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CB Radio Fuse Help!!!!

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taloncb

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Alvord, Texas
I recently inherited a Cobra 29 Limited Edition Nightwatch Soundtracker. This is a used radio and I was told that it was not working. I soldered a 5 amp max fused cigarette lighter plug to the unit, and replaced the inline fuse with a 10 amp. I now know that it should have been 2 amp fuse inline. My problem is that so far it has blown the 5 amp cigarette lighter plug fuse, and after replacing with a 10 amp, it appears to have blown the 15 amp fuse for the cigarette lighter but not the 2 fuses, inline or cigarette lighter plug. I have opened the radio case and have not noticed any damaged wires or damaged solder on the board. I have not attempted any cleaning at this point. Any ideas without having to take it to the radio shop???

Thanks

talon
 

jsikora

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Dec 27, 2006
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Location
West Monroe NY
Polarity Protection Diode. Someone probably hooked it up backwards. Normally there is a $.10 diode right near the power input. Chances are it is shorted. You can check with a multimeter between both power and ground. I have fixed about 10 of these over the years....
 

JayMojave

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Dec 13, 2007
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Location
Mojave Ca
Hello: jsikora has some good points there.

Look at CBTricks.com Cobra Electronics (Dynascan) Radio Index for info on the Corba Radios.

The Diode is placed there to protect the radio from being hooked up backwards polarity wise. So this protects the radio when hooked up with the wrong politarty.

The Protection diode should be close to the 12 volt input wiring.

Good Luck.

Jay in the Mojave


Polarity Protection Diode. Someone probably hooked it up backwards. Normally there is a $.10 diode right near the power input. Chances are it is shorted. You can check with a multimeter between both power and ground. I have fixed about 10 of these over the years....
 

XTS3000

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Nov 4, 2005
Messages
1,078
When are people going to learn. If a device calls for a 2A fuse, and it blows, putting in a larger amp fuse will only cause further damage to the radio. Rather than having 2amps flow through the shorted circuit, 5, 10,15 amps flowing through it only damages it more.
 

taloncb

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Gentlemen, as cobra is so nice as to NOT disclose the ampere size for the fuse on their website OR in the owners manuals, I would ask that you really stop critizing when someone makes a mistake and instead recommend to these companies that they should disclose the size of fuses to use to PROTECT THE INVESTMENT that we have made. But of course it is obvious from your comments that all you really want is to look down on new boots to radio operations and make yourselves feel superior. I admit that I may have made a mistake with the fuse size, but since it is so difficult to find this information, it is an easy mistake to make.

I do appreciate the information on the diode and I know where it is located. I will check that tonight. I may go ahead and take it to my local shop to get looked at and hopefully get it tweaked and peaked at the same time without too much cost!
 

Citywide173

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Feb 18, 2005
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"Polarity Protection Diode" I've never actually heard one called that, but it makes sense. The name I've seen is Zener Diode, but, tomatoes tomatos....if this has blown, it should have effectively cut the circuit where the power enters the radio, and no matter how much reverse polarity, current, voltage or resistance you put through the power cord, it shouldn't be blowing fuses. Something is obviously going directly to ground where it shouldn't be, and I don't think it's the Zener Diode. If the Zener is blown, I'd look at the radio's power connector to be sure someting hasn't corroded to the point where it's grounding out, if the Zener isn't blown, there's a whole multitude of possibilities inside where the problem is.
 
N

N_Jay

Guest
Different beast.

A Zener across the input is used to trim spikes and transients above a certain voltage. (It will also act as a revers polarity protection diode, but has to be big enough to handle the full peak current the properly sized fuse could allow. ;)

Most automotive powered equipment has a regular high current diode backwards across the input so it will "crowbar" and blow (properly sized) fuse if someone 'accidentally" applies power in the wrong direction.

A regular diode is used because it is cheaper and more forgiving to excess current should the fuse be a little slow (or the wrong size) ;) ;)

Just razzing you, taloncb, in fun. My comment was more about the general fact that people always seem to fix overload problems with a "bigger fuse" first. Heck, I have done it once in a while. (Although my favorite is to replace what should be a fast acting fuse with a slow-blow fuse to fix intermittent shorts (like always seem to show up in automotive trailer wiring).
 

XTS3000

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1,078
Gentlemen, as cobra is so nice as to NOT disclose the ampere size for the fuse on their website OR in the owners manuals, I would ask that you really stop critizing when someone makes a mistake and instead recommend to these companies that they should disclose the size of fuses to use to PROTECT THE INVESTMENT that we have made. But of course it is obvious from your comments that all you really want is to look down on new boots to radio operations and make yourselves feel superior. I admit that I may have made a mistake with the fuse size, but since it is so difficult to find this information, it is an easy mistake to make.

