[R-390] R-390 RF Gain Control

[Jacques Fortin]

Hello Kevin,

I believe that your RF gain control have been replaced and that the wrong
taper type was used.
The RF gain original control R123 in a R-390 is a 5K end to end with a
REVERSE-LOG taper, made by Allen-Bradley.
With the knob set at the center of it's travel, you should measure about 500
ohms between the center contact and the CW side one (seen from the shaft
side) and about 4500 ohms from the center contact to the CCW one.
If ever the two measured values are about equal, (± 2.5K) what you have is a
LINEAR taper control and only that suffice to explain why the "control
range" of your RF gain is so small.
I have also encountered another defect related to the R-390/390A controls:
due to their constant use by some operators, the resistive track was
completely worn out.
I still have some 5K RF gain controls measuring in excess of 12K end to end
and AF gain controls in excess of 6K ohms (they have to be 2.5K. LOG taper,
btw)

The problem now is to find replacement parts... I tried very hard to find
new ones, but the only source I found is from Fair Radio Sales selling parts
of "cannibalized" R-390As.
Even there, be sure that they measure properly before using them.

73, Jacques, VE2JFE

-----Message d'origine-----
De : r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> De la
part de Kevin Schuchmann via R-390
Envoyé : 10 septembre 2021 01:58
À : Collins R-390 HF Receiver List <r-390 at mailman.qth.net>
Objet : [R-390] R-390 RF Gain Control

Hi All,
Having only one R-390 around here I have nothing to compare with so I'm
curious how your R-390's compare.

My RF gain control works but pretty much even the strongest signal is gone
by "9" in AGC mode and by "8" in MGC mode.

Is this about normal or should I go looking for a problem?

Thanks
Kevin wa7fwf
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