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Cobra 25LTD Classic Blowing Fuses

Cable Guy

Cable Guy

Packaged by weight, some settling may occur.
Dec 29, 2010
400
343
73
West Tennessee
Hey guys, first post here. Got a Cobra 25LTD Classic that will power on, RX fine, meter works, but when I key up, it blows a 2amp inline fuse. Do you believe this could be caused by the Driver and/or Final circuit?

Thanks,
Cable Guy
 

welcome to fourm
1]id try a 3-5 amp fuse [starting with 3] see if it continues
2]double check make sure it hooked up right.
3]protection diode inside radio maybe bad.
 
If the protection diode is popped wouldn't it blow the fuse when power is connected? I bet the inline fuse is too small a value and should be increased to 5 or 7 amp fuse.
 
Yeh, it the reverse polarity diode was short it would blow as soon as it was switched on.
If it is a standard 25LTD then a 2 amp fuse should be about right.
If modded then its a different story....
 
Hey, Thanks for a fast reply. This radio is a virgin, never been touched until I came along and opened the case. Havent tooled with anything though, just for observation. I agree with protection diode being shot would blow fuses instantly, however, it seems to me like its drawing too much power when under tx. Possibly a short during tx. Leading me to believe its in one of the circuits mentioned. The protection diodes are fine in appearance, havent removed to check tho.
 
Beetle said:
If the fuse blows immediately it's going to be difficult to determine the power level. And just as hard to determine SWR unless the OP has some sort of antenna analyzer.
Click to expand...

I was figuring after he followed the advice of those who said put a larger fuse in he could get that information for us if it that plan worked. If it isn't putting out more power than stock or else has some kind of horrible swr then there must be something very wrong with that radio. My 148gtl doesn't blow a 2 amp fuse.
 
I have a meter comming, tracking says it will be here today, and its a 3 face so i can at least get a quick shot to see them. There is something I forgot to mention. When I got this meter, the inline fuse was bypassed with foil. It worked okay at that time. When I discovered it, I replaced it with a new one. When it DID work, unfused, it would work great, then at random times phase out until you heard nothing. It would not RX or TX and the guage wouldnt move. The power stayed on the whole time and didnt dim the lights. A few *light* taps on the top would make it come right back in. Checked the ground, all good and tight. Reflowed the solder joints from the antenna and other ground spots. Thinking about changing the final from stock to maybe a 1969, but i need to look at the datasheets to make sure if they're compatible.

I will get back to you when I get this meter. Thanks everyone!

CableGuy
 
After you get that meter, sounds like a good one by the way, if you have a digital vom I would first put it in parallel with the fuse of the unit and see how much current is flowing that way. If you are getting less than 5 like that then it would be safe to put you meter in the circuit in place of the fuse and see exactly what the radio is drawing. If it is drawing over 3 I would think that you might have a cap shorting out. And with you update on what you have seen it doing that makes it even more likely that something is failing with heat.
If you can get freeze spray use that to spray on suspect components to see if the current changes. You can use dust off to do that if you make it spray out the liquid. Not quite as good but can do the trick sometimes.
Other thing is to press down on things like tuning cans to see if that makes anything change with the current meter. Could be like you have already thought about being bad solder joints but probably more likely a bad cap with the excessive current draw.
Good luck on the hunt.
 
Okay, meter is in, but is in need of work. Thanks FleaBAY! The Watts meter is shot, the MOD meter works well, and the SWR meter needs a new calibration knob. It will either show full deflection or none during calibration. This will take a few weeks for me to fix. If anyone knows someone who has a Micranta 3 Range Power/ Modulation/ SWR tester that is not working, maybe I can scrap it to repair mine. Anywho, that's all I have.

Thanks,
Cable Guy
 
I have a classic that was working fine on my base station I run a 5a fuse on the base and it turned on worked the whole 9, I put it in my truck with the same power cord and it blew the fuse, so I tried a different hook up and new fuse and it blew it again. I put my President Grant in the truck same power cord new fuse and it worked I talked to my friend then tried the 29 again and it keeps blowing fuses. it basicly melts the wire inside the fires theres nothing left after it blows. the radio doesnt even turn on, nothing happens, as soon as I turn the power on the fuse just blows. I dont know what it is.
 
Robb said:
Probably the protection diode 'D23'. This is a common failure. It happens when someone inadvertently hooks up the power wires backwards. Just takes one second to pop that diode. Easy to fix and replace. Need a 1n4003 to replace the D23 diode on the board closest to the power plug,. Diode will cost you a a nickel or a buck depending on where you buy it.

http://www.cbtricks.com/radios/cobra/29ltd_29ltd_st_29wx_st/graphics/cobra_29ltd-st_29ltd_sch.pdf
Click to expand...

I dont know how to fix it, is it something that can be done cheap by a repair man?
 

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