[R-390] R-390 VFO Question
Barry
n4buq at knology.net
Wed Sep 18 10:59:49 EDT 2024
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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