[R-390] R-390 VFO Question

[Barry]

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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