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RF Barf: does it destroy finals in host radio?

Robb

Robb

Honorary Member Silent Key
Dec 18, 2008
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Silicon Valley CA, Storm Lake IA
Last night, a friend of mine told me that his favorite radio - a galaxy 959 DX - took a dump. No RF output. I told him to hook up his meter and see if there was any output at all - to see if the finals or the driver had failed. He said nothing showed up at all - using a Dosy meter.

At the time of the failure, it was hooked up to a XForce 200HD linear on his base station.

Q: Can the amp 'barf back' power into the finals/driver of a radio and start to destroy parts? And if this is so; what other parts took the hit? I know there are diodes across the output finals in this radio - could they have been popped as well?

More importantly; what is happening in the amp that allowed this to happen - if in fact it is the cause? I know Ham's don't use 10 meter amps because they complain of this problem. What is good for the goose is good for the gander???
 

If the input impedance of the amp is off, it causes a high swr to be presented to the radio. However, if you're paying attention to the SWR meter on the radio you should be able to spot this problem.

If the SWR that the radio sees is too high obviously bad things can happen, especially if the radio was "hot rodded". Sometimes what happens is people have the SWR/power meter between the amp and the antenna and see a good reading from the antenna. However, this doesn't tell the story about what the radio is seeing.
 
Rob KI6USW said:
I know Ham's don't use 10 meter amps because they complain of this problem.
Click to expand...

Not hams that know what they are doing...that's basically nonsense that people have told you.

You do need to tune the input circuit for the band you work to keep the impedance matched to the radio.

With my amp, 10 and 12m share the same input slug so you have to tune up as a compromise and take it easy on the drive. Or, tune for one band or the other.
 
If a device develops a problem, it will tend to destroy it's self before affecting things 'up-stream' of it first. That's a generalization and is never completely true. But, they tend that way. There are things that an amplifier can do that will affect it's driver. Those things are not that common in most cases, and always give hints of what's happening. Seeing and understanding those hints takes several mistakes first. A general statement that the amplifier 'ate' the driver's finals is never a complete answer. Sort of a common excuse, just not a very good one, you know?
How do you find what else is affected? Start checking things closely attached to the affected part and expand from there. Not simple or easy, and you have to know how to do that checking. Not sure about some part of doing that sort of stuff? Find someone who is.
Great answer, huh? Yeah, right. Sorry 'bout that.
- 'Doc
 
The radio is a dual-final radio. I asked him if the SWR was good - he said it was. I am beginning to think that the driver is gone. But I was concerned about the possible causes - to keep it from happening again. I told him to use another linear until anything definitive dhows up. I asked him to check out the patch cords for continuity; he said they are good - he wiggled them to see if they were intermittant. No problem.
 
My 70cm ATV station went down as follows:

Antenna problem caused a mismatch that took one of the 2nd stage PAs. This in turn took out the 1st stage driver. That caused an input impedance mismatch to the amp, which took out the final PA module of the transmitter. I did not open this up as the mimics cost more than an old stock replacement module.

If the amp still works fine, check it out for oscillations. If that is clean, then perhaps the radio was just set too hot and it self destructed.

BTW, Mole has it a bit wrong:

people have the SWR/power meter between the amp and the radio and see a good reading from the antenna. However, this doesn't tell the story about what the radio is seeing.
Click to expand...

I think he means the SWR meter is between the amp and antenna, and then you can’t tell what the radio see’s, except in the case of the DX-959, which has a “good enough” SWR meter built in! I assume the SWR warning light would also indicate a problem, unless that was monkeyed with to not glow under full overpeakage.
 
Yeah C2 said what I was trying to type :) That always happens when I concentrating on work and then post up... I must be getting Doc's CRS syndrome...

...I fixed the post
 
Rf Barf-back comes after coaxial nausea but before potentiometer puke. Poke around in the guts and see if any green goo is leaking out. If green goo is indicated, taste it. If acidic in taste, then wash rig in dishwasher on normal setting. If alkaline in taste, clean goo away and adjust R13 for maximum reanimentensular field deflection.

interview.gif
 
The owner of the radio has ordered new finals and drivers from Hong Kong. My concern is locating the cause so that it doesn't happen again. Or, finding fault with usage and prevent any future mishaps. He bought 8-10 Mitsubishi 2SC1969's for $40 +shipping and 6-2SC2166 drivers for about $30. He wants to spares in case it happens again.

It's his favorite radio out of a dozen radios. I just want to be sure that it stays fixed for him the first time around. That is why I want to know if this kind of thing had other causes.
Thanx all - for all the helpful info and insights...
 
Update:

My friend found the problem and the cause of the radio damage.
It turns out that a 1' patch cable was the culprit. Amazing - how a $6 part was the cause. He said he noticed his SWR light going off when using another radio - when hookep up to the same amp. It didn't show up on the Galaxy 959DX radio that blew up.The Galaxy 959DX SWR indicator had issues - a known defect - and I am guessing that it didn't light up as it should have and masked the problem.

So, he switched the coax patch out for a good one - and no problemo with the SWR light. And - of course - he tossed the offending cable away. Once the finals/driver get here, he will put them in.
So, that is how this story ends...
 
Last edited:
Wire Weasel said:
Rf Barf-back comes after coaxial nausea but before potentiometer puke. Poke around in the guts and see if any green goo is leaking out. If green goo is indicated, taste it. If acidic in taste, then wash rig in dishwasher on normal setting. If alkaline in taste, clean goo away and adjust R13 for maximum reanimentensular field deflection.

interview.gif
Click to expand...

...and you're still alive! You remind me of those old oilfield roughnecks that would smell and taste mud for indications of oil at the mud pit!:p
 

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