Total posts : 45366
You are digging farther into this than I did and that is good. What it means is that I have little to add yet am interested in your observations. My comments which follow are just meant as information as you ponder this.
One thought about the “half the supply voltage” comment in the manual. For amplifiers there are two load lines, the DC and AC load lines. These are usually different though they intersect at the Q point which is the 7.1 volt point in the manual. To get maximum undistorted AC output the Q point may have to be modified away from the intuitive value of 1/2 the supply.voltage in order to center it on the AC load line. Q8 and Q9 form a Darlington amplifier which serves both as a DC and an AC amplifier in that it supplies both the audio and DC power to Q3. If Q3 would not operate down to 0 volts then the minimum AC voltage would not be 0 volts but something higher. It could be the 7.1 volts accounts for this. The statement in the manual about 1/2 supply voltage doesn’t explain this.
On page 6 of the manual, at the top, it says “…a battery pack consisting of eight 1.5 volt cells, a 12 V sealed battery, or other external 12 V DC supply may be used.” So it would seem that 12 VDC is the intended supply voltage, yet all sealed batteries I have used operated at over 13 volts. Perhaps it is not that critical.
Another thing I find interesting about the design is that the bias voltage set by R23 comes from the power supply which would introduce any noise or hum from the power supply into the audio signal going through U1. Since low cost wall warts are usually not regulated and have ripple this seems to be a weakness.
I also noticed that with the bias voltage reduce to 4.7 V (the lowest I could adjust this) the modulation improved. Maybe operating on 6 volts supply not only improves the modulation but brings the unit into the 100 mW compliance as well. I have not tried this.
The RF filter after Q3 seems to be very effective in that I have never observed other than a good sine wave at the carrier frequency from this unit. The modulation envelope (using a sine wave 800 Hz audio test tone) does distort for higher modulation percentages. It would be useful to know if lowering the supply to 6 volts improves this.
Hope these comments help.