[R-390] R-390 VFO Question

Larry Haney larry41gm2 at gmail.com
Thu Sep 19 01:20:12 EDT 2024 

OK Barry,  That is certainly the right approach to take.  I thought you
were already planning on what to do if the screw driver adjustment was not
enough.  I hope it is.

Regards, Larry

On Wed, Sep 18, 2024 at 11:51 AM Barry <n4buq at knology.net> wrote:

> Hi Larry,
>
> I'm sorry but I'm still a little confused.  My intent of adjusting L701
> would be by using a screwdriver on the endpoint adjusting core.  If that
> doesn't give me enough range, then I presume I'd need to start modifying
> the C or L values but would start with just the core adjustment.
>
> Thanks again,
> Barry - N4BUQ
>
> Hi Barry,  I believe that the total range of adjustment of L701 is about
> 7.5 kc (5 turns of wire).  If you needed to only adjust it 1-2.5 kc to get
> it back into range, I'd probably go the route of removing 1-2 turns, but at
> about 1.5 kc per turn, you'd need to remove 3 turns and that makes the
> adjusting range quite small after that.  It's doable, but a little
> restrictive.
> Regards, Larry
>
> On Wed, Sep 18, 2024 at 10:52 AM Barry <n4buq at knology.net> wrote:
>
>> Hi Larry,
>>
>> I'm curious why changing out a cap would be the first line of effort.  Is
>> that because there may not be enough change available from L701 to correct
>> for 4 kc?
>>
>> Thanks,
>> Barry - N4BUQ
>>
>> > Hi Jim, Yes, I've done that and it works quite well.
>> >
>> > Barry,  Since the EP is about 4kc off, I'd suggest using the capacitor
>> > adjustment method because removing 1 turn off of the L701 is not going
>> to
>> > be enough.  Removing 2 turns really limits its adjustment range.  Be
>> very
>> > careful with those 10 pf caps inside the pto, they are very fragile.
>> You
>> > should be able to calculate fairly close as to the value to put in
>> series
>> > with a 10 pf, and give it a little wiggle room for adjustment.
>> >
>> > Regards, Larry
>> >
>> > On Wed, Sep 18, 2024 at 9:07 AM Jim Whartenby via R-390 <
>> > r-390 at mailman.qth.net> wrote:
>> >
>> >> 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 <
>> >> n4buq at knology.net> 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 <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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