Ion Petroianu, VA3NOI
To calculate the frequency of a Clapp oscillator you should use the information found here:
Now, BitX is using a VFO that oscillates between, roughly 4.5 MHz to 5 MHz.
The C0 in the formula above is the combination of C96 in series with MV209 and those two in parallel with C93, C94 and C95.
The equivalent capacitance of a series of capacitors is calculated as follow:
C eq series = (C1 x C2)/(C1+C2)
The equivalent capacitance of a parallel group of capacitors is calculated as follow:
Ceq parallel = C1+C2+C3+C4
For our circuit C1=C93, C2=C94, C3=C95 and C4=(C96x MV209)/(C96+MV209)
The nominal value of the inductance is stated on the schematic: L4=9 uH, but with the capacitors as per schematic I am expecting more like a 6 uH inductance. I used that value for may calculations.
In my calculation, for the minimum VFO frequency, using the nominal values in the schematic, I ended up with a frequency range of 4.7MHz to 5 MHz, which looks consistent with a band coverage of 7 MHz to 7.3 MHz.
With all those in mind, I think the best option for you is to place a second varactor diode in parallel, add another NPO capacitor of 220 pF or 300 pF in parallel with C96 and verify the new frequency range of your VFO.
It calculates as 4.35 MHz to 4.85 MHz, or a band coverage of 7.65 MHz to 7.15 MHz for 220 pF.