[R-390] R-390 VFO Question

[Bob Camp]

Hi

Motorola most certainly made their own PTO’s for the radios they supplied. Long ago I talked to the folks who did the linearity adjustments on them. They still had (not so) fond memories of doing those adjustments. 

Bob

> On Sep 17, 2024, at 11:29 PM, Barry <n4buq at knology.net> wrote:
> 
> 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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