[R-390] 4 kc filter

[Dallas Lankford]

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> Has anyone tried replacing a single mechanical filter with a readily
> available crystal filter, preferably one that does not cost an arm and a
> leg?

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Many crystal filters are 2000 ohms source and load, so you would have to
impedance match, probably with two mini IF transformers like I did.
Depending on the size of the crystal filter, there might not be enough space
to do a nice job.  Just for fun I put a NTK 3 kHz BW ceramic filter (no
longer available) in one of my IF decks.  I impedance matched with two
Mouser 43IF303 20K:5K ohms, 6:1 turns.  Mouser doesn't carry this part any
more, but the 42IF103 or 42IF303 should work.  I didn't expect them to do a
good job of impedance matching, but it worked fine.  For my first try I
mounted a PC board with filter and IF transformers in the hole where the 16
kHz mechanical filter had been.  But filter leakage made the filter
ineffective.  With two PC boards soldered to a copper flange, and a
rectangular hole cut in the flange which flush fit the filter, and the
flange bolted to the R-390A IF chassis, like an N type filter, filter
leakage was greatly reduced.  If I did it again, I would use a 3.3 kHz BW
torsion filter with a metal case (occasionally seen on Ebay) to eliminate
residual leakage.  For 4 kHz , I believe Sherwood Engineering still sells
the 15 resonator Murata CFS-455J ceramic filter, typically 4.3 kHz @ -6 dB,
7.0 kHz @ -60 dB.  I don't know of a 4 kHz BW crystal filter.

Dallas