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Old 03-04-2007, 18:02   #16
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The line isolator you describe is a common-mode choke, and will reduce stray shield currents in the coax to the tuner, without affecting the RF signal carried by the coax. It is also called a "balun" (short for "balanced-to-unbalanced").

I like the idea of the isolator, and it should help reduce any RF ground-loops. But, I still think you need a good ground on the radio. because otherwise you are (by using the isolator) floating the chassis. The 12V power leads may provide an adequate ground, but then again, they may not. If the chassis is floating then you are much more likely to see interactions and interference from the other cables that connect to the radio.

I am still puzzled as to how a line isolator such as yours could cause the problem you have by being defective. It works fine on the lower frequencies, so there aren't any shorts or opens. The only other thing that could be wrong is either an overly-lossy ferrite, or one that isn't permeable enough (a cracked core could do this). But this shouldn't affect the RF signal, only whatever common-mode signal there is. And it certainly shouldn't make a difference if you are just keying the mic but not transmitting a signal.

If it were my system, I would keep the isolator and ground the radio (as an experiment, anyway).

It could still be a broken radio, of course.

For what it's worth, I am an ex-engineer, have an advanced ham license (wb6cxc), and used to design VHF transceivers. My last job was designing 10 and 40 Gbit/s electrical/optical systems. This doesn't mean I'm correct, but at least I can talk the talk.
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Old 03-04-2007, 18:29   #17
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And good talk it is, Paul. Thanks for those explanations.

I, too, am puzzled by how the isolator/balun could cause the symptoms first noted.

My bet is that the problem is the radio. Now, if Larry were around these parts I'd just substitute a radio or two. Currently have over 20 of them in the shack....running out of room to move :-))

73,

Bill
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Old 03-04-2007, 20:49   #18
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What is the model of the isolator? Is it the T4-500?
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Old 03-04-2007, 21:30   #19
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Antares,

Just to be complete, is your ammeter an analog one (moving needle) or a digital one? An analog meter will not respond to stray RF, so it should be displaying accurately. A digital one *may* be sensitive to the transmitted RF, and may give bogus readings. Bypass capacitors and/or ferrite chokes will usually help.

Given the circumstances, I kind of doubt that the meter is the problem, but you never know... Have you tried to make a radio contact with the radio acting up? A signal report may give you a clue. If there is bad RF feedback occuring, it will likely cause the transmitted signal to sound bad.

On the other hand, I'm running out of hands! Too many possibilities. Try to narrow it down (as was suggested), or take the radio and tuner in to a shop and have them bench-tested.
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Old 04-04-2007, 09:23   #20
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Hey, this is certainly illuminating. The isolator is a T-4. I took it out last night and things got better, but I'm not convinced everything is right. The isolator checked ok for continuity. I'm thinking it did it's job and removed a path the real problem was using in a way that minimized symptoms. I was able to get a SWR meter but did not have time to install it. That's probably a weekend project. Given the nature of what is happening it makes sense to start with Bill's list and work through the entire system.

Back to Paul's question, the meter is digital. It is not working in a "strange" way and I even see the volts drop when the current peaks. That tells me I need to check out every connection for resistance as the wire sizes should be adequate. I'll report back when I make some progress.
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Old 04-04-2007, 09:33   #21
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Thanks for the correction. As Bogart said in Casablanca, obviously I've been misinformed.
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Old 04-04-2007, 11:01   #22
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My favorite all-time all-time film!

No problem; hey, we all miss a beat once in awhile. I'm finding that "senior moments" come along more frequently than they used to :-)

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Old 04-04-2007, 12:34   #23
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Get ten "senior moments" punched on your AARP card, and you get the 11th one for free?<G>
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Old 21-04-2007, 00:19   #24
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Antares,

Did you ever figure out what was going on with the radio?
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Old 23-04-2007, 16:33   #25
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I haven't had any quality time to work the issues the last couple of weeks. I have gathered a SWR meter and dummy load so at first opportunity, I will get "scientific" and see what shows. The little I have been able to play with it still shows eradic behavior. I'll report in as soon as I have the next step accomplished.
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