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Destroyed CB finals?

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needairtime

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Has anyone actually blown a CB final, whether by forgetting to attach an antenna or running with crappy SWR? How did you do it?

Curious as how often this actually happens...

Note that I would expect that modern CB transceivers have SWR protection so it's tougher to blow finals. Also note, please don't post about linear amplifier finals, they "don't exist" in the CB world.

I was studying final amplifier schematics and trying to learn how they're designed and their weaknesses. Due to the low power of CBs I was thinking that though the problem is still there, it may not be so bad compared to higher powered ham equipment or their linear amplifiers. But even so, it may still be designed at the limits and having a bad SWR would definitely destroy finals.
 

mmckenna

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I'd love to hear it, too.
Not "I knew a guy who's barbers next door neighbor once met a guy who did it".

At 4 watts and taping the PTT key down, I think it would take a hell of a lot of time to cause any damage, if any at all.
Most CB users can't even spell SWR, never mind know how to check it, so if it was a big issue, you'd probably see a lot more damaged radios out there.
 

zz0468

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I suppose it all depends on the nature of the mismatch. The real problem is not so much one of power dissipation, like 4 watts out reflecting and causing 4 watts of additional heat needing to be dissipated.

The problem is that some mismatch conditions can create really high RF voltages. A 4 watt radio could transmit into a reactive load that creates several hundred volts of RF that happens to appear between the base and collector junctions of the final transistor, and that could easily blow the final transistor.

It could also happen before the protection circuits have a chance to react. Look at the transistor specs. How many microseconds will a 12 volt transistor sustain 200 volts peak RF voltage at it's junctions before it fails?
 

needairtime

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It definitely still depends on how close to the limits the design is. Bad SWR and you can still take out a final, but if the final is well heatsinked and the transistor has a low thermal impedance to it, I'd imagine they should generally be able to take the heat.

I have an early 70s 23-channel radio (Alaron) that the owners manual that just says you "must connect to 50Ω antenna" but doesn't mention anything about destroying finals without such. It appears to be a 2SC1678 TO-220 that can dissipate 10W with proper heatsinking.

My 1W/4W CB HT (Uniden) I'd imagine has another issue: it's common to remove the antenna and hopefully designed robust enough to accidentally transmit without the antenna, though uses a 2SC2166C final (TO-220, 12.5W limit). And my 2.5W 2m handheld transceiver likewise has the same problem, but appears to use a really wimpy 2SC1947 (TO-39, ~3.5W limit)... Same potential issue.

And none of these have SWR protection by looking at their schematics, at least like what my 15W VHF / 10W UHF radios use. I haven't found my 35W UHF radio's schematic but I suspect it's similar to the 15W/10W radios.
 

needairtime

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Once again, theoretical vs actual destruction :)
But true that voltage is a serious problem (though I'm still trying to figure out how to get 200V with a final transistor connected to 12V) but this would be an issue with non-CB radios as CB is limited to 4W.
 

prcguy

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I've tried to burn up many CBs by wrapping a rubber band around the mic button and letting them transmit endlessly with no antenna and I have never been able to break one. In my early career I've had my hands on thousands of CBs running running quality control on new radios from Asia and repairing factory failures and customer radios. Blown final transistors were not that common.

The few final transistor failures I've heard of had no problems with antennas and there was no obvious reason for the failure. It may have been simply a bad part from the factory that finally gave up. There has been millions of CBs sold in the US and you don't hear about many actual PA transistor failures, just people warning about it.

There were some radios in the 60s that were known for delicate transmitters, but that was well before we had rugged RF transistors like today. Besides the transistor dissipating extra power and generating a little more heat from a bad match, when the point of failure in the antenna system is at a perfect point down the coax to cause the VSWR to be at a high voltage point right at the transistor, it would be more susceptible to damage.
 
