Cobra 148 NW ST Blowing Fuses Instantly

[vdubb16]

my grandfather handed down some misc things when cleaning out his shop. one being this CB. However he believed it to have something wrong with it but didnt recall what. Upon hooking it up i checked the inline fuse "5A" to find it blown. so i replaced and it blew immediatly. I tried a 10A to be sure it wasnt too small of a fuse. and it blew aswell.

i pulled the case off to check for any visible problem. but to be honest im not that kind of in depth electronic savy person to tell for sure if a resistor is blown or sure to recognize something wrong on a circuit board. i took some quick photos however cannot load them on the work govy computers. so i have them in an email if someone would be willing to post them for me.

and if anyone can tell me where to start looking or suggest what to do, it'd be greatly appreciated.

 

[VE5JL]

That sounds like something is shorted out like a transistor. Take it to a shop and I'm sure they can fix it cheap.

 

[vdubb16]

Yea i assumed that would be my best bet. i find it hard to believe that there would be a shop around this area with any knowledge on it. "Carteret County NC" id probably have better luck finding someone knowledgeable who could repair it at their house. ill try the NC section of the forum. Thanks!

 

[zzdiesel]

Reminds me of the time I hooked one's power leads backwards and it did the same thing. Seems like it blew a diode bridge.

 

[GMCKID45]

blown fuse

It is prob the protection diode look for a round black diode with a white stripe near the power cord between the pos and neg on the pc board just cut it or unsolder it u can buy a new one at radio shack for about a 1.00 hope this helps

 

[vdubb16]

I read that doing such will more often than not cause damange somewhere. However i was using what i believe is the originally equipped cigarette lighter power cord plugged into a workin cig lighter in my vehicle. it hasnt been cut or spliced, no nicks or soft spots it the cable. so i counted that out as the cause of my problem.

 

[zzdiesel]

You said it happened to your Grandpa a long time ago. He or someone may have used the other connector that connects into the car's wiring. It's just a thought. It needs work done to it.

 

[vdubb16]

thats very well possible. ive been searching for a local place to service it without much luck. i doubt the local shop who does our communication equipment for public safety would touch it. but i guess i can contact them to see if they know any place that does citizen band radios

 

[jackj]

Either the audio IC is blown or the final RF amp is blown. Both are fairly cheap but not easy to replace, at least the audio IC isn't. Most likely someone hooked it up with the polarity reversed. That used to be pretty common as a lot of semi-tractors used 12 volt positive ground electrical systems. I had a CB repair shop back in the middle 70's and made a LOT of money fixing shorted CBs.

 

[poikaa]

No doubt it was hooked up backwards, that is, reverse polarity. There is a diode that is designed to short out when this happens and then the fuse will blow. If the diode does it's work then the radio is protected.
You should not try a larger fuse as this lets more amperage pass into the radio and may bypass the reverse polarity protection diode. If this happens then the damage can be more extensive and in some cases non-repairable.
I just repaired a Grant XL with the same problem but the damage went farther as someone used a larger fuse. The AM regulator and driver were damaged, about 7.50 in parts but the labor will be at least 50 dollars. He was lucky! Could have been a dust bin candidate!
There is a 100% cure for this, by adding a full wave diode bridge the radio will never be reversed polarity again. The price of this is that the added diodes or bridge will cause a voltage drop of 3.8 volts. Not a big deal but saves the radio.
Too bad I was not closer as I can do the repair. I live in Michigan's upper peninsula.... Yes I'm a YOOPER!

73 Rod KB8DNS aka poikaa

 

[vdubb16]

No doubt it was hooked up backwards, that is, reverse polarity. There is a diode that is designed to short out when this happens and then the fuse will blow. If the diode does it's work then the radio is protected.
You should not try a larger fuse as this lets more amperage pass into the radio and may bypass the reverse polarity protection diode. If this happens then the damage can be more extensive and in some cases non-repairable.
I just repaired a Grant XL with the same problem but the damage went farther as someone used a larger fuse. The AM regulator and driver were damaged, about 7.50 in parts but the labor will be at least 50 dollars. He was lucky! Could have been a dust bin candidate!
There is a 100% cure for this, by adding a full wave diode bridge the radio will never be reversed polarity again. The price of this is that the added diodes or bridge will cause a voltage drop of 3.8 volts. Not a big deal but saves the radio.
Too bad I was not closer as I can do the repair. I live in Michigan's upper peninsula.... Yes I'm a YOOPER!

73 Rod KB8DNS aka poikaa

i tracked down a local tech, and as u say is dead on. He said theres no further apparent damage that he could see. other than the polarity diode. but he was going to send it off to be repaired.

 

[poikaa]

Here is a diagram of the use of a full wave bridge. It does work well but has a voltage drop across all the diodes. Input is on the left and can be reversed DC at the terminals, the diodes will"steer" the polarity correctly so it will be correct.

73 poikaa