Icom 7300 ALC Mod for AM - Works on Some But Not All?

[AJ1G]

Just tried the 9V battery and 500 K pot dongle mod technique on my IC-7300 to improve AM modulation peak percentage and it has no effect on AM operation. Kevin K2XAM is running it and to works fine on his 7300, he can crank down the carrier by about 50% with about -2.8 V across the ALC jack to get improved positive peaks.  

I built the same circuit and it's not working. Voltage measurements at the ALC jack with the dongle mod disconnected show a +0.1 or so volts at standby that increases to about +0.3V DC at 100 watts steady state output on key down CW and somewhat less on SSB voice peaks on AM.  Cranking in negative voltage from the dongle when it's connected only slightly reduces these positive voltages and has no effect on reducing transmitter output power as desired. Kevin is getting good results with -2.8 V or so across the ALC input jack, and can even crank in more negative voltage to bring output down to zero.

I'm thinking that this may be due to some ALC circuit mods made over the 7300 production run that were apparently implemented to address complaints that the radios were often not hitting full PEP output power in SSB.  My 7300 is a relatively recent one, purchased last year, and never has had a problem hitting full output on SSB voice peaks. Talking with Kevin on this, his is apparently an older production model and he has noticed that it doesn't hit full output on SSB peaks.

I'm thinking that my 7300 ALC jack is presenting a much lower input impedance than Kevin's and the high source impedance of the battery through a 500K pot voltage divider isn't capable of cutting the mustard on my radio.

Can anybody shed further light on this situation and or have experience using the ALC mod on recent production IC-7300s?

[Pete, WA2CWA]

FYI: Did you know that this mod initially had nothing to do with AM. Kenwood introduced the mod back in the early 90's for customers that wanted to run their hybrid TS-520's/830's, etc. transceivers with low power in the SSB mode.

I've tried the mod on my IC-756 PRO II, IC-7000 and I think my IC-706MKIIG  in the AM mode and never found any real advantage. However, I did find with the mod the increase in positive peaks, which according to contacts at the other end didn't increase or change any audio quality, but did find that I was more prone to over driving an amplifier. I also worried about the additional stress on the driver and output transistors. They weren't cheap to replace. The mod box now sits, I think, in my useless project box collection.

The low level AM modulation circuitry that's employed in many of the Icom rigs and now, I believe similar circuitry in Yaesu and maybe some of the Kenwood rigs was described somewhat by Bacon, WA3WDR, many years ago. Bacon called it "reverse carrier control".
It's documented at the bottom of this web site:
Amplitude Modulation ("AM")
using natural asymmetrical voice
a joint effort by John, WA5BXO  Bob, WA3WDR (Bacon)  Tim, W5TOB  Don, K4KYV
https://www.qsl.net/wa5bxo/asyam/aam3.html

There are some definite advantages in not adding the mod. I don't have to worry about splatter or over modulating in transmit even with a linear and even, for whatever reason, I yell into the microphone.

[AJ1G]

Thanks Pete for the reply.  In searching on line for info on this subject today, I came across a recommendation that the simplest tweak for the 7300 to generate a better AM modulation wave form is simply go into the option screen of the MULTI control while in AM mode and crank down the percent of output power from the normal 100 percent, (which automatically provides you with a nominal 25 watt carrier on AM), to an apparent "sweet spot" at around 60 percent output, which gives you an indicated 12-15 percent carrier on the 7300 power peak reading output meter, peaking to about 45 watts with modulation. The mod waveform does indeed look better on the scope, can't say I can discern much improvement audibly from some remote SDR airchecks, and the reduction in carrier power certainly did not significantly reduce readability or signal strength.  Next step is to see how it modulates at the very low power levels required to prevent over-driving the grid driven 813 Class AB1 Central Electronics 600L amplifier beyond 200 watts plate input prescribed in its manual for "monologue" i.e, Old Buzzard, AM transmissions.  The 600 L generally only needs a couple of watts drive on AM on 75 meters. If the envelope isn't as good at those driver power levels, a resistive padder will be used to try to keep the 7300 output in the "60 watt sweet spot".

