[R-390] R-390 VFO Question

Wed Sep 18 10:13:48 EDT 2024

Hi  Barry,  Since the 390 PTO is mechanically the same as the 390A PTO made
by Collins (has the corrector stack on the inside), the document I wrote on
the Collins 390A PTO should be applicable to the 390 PTO.  Here's a link to
it in the tutorials section: R390A My Collins PTO Observations
<https://www.r-390a.net/R390A_My_Collins_PTO_Observations.pdf>.  I think
Collins was the only manufacturer of the 390 PTO.  FYI - removing a turn
off of the EP adjustment coil, L701 reduces the EP adjustment range.  This
can be a problem in the future when readjustment is necessary.

Changing the ratio of L to C can put a linearity bubble in the middle of
the range.  Small changes should not be an issue.  Also, removing the
covers off of the PTO changes the operating frequency quite a bit, but is
restored when replaced.  And, adjusting  the 'stack' is quite challenging.
Unless something specific has happened to it, its current state should be
OK.

Working on the internals of these can be frustrating and time consuming -
be prepared.

Regards, Larry

On Wed, Sep 18, 2024 at 4:58 AM Barry <n4buq at knology.net> wrote:

> That first line should have been:
>
> Regarding the tutorials, I'm wondering which, if any, might apply to the
> PTO in the R-390/URR.
>
> Thanks,
> Barry - N4BUQ
>
> > Hi Larry,
> >
> > Regarding the tutorials, I'm wondering which might apply to the PTO in
> the
> > R-390A/URR.  I see most(all?) of them are regarding the COSMOS units but
> am
> > wondering which PTO is in the R-390/URR and whether any of the
> > rebuild/linearity documents apply to that one.  Were all the R-390/URR
> PTOs
> > made by Collins and no COSMOS in that version?
> >
> > Mine is running just a bit fat - maybe 1.5 kc end-to-end.  I have the
> actual
> > numbers written down and can post that when I get back to the
> workbench.  From
> > what I remember, removing a turn shortens the end-to-end but perhaps
> additional
> > C would work as well.  I don't know what effect that might have on
> linearity
> > but I don't think it should.
> >
> > The R-390A/URR PTO has an endpoint adjustment.  Does this exist for the
> PTO in
> > an R-390/URR?
> >
> > Thanks,
> > Barry - N4BUQ
> >
> >> Hi Barry,  I thought what you wrote makes sense and is correct.  It's
> good
> >> that you are understanding how it all works.  It makes diagnosys so much
> >> easier.  Good going.
> >>
> >> If you're interested in more reading on the 390A PTO's, there are 3
> docs on
> >> our website in the repair tutorials section by Tom Marcotte, Jim Miller
> and
> >> myself.
> >>
> >> Regards, Larry
> >>
> >> On Mon, Sep 16, 2024 at 5:51 AM Barry Scott <72volkswagon at gmail.com>
> wrote:
> >>
> >>> Okay, Larry.  Thanks for the reply.
> >>>
> >>> On further reading, the VFO is a Hartley design and, given that the
> >>> frequency formula for a Hartley is an inverse function of the LC
> values,
> >>> presuming that at xx 000, the iron core is "out" of the coil and a
> >>> clockwise turn of the KC knob causes the core to be pushed further
> inside
> >>> the coil increasing the L, then the frequency would indeed drop with CW
> >>> motion of the knob.  Sorry for the awkward way of stating that but I
> think
> >>> it makes sense to me.
> >>>
> >>> Thanks again,
> >>> Barry - N4BUQ
> >>> Thanks,
> >>> Barry - N4BUQ
> >>>
> >>>
> >>> On Sun, Sep 15, 2024 at 11:10 PM Larry Haney <larry41gm2 at gmail.com>
> wrote:
> >>>
> >>> > Hi Barry,  You are absolutely correct in your deductions.  When the
> KC is
> >>> > at its lowest of 000, the vfo is at its highest (3.455 mh).
> >>> >
> >>> > Regards, Larry
> >>> >
> >>> > On Sun, Sep 15, 2024 at 7:23 PM Barry Scott <72volkswagon at gmail.com>
> >>> > wrote:
> >>> >
> >>> >> Looking at the simplified schematic for the 3rd mixer (V205), the
> output
> >>> >> from the 2nd mixer (V204) tunes from 3 to 2 MC and the VFO tunes
> from
> >>> >> 3.455
> >>> >> MC to 2.455 MC yielding a constant 455 kc mixer product.  Is it
> correct
> >>> to
> >>> >> interpret those numbers to mean that if the counter starts at XX
> 000
> >>> and
> >>> >> the KC control is rotated CW to XX +000 the VFO's output frequency
> >>> starts
> >>> >> at 3.455kc and falls 1000 kc for 10 turns CW on the KC knob?
> >>> >>
> >>> >> I'm asking because I want to know what the frequency of the VFO is
> >>> >> supposed
> >>> >> to be when the dial is at XX  000 and I presume it's 3.455 kc but
> >>> wanting
> >>> >> to make sure.
> >>> >>
> >>> >> Thanks,
> >>> >> Barry - N4BUQ
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