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needairtime

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Incidentally as the SWR protection circuitry in modern high wattage transceivers seems to be a control system. This means multiple keying is much worse than one very long key as far as I know...
 

zz0468

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I've tried to burn up many CBs by wrapping a rubber band around the mic button and letting them transmit endlessly with no antenna and I have never been able to break one.

Did you ever try it with an inductive load, as opposed to an open or dead short? That inductive kick can be a whopper.
 

rescue161

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I blew the final transistor in a Uniden Grant XL, but not in the fashion. I was aligning the radio to spec, so the lid was off. After I finished, I was going to disconnect everything, but before I did, my grounding strap got tangled around the lid and it was pulled onto the PCB. The lid shorted across some components and magic smoke sprang forth.

After I replaced the transistor, I aligned the radio again with the lid placed firmly under the bench. I still have that radio and it still only puts out 12 Watts on SSB, but it is tuned into the 10 meter band. I actually made a contact from North Carolina to England using 12 Watts into a 10M Ringo Ranger!
 

n9mxq

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Only "blown up" final I've ever seen, was when I was interning at the local CB shop in the late 80s. Finals in a Cobra 2000GTL had chunks missing..

Guy had a close lightning strike to a tree 20 yards from his antenna... Finals weren't the only part that was "blown".
 

prcguy

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I've shorted the antenna connector at some point trying to blow up a radio but but nothing beyond that. Its possible some installations with the perfect length of coax and an open or shorted antenna has damaged a radio but its still a rare case.

Think of how many CBs are out there and some of the people who own them with zero knowledge on electrical stuff. If CBs were sensitive to VSWR damage there would be a whole lot of burned up radios out there and I just don't see them.

Did you ever try it with an inductive load, as opposed to an open or dead short? That inductive kick can be a whopper.
 

DJ11DLN

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Only one I heard of (way back when I still fiddled with CB) I am pretty sure was a bad part. Friend of mine had just gotten a new Courier 40D. Great radio. 2 weeks in he lost most of his TX. Had a good antenna setup and SWR was not above 1.3:1 anywhere in the band. He asked me to bring my meter over to see if it agreed with his and it did. They made a minor fuss over warrantying it, must have thought he was TX'ing into a bad antenna but that wasn't so. He hooked up his old radio and all was good, no lightning strikes during the 2 weeks he'd run the new one. Once he got it back he ran it for years without a problem.

There used to be a lot of talk about so-and-so blowing his finals because of a pin in the co-ax or something. But I always found it strange that "so-and-so" was always at least 3 people removed from the teller of the tale...and the broken radios just never seemed to turn up!
 

FiveFilter

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Funny this topic pops up now. I just ran an involuntary test about just this type of thing. To wit:

Two days ago I was in a hurry to get on the road. Since I was going on a 600 mile trip, I wanted to bring a CB to help avoid traffic problems, but unfortunately the CB was not in the car I was going to use. So, I quickly grabbed my travel kit comprised of a plugged-up Uniden 520 radio, an Italy RL 203P amp and an SWR meter, brought it out to the car and plugged it to dedicated 12vdc ports in the car. I then put a magnetic Wilson 500 antenna on the roof of the car and promptly forgot to hook the thing to the SWR meter and thus to the transmitting equipment. Bad ju-ju.

As I always do, after starting the engine, I mashed the transmit button on the mike just to make sure my SWRs are OK. I do this because you never know what could go wrong. Well, for the first time, and to my surprise, the SWR meter reported really, really bad values. Of course, to check my sanity and make sure I was testing everything right, I repeated the process several times, with and without the amp, and consistently the meter pointed only to failure of both the radio and the amp achieving decent SWRs. Then, in a eureka moment, I realized that I hadn't plugged the antenna into the system. Well, I thought, that's an easy way to throw your money away.

