[R-390] R-390 RF Gain Control

[Jacques Fortin]

Hello to all,

Despite that it can be done to modify the TRANSFER FUNCTION (Vout vs Vin) of
a linear potentiometer with the use of a fixed value resistor, nothing can
be done to change the behavior of a linear RHEOSTAT (variable resistor with
2 connections) by using any arrangement of fixed-value resistors connected
to it.

The fundamental reason why a REVERSE-LOG rheostat is needed in a R-390/390A
is the fact that the RF gain control is connected in the ground return line
of the RF and IF stages, varying their bias to the control the stages gain,
and in AGC mode this runs in parallel with the AGC within the receiver.
Because that the relationship between the RF-IF stages gain and their common
bias line is non-linear, this is why a REVERSE-LOG control is needed.
This is the same in many other receivers: the HRO series are another
exemple.
In MGC mode, the AGC line is grounded (or fixed in voltage in some
receivers) and the RF gain do it's job alone.

Why this "compound" gain control scheme was chosen for the R-390/390A have
to do with the SQUELCH function that is present in the R-390 (and can be
"activated" in the R-390A) for which the RF gain act as the SQUELCH TRESHOLD
control.
Some other receiver architectures uses a LINEAR potentiometer and a negative
DC source to take control of the AGC line in "manual gain" mode.

I have already suggested that if no REVERSE-LOG, 2W rating  potentiometer
can be found, (one day, the Fair Radio stock will be exhausted..) that a 5K
ohms, 2W  multi-turn can be used.
Another trick (please don't kill the messenger) is to use a 5K ohms
LOGARITHMIC potentiometer but, in this case, the RF gain will work in
reverse from the original design intent, meaning maximum gain a Zero,
minimum gain at "10".
Search for KA5021S28-ND on Digi-Key for ex.: 106 parts available vs zero for
the KB5021S28-ND, which is the "true" replacement.
However, in both cases, the control range will be wider than the one
obtainable with a one-turn LINEAR potentiometer.

73, Jacques, VE2JFE