[W3SLK]

Chris, does the IC-7300 suffer from being 'spike' prone? I remember reading with some of the earlier solid-state equipment driving amplifiers that regardless how far down you crank your power, you still manage a 100W spike within the first millisecond. I'm not sure if that applies to the -7300 or not.

[AJ1G]

I’m not aware of any spiking issue with the 7300, I’d imagine a Google
search on the subject would flush out any issues if there are.  I used to have a random
but annoyingly frequent plate overcurrent trip shutdown of the 600L amplifier mainly on SSB which turned out to be due to an ultrasonic noise pulse associated with the Central Electronics 20A exciter VOX relay driver 12AT7. Somehow the tube was having an internal breakdown of some sort that was generating ultrasonic noise spikes that eventually I realized were actually visible in the station mod
monitor scope trace, when I happened to see one on the scope coincident with a plate trip. Then I just kept an eye out for them, and sure enough they were visible every time a trip occurred. They l apparently were getting into the audio chain as part of the modulation. I can’t recall how I localized it the VOX circuit but changing the affected 12AT7 eliminated the problem.

[KD6VXI]

I would be interested in the software revisions you are both using.

--Shane
WP2ASS / ex KD6VXI

[Pete, WA2CWA]

You might consider modulating the rig by connecting into the rear ACC connector. This avoids all the audio determining/shaping components between the front panel mike input and the modulator circuitry. I haven't used the front panel mike connector in my IC756PROII nor my IC-9700. 

For the IC-7300, page 18-2, gives connection pins on the ACC connector. In pin 11 is modulator input, pin 2 is ground. Input Impedance is 10K. Input level is 100mv but you can go to the Connections Set Screen and change that.

[AJ1G]

Thanks for the info re feeding in external processed audio.  That’s next!  Got the dongle working just like
K2XAM now, think I may have had a sketchy corroded connection of the wire to one of the buttons on the old
9V battery connector.  That along with the high source impedance using a 500K pot.  Checked with what K2XAM’s voltage off the pot was with it disconnected from the ALC Jack for the setting he ended up at: -9.2 volts!  My sweet spot came in very close to his, when measured at the ALC jack it was about -2.2 volts, with about 12-15 watts vice 25 watts carrier.  Totally eliminated the downward carrier shift, however modulation envelope still looks well shy of full 100 percent.  Working on that by dropping
carrier down a bit more with the dongle, still have more than adequate drive to my grid driven AB1 813 Central Electronics 20A to get it to 200W input limit/nominal 100 W carrier out.

Shane as far as software version, no clue.  Like Joe, I’m
pretty much still an analog guy in a digital world.

https://youtu.be/6YkAnv8inQE?si=OFtrVLZgXzW8Q4p3

[KD6VXI]

Being that it was an installer error (lol, we've all been there), I could care less about software revisions now.

I was leaning towards either AM or ALC updates between revisions causing your problem.

Turns out it was a physical error


Just glad ya got it working.

--Shane
WP2ASS / ex KD6VXI

[AJ1G]

Wasn’t so much an error as making a bad pick from the connector junk box, the black pigtail
connection to the connector button under the vinyl cover sheath was apparently hanging on by a thread, and eventually fell off.  With no load on the pot I was seeing voltage change but under load it was adding insult to injury with respect to high voltage source impedance.

[W1GFH]

Chris, I tried the battery trick on my Icom and got tired of changing batteries, so I found this Chinese 120VAC to variable 1.25 to 12VDC @300ma regulated power supply kit (about $15 on Amazon or eBay). Just reverse the output leads and there's your adjustable negative bias voltage, complete with an LED display. The plastic case makes adding a toggle switch for AC power on/off easy. As you may have discovered, fine voltage adjustments are tricky. So I replaced the kit-supplied pot with a beefier 10-turn precision pot, and it helps somewhat.

Even with a regulated and observable bias voltage being fed into the ALC, the Icom's output will sometimes, after a few transmissions, inexplicably increase by 5-10 watts, requiring a quick readjustment of the pot. My theory is that some component in the Icom's ALC circuit is changing value as it heats up. But I don't really know.

