[R-390] Rf gain control

[Jacques Fortin]

A "thump" in the RF gain when turning the pot may indicate a worn-out track
inside.
I have one such misbehaving one, which measures 10k ohms end-to-end instead
of the stamped 5K value of the housing.
When opened, the damaged carbon track can be seen.
If anyone search for replacement, remember that a reverse-log taper is
needed there (not easy to find).
Same comment for the audio pots: if ever they measures higher than 3k ohms
end to end, suspect a worn-out track inside also.
These pots are also a part of the biasing network for V601B, so detraction
from the design value change the V601B operating condition.
And they have to be log (audio) taper if any replacement is contemplated.

73, Jacques, VE2JFE