74HC04 Franklin Osc based AM Transmitter (Cool circuit!)

Hello again,
This is an interesting circuit. I have been playing with LW and MW AM transmitters.

My goal is to have about 6 AM transmitters, around 50mW, and modulate them from old retro-style MP3 players. The MP3 files will be "Radio Luxemburg", "The Light Program", the "BBC Home Service", VOA, Radio China and anything else I can find on the internet.

I have a small collection of vintage AM receivers, including a Vidor Lady Margaret. I have a Skantic 230V radio and a really well made DUX (Swedish transistor) radio, among others. It would be really cool to have vintage wireless sets that still receive vintage wireless programs?

Unfortunately this is a low priority for me as I have other things to do, when I have the time.

As regards the circuit, the only question is the cap/resistor 100pf/10K that are coupling the output of the driver to the 2N2222. I built an HF CW transmitter n´many years ago using the 7400. I used a series resistor and decoupled the base of the output transistor to ground through a capacitor. This was to cause the square-wave to become triangular, which is closer to a sinusoidal waveform. The 100p in your circuit may possibly pass leading and trailing edges of the square wave. Have you looked at the spectral purity on your analyser?

Other than this I think it is quite clever.

One small suggestion for AM, is that of using low-level modulation. If you modulate a driver stage to, say, 25%, and then feed that into a Class-C power amplifier, it will have the effect of increasing the modulation depth. This will eliminate the distortion problems when you try to modulate a power amplifier to 100%.

Of course, this method of modulation has it's dangers, in tht it is easy to over-modulate, and the PA stage must have a regulated Class-C bias to keep the "modulation multiplication" a constant. But it is just a suggestion.

Very best regards from Harry - SM0VPO