[R-390] R-390 VFO Question

[Barry]

Valid point about the frequency change with increased inductance.  I just happened to look at the Hartley and used that as an example but it certainly applies to LC circuits in general.

Thanks,
Barry - N4BUQ

> 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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>>>>> 
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