I can also vouch for Pete's suggestion to feed audio into the rear ACC connector. It does make a noticeable difference. If you have one of those inexpensive Behringer mixers with 3 bands of basic EQ, you can "force feed" some mid and high frequencies into the rig by adding +3-5db at 2.5K and 10K. The modulator can't hear 10K but it does actually hear the lower harmonics, and will give some extra energy at 3-4K or so for presence. No need for a fancy mic, any kind of a decent range dynamic like a Shure SM58, Behringer, Pyle, etc. will make the transmitted audio sound pretty decent.

[AJ1G]

Installed a pigtail to bring external audio directly into the modulator as recommended by Pete WA2CWA.  Definitely can hear improvement in the audio relative to using the stock hand held microphone and internal audio processing. However, neither the internal audio or
using external audio seems to be able
to hit anywhere near 100 percent either positive or negative based on what I can see my o scope modulation waveform.  Has anyone been successful in getting full AM
modulation on a 7300, and if so how?  I did add a phase flipping line to line isolation transformer to the external audio chain which improved things a bit “in phase”. but what I’m seeing
on the scope is still a bit underwhelming despite total elimination of negative carrier shift by use of the ALC dongle.

[AJ1G]

One gotcha to watch out for for those using
external audio injection - if you use toggling the TRANSMIT button for transmit/receive switching instead of the PTT on the hand held stock microphone, a transmit timeout cutoff function with a default setting of
3 minutes comes into play.  Was quite unnerving the first few times it kicked
in on me.  It can be turned off or changed through the settings menu.  Brought me a few “Hey, did you
just have a DYY type crap out?” comments.

[W1GFH]

Chris, I forgot to mention, the Icom transceivers have historically not been able to produce 100% modulation on AM. Approximately 90% is the limit, per W1AEX findings. I don't know if SDR rigs like the 7300 have changed that. But, a bit less than 100% modulation is only a handicap at low power when others are struggling to copy you. When the rigs are followed by an amplifier and producing a strong signal, the combo of near-100% modulation and force-fed EQ can create the sonic illusion of a very respectable boatanchor AM signal.

[kb3ouk]

The jumpy output when externally controlling the ALC was an issue I used to run into on my 718 when I was still using it pretty heavily for AM. I think mine was even more random, it might climb one time add drop the next. I think at one point I might've came to the conclusion that I was picking up RF on the cable going into the ALC jack so I put some ferrite on the cable and that seemed to help it a little bit but it still wasn't completely stable.

[W1GFH]

AJ1G using external audio. Screenshot off the Web SDR. Wow...I guess the IC7300 really does "brick wall" at 3 KHz on AM.

[ki4nr]

Brickwall is the name of the game on the new Icoms with 3khz AM transmit and their annoying "corporate bandwidth" of 2.8khz on transmit SSB. But with a little EQing and feed the audio in the rear mod input you can get
pretty nice AM audio out of the rig.

The real issue with all the new Icom's is the AM receive, it's super restricted in the bass response. They all roll off at 200 hz. The 7300 is down about 27db at 100hz, so if you have nice external speakers and like to listen to full broadcast audio, either from shortwave station or AM ham radio you’re out of luck. It's not just the 7300 all the new models are the same.

Interestingly I have two older Icom's the 756pro and 746pro, they both have no low end restriction and are flat to 100hz and roll off about 3 db at 50hz and sound super. Look at the spectral graph below for the 7300, the bass response is the same for all three AM bandwidths.

[KC2ZFA]

yesterday AJ1G sounded better when he moved to
7290 and called CQ…but man, turn off them microphoneys.

Peter

[W1GFH]

yesterday AJ1G sounded better when he moved to
7290 and called CQ…but man, turn off them microphoneys.

Loud audio feedback, yes it sounded like he was trying to transmit with the 7300 and listen on a separate receiver at the same time.

Interestingly I have two older Icom's the 756pro and 746pro, they both have no low end restriction and are flat to 100hz and roll off about 3 db at 50hz and sound super. Look at the spectral graph below for the 7300, the bass response is the same for all three AM bandwidths.

Yes my 756 Pro will go down to 50 Hz and go a little bit above 3000 Hz using external audio into the ACC. It looks like the 7300 will not do that. Not sure why Icom chose to restrict bandwidth on AM, knowing customers generally want more fidelity in phone modes.

