[R-390] R-390 VFO Question
Bob Camp
kb8tq at n1k.org
Wed Sep 18 10:03:00 EDT 2024
Hi
Using a impossible extreme case to keep the math easy:
If your PTO covers (say) 900KHz rather than 1000 KHz, there will be a 100KHz “gap” that you can not tune to.
Any significantly “too small” range would have the same impact. (Yes, there is a bit of extra travel at the ends of the range so this does not get totally insane ….).
Bob
> On Sep 18, 2024, at 9:54 AM, Barry <n4buq at knology.net> wrote:
>
> Would a significant difference in span between bands be due to the first crystal oscillator? Otherwise, I'm not sure how that would occur.
>
> Thanks,
> Barry - N4BUQ
>
>> Hi
>>
>> As long as the end points still line up so you have a 1000 KHz span with one
>> being at 2955, the radio should be in reasonable shape. Having a “gap” between
>> bands ( = a < 1000 KHz span) would be a PIA …. Fortunately that rarely is the
>> case.
>>
>> There are still piles of R-390A’s sitting here or there ….
>>
>> Bob
>>
>>> On Sep 18, 2024, at 9:39 AM, Barry <n4buq at knology.net> wrote:
>>>
>>> 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
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