Please do not expect to learn anything as I do not know.
It has to be written for myself.
This kit from the amazing Dutch Jostykit company is usually used for its FM coding capability.
But it seems the real gold is the quality of the FM oscillator.
It is just one transistor. It is p n p and mabe p n p versions of a complimentary transistor may have 1/4 of the bandwidth if checked on datasheet. This might be a deliberate choice by the designers so that even if harmonics are created they are diminished?
There is a nice varicap (variable capacitance diode) implementing the frequency modulation.
Very neat circuit.
The schematic can be found on this nice webpage:
https://www.circuitsonline.net/forum/view/106114/last
All it should need is preemphasis for flat frequency response by a series resistor in parallel with a small capacitor connected at audio input. Values should be chosen according to continent standards, 50μs, or 75μs.
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