Total posts : 45366
This project is on my bench now waiting for me to get inspired to do some more.
The power in question is the DC power delivered to the final amplifier, Q3 with no modulation. I measured this by measuring the DC voltage from the drain to the source (ground) using a digital voltmeter on the DC setting. I measured the drain current by measuring the voltage across R34 and dividing by 10. The power is Vds x Id.
The current through Q3 is set primarily by the conditions on the gate and the effective way to modify the power is to change the DC voltage from drain to source. This can be done in two ways. Increase the resistance of R33 and R34 and/or lower the DC output of Q9 by means of R23. So your pot idea would seem to be OK providing the pot can handle the current but it will still require quite a high output voltage capability from the audio system which could clip the audio peaks.
I think a better approach is to change R21 so the voltage set by R23 will be lower than the present minimum (about 4.5 volts) at the R34 test point. This will give the audio amplifier system more headroom and should prevent distortion on the modulation peaks. This was evident when I lowered the test point voltage from 7.1 to 4.7 V.
Unfortunately, the power input and the audio performance are not independent and adjustment back and forth will probably be needed to find the best modulation performance along with the unmodulated input power being 100 mW.
The RF output power measurement was done using a digital oscilloscope which measures RMS voltage. The power out is Pout = Vload^2/Rload. One can also do this with the peak to peak voltage reading from a scope by converting the peak to peak to RMS. Don’t trust a digital meter for this since most of them do not respond to frequencies much above 10 or 20 kHz. One can build a simple diode rectifier capacitor filtered probe and use a DVM on DC to get relative voltage measurements however.
I will try to do more on this later. If I can get my unit to 100 mW with close to 100% undistorted modulation I will seriously consider putting it back on the air. I think the robustness of the output circuit to changing antenna load and the effective filtering of the RF output are real pluses. I have an Alesis Nanocompressor which I use for my FM audio chain and this could serve as well to control the audio into the AM-25 to achieve high average modulation without the danger of overmodulation.