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Cobra 25LTD Classic Blowing Fuses
- Thread starter Cable Guy
- Start date
I found out the problem after talkin to someone who knows cobras. if you zoom into it a bit. theres a short that burnt a little section of the board
Attachments
What would you expect to see by putting a meter (doesn't matter if it's digital or analog) "in parallel with the fuse"? Why not remove the fuse, put the meter on its highest dc current range, connect the meter IN PLACE OF the fuse, and see how much current the radio is pulling? You can make dc voltage checks with the fuse in place, but I'd expect a very low reading if the fuse was good, and pretty close to full supply voltage if it's open.
looks to me like the protect diode is shorted. the diode needs to be changed and the trace repaired before you burn up more parts in the radio.
A stock never had the hood up cobra 25 should have a 5 amp fuse in the line one that has been peaked & tuned should have a 7-10 amp fuse. It should cost you less than 20 to have the Diode replaced. Seems a shame to pay that much for a 1.50 component that takes less time to replace that it does to remove the bottom chassis cover.
Fixing that radio is a breeze.
As Sonoma said, you will need to fix the open trace by sanding both sides of the break with a small piece of fine or medium grit sandpaper first; then apply some solder with a 20 watt soldering iron to close it.
( in red circle pictured below)
Then replace D23 with a new 1n4003 diode. Diodes have polarity; so pay attention to the line that circles one end of the old diode and put the new diode in with the same orientation.
D23's purpose in that radio is to protect the rest of the circuit from being fried in the event that the radio's power wires have been reversed. But when it is replaced, as it must be, it will continue to protect the radio from any future reversed polarity charge if/when another mistake is made. This is also the first lesson you get in radio electronics; as this is easy compared to how complex it can get elsewhere on its circuit board.
Consider this as a test and your initiation into radio.
This is not rocket science.
Be glad that it is a simple fix that won't cost you time and money. In comparison, if you were to ship your radio to a shop to do this repair, it will cost $+25 for the shipping both ways and the repairman will want $30 for his time plus parts. If you were to buy a cheap soldering pencil kit (solder pencil, solder, solder wick for old solder removal, and the solder stand). you could do that for about $25 and then buy a diode for $1. The difference is $30; not to mention you will have a soldering pencil and have the satisfaction of fixing your first radio. Up to you . . .
If you just fix the open trace on the board w/o replacing that diode, it will just do the same thing it was doing before - popping fuses; or even open up that same trace again. You must also replace that diode. Once that diode gets a reversed voltage, it is toast and is no longer any good.
Fixed lots of radios that had this problem; quite common and easy to remedy . . . go for it . . .
As Sonoma said, you will need to fix the open trace by sanding both sides of the break with a small piece of fine or medium grit sandpaper first; then apply some solder with a 20 watt soldering iron to close it.
( in red circle pictured below)
Then replace D23 with a new 1n4003 diode. Diodes have polarity; so pay attention to the line that circles one end of the old diode and put the new diode in with the same orientation.
D23's purpose in that radio is to protect the rest of the circuit from being fried in the event that the radio's power wires have been reversed. But when it is replaced, as it must be, it will continue to protect the radio from any future reversed polarity charge if/when another mistake is made. This is also the first lesson you get in radio electronics; as this is easy compared to how complex it can get elsewhere on its circuit board.
Consider this as a test and your initiation into radio.
This is not rocket science.
Be glad that it is a simple fix that won't cost you time and money. In comparison, if you were to ship your radio to a shop to do this repair, it will cost $+25 for the shipping both ways and the repairman will want $30 for his time plus parts. If you were to buy a cheap soldering pencil kit (solder pencil, solder, solder wick for old solder removal, and the solder stand). you could do that for about $25 and then buy a diode for $1. The difference is $30; not to mention you will have a soldering pencil and have the satisfaction of fixing your first radio. Up to you . . .
If you just fix the open trace on the board w/o replacing that diode, it will just do the same thing it was doing before - popping fuses; or even open up that same trace again. You must also replace that diode. Once that diode gets a reversed voltage, it is toast and is no longer any good.
Fixed lots of radios that had this problem; quite common and easy to remedy . . . go for it . . .
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good job Robb, could not have been done any better. he should be able to figure it out with this layout you did.
thank you for the info, ill have to go to radioshack and see if i cant get the kit and parts. also my friend gave me a torn apart 29 classic. its missing a few things. i was wondering if there was a book or something i could get to learn to build/fix my own radios?Fixing that radio is a breeze.
As Sonoma said, you will need to fix the open trace by sanding both sides of the break with a small piece of fine or medium grit sandpaper first; then apply some solder with a 20 watt soldering iron to close it.
( in red circle pictured below)
Then replace D23 with a new 1n4003 diode. Diodes have polarity; so pay attention to the line that circles one end of the old diode and put the new diode in with the same orientation.
D23's purpose in that radio is to protect the rest of the circuit from being fried in the event that the radio's power wires have been reversed. But when it is replaced, as it must be, it will continue to protect the radio from any future reversed polarity charge if/when another mistake is made. This is also the first lesson you get in radio electronics; as this is easy compared to how complex it can get elsewhere on its circuit board.
Consider this as a test and your initiation into radio.
This is not rocket science.
Be glad that it is a simple fix that won't cost you time and money. In comparison, if you were to ship your radio to a shop to do this repair, it will cost $+25 for the shipping both ways and the repairman will want $30 for his time plus parts. If you were to buy a cheap soldering pencil kit (solder pencil, solder, solder wick for old solder removal, and the solder stand). you could do that for about $25 and then buy a diode for $1. The difference is $30; not to mention you will have a soldering pencil and have the satisfaction of fixing your first radio. Up to you . . .
If you just fix the open trace on the board w/o replacing that diode, it will just do the same thing it was doing before - popping fuses; or even open up that same trace again. You must also replace that diode. Once that diode gets a reversed voltage, it is toast and is no longer any good.
Fixed lots of radios that had this problem; quite common and easy to remedy . . . go for it . . .
View attachment 14369
Start small when learning to fix radios. I am no guru or tech; yet I have fixed just about every radio that has come across my bench so far - with few exceptions. You will find that the principles of radio operation are similar among brands and you can expand your knowledge gradually with each experience.
Not trying to take away business from shops. But learning the simple fixes, such as mic wiring, protection diodes, cleaning pots and switches, and even replacing meter lamps is a fine way to start out. You will need a decent DMM (digital multimeter) and soldering tools to start out with.
But as your experience grows, you may well move up to getting some lab grade gears, such as a scope, freq counter, and perhaps a signal generator.
There is a lot of info available on this site and the internet in general to find common problems as well as service manuals and schematics to work with.
Forget about building your own radio, as this isn't really cost effective for now. You can acquire lots of non-working radios for very little money and have a go with them. Sometimes people toss radios out because they have a blown protection diode alone. So, if you fix your radio; then you have also taken the first important step . . .
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What diode on the 25 LTD? I can't find D23 on the 25 model. It immediately blos fuse partial melted black wire
If you're looking for the reverse polarity protection diode it's D17, which is an IN4003. I got that info from here:
Cobra 25LTD
Links Owner's Manual Alignment Alignment Points Voltage Chart (Very Early Version) Voltage Chart Circuit Theory Diagrams Schematic Diagram (Very Early Version) Schematic Diagram (TA7222AP Audio Amp Version) Schematic Diagram (YD1022 Audio Amp...
There's three versions of the radio listed there, but they all have D17 in the same place.