Reversed Polarity BIG Cobra 29LTD Classic...

[Sci_Fi]

Anyone experienced this issue and seen it through fixed? Any help would be appreciated.

I really loved this radio, it had swing kit, echo board, and modulation way over 100% BIG BAD SUPER AUDIO, Mod limiter Out! Set mike gain to perfect and used a secret Mic! This radio was KING! Too bad its dead now... ...
I would really love to get it running again I'd hate to strip it for parts any help appreciated... I went and bought a brand new stock Uniden PC78XL want to see what these are like. Heard its the same exact board layout, pretty much the same radio with a few more features...

Original Radio Make and Model: Cobra 29LTD Classic
Manufactured Date: Feb 08

What is working: Nothing Applied power, seems dead.

What has been tried: Tested with multimeter, Seems to be a short on postive to negative leads, instantly blows fuse.

Did it suddenly happen ?: Sure soon as hooked the polarity in reverse before i marked it! Lesson Learned...

Anything else that may be of help?: I feel like an idiot. I read somewhere else that somone said it is fixable if you replace a certain diode, but which one.
Than he said it will only work if the fuse used was under 5amp fuse. Guess what lucky me. I used a 6amp fuse, since I didn't have another 3 amp, and just wanted to get on the Air. Another Lesson Learned BIG MISTAKE...

Hopefully this helps other people out reading this so they don't make my mistakes... lol Though i'm sure they won't go looking for help untill its too late lol!

SCI-FI! on this end, got the biggest baddest station in the Nation! Keep'in them Mud Ducks Back Quite, Were Back Out.

[zodiac]

The diode you are looking for is very close to the red 13.8v input wire, it's black with a silver band at one end.
If you de-solder one end and lift it from the board you should be able to switch the radio on and get power without blowing the fuse BUT make sure you have the correct fuse in first, if it powers up then replace the diode with a new one.
Dave.

[Sci_Fi]

I see 2 diodes.. pictured here... which one is it?

From what you said i figure its probably the one on the left, but please check me to be sure... Does it matter which side i un-solder?
would it be normal to replace both? I cant find my meter yet...

Click this to see what i mean...
http://home.comcast.net/~jnb16/cobra_diodes.jpg

[zodiac]

I think it's the one on the left but check both of them as I don't know the radio.
You can lift any end as it will take the diode out of circuit.
It sounds like the diode has gone short circuit to keep blowing the fuse.

[Sci_Fi]

thx so much!!! I'll check it when i get home an let you know how it turns out...
_Nick_

[Sci_Fi]

Perfect! Replaced the diode on the left! Works 100%! I'm so thankfull! YOU ROCK! Time to piss off the neighbors again!

[spikeselectronics]

One thing that I would like to add is: location of the rectifying diodes for cobra 29 series (excluding 29 xlr) is D8 and D23. One way to resolve accidently hooking up your radio is to modify (pay attention you gurus out there, because this applies to all radios that use rectifying diodes) the circuit like this: On both places pair up 2 1N4001 rectifying diodes in a pair, but with banding opposite of each other. What this will do is in effect allow you to hook up your radio no matter the polarity of the wires (which means red to ground and black to hot, and vise versa) and if will function properly. Only draw back to this mod is that you will lose, because of the gating action of the diode, 7/10th of one volt per diode across the junction (meaning .7 times 4 equals 2.8 volts off of the original 13.8 volts leaving a remainder of 12 volts total input), but you can compensate by retuning for the voltage drop. I hope that this helps everyone out.

[rpaulickjr]

Im hoping to try the same thing soon to mine, I was given a pc78xl that was hooked up wrong and the owner didnt want to fix it. He said that it blows fuses when hooked up ever since he reversed the wires. The diode is not burned so I should be able to get a new one if the number is still readable right?

[lbcomms]

The diode is not burned so I should be able to get a new one if the number is still readable right?

Quite often the number on the component is different from its actual part number.
Anyway, it doesn't matter - just get any one of the below types:

1N4001
1N4002
1N4003
1N4004
1N4007

They are very common, you should be able to get at least one of the above from any electronic component stores (even radio shack!)
Sue

[lbcomms]

Only draw back to this mod is that you will lose, because of the gating action of the diode, 7/10th of one volt per diode across the junction (meaning .7 times 4 equals 2.8 volts off of the original 13.8 volts leaving a remainder of 12 volts total input), but you can compensate by retuning for the voltage drop. I hope

This is one way to reduce your output markedly - power is the square of the voltage, so a small voltage drop can cause a big drop in power.
Losing nearly 3 volts could easily reduce your peak RF output by half!

There are also other issues with this mod. The diodes will dissipate heat (on a higher powered rig, this could mean heatsinking them). This wasted energy should be going into your antenna, not heating up the inside of your radio.
No manufacturer of DC powered products does this, and for good reason.

A MUCH better idea, if you want to avoid reverse polarity damage, is to use a relay and ONE diode instead.
Put the contacts in series with the red 12V wire inside the radio.
Wire one side of the coil to the black (negative) wire.
Connect a diode - the marked side (cathode) to the other side of the relay coil, and the other unmarked side (anode) to the red 12V wire inside the radio on the power connector side.

Result:
- with correct polarity, the relay closes and lets the radio be powered.
- with reversed polarity, the diode prevents the relay from energizing, this stops the power getting to the radio.

The voltage drop is extremely small (less than 0.1volts) compared with the four diode method. No power losses or unwanted component heat.

Cheers,
Sue