The BFO is a crystal oscillator. The oscillator, connected to pins 6 & 7, is changed in frequency with inductance and capacitance plus or minus 500-1500 Hz. The output of the product detector is an audio frequency signal.
The BFO circuit is designed to work with fundamental mode crystals up to 20 MHz. The "BFO ADJ" will be more critical as you raise the frequency above 10 MHz and tune broader and broader as you go down from 10 MHz.
C27 and C28 need to be certain values in order to give the best output for the higher frequency crystals. The formula for C27 is 100/F. The formula for C28 is 1000/F. The capacitor values are in pf and the frequency (F) is in MHz. If a non-standard
value results from the formula, use the next highest standard value capacitor.
To fine tune the BFO, changing the value of C28 and RFC2 will give the best results. Move the value of C28 10pf at a time for lower values (10 to 100pf) and 50pf at a time for higher values (100 to 500pf).
For low frequency crystals, RFC2 may be replaced with a wire and the capacitor used alone to pull the BFO crystal.
High frequency crystals (above 9 MHz) will usually always require an RF choke. Change the value of the choke 15 to 20uH at a time to fine tune its value.
The rule is a capacitor will lower the crystal resonant frequency and an inductor will raise the resonant frequency. Modular RF chokes will work fine as will inductors wound on T37-2's or T37-6's.