[R-390] R-390 RF Gain Control

[Jacques Fortin]

Steve, In the case of a volume control, I agree that this can be done.
That will work only if the amplifier stage preceding the volume control is strong enough to drive the "mocked logarithmic potentiometer".
Say, if you have a 500k ohms linear pot with a 50k resistor connected between the slider and the CCW end, the Vo vs Vin function will be logarithmic, or at least close to: ~10% output at mid-travel, 100% output at full CW position.
But, in the case of the R-390/390A RF gain control, we need a RESISTOR that behave as having 500 ohms when the control shaft is at mid-travel and 5000 ohms when it is at the CCW position.
I know that it can be hard to figure, but there is no free lunch here.

Some other solutions can be used: 
1- A multiturn LINEAR 5000 ohms potentiometer.
    Better behavior than a 300 deg. Linear potentiometer, but it's a multiturn, so...
2- A 10 or 12 position switch with logarithmically scaled resistors: this has been used in some WWII receivers, like the Canadian Marconi CSR-5A.
    But there is no intermediate "gain" position between the steps...
Replacing a REVERSE-LOG potentiometer with any given arrangement of a linear potentiometer combined with fixed values resistors ?
I do not want to hurt anyone, but I do not believe that this is possible.

73, Jacques, VE2JFE

Good call - I have done this in repairing an automobile radio from the sixties.  It works.
- Steve  W7SJT