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Author Topic: 756PROIII tuner noise  (Read 925 times)
N7OE
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Posts: 7




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« on: April 11, 2007, 09:48:42 PM »

I recently picked up 756ProIII used.  Looks and works well so far, however the internal tuner makes a sort of ominous "buzzing" sound when tuning on 6m.  Or more specifically, when I try and tune a long wire or HF dipole on 6m.  Yes, I suppose the built-in tuner just isn't meant for such things, but I wonder what I'm hearing...?  Basically, 20m and up, if I press and hold the "Tuner" button, the power kicks on, I hear a few relays click, the fan comes on, I see the SWR meter going down...sometimes *up* then down, and then everything is done.  But on 6m, this all goes on for about ten seconds or so with a high average SWR, and then I hear about a second or two of this "buzzing".  It's not loud, but then it doesn't sound particularly good either.  Any thoughts on what I'm hearing?  Is is just some stepper motor, or is it (gulp) arcing?  Hopefully not the latter!  I suppose if it were this easy to make the radio smoke itself, Icom would deserve some major demerits!  Maybe I should have some more faith in them :-)  Anybody else hear a sort of buzzing when using the 756ProIII's (or II's, or I's, or non-Pro's) built-in AT?

73,
Aaron, N7OE
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AA4PB
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« Reply #1 on: April 12, 2007, 05:06:48 AM »

What you are hearing is the relays switching trying to find a match. The tuner in the PRO, like most built-in tuners, is only designed to "trim" the match on antennas having an SWR of about 3:1 or less. It's designed, for example, to match a 40M dipole resonant at the center of 40M to the band edges. It is NOT designed to match a long wire or an HF antenna on 6M or any other antenna on a band for which it is not close to resonance.
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AA4PB
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« Reply #2 on: April 12, 2007, 05:08:30 AM »

By the way, continually trying to match an antenna that is way out range of the tuner's capabilities can result in damaging the tuner in your PRO.
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WW5AA
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« Reply #3 on: April 12, 2007, 07:01:19 AM »

I would wonder about the fan coming on when tuning. I easly tune my doublets and inverted "L", (160-6 meters) with the internal tuner. The fan in the Pro-III only comes on while doing some real heavy rag chewing.

73, de Lindy
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N7OE
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« Reply #4 on: April 12, 2007, 08:05:00 AM »

Thanks, folks!

Regarding the fan coming on:  I'd been doing some transmitting before this, so things were probably already pretty warm.

Regarding the 'continually trying':  I won't do that!  I tried tuning twice; the first time I wasn't sure if I heard the radio or something else.  I tried again to see if I'd hear the same thing.  I did.

If it is a relay, then it is chattering pretty darn fast!  Far faster than a TX relay in 60WPM QSK, HIHI :-)
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AA4PB
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« Reply #5 on: April 12, 2007, 09:51:50 AM »

The spec on the PRO III tuner is 16.7 ohms to 150 ohms (even less on 6M). That's a 3:1 SWR. If you can tune a doublet and inverted-L with it on all bands 160M thru 6M then I suspect you must have a lot of loss in the antenna system (perhaps a long coax run). These antennas, especially the inverted-L, will normally have impedances of 1000 ohms or more on some bands. That's an SWR of 20:1 assuming a 50 ohm feed.

Loss in the system will keep the impedance closer to 50 ohms BUT much of the power will be disipated as heat in the loss rather than being radiated by the antenna. That's how broad band antennnas using a terminating resistor (like the B&W) work.
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WW5AA
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« Reply #6 on: April 12, 2007, 01:57:25 PM »

Bob,

Just to use my doublet as an example, by using proper feeding of 450 ohm ladder line and matching sections of 600 ohm feeders properly spaced and using the shield of 75 ohm coax for the elements, the 177' doublet has a worst case SWR at 7125.0 of 425+j110 (9:1). Part of the matching is use of a 75 ohm coaxial balun 10-6" coils then 3' to the tuner.  I have not even looked at the figures for the Pro-3 tuner since I use the LDG AT-1000 or IC AT-500 auto tuners with the amp. However as said it will tune to 1.5 or less from 160-6 meters. Thanks for getting me to think about this, as my 746 internal tuner could not tune much beyond 3:1.

73, de Lindy
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AA4PB
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« Reply #7 on: April 12, 2007, 05:27:14 PM »

Lindy,

The radio side of your external tuner shows an SWR of 1.5:1 or less from 160M thru 6M. In addition, you have a matching section on the antenna to limit the impedance range of the antenna. I think that's a totally different animal than what he was talking about.

What I assume he was talking about was connecting the end of a long wire to the radio and trying to match it on all bands with the radio's internal tuner. Most radios (the Elecraft may be the exception) just don't have the tuning range to handle anywhere near that. Certainly the PRO III will not and there is a very good chance that the tuner will be damaged if you try.

Certainly you can tune a doublet, an inverted-L, or a long wire to any band with the proper tuner but you won't do it with the tuner built in to most rigs.

73, Bob
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W0FM
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« Reply #8 on: April 16, 2007, 02:32:27 PM »

I don't own a PRO-III so I may be way off base here, but your statement that you "press and HOLD" the tuner button worries me.  Most every rig I've had with an internal tuner, you simply "bump" the tune button and the radio seeks a match without further encouragement.  The tuner typically then shuts off at the best match or upon not being able to find a match.

Check your instruction manual and see if the tune button is supposed to be held in during tuning.  I don't think it is.  If not, you may be stressing the poor thing out by forcing it.

Perhaps someone else who owns a PROIII can clarify that for you (and ME).

Good luck,

Terry, WØFM
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N7OE
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« Reply #9 on: April 16, 2007, 04:37:36 PM »

I didn't mean press and hold *indefinitely* ;-)  You hold it down for about a second or so, and this initiates a tune.  Otherwise, it will tune automatically when you key the radio (if the SWR is high).  Just poking the button turns the automatic tune function on or off, but does not actually initiate a tune.  This is pretty typical of Icom rigs.

I should add:  I got a note back from Icom about this, and they offered that I was probably just hearing the relays disengaging as the tuner "gave up", as a prior post suggested.  Didn't say anything about any harm coming to the radio, however.  I'll consider myself "satisfied" :-)
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KB1NXL
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« Reply #10 on: April 23, 2007, 10:49:43 PM »

no tuner is perfect. you don't mention what bands your antenna's resonant on.  I agree that you can't rely on the internal tuner to match every antenna. that's why there are literally hundreds of tuners out there in Hamland. The buzzing is most likely the tuner's way of saying 'can't find a match i like, don't transmit!"  and i hope you are doing the tuning at <10 watts out, right?
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