[R-390] R-390 VFO Question

Wed Sep 18 08:28:48 EDT 2024

Hi Bob,

I see where the PTOs for the R-390/URR and R-390A/URR both have end point adjustments.  L701 performs that in both.  The schematic I see for the R-390A/URR conveniently labels that as such on the schematic.  RM 11-5820-357-35 calls it out on page 12, Paragraph 12-b defines the function of that coil.  I just wasn't seeing that last night.  I sure wish I had a searchable PDF for the R-390/URR's service manual.

Thanks,
Barry - N4BUQ

> 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
>>>>>> ______________________________________________________________
>>>>>> R-390 mailing list
>>>>>> Home: http://mailman.qth.net/mailman/listinfo/r-390
>>>>>> Help: http://mailman.qth.net/mmfaq.htm
>>>>>> Post: mailto:R-390 at mailman.qth.net
>>>>>> 
>>>>>> This list hosted by: http://www.qsl.net
>>>>>> Please help support this email list: http://www.qsl.net/donate.html
>>>>>> 
>>>>> 
>>>> ______________________________________________________________
>>>> R-390 mailing list
>>>> Home: http://mailman.qth.net/mailman/listinfo/r-390
>>>> Help: http://mailman.qth.net/mmfaq.htm
>>>> Post: mailto:R-390 at mailman.qth.net
>>>> 
>>>> This list hosted by: http://www.qsl.net
>>>> Please help support this email list: http://www.qsl.net/donate.html
>>> ______________________________________________________________
>>> R-390 mailing list
>>> Home: http://mailman.qth.net/mailman/listinfo/r-390
>>> Help: http://mailman.qth.net/mmfaq.htm
>>> Post: mailto:R-390 at mailman.qth.net
>>> 
>>> This list hosted by: http://www.qsl.net
>>> Please help support this email list: http://www.qsl.net/donate.html
>> ______________________________________________________________
>> R-390 mailing list
>> Home: http://mailman.qth.net/mailman/listinfo/r-390
>> Help: http://mailman.qth.net/mmfaq.htm
>> Post: mailto:R-390 at mailman.qth.net
>> 
>> This list hosted by: http://www.qsl.net
> > Please help support this email list: http://www.qsl.net/donate.html