unit_399:
The AM Limiter affects the positive peaks before it affects the negative peaks. Pull it out and positive peak will go over 100% before the negative peak does. I suggested - if you read the whole thread - you will find that I said pretty much what you did, that to affect the negative peak it should be done in the AM Regulator circuit and not the audio circuit.
I also did say that audio compression - speaking in general terms - is not correct because it acts more like a limiter (which by definition 'limiting' uses a 4:1 ratio with a fixed 'threshold') by the way the radio's circuit was designed for. It isn't real audio compression in the sense that it can be tailored to soft/hard knee compression, variable ratios of compression, variable threshold, or with variable attack/decay times as a real compressor can do. Radios use a fixed limiter setting by design and effect. Cheap; but not the same quality as real outboard audio processing would work.
So some compression is added to the negative peaks by way of installing a diode in the AM Reg. Xit13 asked the forum how he did it at first, and I guessed it right. I mentioned an electrolytic cap in the audio circuit because most people do it that way. Xit13 also changed the resistor value (R228) in parallel with the diode in the AM Reg, probably why he went to 180% instead of 140% that he was getting before with the cap mod on the audio circuit alone. The standard NPC mod for this radio - as done in the popular sense - is to clip the AM Limiter out, put a cap on the audio circuit, and volt the final.
At least, that is what I can can gather from it all . . .
*EDIT*
Xit13 might also consider putting a 1.5k ohm resistor on the center lead of the AM Limiter, which should reduce the effect of the limiting action in circuit. From what I understand, it should raise the AM Limiter's (Q24?) 'threshold' point before limiting can occur. But since he is already seeing 180% modulation, I don't see why he would need to. I'd be interested in seeing if he could get 200% if it is possible; but he has done plenty already on this forum and his work is much appreciated. Fair question - though . . .
