[R-390] R-390 VFO Question

[Barry Scott]

Hi Larry,

I remember seeing a lot of documentation for the repair and "linearization"
of the R-390A PTOs from years ago.  It's a good idea to go back and review
those.  The R-390's PTO has slight differences but I think the mechanical
aspects are probably very similar to the R-390A's PTO.

Other than a cursory test, I haven't done much to check the linearity but
will probably get into that once I get the radio fully back together (which
shouldn't be long now).

Thanks again,
Barry - N4BUQ


On Mon, Sep 16, 2024 at 8:37 AM Larry Haney <larry41gm2 at gmail.com> wrote:

> 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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