I then hooked up the Wilson to the SWR meter and thus to the rest of the system and things immediately looked normal, with the SWR at about 1.1 from the 2-watt radio solo and 1.3 with the 60-watt amplifier. But there's more:

After I got back from the trip, on which my equipment seemed to be working well, I decided to verify that it actually was working as well as it should be. Since my car was parked next to a metal building, I inserted a 50-ohm dummy load in place of the antenna and an SWR/watt meter in placer of the SWR meter in order to test the transmitting wattage from my equipment. When I keyed the mike, the SWR meter and the watt meter both were acting funny, quivering and then moving in tandem back and forth and doing all sorts of things that made me fear that I had screwed up my equipment from the original sin of transmitting without an antenna. After fiddling around with this a while and finding only unsatisfactory and totally erratic results, I went inside for a cup of coffee feeling sad and mad and stupid and, well, you know what I mean.

After the coffee perked me up a bit, I returned to the car to begin troubleshooting. I figured the easiest first step was to check the dummy load device, which I had bought a year or so ago at a cheap price from ebay and seemed to work just fine on a couple of projects, but you got to start somewhere. So, I removed the dummy load and screwed on the Wilson antenna coax and, guess what, my SWRs suddenly looked fine and the watt values seemed healthy. I confirmed that my radio actually was producing its nominal two watts deadkeyed into the amp, and the amp was producing its nominal 60 watts deadkeyed into the antenna, and that ole Democrat refrain "Happy Days Are Here Again" began playing in my head, and I'm no Democrat.

So, I threw away the cheap dummy load like a saved cripple his crutches, and I began to hope that the 15 or 20 times the radio and amp were keyed to bad loads have not caused harm down the road. All I know is that at this juncture, things look OooooKkkkk.

So, if you, dear reader, are still with me, the moral of the story "seems" to be that yes, transmitting without a proper antenna load may be bad, but if you are watching your instruments and don't mistreat your equipment too excessively, then you can get away with it, at least, maybe, sometimes.
 
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wyShack

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In my experience, most of the blown finals happened way back. In the 60s and 70s, solid state finals were new, and expensive. Early model solid state radios of that time period often did not have SWR protection built in and the finals cost was a factor so the final was designed without much margin to save costs. Labor costs were relatively lower so each radio could be 'tuned'before it left the factory.

Today's radios are mass produced and labor is the thing to avoid-the design has lots of tolerance and few adjustments are 'built in'. Using a output transistor that can take mismatches and adding protection circuitry adds almost nothing to the end cost so that's the way they are designed.

73
 

needairtime

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That probably makes sense, just the old radios had issue.

I suspect design tolerance is probably the larger factor for the apparent indestructibility of 'modern' 4W CB's where 'modern' seems to be the 80s or newer. At least studying the SWR protection circuitry on higher power VHF and UHF circuitry, it added a non-negligible amount to the cost of the radio. However, it seems to be absent on the lower power radios that I have, including CBs.
 

helvis

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A friend and I were curious about this too so we left a cobra 29 keyed for 12 hours without an antenna hooked up. The radio chassis was warm to the touch the next day. We checked the output, 4w, PEP was 10w and audio integrity was all the same before and after the test. Good reports before and after.
 

needairtime

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Yes, thanks for doing the experiment. I suppose Cobra is one of the fairly well established mass producers so they must have something going to make sure they don't get failures out the door and fewer returns due to user error (forget to connect antenna).

But in any case it's still good warning to never transmit without an antenna, after all, not sure why one would have a 2-way radio without an antenna hooked up, lest people start forgetting there's an antenna inside peoples cellphones...
 

needairtime

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Oh BTW:

I was looking at some transistors that are used for finals - was speccing out how much it'd cost to build a small linear for amateur/HF...

I noticed some of them (VHF) actually are tested for 20:1 durability VSWR out of the factory!
And get this: I saw a couple transistors (HF, which includes CB) that are tested infinity:1 VSWR out of the factory!

Not all transistors are tested for durability like this but it seems some are, which would explain why we don't see many final failures.
 

FiveFilter

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That's interesting stuff!

Do they specify the transmission wattage when specifying the VSWR tolerance?

I have no idea how transistors are rated.
 
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