[R-390] R-390 VFO Question

[Bob Camp]

Hi

*Any* L/C oscillator (VFO or whatever) goes down in frequency as you add
inductance to the tank. Indeed crystal oscillators work the same way. (add 
series inductance and the frequency goes down). 

First step on linearity is to get the end point span so it’s near right. That may
well involve a bit of electrical fiddling ahead of the mechanical stuff. Most of
these PTO coils have shrunk a bit over the decades ...

Bob

> On Sep 16, 2024, at 9:45 AM, Barry Scott <72volkswagon at gmail.com> wrote:
> 
> 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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