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ICOM 756 (non-Pro model) on AM




 
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Author Topic: ICOM 756 (non-Pro model) on AM  (Read 23498 times)
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wb4iuy
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« on: June 21, 2010, 09:03:45 PM »

I've tried using my IC-756 on AM from time to time, as I enjoy this mode on 75, 40, 20, and 10 meters. Unlike my Yaesu 901 and Heathkit equipment, the output power of my 756 Icom would drop considerably when I would modulate the transmitter with voice. In order to get decent power out, I would have to run my amp harder than I care when operating AM.

When operating at 25 watts carrier from my 756, I was producing about 25 watts pep when modulated at 100% (because the power output would drop to about 8 watts!). My amp (AL-82) would run about 350 watts carrier, and would fall back to about 85 watts or so when I would speak (still about 350 watts pep).

I built the little curcuit from W8JI's page this afternoon. I can now operate the rig at about 10 watts carrier, which drives the amp to about 150 watts. When the 756 is modulated the amp produces about 600 watts pep, but runs very cool.

Listening to it on my Yaesu 901, it seems much cleaner, but I can't hear any difference on my Heathkit SB-301. I guess this is partly determined by the detector used in the receiver of the receiving station. The amp is much happier, though, and runs much cooler. I can run a much higher PEP with a lot less heat!

I've posted info about it on my AM Mode page at:

http://www.ipass.net/wb4iuy/radio/wb4iuy2l.htm
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W1AEX
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« Reply #1 on: June 26, 2010, 07:04:45 PM »

Dave,

Thanks for posting this information. I built the little circuit you referenced on your page and it works beautifully with the IC-756 Pro III. It completely removes the gritty ALC artifacts from the audio and allows the 756 series to actually develop positive peaks when it's modulated. I now see upward movement on an average reading power meter when pushing audio on AM. Nice find!!!

Rob W1AEX
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Jim, W5JO
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« Reply #2 on: June 26, 2010, 09:14:01 PM »

I wonder what the deal is?  I have a 756 Pro and it does not do that.  When I hit the AM button the PO goes to about 35 watts and peaks near 100 on the PEP meter.  The meter on the radio also increases with modulation.  It has been a while but I don't think it will allow AGC on AM, at least it doesn't show on the menu buttons.

I have an AL 1200 and found that on SSB if I hook the AGC line between the two, set the AGC on the 1200 according to the book, the radio pumps the audio and sounds terrible.  So I disconnected the AGC line and use a scope to set the power and mic. level on SSB and AM which rids the system of that tendency.
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W1AEX
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« Reply #3 on: June 26, 2010, 10:45:32 PM »

Hi Jim,

The meter on my Pro III behaves as yours does, and an external PEP meter will deflect in the positive direction as well. However, an average reading wattmeter, or a scope, will reveal the truth. The best way I can describe what normally happens is by video. Take a look at the output of my Pro III with an average reading wattmeter when the internal ALC is allowed to do its thing on AM:

http://www.youtube.com/watch?v=nFCSbsqZyz0

Now look at what happens with a stable negative ALC voltage fed in through the back panel, as Dave described:

http://www.youtube.com/watch?v=IMinfBlvnN8

If your 756 Pro doesn't have the ALC curse, count yourself lucky! I have never heard any 756 series rig on AM that didn't exhibit the gritty artifacts caused by the internal ALC clamping the peak power output to the carrier power level.

Rob W1AEX
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Pete, WA2CWA
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« Reply #4 on: June 27, 2010, 04:14:06 PM »

The circuit that W8JI shows is nothing new. This same type of circuit was touted back in the late 70's and early 80's for use with a number of Kenwood rigs like the TS-820, 830, etc. for reducing the output power in the SSB mode. I think Kenwood even issued a service blurb or "operating hint" way back then showing this circuit.

The real question, why is it important that: "I now see upward movement on an average reading power meter when pushing audio on AM". 

A very good tutorial on AM operation with the 756 PRO series can be found here http://www.ab4oj.com/icom/dsp_am.html and here, http://www.ab4oj.com/icom/pro_notes.html#am

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« Reply #5 on: June 27, 2010, 04:24:58 PM »

I have never heard any 756 series rig on AM that didn't exhibit the gritty artifacts caused by the internal ALC clamping the peak power output to the carrier power level.

Rob, is that what the transmitted "grit" has been traced to ?

An ICOM representative I called on the phone denied there was a problem among examples of the Pro II that were in production at the time.

