In the previous Post, I described that I have sporadic overtones in the Fork measurement which slightly shifts the base frequency.

The main Feedback is this (taken from the Inspiration's Project):
There, R4 and R5 together with C2 and C3 form a Bandpass filter. Mainly I suspected the large C3 (High-Pass) to be too big.

So I used my "trusty" DS203 oscilloscope and attached Probes to ForkOut (Yellow) and Pin 3 of the Tuning Fork Inductor (Blue). It is labelled 47^3 because of the 47 nF Capacitor at C3.

Notice the sharp edges  at the low and high spots of the Yellow trace, as soon as the blue trace (the driving coil) either Turns on or the first step-off. (I don't know why it does the first step down. Well, I am not an Electrical Engineer...) 

Then I tried different Values for C3. From Top to bottom: 47^3, (10+22)^3, 10^3, and 68^2.

You can notice the timing of the sharp edges being later with a smaller value, and also the drive of the Fork is more smooth with a lower value. Probably the Transistor was overloaded, but I don't know (Not an EE...)

The Final Fork signal, after choosing a single 10nF Capacitor for C3 and moving the Fork until the trace looks optimal, I am much happier with the ForkOut signal, because it looks much more like an actual sine wave:

Well, now off to a new test run for logging :)