fine tuning capacitor VC3 is in parallel with the main kHz tuning
capacitor VC2. By adding a capacitor CX in series with VC3 it
reduces the range of the fine tune control, see Table.
can be seen from the table the effective fine tuning range changes
from the low to high end of the main kHz dial. If you fit a 10pF
capacitor the fine tuning range is approximately halved. If you use
a capacitor value less than 10pF - e.g. 3.3pF - you may experience
too little range at the top end of the main kHz dial.
The type of capacitor is also important as its stability will
influence the overall performance of the receiver. Two types of
capacitor offer good stability, Silver Mica and NPO
(Negative-Positive-Zero) Ceramic. Silver Mica capacitors offer
excellent temperature and long term ageing stability. However, they
are expensive and becoming difficult to obtain. NPO ceramic
capacitors are very stable over wide variations in temperature and
time. They are relatively inexpensive and are available from the
majority of electronic component suppliers.