[R-390] R-390 VFO Question
Barry
n4buq at knology.net
Wed Sep 18 22:45:59 EDT 2024
I took the PTO out and rigged a setup where I can rotate the shaft exactly ten turns while monitoring the output frequency.
Thinking I needed less L, I tried rotating the endpoint adjustment 1/4 turn CCW. That decreased the overall frequency change by a small amount which was the wrong direction I needed. I then rotated it 1/2 turn CW (effectively 1/4 turn CW from the starting point) and that got me to about 998 kc on ten turns. Another 1/4 turn CW and I got exactly 1000 kc on ten turns.
I don't know what the linearity is yet but I can check that at 50 kc and 100 kc intervals while I still have it out.
Thankfully, it was quite easy to do this.
Thanks to all who replied.
Barry - N4BUQ
> Sorry. The first line of that last paragraph had kc where it should have been
> MC (or Mc or MHz or...):
>
> I don't how the counter now shows 499.7 for the VFO set at 2.9550 MC and,
> conversely, the VFO frequency at 500.0 is now at 2.9548 MC.
>
> Thanks,
> Barry - N4BUQ
>
>> Previously, I mentioned I have the endpoint data.
>>
>> VR Counter VFO Frequency (in MC)
>> ---------- ---------------------
>> +001.4 2.4550
>> 499.7 2.9550
>> -997.4 3.4550
>>
>> That makes the counter span for 1000 kc of the VFO to be 1000 + 1.4 + 2.6 = 1004
>> kc.
>>
>> Switching to zeroing the VR counter at both ends:
>>
>> VR Counter VFO Frequency (in MC)
>> ---------- ---------------------
>> +000.0 2.4566
>> 500.0 2.9548
>> 000.0 3.4524
>>
>> That makes the VFO span for 1000 kc on the counter to be 3452.4 kc - 2456.6 kc =
>> 995.8 kc so 4.2 kc short. Hopefully the endpoint adjustment still has that
>> much left in it and I won't need to open the can and perform any surgery.
>>
>> I don't how the counter now shows 499.7 for the VFO set at 2.9550 and,
>> conversely, the VFO frequency at 500.0 is now at 2.9548 kc. Something must've
>> moved just a tiny bit since I set it.
>>
>> Thanks,
>> Barry - N4BUQ
>>
>>> 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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>>>>>>>>>>>>>
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