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Post by gator7 on Sept 5, 2016 13:23:09 GMT -5
I have a Cobra 29 LTD that the fuse blows on power up? Can you give me idea where to start looking?
Thanks, gator
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Post by KneeBiter on Sept 5, 2016 13:56:32 GMT -5
Did you check the saftey diode across the power leads?
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Post by gator7 on Sept 5, 2016 14:21:28 GMT -5
Not yet, that's my 1st stop on this one.
Thanks,
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Post by KneeBiter on Sept 5, 2016 14:23:28 GMT -5
Just clip one side and lift it. Then try. If it works just replace it.
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Post by gator7 on Sept 5, 2016 14:45:02 GMT -5
Okay, I will try that. Thanks again.
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Post by 2600 on Sept 5, 2016 15:19:52 GMT -5
Hmmm. You said "Fuse blows on power up".
Does this mean it won't blow the fuse with the power switched off?
A shorted reverse-protection diode will trip the fuse whether the power switch is on or off, either way. It's connected 'upstream' from the power switch.
If the fuse only trips with you click the power switch to the "on" position, the problem is downstream from the protection diode.
73
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Post by gator7 on Sept 5, 2016 15:24:27 GMT -5
I think that is the issue, the diode is intact. And yes it blows when you turn on the switch. I'm out of fuses at the moment, so I have to make a run to radio shack.
Thanks,
73
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Post by 2600 on Sept 7, 2016 13:41:01 GMT -5
Best way to go any farther with this fault would be to set your meter to continuity test. Clip the leads to the plus and minus pins of the power socket. When you turn on the power, the meter will show a short.
Next, take the diode D8 loose. Only have to lift one end from the foil pad and pull it clear of the hole. If it cracks in half, that means that D8 was in line with the short, and has gone bad. This points to the final and/or driver as the shorted component or components. If the short indication on the meter goes away, the next step is to unsolder and lift the center pin of the final transistor, so that it's not touching the circuit-board foil.
Hook one side of the meter to ground. Best place is the negative-side pin of the power socket. Touch the other probe to the now-loose center pin of the final transistor. If you show a short, this was the problem. A new final will only fix the problem if you also replace D8. Even if it doesn't fall apart, you can be sure that the overload has damaged it. A small resistor next to the final will also be burned in a lot of cases. That will have to be replaced along with the final.
And if the short is still present on the power socket with one end of D8 pulled loose, the problem is elsewhere.
Let us know what this procedure reveals, and we can take it from there.
One last detail. How large a fuse was put in line with this radio? If you have NEVER used a fuse bigger than 2 Amp, congratulations. This limits the damage when a fault occurs. And if you did the "slip-seat" trick of wrapping foil around the fuse, the list of collateral damage could be pretty long.
73
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Post by gator7 on Sept 8, 2016 17:30:36 GMT -5
2600
Thanks for the great info, the fuse is a 2 amp only. So that is a good thing. I hope to track the fault out in the next few days.
Gator
73
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Post by gator7 on Sept 9, 2016 9:18:48 GMT -5
I was getting ready to start on trouble shooting, and I found the final is a ERF2030 that someone replaced the original part. Is this transistor known for issues? I know it replaced the 1969. But is it ok to leave in place if it is still good?
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Sandbagger
Administrator/The Boss 
Posts: 6,247 
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Post by Sandbagger on Sept 9, 2016 9:31:17 GMT -5
I was getting ready to start on trouble shooting, and I found the final is a ERF2030 that someone replaced the original part. Is this transistor known for issues? I know it replaced the 1969. But is it ok to leave in place if it is still good? Looks like someone replaced the normal bipolar transistor final with a switching MOSFET. The biggest problem with doing that is creating a stable (and temperature compensated) bias circuit to keep the part within a VERY narrow linear range that allows the part to function as an amplifier. Most likely the bias went out of that range and caused the part to short out. For best reliability, I'd try to find the correct part and restore the factory configuration. IMHO, the one or 2 watts more that MOSFETS alleged give you, it's not worth the hit in the reliability that the mod will give you. |
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Post by gator7 on Sept 9, 2016 9:37:20 GMT -5
Well that helps to know that. I had a feeling that it was not the best replacement. I've had a few Cobras with the 1969 trans and didn't have any issues.
Thanks Sandbagger.
73
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Sandbagger
Administrator/The Boss
Posts: 6,247
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Post by Sandbagger on Sept 9, 2016 17:28:48 GMT -5
Well that helps to know that. I had a feeling that it was not the best replacement. I've had a few Cobras with the 1969 trans and didn't have any issues. Thanks Sandbagger. 73 1969's were not the stock final for those AM-only Cobras. The 1969 was a performance upgrade, since that was the final in the SSB radios, and it was good for 25 watts. 1969's, unfortunately, are becoming harder and harder to find since they've been discontinued. If you can find one for a reasonable price, that would be the way to go. I'm not a big fan of using switching MOSFET's as an RF amplifier. Yes, they're cheap (which comes in handy when you blow them out frequently), and in some cases they put out a bit more power. But they're just too unstable. |
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Post by BBB on Sept 9, 2016 18:00:47 GMT -5
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Sandbagger
Administrator/The Boss
Posts: 6,247
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Post by Sandbagger on Sept 9, 2016 18:26:15 GMT -5
Once upon a time, I had one of those. Has the 858 PLL chip (=lots of channels) and an audio chain similar to the SSB version. |
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Post by gator7 on Sept 9, 2016 20:25:23 GMT -5
Yes I'm aware that the 1969 was not the stock final, I was commenting on the final in the radio now replaced the 1969.