I do appreciate the information on the diode and I know where it is located. I will check that tonight. I may go ahead and take it to my local shop to get looked at and hopefully get it tweaked and peaked at the same time without too much cost!
The Cobra 29 LTD Classic I have w/original power cord has a sticker right on the fuse holder. Says: "2 amp MAX"

If you get an aftermarket powercord, these are not labled due to different power requirements of the different radios that use this style of connector. Most aftermarket CB power cords have a 4amp fuse.

Good luck
 

Citywide173

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Feb 18, 2005
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Attleboro, MA
Different beast.

A Zener across the input is used to trim spikes and transients above a certain voltage. (It will also act as a revers polarity protection diode, but has to be big enough to handle the full peak current the properly sized fuse could allow. ;)

Most automotive powered equipment has a regular high current diode backwards across the input so it will "crowbar" and blow (properly sized) fuse if someone 'accidentally" applies power in the wrong direction.

A regular diode is used because it is cheaper and more forgiving to excess current should the fuse be a little slow (or the wrong size) ;) ;)
OK, that makes sense, having only worked with the Zener in the BC-760XLT (boy was that a racket $25 repair for a $0.37 part and 3 minutes of labor), and a few Pico fuses in Motorola and Whelen stuff, I wasn't aware of the larger diode in most automotive stuff.

Although I don't see where it would be different though. If the diode has blown, it should still interrupt the circuit, and fuses shouldn't be blowing, which would indicate that this isn't the problem, and something is going directly to ground. If the diode is blown, it's somewhere in the cig lighter plug, power cord or connector, if it isn't blown, it's somewhere in the radio.
 
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N_Jay

Guest
The purpose of the diode is not to "blow".

The diode provide a near short circuit to the reverse current and (in theory) the (properly sized) fuse blows first and the diode is unharmed to do its job again another day.

Lucky for us, most diodes fail 'shorted' forcing replacement.
If it failed 'open' it would leave the circuit unprotected for the next time it was hooked up backwards.
 

Thayne

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What I hate is when the diode stays shorted and then somebody hooks it right to a battery & it burns the trace right off the board--- :roll:
 
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N_Jay

Guest
What I hate is when the diode stays shorted and then somebody hooks it right to a battery & it burns the trace right off the board--- :roll:
How does that happen with a (properly sized) fuse in place?

Oh, Never mind!:lol:
 

taloncb

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Apr 10, 2009
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Location
Alvord, Texas
One of the odd things I noticed is that there was a copper wire wrapped around one of the mounting bracket screws. Of course I notice this after I posted here and had the fuse problems. Naturally I removed this wire. I know that in and of itself it should not have had anything to do with this problem, but is sure as hell didn't belong there! I have had a CB for many years, and have never had this fuse problem and that is where I am at a loss. Normally I hard wire with an inline fuse to the battery, but this time I did not as I want to move it between vehicles. Let me give you the complete rundown. I purchased an after market replacement accessory plug that was marked 5 amp max. It had a 5 amp fuse in it. I soldered it to the wiring, using the wiring diagram just to be sure I was right lol. I did not check to see if the radio was on. When I first plugged it in, the radio came on for approximately 1 sec and shut off. I checked the fuse in the accessory socket and it was blown. Unfortunately I only had 10 amp fuses, and I installed one in the accessory plug. I then turned it on again and this time it blew the inline fuse. I replaced it with a 10 amp and did it again, and this time it blew the 15 amp panel fuse for the cigarette lighter in the car. I relaced that fuse and took the radio out of the car. This is where I am....LOST!!!!!! lol and since we have nearly a foot of snow on the ground now, I thought I might work on it a little...then changed my mind after reading the posts about the diode. Also, I visually checked the board and found no degredation anywhere, and all contacts appear clean and in good shape. I did not smell any type of "electronic" smell that would make me think of a short.

BTW thanks for all the info, you guys are great!
 
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N_Jay

Guest
It could be the diode (or something else in the radio) just gave up the ghost.

Are you sure it came on, or just the light? Maybe you have it wired backwards?

Or your plug is bad internally, or when you soldered the wires you melted something and made short.

or, or ,or . . . .
 

ArtU

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I replaced about 100 or so of these polarity protection diodes a year when I was young and worked at a CB shop as my first RF tech job. Yes they are designed to CROWBAR and also to SHORT on large amps so saving the radio.

I also saw PCB traces burned off from 20amp fuses put in. I would replace those traces with old fashioned point to point wiring.
 
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