[R-390] R-390/URR Antenna Trimmer Position

[Barry Scott]

Another list member, Don Heywood, let me know that, like the R-390A, the
shaft on the R-390 also has a red dot which is used to align the
capacitor.  It's also mentioned in the Pearls document.

With the dot pointing to the 3:00 position, the knob should point to the
12:00 position.  I just looked and, indeed, that dot is on mine (and
somehow, I had the knob pointing to almost exactly where it should be
pointing).

I still don't know where that's described in the manual but apparently it's
there somewhere.

Thanks,
Barry - N4BUQ


On Sun, Sep 22, 2024 at 9:50 AM Jacques Fortin <jacques.f at videotron.ca>
wrote:

> Hi Barry,
>
> Well... the problem will arise when you will try to align the Antenna
> circuit RF transformers...
> During that operation, the ANT TRIM position is specified to be a "0" (ANT.
> TRIM caps meshed halfway).
> I vaguely remember that -4 position relates to fully meshed ANT. TRIM caps
> and that I had "marked" the related shaft position when I worked on mine.
> But trying to determine the caps position when the whole RF deck is in
> place
> is... well.. not that easy.
>
> I have an idea, but it will require the use of a handheld capacitance
> meter,
> plus the removal of T201...
> The logic behind is that both sections of the ANT. TRIM cap are connected
> to
> T201 pin 5 when the MC control is at 00.
> With the T201 out of the way, a capacitance meter can be connected to the
> T201 socket between pins 4 and 5 and the measured capacitance value looked
> at when the shaft is rotated.
> There should be a maximum and a minimum capacitance value measured.
> When the value is maximum, the caps are fully meshed and that is the "-4"
> position of the control.
>
> I never tried to do it that way.
> So I do not know if that make sense overall... or not !
>
> 73, Jacques, VE2JFE in Montreal
>
>
> While I had the RF Deck out to work on the gear train, I rotated the
> Antenna
> Trimmer and left it at some unknown rotational position when I installed
> the
> RF Deck back into the cabinet.
> I'm now trying to determine the rotational position of the Antenna
> Trimmer's
> shaft and where that position should be located with respect to the
> markings
> on the front panel.
> I realize the trimmer can just be rotated to where the signal is best and
> the numbers may not really have much meaning but I'd think that some
> position (say, 12:00) should correspond to a given mesh value of the cap's
> plates (e.g. when the knob is at -4, the plates should be fully meshed,
> etc.).
> The only reference to that that I've found are the paragraphs that deal
> with
> the removal and installation of the front panel.  The manual states to
> position the knob at the -4 position before removing the front panel and
> then, when replacing the front panel, replace the knob (presumably at the
> -4 position).  That's fine but if the Antenna Trimmer is rotated while the
> front panel is removed, then it isn't clear to me how to replace the knob
> properly w.r.t. to the capacitor's plates.
> Anyone know?
> Thanks,
> Barry - N4BUQ
>
>