busted open if so they need replaced also.