Icom IC-718 on AM

[W3FJJ]

This is pretty standard set up for audio level on any ricebox on AM.  I don't think Icom has
figured out how to rate their rice boxes  on AM. This rig should not be run at 40 watts carrier,
but 25 watts max to alow audio peaks to reach 100 watts pep output. You may even noticed a upward
swing in power out on voice peaks..

This inexpensive transceiver works at a rated 40W output on AM.   The audio compressor must be turned off. Back off the mike audio gain to about 50%.   While transmitting, set to monitor the output power.  If the power drops out on audio peaks, back off from the mike or lower the audio mike gain until it does not drop.   This is the setting that the ALC will not cut in.  You can also monitor the ALC while adjusting the mike gain until it does not change on audio peaks. 

[Ed WA4NJY]

              I am thinking of getting a 718 for casual AM use.  Intend to use an AL-811 amp
     with it. Was using an IC-706MKII, but that nasty spike keeps me from using the amp.
     Best info is that the 718 does not spike. Receive filtering could be too wide. Might wire
    in a seperate receiver connector. ICOM has a 3.3kc filter(pricey). With split VFO, transmit
    AM on one and receive 3.3 SSB on the other.

              Also, Jupiter does well on AM except I can't get rid of the RF problems when
    running power. Hope the 718 is tolerant of that.

               Lemme know what info is out there.
                                                                       Ed Purvis
                                                                          Bradenton, Fl
         

[K3ZS]

The IC-718 seems to run fine at 40W output power on "AM".  I think if the finals can handle the duty cycle, this is OK since it is SSB with carrier added.  The 4 times PEP vs. carrier power applies when both sidebands are part of the total power.  With my old tube SSB tranceiver (Tempo One) you could not run more that 25W of carrier, but I believe that was a duty cycle limit.  Any comments on this or I am I missing somthing?

[Pete, WA2CWA]

The IC-718 seems to run fine at 40W output power on "AM".  I think if the finals can handle the duty cycle, this is OK since it is SSB with carrier added.  The 4 times PEP vs. carrier power applies when both sidebands are part of the total power.  With my old tube SSB tranceiver (Tempo One) you could not run more that 25W of carrier, but I believe that was a duty cycle limit.  Any comments on this or I am I missing somthing?

In many respects the 718 is a scaled down version of the 706, but with larger cabinet, display, knobs, etc. I can't remember if they use identical final transistors. Although Icom has stated in many of their current rigs 40 watts AM max., some distortion products do/or can appear on the output signal in the AM mode. Icom technical support stated to me at Dayton two years ago, the 40 watt AM max is for a "very short" duration of time. They did not recommend long key down in the AM mode at 40 watts. As stated in an earlier post, 25 watts or less, is recommended and encouraged, for most of Icom's rigs that have AM capability. I seem to remember the finals were not cheap.

[K3ZS]

The fan starts running when running 40W.  I usually don't use the 718 for long on AM.  Just to make a quick call while the tubes on the AF-67 warm up.  I will run 25W from now on.   The rig has a built in RTTY modulator and runs 100W with 100% duty cycle.  I never hear the fan run when on SSB.  The fan seems to be variable speed.  Maybe the trick is to stop transmitting when the fan starts picking up speed when running any more that 25W.

[ka2zni]

 Is there not a way to defeat the ALC circuit atleast in part for more than casual AM use so the nasty backwards spike is gone and true forward modulation can be achieved? Surely someone out there has figured some way to do this??

Thanks in advance...

[WA3VJB]

Drop a line to  Dave, W3LOW near Scranton.

I worked him last night 3819Kc, and he was using one of these and sounding pretty good.
Heil ICM mic on a Heil boom.

None of the digital artifacts/dirt that otherwise are a dead giveaway to other Icom models.

Initially, yes, he just barely touched the ALC on voice peaks and it really sucked down the audio. After he backed it down 10 percent or so, it was clean, and the dynamic range was good. 


http://www.qrz.com/w3low

[Pete, WA2CWA]

If you are seeing a downward movement on your analog SWR meter when modulating an ICOM rig on AM, you might want to read these postings from December 2005:
http://amfone.net/Amforum/index.php?topic=6429.0

And even better information on AM in ICOM DSP type rigs:
http://www.ab4oj.com/icom/dsp_am.html

Downward spikes don't exist unless you have some other problem. However, on an analog mechanical meter, as you modulate the rig, you will have upward (at max. carrier) and downward movement of the needle pointer as you modulate the Icom rig in the AM mode.

You can also apply a variable negative voltage (from roughly 0 to - 5 volts) to the ALC terminal and ground. Key the rig, and while you are modulating the rig on AM, adjust the variable control for minimum downward swing on your analog meter. However, in most cases, this does nothing to the over all quality of your audio.

[WA3VJB]

It's possible the 718 is cleaner on transmitted AM than the 756 Pro II and III models because it does a better job handling higher audio input levels.  I had asked an expert on the DSP based Icoms to help shed light on the distinctive and unattractive sound the 756 makes when used on AM, and to share the unreceptive level of interest a factory rep responded with when I telephoned Icom about this widespread and audible problem.

Although two questions are left unanswered (why this digital dirt can be heard at all amplitudes, and whether there are input sensitivity differences between the 718 and 756), this is one of the more relevant answers to explain what could be causing the noise.

