[R-390] R-390 RF Gain Control

[Kevin Schuchmann]

Jacques,
  Well you solved the problem on the first try. It is a allen-bradley 5K pot and it is clear that it has been replaced at some time and it is linear.

So the hunt is on now for the correct part, I will check with Fair Radio Sales.

Thanks
73 Kevin wa7fwf

‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐

On Friday, September 10th, 2021 at 8:17 AM, Jacques Fortin <jacques.f at videotron.ca> wrote:

> 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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