Since the trend is a reduced frequency range, something has obviously increased
in value internal to the PTO. There are two inductors and three capacitors in
the circuit. The inductors have two ferrite cores so there are seven
components in total that determine frequency. I have the feeling that the
ferrite permeability has increased over time perhaps due to shrinkage. I don't
see how either coil can have increased in inductance without causing binding
with their respective ferrite cores. Of course it is still possible that any
or all of of the tank capacitors could have increased in value.
I assume that the two 10 pF capacitors have either a positive or negative
temperature coefficient of different slopes to compensate for the operating
temperature of the PTO. Lifting one of the two 10pF caps and placing, for
example, a 100 pF NPO in series with it will subtract 1 pF from the total tank
capacitance and raise the operating frequency without significantly changing
the temco. This should preserve the end point coil inductance range. Has
anyone tried this instead of removing a turn on the endpoint coil?
Jim
Logic: Method used to arrive at the wrong conclusion, with confidence. Murphy
On Wednesday, September 18, 2024 at 10:04:39 AM CDT, Barry
<[email protected]> wrote:
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 <[email protected]> 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 <[email protected]> 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 <[email protected]> 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 <[email protected]> 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 <[email protected]>
>>>>>>>>>> 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 <[email protected]>
>>>>>>>>>>> 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
>>>>>>>>>>>> <[email protected]>
>>>>>>>>>>>> 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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