But instead of taking a sound file I had recorded for consideration, he directed me to paid advertising in one of the "HAM radio" magazines, where ICOM explained they had repaired a problem with audio grit on receive.

In some correspondence, I did gain some possible insight into the factory's thinking.
(reposted with permission)

Quote
The "rumble" in the receiver was due to modulation by the NB of the +8V rail
feeding the low-level receive audio amplifiers. It affected the IC-756Pro,
and was corrected in the Pro2 and Pro3.

http://www.ab4oj.com/icom/756pro_nb/main.html

With any system involving A/D and D/A conversion, quantising distortion will
occur at high signal levels. This will manifest itself as noise on
modulation peaks. As the ADC and DAC used in the Icom radios are 24-bit
devices originally intended for high-grade audio applications, I do not
believe quantising distortion will be significant as long as the devices are
not overdriven.

Icom will not treat this as an action item unless a significant number of
user complaints are received. Given the minority status of AM in the ARS
nowadays, and the fact that A3E is not even an authorised emission for other
radio services in which these radios may be deployed, I am not sanguine that
there will be any change.

Cheers for now, 73,
Adam VA7OJ/AB4OJ

And I let it drop at that.

But with the prominent "rumble" I consistently heard on stations transmitting with the 756 series, I suspected it was a design shortcoming, not maladustment or isolated incidents.  

Your readout is the first I've heard with any specific cause.

Even better, for those who run these rigs, that you've successfully proven out a fairly simple cure ...

Quote
It completely removes the gritty ALC artifacts from the audio

As a listener, I thank you for that.
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W1AEX
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« Reply #6 on: June 27, 2010, 05:11:34 PM »

The real question, why is it important that: "I now see upward movement on an average reading power meter when pushing audio on AM".  

Yes, the idea has floated around for quite some time. I have used a similar setup with older sideband transceivers that exhibited power overshoot when they were keyed.

Regarding the significance of downward deflection on an average reading meter, it is a matter of cause and effect. The downward deflection simply indicates one effect the internal ALC has on the behavior of the 756 series on AM. Another more unfortunate effect is that it is impossible to reach much more than 60 to 70 percent modulation, as the ALC clamps the positive peaks and holds them down to whatever power level the carrier is set to. The most unfortunate effect to those on the receiving end, is the gritty sounding component that Paul mentioned.

Which would you rather listen to?

http://members.cox.net/w1aex/stocktofrg7700.mp3

http://members.cox.net/w1aex/vstofrg7700.mp3
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« Reply #7 on: June 27, 2010, 05:24:32 PM »


Is voice shaper software processing being used with first recording above?
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« Reply #8 on: June 27, 2010, 06:12:44 PM »

Pete: No, that is completely stock audio. Also, to be fair, that first file was not recorded on the same receiver, or at the same bandwidth as the second file. To equal the playing field, I uploaded a clip made with the same receiver and bandwidth setting as the second link. Note that in the second file, the Pro III, running with external ALC and audio processing, is reaching approximately 90% modulation on peaks fairly cleanly. In the first file, the Pro III running stock audio and internal ALC is reaching approximatelly 60% modulation on peaks, and has come to the point where driving it harder results in heavy distortion.

Paul: If you listen to both files with headphones, you can hear the gritty artifacts that mimic the audio when the internal ALC is active. That effect is completely gone when the external ALC is used. It seems like the clamping action of the internal ALC is indeed the culprit, but I'll let people come to their own conclusions. I simply like the fact that it looks much healthier on the scope, and sounds less aggravating to the poor guy on the other end!
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Jim, W5JO
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« Reply #9 on: June 27, 2010, 09:06:31 PM »

The meter on my Pro III behaves as yours does, and an external PEP meter will deflect in the positive direction as well. However, an average reading wattmeter, or a scope, will reveal the truth. The best way I can describe what normally happens is by video. Take a look at the output of my Pro III with an average reading wattmeter when the internal ALC is allowed to do its thing on AM:
Rob W1AEX

I believe all SS rigs use negative modulation tehcniques, so you are seeing is normal for them.  It also manifests itself on an average reading meter by downward movement of the meter. 
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wb4iuy
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« Reply #10 on: June 28, 2010, 08:11:18 AM »

It seems that some of you are deep into the technical aspects behind this... I simply ran several blind on-the-air a/b tests, and every report given was better with the alc source circuit operating. Just my $.01 worth :-)

Dave WB4IUY
www.WB4IUY.net
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