Dear Mr Allison many thanks for the detailed info. The skert or shape of 12 MHz filter is too great as you mentioned. Thus mooving BFO higher will give you not only carrier?
but some ather side band products. What I did, having spectrum analiser- did some culculations for 4 crystal Chebyshev 1 dB filter and step by step with 8 crystals moved with vary cap 10/150 pF from central to the end trying to get good responce. Result: 2607 Hz filter/-6/-3.17 losses/not more 1 dB fluctuations in band. Shape is a little better 1.6? = 2607/4000 Hz? -6/-60.
By the way 4-pole 45MHz SMD filter is much better the original one. Transformes for this filter - 43-2402 binocular with 3t/10t. 12 kHz/-6 dB/2 dB losses.
Best regards George RX3ARG/UR4CRG.?
George