ALC / AMC Explanation

[RichardHartman]

Good morning,

Can someone please explain what exactly these two circuits do? All I know about them is people do mods to them to get/show more swing and I know that's not good. I'm not wanting to do mods to these circuits I just want to understand what it is they actually do. Is the mic limiter also part of one these circuits?


Thank you,

Richard

 

[Handy Andy]

ALC and AMC, the two are similar but not EXACTLY applicable to each other.

Both are a type of feedback, to reduce or attenuate power AFTER the processing has occurred, but to prevent further degradation over-amplifying does to the signal in the POWER AMPLIFIER.

So if you agree to the above statement...then

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If you have a question or two about this, then realize too, where you posted this - in a CB forum...

The two processes are similar, but one uses the RF power output levels as a means to control the RF power Output by controlling the level of the input signal - AFTER processing the Input signal.

The AMC however, does not limit RF power - only the Audio Power AFTER the input signal is amplified and processed - BEFORE it goes to the Power Amplifier.

This may generate a discussion - but the above is how I see ALC, as a form of RF power Regulation - while AMC is a from of Audio Power Regulation - not RF.

 

[nomadradio]

The one thing Andy left off is why.

They prevent you from overmodulating the radio. The distortion this brings about will create interference on other channels. This is the main reason the regulations say you can't legally sell a CB unless it has that feature. And a desire to be "louder" is what prompts such attention to the details of defeating them.

73

 

[M0GVZ]

Short version....

They try to compensate for people who wind up the mic gain too high to try to minimise how much of a splattering signal they transmit.

 

[RichardHartman]

I am having an issue with cb and the oscilloscope to get a clean sign wave. Could this due to the ALC/AMC modification? I am currently working on a Robyn 520D I don't see any modulation modifications that stand out except diode D46 lifted.

 

[Handy Andy]

Yup, that can do it...

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Firstly thank you for this picture.

Secondly, yes, but in SSB mode. It looks at "peak" and R171 "swamps" or changes the sensitivity of the Diode - notes the 0.22V in TX - but notice the MINUS " - " symbol.. It's looking for spikes and the diode conducts really heavily when the RF wave ever gets to a point where the negative peaks of the RF signal go below the Detectors (R171 and D46) reverse biased condition and then the Gate conducts because the Diode's orientation in the circuit is now forward biased.

L25, C154 CT7 - form a tunable trap to sense RF, L25 takes the DC the Diode makes in the detection - and keeps the signal level low enough to keep the FET that 2SK68 (really old unit) from blowing up (perforation)...

Did not know if you were aware of this, but a circuit like this can also function even when it's not supposed to. The Diode used in this fashion will make a small current appear and have power to affect the Limiter amp located in another section of your Sams manual.

• See Block 2 it shows 9.50V ?
• With the discrete parts (not the Diode, remove it for a moment mentally) The FET conducts to ground when the signal approaches a threshold - with out the Diode - as a level of peaks in the RF output envelope.
• Now, put that Diode back in, it helps to rectify these peaks, and when these peaks Go Negative, that Diode helps to make this circuit more sensitive to these negative peaks.

When the power is off, this circuit is not, it's still working - sensing the levels of RF power and since the diode can trigger the Gate into functioning - that DC power from Block 2 doesn't need to even be on, it can send a signal to ground one of two ways, The line being more positive than ground - or the FET itself is DAMAGED and the Gate is perforated (shorted) into the substrate, it's not insulated from it anymore - so the signal the Diode makes, can now go back into the line that's labeled MOD SSB and affect the limiter amp.

So if you're having trouble, we can suggest some ideas but what radio is this in? (520D?)

 

[RichardHartman]

Thank you Handy Andy I love the knowledge you share.

I am working on a Robyn 520D.
A beautiful garage find. I cleaned it up recapped it and she fired up.

I started from the synthesizer alignment procedures in SAMS manual and worked all the way until the 100% mod in TX alignment. The oscilloscope was terrible and on air reports were reported as having tons modulation overdriving/clipping. On my test radio I could here that the hand mic was so strong and sensitive it was like having a power mic turned up all the way.

I was able to lessen this problem by dropping the AM carrier to 2 1/2w and then I had to turn the mic gain VR6 all the way down then just forward slightly. Then VR7 doesn't seem to have much influence. Continuing to have about 17w on SSB

This arrangement I was able to achieve a good clean sinewave and the reports are great now. I think I was able to negate what D46 lifted was doing by turning those items down. I would like to put D46 back in circuit but I wanted to check in on the forum to gain some knowledge of what these actually do.

Now that I have a clean output signal should I leave it alone or should I return D46 back inline?

 

[Handy Andy]

That approach is up to you.

I would test the Diode - even swap it out for another - use a 1N4148 PN type, not a Schottky - for it can clamp way too tightly and limit the modulation going the other way, as in hardly any.

But also too, the 520 may have a mod or two still in it - so look for them, if you had that type of screaming audio - I remember several Bases that used the 520D and many worked great, some were a bit - if not a tad - maybe an avalanche or two, above the modulation level they should be - but on those loud ones the user rode the mic gain and the cranked up Power Mike (D104 types usually) controls to handle their neighbors knocking on their door.

Its' entertaining to hear them say, "Oh I run a clean station" only about 2 minutes into their conversation you hear banging on their door in the background - so nothing new to see this radio turned up a bit more than it should be.

Usually when you turn down the wattage, the Audio goes up - more room to breathe so to speak. So if you have to turn the wattage down to clean up the audio - then the radio certainly needs some more work - it seems that they messed with more than just the power settings.

There may be several tweaks in there you'll have to dig out and replace with proper parts. Nothing big, but - remember this is a Labor of Love when it comes to making the choice to to keep it or get rid of it - because as you rebuild such a "Masterpiece" Robyns were USA-made and this is one of their last ones to make it off the assembly line.

Don't get frustrated, enjoy the journey, Rome wasn't built in a Day,