Low frequency in this VCO design
You built it well and measured the frequencies accurately. The value of C1 was originally probably 1nF. The signal is a bit asymmetric because the 57k resistors were originally probably 51k. 57k is not a standard value.


I suggest adding 4 more resistors to make the amplitude of the rectangle and the triangle the same.

With C1=1nF, the frequency can be set in the range of 200Hz ... 5kHz.