[R-390] R-390 VFO Question

Barry n4buq at knology.net
Wed Sep 18 11:04:15 EDT 2024


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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