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Old 08-23-2018, 11:39 AM
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Default Cobra 29ltd classic blown protection diode

Hey everyone, new to cb radios, just found this radio at the local Arc thrift store for cheap. Looks un-used or at least very lightly used, got it home and hooked it up to my car battery with some Jumper wire (no fuse, I know now) and it turned on and seemed to work fine, then it started smoking as it does so I unplugged it and let it sit for a day, plugged it back in real quick because I wanted to see if it still functioned and it did so I unplugged it and took the cover off, blew the protection diode so I have a new one on order, is there anything that could have caused this that I should be looking at or troubleshooting? I do not have a hand Mike or antenna hooked up so I was not transmitting. I just want to make sure that if I replace the diode it won't happen again and possibly get past the diode and destroy the radio next time. I hooked the positive and negative according to the + and the - stamped on the back of the radio so I don't think it was a reverse voltage unless it was set up wrong.

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Old 08-23-2018, 12:39 PM
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The protection diode is there to protect the radio if it gets hit with reverse polarity.
To work correctly, there needs to be a fuse installed. The diode shows up as a short when it's hooked up backwards and should cause the current consumption to jump high enough to open the fuse.

By hooking it up with out a fuse, it's possible that you have damaged something internally. What, I don't know. Replacing the diode on it's own isn't going to fix that.
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Old 08-23-2018, 12:46 PM
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Ok so with that in mind, would the radio function/turn on at all if the polarity was reversed? Is it possible that the + and - stamped on the case is wrong? I looked thru the board and haven't seen anything else that looked bad, but that means nothing really.
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Old 08-23-2018, 1:10 PM
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Quote:
Originally Posted by Murdoc1905 View Post
Ok so with that in mind, would the radio function/turn on at all if the polarity was reversed? Is it possible that the + and - stamped on the case is wrong? I looked thru the board and haven't seen anything else that looked bad, but that means nothing really.
Depends on how much damage was done. That depends on who hooked it up backwards, if they had a fuse installed, what size the fuse was, etc. etc, etc…

Unlikely the case is labeled wrong. I believe the plugs are polarized so they cannot be plugged in backwards. I'm guessing someone just wasn't paying attention. It happens, I did it once while distracted with a $1800 Motorola. I ate that crow and moved on.

The radio could be fine, or there could be internal damage that you won't be able to visually see. Only way to know for sure is to replace the diode and hook it up again. Just make sure you have a fuse installed.
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Old 08-23-2018, 1:59 PM
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Originally Posted by mmckenna View Post
Depends on how much damage was done. That depends on who hooked it up backwards, if they had a fuse installed, what size the fuse was, etc. etc, etc…

Unlikely the case is labeled wrong. I believe the plugs are polarized so they cannot be plugged in backwards. I'm guessing someone just wasn't paying attention. It happens, I did it once while distracted with a $1800 Motorola. I ate that crow and moved on.

The radio could be fine, or there could be internal damage that you won't be able to visually see. Only way to know for sure is to replace the diode and hook it up again. Just make sure you have a fuse installed.
I kinda figured, just wanted to make sure. I would completely be the one to blame if it is broken, but hopefully it only made it to the diode. We will see in a month when I get my China direct parts lol.
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Old 08-23-2018, 6:04 PM
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I've worked on a number of radios that were hooked up backwards and the amount of damage can vary. Some radios will blow the fuse and the radio is fine with a new fuse. Sometimes the reverse protection takes the brunt of the current and shorts doing its job and sacraficing itself and protecting the rest of the radio. If your lucky, replacing the diode may leave you with a working radio.

In most radios, things like final amplifier transistors, audio ICs, regulator chips, etc, are connected directly to the 12V buss and they can be destroyed along with the reverse protection diode, especially if the diode opens on failure.

For now simply cut the lead on the protection diode and see if the rest of the radio is working. If so consider yourself lucky and put in a new but bigger diode and a fuse.
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Old 08-24-2018, 12:01 AM
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I have completely de-soldered the diode and hooked it up properly this time
I can imagine what I did was look at the back of the radio upside down then when I went to plug the power in I looked at the front and turned it right side up and proceeded to hook the power up backwards. I keep thinking the speaker points up which is messing with me. I don't have an antenna or a mic to test it, but I'll see what I can do to remedy that.
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Old 08-24-2018, 12:09 AM
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No smoke is always a good sign.
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Old 09-04-2018, 1:39 PM
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Not lookin too good, hooked a uniden mic up and an antenna to it to see if it would work, attempting to calibrate it and the SWR meter won't move. In order to hear any static the squelch needs to be all the way on and there is a hard drop off when it's turned back some.
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Old 09-05-2018, 11:34 AM
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Nevermind about the squelch thing lol. Been messing around with the little uniden 520xl I picked up and realized that's how it's supposed to operate. But the SWR gauge is still not budging. I'm gonna try hook it up in a few days and see if I can even hear any traffic on my way to work.
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Old 09-12-2018, 7:57 PM
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Ok, so I soldered the new diode in place, double checked the polarity of the wires as I hooked them up, I'm getting smoking coming from the protection diode... According to the - >|- diagram on the board I soldered it in the right way. So I'm at a loss. I had a 5 amp fuse that did not pop, I'm wondering if there was a reason this was in the thrift store. Lol.
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Old 09-14-2018, 8:32 AM
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Uhh shorted final output transistor perhaps. Might try unsoldering it and then see if the diode/fuse blows. Its been a long long time since I worked on a CB, but if memory serves if the final RF output transistor is shorted it will blow the fuse.
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Old 09-17-2018, 2:00 PM
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Is the the transistor you are referring to?
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Old 09-18-2018, 12:52 PM
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Yes.
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Old 09-18-2018, 7:44 PM
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Yep, it looks like that's bad as well.
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Old 09-18-2018, 11:38 PM
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Shorted out? Use diode test on multimeter?
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Old 09-18-2018, 11:58 PM
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Yeah it's shorted, got some on order, I imagine this isn't the only thing fried now and it might be futile to try to replace things one by one
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Old 09-19-2018, 7:05 PM
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Did you remove the shorted RF PA transistor? Did that clear the short on the DC input? Then see if the radio will receive.

Before you go and replace things only to find other fried parts.
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Old 09-20-2018, 11:45 AM
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Quote:
Originally Posted by cmdrwill View Post
Did you remove the shorted RF PA transistor? Did that clear the short on the DC input? Then see if the radio will receive.

Before you go and replace things only to find other fried parts.
If thats the pictured transistor above, yes I removed it. I didn't know I could power the radio with the transistor removed so I'll try it soon is there a safer way for me to check if there's a short than just plugging it in and waiting for smoke?
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Old 09-20-2018, 4:58 PM
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A safer way to power up a radio. Get a tail light from/for a car/truck put the lamp in series with the DC 12 volts going to the radio. If there is a short or excessive current draw the lamp will light.

Trouble Shooting 101.
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