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FT-900 Negative Swing on AM ??

Moleculo

Ham Radio Nerd
Apr 14, 2002
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that is a post from 2009.thats 13 years ago

Necrothread. But I figured out why this happens with ham rigs in the years that have passed. If the radio wasn't golden screwdriver'd , then the problem is usually caused by a microphone / audio chain input that has the phase 180 degrees in reverse of what it should be. There are a number of things that can introduce this phenomenon including impedance mismatches, cabling, adapters, etc. One way to solve the problem is by using something like the W2IHY iPlus, which is really an audio switch to allow you to use one mic / speaker combo with up to 3 rigs. But it also contains a phase switch which will change the audio output of the device by 180 degrees to solve this.

In my older post about the Icom 746 Pro doing the same thing, I later added the W2IHY audio stack, tried the audio phase switch and the problem was solved. I also experienced this on AM with a few other rigs since then.
 

kc4eoe

Member
Jun 18, 2018
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that is a post from 2009.thats 13 years ago
So? When researching the web for certain info, many times a thread will still remain open and just waiting for someone like me to make further comments. You leave the back door open, someone will sneak in....just sayin'!
 
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Greg T

945 (Jazz Singer) Upper Peninsula of Michigan
Sep 18, 2014
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I'm no RF guru so I can't explain it, but I've owned probably 6 or 8 Yaesu radios. Mostly 757GXII, but also my FT-990. They ALL swing backwards when reading average power. PEP is another story with huge forward swing. The internal meters all show forward swing on either AM or SSB, but external meters will show back swing on average AM power. I guess it's their personality.
 
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GlocknSpiel

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Mar 19, 2022
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If the thread is still open, it should be fair game. A firearm forum I go to every day (GlockTalk) never closes a thread, and about a year ago, they "upgraded" their software.. You should see the vampires it's built-in algorithm drags up as 'suggested reading" now! But yeah, 2009 is ancient. ;)

One thing that most radio guys ignore is how they're FEEDING their rigs. Their radio may require 25A at full power output, yet they might try to use a 15A supply, thinking that's adequate.
But even if they have an ample 30A supply, they might ignore the size of the power wires coming from it, to their radio, trying to get 25A through a 10FT run of #14 wires. Both of these can cause the rig to "swing backward" -- sometimes, even dimming the meter lamp when they modulate the rig.
These are the first causes to rule-out in the setup.
 
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GlocknSpiel

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Mar 19, 2022
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SW Florida
Ham radios are not really known to be good AM performers.
That used to be very true, and across the board with manufacturers -- but not in TX, because most pre-70's era ham transceivers were designed to be AM only in the first place. Even then, the radios that were capable of operating in SBB suffered on the receive sensitivity end of things anyhow. This was also normal because of the wider operational bandspread of ham radios... They have to work everywhere. But with most radios built after the 90's, this ALSO is actually no longer the case.

On 'modern' ham rigs, it's totally dependent upon the individual radio. And everything depends upon how they're aligned/set-up. I had an Icom IC-706MK2 that out-performed my RCI2970 in all respects on 11M. This was because of where I had the 2970 aligned to perform best (which happened to be the SBB portion of 10M).
Regardless of it's designed operational frequency range, every radio requires enough voltage and current to keep from "swinging backwards" in-TX mode ---- and this is the one requirement most often overlooked by us all.
If you want to see a meter swing-backward, just "underfeed" the rig. It works every time.
 
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