Destroyed CB finals?

[needairtime]

Transistors are rated for how much power they can stand before their internals fry no matter what you do (assuming you can keep it otherwise cool from the outside.) While this doesn't have anything to do with RF emitted power, it depends on the ultimate efficiency of the design to relate wasted power to usable RF power.

 

[FiveFilter]

I was wondering if the transistors are being run at full specified power while undergoing the VSWR endurance level tests, or at some fraction thereof. I assume it could make a difference in their survivability under high VSWR levels.

 

[Ensnared]

Fried

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.

Without realizing the PL-259 was disconnected, I keyed the microphone and fried the finals according to the technician who fixed my radio.

 

[zz0468]

I was wondering if the transistors are being run at full specified power while undergoing the VSWR endurance level tests, or at some fraction thereof. I assume it could make a difference in their survivability under high VSWR levels.

It's not really a matter of power dissipation, any potential failure would be more likely to be from high RF voltages ( which can get quite high) exceeding the break down voltages of the transistors.

 

[needairtime]

Without realizing the PL-259 was disconnected, I keyed the microphone and fried the finals according to the technician who fixed my radio.

What radio was this (so we can avoid, or buy one to study )

 

[mastr]

Final transistor replacement

In the late 70's- early 80's I replaced several final transistors a month
for the local CB community. My recollection is that most of the radios I
repaired had either been "tuned" by someone else, or used with a sweep
tube amplifier lacking a tuned input circuit.

 

[wyShack]

The two ways high VSWR is likely to 'blow' finials are thermal(overheating) and voltage breakdown(too 'high' a voltage 'across' the finial) When transistors first became available for use as CB 'finials' they were 'cutting edge' and as such often were operated right on the edge of spec (example a 5 or 6 watts transistor) for cost reasons (back then they were 'pricey). High SWR could and would 'push the envelope' -sometimes to failure. Nowadays I would guess most finals are designed with transistor that just 'loaf along' at the 4 W output as the devices are cheap enough that the 'extra' is cheap insurance.

Most higher powered gear (like HF amateur gear) has protection circuits as the cost of overspecing the finial is more than adding the circuit.

Today's radios are built to avoid a trip to the shop-which is why there are few shops.

 

[FiveFilter]

Yep, modern radio equipment must be designed and built to take some level of too-high SWR.

In my post #13 above, I describe torturing a little Uniden 520 radio and an RL203P amplifier with no loads and faulty loads which shot SWRs to the far, red end of the range, yet they are still working up to spec as determined by subsequent testing and usage.

Hoo-Ray!

 

[KD0PEZ]

I've never blown a final per se. At most I came into a bunch of old CB gear once. 4 of them were Royce 1-604's from 1977-78. One of them transmitted but very low wattage. So low you couldn't hear it past 50 feet. It didn't even register on my meter, but it was clearly keying up as determined by a "test receiver" radio i had in the shop. The driver was doing all the work, so I understand.

One thing to keep in mind, most finals are secured to the radio case which can act as a big heat sink to take some of the heat off the final. I suppose if someone didn't re-secure it and if the SWR was too high and the final had no way to dissipate the heat, a fried final would be inevitable.

 

[zz0468]

...and if the SWR was too high and the final had no way to dissipate the heat...

IF a high SWR could kill the finals, and evidently that's a pretty big IF, it wouldn't be the heat that does it. It would be high voltage induced from a complex reactive load that would damage the transistor junctions.

 

[KD0PEZ]

IF a high SWR could kill the finals, and evidently that's a pretty big IF, it wouldn't be the heat that does it. It would be high voltage induced from a complex reactive load that would damage the transistor junctions.

Good to know

 

[needairtime]

Speaking of finals and timeline, since 40-channel CB was authorized by the FCC in 1977, this is pretty close to the time where apparently all CBs were made with high reliability final output transistors, so all 40-channel CBs should be fairly resistant to "antenna not connected" damage?

So I should watch out on my 23-channel CB, though there wasn't any big bold print to not transmit without an antenna else it will cause damage not covered by the warranty in the manual... There was a side blurb to simply not transmit without an antenna but no warnings about damage. However this is a fairly "late" model 23-channel (I suspect middle '70s) so transistors have started getting better by then.