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

[Jacques Fortin]

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