2SC1970 makes FM transmitter

This FM transmitter is made of unused disassembled parts. It can be used to transmit music or voice signals by frequency modulation, and can be received by an ordinary FM radio at a distance of 2km.

The circuit of the whole machine is shown in the attached figure. The external audio signal is amplified by the transistor Tr1 and then loaded on the diode D1. Tr2, the crystal oscillator, the coil T1, etc. form a 25MHz high-frequency oscillator. The modulation of the audio signal does not use a variable capacitance diode. Instead, it uses a common rectifier diode V06G instead. V06G and the 25MHz crystal oscillator are both removed from an old computer. The 25MHz frequency modulation signal is coupled to the lower stage through the coil T2.

In order to obtain 100MHz FM signal output, Tr3, Tr4, coil T3 and other components form a frequency multiplication stage. A 50MHz signal is obtained and coupled to the next stage via coil T4.

T5, T6, coil L5, etc. multiply the input signal again to obtain a 100MHz FM signal and couple it to the excitation stage through T6.

The high-frequency power amplifier uses a high-frequency amplifier tube 2SC1970. Its parameters are 40V, 0.6A, power 1.3W, frequency 175MHz. The collector current of this stage is 0.15A.

The high frequency output signal can be connected to the antenna for external transmission. The measured output power is 0.25W on a 50 load resistor. It can be stably received at a straight line distance of 2.5km.

(Editor's note: Please comply with relevant national laws and regulations when using radio transmitters)

Note: You need to merge the two pictures to get a complete circuit diagram