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Post by gator7 on Sept 26, 2016 10:51:33 GMT -5
I have not been able to find the original p/n for the final. Anyone know what it is?
Thanks,
73
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Sandbagger
Administrator/The Boss
Posts: 6,247
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Post by Sandbagger on Sept 26, 2016 11:37:04 GMT -5
I have not been able to find the original p/n for the final. Anyone know what it is? Thanks, 73 Most of those Uniden-based AM radios used a 2SC-2078 for the final. |
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Post by 2600 on Sept 28, 2016 16:43:49 GMT -5
If the radio was modified to use the ERF2030, the final transistor's input circuit has been modified. The 2030 is not the same kind of transistor as the factory type.
Original types used are 2SC2166 and 2SC2078. The 2SC1969 was an 'upgrade' part. Not the same, but compatible.
And if the radio was not modified correctly, this just muddies the water.
Take the center pin of the ERF2030 loose and see if this clears the short. If the radio now powers up and receives okay, that would be a good sign.
Remember that the diode D8 will be damaged from the overload and must also be replaced.
A new ERF2030 will only work if the radio was modified correctly to use this part.
An original-type 2SC2078, 2SC2166 or equivalent will only work if the factory-original input circuit is still in the radio.
If it was correctly modded for the 2030, that original circuit is not there, and those won't work.
And if someone botched the ERF2030 modification, that's the "muddy water" I mentioned.
73
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Post by gator7 on Sept 30, 2016 8:10:49 GMT -5
That sounds like it might ugly. I will let you know.
73
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Sandbagger
Administrator/The Boss
Posts: 6,247
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Post by Sandbagger on Sept 30, 2016 12:56:50 GMT -5
That sounds like it might ugly. I will let you know. 73 This looks like a job for the junk box of filleted radios. Guaranteed to find the parts you need  |
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Post by gator7 on Sept 30, 2016 22:51:20 GMT -5
I hope not, it is a nice 29
73
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Post by 2600 on Oct 1, 2016 12:59:51 GMT -5
Umm, 'was' a nice 29?
Sounds like it's time for a web page (or video?) showing how to unwind a MOSFET conversion in your Cobra 29, and reinstall an original-type final.
A kit of the parts that will be missing from the radio would be a cool idea, too.
Hard part is to predict what was done to modify it in the first place.
It's always possible that the modification was done right, and a high SWR blew out the MOSFET. A new D8 and IRF520 might get it back on the air.
Maybe.
73
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Post by gator7 on Oct 1, 2016 23:38:23 GMT -5
Roger that on the maybe. We shall see.
73
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Post by BBB on Oct 4, 2016 17:08:54 GMT -5
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Post by gator7 on Oct 5, 2016 16:31:33 GMT -5
I'm not looking to buy, I'm trying to unload my excess radios. I have made some progress in that direction. But eBay is a real pain, so I will see if I can get this one back to life. And make a few more $$ I hope lol
But right now I'm dealing with a hurricane in the hood. That will keep me busy for a few days.
73's
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Sandbagger
Administrator/The Boss
Posts: 6,247
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Post by Sandbagger on Oct 6, 2016 6:21:44 GMT -5
I'm not looking to buy, I'm trying to unload my excess radios. I have made some progress in that direction. But eBay is a real pain, so I will see if I can get this one back to life. And make a few more $$ I hope lol But right now I'm dealing with a hurricane in the hood. That will keep me busy for a few days. 73's Keep safe down there. Latest projections show that there's a possibility that this thing may loop around and hit Florida twice. Hopefully that won't happen. |
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Post by gator7 on Oct 7, 2016 14:52:40 GMT -5
Thanks Sandbager, we may know tomorrow night if it is going to loop. I'm without power now, hope we get it back in a few days or sooner.
73
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Post by BBB on Oct 8, 2016 9:12:21 GMT -5
I just sat thru Matthew while visiting relatives in West Palm Beach. It stayed off shore. No biggie, but a wobble 40-60 miles to the west would have sucked big time. They take storms of that caliber very seriously down here 🌀
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Sandbagger
Administrator/The Boss
Posts: 6,247
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Post by Sandbagger on Oct 8, 2016 17:50:28 GMT -5
I just sat thru Matthew while visiting relatives in West Palm Beach. It stayed off shore. No biggie, but a wobble 40-60 miles to the west would have sucked big time. They take storms of that caliber very seriously down here 🌀 Some people pick entirely the wrong time to go visiting...... I guess that also explains your absence from the Roundup....  |
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