Transistor tend to be fairly sensitive to heat. And most good oscillator
circuits have large capacitors in shunt with the transistor so that
changes in the transistor have minimal effect on the frequency. I think,
I'd first change the transistor going to a VHF type that has a much
smaller geometry in the chip, and thus a much smaller capacitance to
change. Probably a 2N5179 or 2N2857 or 2N918. There are PN5179, and
PN918 in plastic packages. It could be that the 2N4124 being a general
purpose transistor that the maker has substituted a larger geometry chip
with lots of gain for the original the circuit was developed with.
You could try to compensate for the transistor by make some of the N
type capacitors be thermally coupled to the transistor, e.g. stick a N
type capacitor TO the transistor case with Q-dope or duco cement. I
think going to a low C transistor is far easier and should be adequately
effective.
73, Jerry, K0CQ
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