[AJ1G]

Sorry about the feedback.  I
Might have had my direct off air monitor headphones (1N34 detector into a small audio amp turned up
to high on the transmission you heard, or my receiving
audio system Sony AV receiver gain was too high, sometimes gets RFI in that condition.
 
Those ops on 40 with high signal levels and very low noise yesterday were very helpful is hearing and seeing how the 7300 is working with external
audio and the ALC locked down to eliminate the Heartbrake of Downward Modulation.

[Pete, WA2CWA]

I actually would not call it "downward modulation".

You're just modulating within the upper and lower limits of the carrier. Operators may have external in line wattmeters or SWR bridges in the "Forward" switch position, and when talking into the microphone, see the meter pointer move in a downward direction. That's not a bad thing from a design standpoint.

But if you've been around for awhile and have had vacuum tube transmitters, and your RF output indicators  or your plate current meter moves in a negative direction, it's normally an indicator of a problem within the final or driver circuitry.

  But many AM operators who mitigate from a tube transmitter to a current solid state rig, "wonder where are my positive peaks"  Why is my RF meter going in the wrong direction when I modulate the rig 

[Jim/WA2MER]

Not sure why Icom chose to restrict bandwidth on AM, knowing customers generally want more fidelity in phone modes.

The answer to your question is twofold. First, Icom engineers employed good practice in the design of their radios. Given that the FCC stipulates that the BW on the HF bands should be limited to that necessary for a communications quality phone emission for the modulation type employed (a paraphrase), their radios accomplish that. "Communications quality" audio (often used on the air as a pejorative term) has generally been accepted to be in the 300 - 3000 Hz range. Is that range acceptable? It is to me, but apparently not so to others. The FCC didn't specify a regulatory BW limit but instead allowed for significant flexibility in interpretation of the requirement, and that's a good thing. People like me who don't worry very much about fidelity can use our rigs as designed, and wannabe AM broadcast stations can do their thing as well. Everybody wins.

Second, whether or not Icom "knows" that we generally want more fidelity is secondary to the question of whether they care, given that their radios are primarily deigned for and aimed at SSB and data users. Like it or not, accept it or not, we AMers are a niche market who will occasionally be thrown a tidbit every so often. I'm happy that current model offerings still have AM at all.

[ki4nr]

Icom radios on AM vary in performance. The older DSP radios are wide on AM transmit and receive. Some of the newer model radios are not as wide as the older ones but decent, then the newest SDR rigs are super narrow.

Now on SSB,  All Icom DSP rigs from 1995 and on are 2.8khz bandwidth on transmit. Top of the line or entry level model doesn’t matter, you get what I call the "Icom Corporate bandwidth" on SSB of 100hz to 2900hz.

The other brands on SSB and many on AM, Yaesu gives you 3khz with simple mod to 4khz, Kenwood's to 4khz, Elecraft I think to 4khz, Flex and Anon SDR are 20hz to 20khz all modes. Some Ten Tec to 3.9khz. Yaesu has parametric EQ, Kenwood has graphic EQ, Same with the others.

Crummy Icom SSB 2.8khz bandwidth and simple primitive Bass & Treble controls is what you get from them since 1995

The marketing people at Icom know big time that hams care very much about the sound of their station and see what the other manufacture's are doing and read the forums. I'm 100 percent sure they like to change all that in their rigs. But certain higher up's, I imagine either Mr. Inoue or Mr. Harima just do not like or think nice audio belongs on hammy radio ..LOL

[AJ1G]

I actually would not call it "downward modulation"

Well, call it downward modulation or negative carrier shift, or something else, what an averaging wattmeter, shows during as designed/built IC-7300 AM operation, is an up to 50% decrease in forward power meter deflection while modulating.  Might even be more when you consider the ballistic limitations of the wattmeter needle.  Doesn’t occur in a plate modulated AM transmitter, and I’ve never seen such a severe case of it in a solid state AM transmitter.  Clamping the ALC with the battery and pot dongle totally eliminates it, and I’m seeing close to 100% clean modulation at least with test tones with the ALC clamped.