[R-390] USB & LSB Filters and BFO Fine Tuning

[Dallas Lankford]

For several years I have had some high impedance USB and LSB filters which
require 130 pF resonating capacitance.  These seemed like ideal USB and LSB
filters for the R-390A, except for the work involved in removing the
original filters and installing these, and these did not have flanges.
Flanges are essential, otherwise RF will leak around the filters.  Making
and fitting the flanges to the filters turned out to be easier than I
thought.  Details of this mod can be found in the FILES section of the YAHOO
R390 group.  The obvious advantage of this mod is that you can set the BFO
PITCH and forget it.

There are several reasons why an unmodified R-390A is not a great SSB
receiver.  Fortunately, most of these have been addressed in the past.  The
KCS tuning rate has not, mainly because there seems to be no way to add fine
tuning to the KCS tuning.  The BFO tuning rate has also not been addressed
generally, though there were a few R-390A's with BFO reduction tuning gizmos
with a counter dial.  I never used one, so I can't say whether it solved the
SSB tuning problem or not.  My BFO tuning mod uses voltage variable diodes,
and works very well.  The tuning range is 200 Hz (for 1 turn), which is
equivalent to a 30:1 reduction in the BFO PITCH.  You tune in a SSB signal
as well as you can with the KCS knob, and then clarify the signal with the
BFO fine tuning knob.  The knob is attacked to a pot which is installed in
the DIAL LOCK hole.  This mod can also be found in the FILES section of the
YAHOO R390 group.