[R-390] R-390 VFO Question

Barry n4buq at knology.net
Wed Sep 18 09:39:24 EDT 2024 

I set the PTO's frequency per the manual (i.e. set the VR counter to any 500.0 reading and adjust the PTO to 2955 kc.  The endpoint on the low side is different from the endpoint on the high side so I'm presuming I have just a bit of non-linearity; however, I think if I get the endpoints set to an even 1000 kc, then any non-linearity will hopefully be minimal.  I'm not all that concerned if the minor divisions aren't exact and will live with it as long it's not too much.  The trouble to tweak that may be much more effort than it's worth to me.

I wish I could've seen some of those "mountains"!  I've seen pictures of the stacks of blue-striper R-390A/URRs that sat out in the rain.  Sad.

Thanks,
Barry

> Hi
> 
> The gotcah is that the internal adjust coil may not have enough range to get the
> PTO back to where it needs to be. You may need to get a bit creative. Better to
> do this *before* any of the other work.
> 
> This is not a new issue. Back in the day, the military spent a lot of time and
> money swapping out PTO’s. It was cheaper / easier to do that than doing a full
> rebuild. There are stories of “small mountains” of PTO’s building up behind
> repair depots as a result. Like any story, the size of those mountains likely
> got bigger and bigger with each telling of the story :) :) :).
> 
> Bob
> 
>> On Sep 18, 2024, at 8:28 AM, Barry <n4buq at knology.net> wrote:
>> 
>> 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

More information about the R-390 mailing list