First can someone explain to me how this FM transmitter circuit work? enter image description here

Second, after replacing Q2 with a blt81, I got a Frequency drifting problem. Once it's running for a while, I need to re-tune my radio. The transistor is not hot none warm to touch, and the power supply is 6v regulated, and it's tuned into around 88mhz.

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    \$\begingroup\$ Your oscillator is running open-loop, so it will naturally drift over time. Modern designs typically use a phase locked loop frequency synthesizer to avoid this. \$\endgroup\$ – Peter Nov 2 '15 at 6:29
  • \$\begingroup\$ This circuit is one step above the single-transistor oscillator in the foodchain of simple FM transmitters. next step: llok for one that uses a crystal + varactor dioded in the oscillator (and, as your current circuit, a buffer stage after the oscillator). \$\endgroup\$ – Wouter van Ooijen Nov 2 '15 at 6:36
  • \$\begingroup\$ I see weaknesses in that design, where did it come from. I ask because I don't see any benefit in trying to explain how something works when the designer appears to be an amateur. \$\endgroup\$ – Andy aka Nov 2 '15 at 8:45
  • \$\begingroup\$ Actually this circuit works very fine. I have been listening musics from it for hours. It only became problematic when I change Q2. I changed Q2 because I want to improve the range of this radio, then make a pcb out of it. The circuit really works very good. This kit came from China, so I guess someone is China designed it. \$\endgroup\$ – Atmega 328 Nov 2 '15 at 10:20
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    \$\begingroup\$ Is Q2 running warmer than the original and heating the oscillator? That would definitely cause drift. \$\endgroup\$ – Brian Drummond Nov 2 '15 at 12:44

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