I found his point especially interesting that Icom won't deal with such a problem if it considers the number of complaints small. That's in stark contrast to other manufacturers who have developed DSP radios.  Flex Radio, the makers of the SDR1000, have taken individual, specific suggestions from users and swiftly implemented corrective action or improvement in response. That's the kind of company I can support, were I to find myself in the market for a contemporary transceiver (not likely).

Hi Paul,

Many thanks for the note. Actually, I wrote my article from a purely
academic standpoint; my amateur AM career ended rather suddenly at the end
of 1963, never to resume.

The tests I ran on AM prior to writing the article yielded a very clean
display on my HP spectrum analyser. The test conditions were:

Po (resting carrier) < 25W.

%Mod < 90%.

Test tone: 1 kHz at 3 mV rms, injected into Mic socket.

Casual listening on a separate receiver did not reveal any audible artefacts
during test-tone or speech modulation - at least nothing that I could hear.

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

[Pete, WA2CWA]

It’s understandable that Icom won’t acknowledge a possible design flaw if the number of complaints are minuscule in comparison to the number of rigs they sell. Other than your complaint about “756 digital dirt” on this forum, QRZ, eham, etc., I cannot recall any vast support of this possible problem. As Adam has pointed out, if you overdrive a device, distortion might be the result, but this anomaly is not limited to just DSP type rigs.

As far as the Flex products, the basic hardware “black box” hasn’t changed much in about a year. There were bias changes implemented, I believe, after the ARRL Product Review (don’t remember if it was after the first or second review). However, there have been loads of hardware problems with connectors, cables, PC hardware, processors, and peripheral type equipment that make the Flex work. Further, software changes, tweaking, and massaging seems to be an on-going exercise almost on a daily basis. With these types of activities, the threat of other bug introduction into the software is a notable reality. Fortunately, many owners of Flex products enjoy the ability to diddle with the open source software, so when bugs, requests for changes to existing versions of software, and new feature requests are dialoged, there’s an army of free enthusiastic users jumping into the software fray to find a solution. Icom, on the other hand for the majority of their current amateur products, have their software feature and control imbedded in firmware, which is not easily changed. They also will not release the software to the amateur market for these products.

However, as time matches on, I suspect that we start seeing more “middle of the road $$” rigs entering the market that will have the ability to implement changes to the software through downloads from the manufacturer. Ten-Tec and Elecraft have been doing this for some time now, and I believe Icom and Yaesu’s high-end rigs also have this feature.

[va7oj]

Firstly, I would like to thank the moderators for accepting me in the group.

Here are the results of a follow-up test:

Yesterday, I ran an AM  listening test with my IC-756Pro3 at 25W resting carrier and 90% modulation , using an ME-61/GRC field-strength meter with a good-quality headset as a simple tuneable demodulator. I then repeated the test at 250W resting carrier, with the Pro3 driving my Yaesu Quadra MOSFET amplifier.

In neither case could I hear any artefacts. The recovered audio sounded smooth and clean in both tests. This tends to confirm my earlier tests with the spectrum analyser.

When operating AM with the IC-756Pro series, it is important to set resting-carrier P_o to 25W max. In addition, Mic Gain should be set such that the sidebands are 7 dB below the carrier, as displayed on the spectrum scope (90% modulation). This is discussed on my website:

http://www.ab4oj.com/icom/pro_notes.html#am

Best 73,
Adam VA7OJ/AB4OJ

[Pete, WA2CWA]

Thanks Adam and welcome aboard.
Your results confirm a similar test I ran on my PRO II last year after one of our previous discussions on one of the Reflectors. I also agree that keeping the resting carrier to 25 watts maximum is an important requirement to maintaining good audio quality on AM.

[WA3VJB]

Adam,
Glad you're here.

I'll send along some MP3 that highlight the digital dirt that I and others have heard on the Pro II when used on AM  to see if you can make a determination of where it comes from.

It is pronounced enough that when I hear it, I can correctly guess most of the time that it's an Icom being used.

Curiously, the 718 does not consistently present the problem that I can hear on the 756 series. I worked a station this past weekend using one and will post a recording showing how cleanly the audio came across.

Meantime, I am curious whether you can create this noise if  you could repeat your tests with a higher carrier level, higher audio drive, or some other deliberate mis-adjustment, to shed light on when and if  the problem shows up.

Could you send me some recordings to consider ?  Or post them here ?

It could be a matter of poor adjustment by operators of the stations I've encountered, which in turn may reflect a less forgiving nature of the 756 compared to other contemporary transceivers when used on AM.

[Steve - WB3HUZ]

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.

I found this comment interesting. I'm not sure if it's true. If so, we should all hear similar distortion on our CD players, nearly every radio and TV station, and any sound that comes out of our computers. All these use A/D and D/A converters in the signal chain, in some cases several!

I think there's more to the problem with the Icom radios than quantization error. To my ears, 16-bit audio from some older computer systems sounds better than the 24-bit Icom stuff. If it was all about quantization errors, then the 24-bit stuff would sound better.


Since this discussion is moving away from how to make the 718 sounds good and more to a general discussion, I'm splitting the topic and moving part of it to the Technical section.