ICOM 761 ALC cure for poor AM

[ns7h]

I have followed the discussions here and on the air regarding injecting an ALC voltage in older Icom transceivers to improve AM modulation quality.  I have an Icom 761 with no modifications and tried the circuit with no success.  The manual has an ALC range of 0 - -3 volts.  The adjustment using mic gain for SSB works, but is out of the ALC range indicated on the multimeter for AM regardless of power, mike gain, or injected ALC voltage.  I know the Icom can do better....The modulation monitor shows little positive modulation and on the air reports indicate muffled, basey modulation.

Any experience on this issue for the IC 761?

Thanks

NS7H

[Pete, WA2CWA]

You should state what microphone you are using.

I've been using an IC-756 Pro II on AM with great results.
However, I haven't used the front panel microphone input since I've owned the rig.
I always use the Modulator input, Pin 4 and Pin 2(ground) on Accessory Socket 1 which connects directly to the modulator. This bypasses all the audio "tailoring" circuitry between the front panel microphone connector and the modulator.

Microphone into a equalizer like the W2IHY box, or something similar, into pin 4 and 2 will add life to your audio. It will also work well on SSB.

I also never use the negative voltage mod into the ALC pin. FYI: this mod was originally introduced by Kenwood back in the 80's to lower the RF output on SSB for their hybrid transceivers starting with the TS-520. Back then, there was a movement for QRP SSB.

If you want to read the advantage of the Icom design which is also used in some Yaesu rigs go here
https://www.qsl.net/wa5bxo/asyam/aam3.html
and scroll all the way down to the article by Bacon, WA3WDR

Technically, with this type of modulation and a linear, you can run key down a 1500 watt carrier on AM and still be considered legal. Positive peaks might have been cool back in the "good old days" with vacuum tube rigs, but its really not needed with today's solid state rigs.

[ki4nr]

Sounds like your 761 has had the golden screwdriver in it. I had a 761 and 765 and the ALC negative voltage mod circuit works perfectly. No positive peaks tell me the balanced modulator ( IC-9 on the main  board) is over unbalanced and has no headroom left.

This is the way a properly operating stock 761 works on AM. First when the power control is at max , you should get 40 watts carrier on AM, no modulation. If not adjust R-84 on the Main board so that it does. Next the meter switch set to ALC, key the radio and at the 40 watts output you should see the ALC sitting at Mid scale ( S-9) again testing with no modulation. If not adjust R-159 on the main board so that it does. R-159 controls the amount of DC is injected in the modulator to unbalanced it for the AM carrier and max positive peaks. Now all this is under the assumption that the ALC meter calibration is correct and no one has played with that adjustment pot.

Now here is the quick and easy way to do all this for best AM performance, which is different from the way Icom says.

First make sure the radio on AM does the 40 watts with power control at max. If not adjust R-84.
R-159 control the power also and if you turn it up and down you will see that. Adjust it so you just get the 40 watts, then advance it another few degrees rotation on the pot. If your ALC meter is working right you should see a little ALC indication, On the S scale about S-4 if ALC calibration has not been played with.

Connect your ALC negative setup to the back of the radio. Key the rig and you should be able to adjust the power from zero to 40 watts if all is working correctly. Now set the carrier at 25 watts, no audio.

One thing, the front panel power control must always be at Max. All AM power adjustment must be done by the Negative ALC mod circuit !!

This is very important for all this to work right !! You want the radios internal ALC threshold to be at 40 watts detection on AM (the front panel RF power control is what controls that)  and the radios IF strip to be at maximum gain.

Advanced the mic gain, Now you should be getting good audio and large positives peaks ( way over 125 percent ) at the 25 watt carrier level. You can run more but 40 watts would be the limit before the final amp runs out of headroom or the ALC in the radio starts to activate and knock the power back. Icom was nice enough to provide a change in the ALC time constant on AM to change the ALC from peak reading to RMS to help. This is provided by Q-17, C-44, R-65, R-66.

Since the radio use's the SSB balance modulator for AM it will at the 100 percent negative half cycles phase reverse and show wavelet distortion, unlike a plate mod transmitter which will pinch the carrier off. This in normal operation on a four quadrant multiplier gilbert cell. The way to make it work great is to use a compressor & clipper and apply the audio to the mod input on the rear of the radio. When setup correctly the radio will rival the best plate modulation transmitters.

The poor audio your getting is just a function of the mic your using. If its a studio mic it needs pre emphasis of about 10db of boost at 3 to 4 khz and maybe a little bass cut around 200hz. Using a EQ is absolutely must when using any studio mic on AM to sound good. The radio itself has a decent frequency response from about 80hz to 3.8khz mainly limited by the fact it transmits thru the AM receive murata filter. Also there is a undocumented mic tone control pot in the mic preamp circuit that could be misadjusted to cut the high frequency, be sure to check that and set for maximum treble response. The pot is labeled R-155 on the schematic.

Of course a D-104 into a 5 to 10 meg load FET buffer will fix all that easily without any EQ and sound excellent. But you still need the compressor and clipper combination to make it work best. You can just use a compressor and that will work fine, but adding the clipper adjusted for about 2 to 3 db max action sounds super. The audio drive into a four quadrant multiplier must be held precisely at 100 percent negative audio so it does not phase reverse, which is easy to do with a op-amp diode clipper.