[R-390] R-390 VFO Question

Larry Haney larry41gm2 at gmail.com
Wed Sep 18 14:39:46 EDT 2024 

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
> >> >>>>>>>>>>>>>
> >> ______________________________________________________________
> >> >>>>>>>>>>>>> R-390 mailing list
> >> >>>>>>>>>>>>> Home: http://mailman.qth.net/mailman/listinfo/r-390
> >> >>>>>>>>>>>>> Help: http://mailman.qth.net/mmfaq.htm
> >> >>>>>>>>>>>>> Post: mailto:R-390 at mailman.qth.net
> >> >>>>>>>>>>>>>
> >> >>>>>>>>>>>>> This list hosted by: http://www.qsl.net
> >> >>>>>>>>>>>>> Please help support this email list:
> >> http://www.qsl.net/donate.html
> >> >>>>>>>>>>>>>
> >> >>>>>>>>>>>>
> >> >>>>>>>>>>>
> ______________________________________________________________
> >> >>>>>>>>>>> R-390 mailing list
> >> >>>>>>>>>>> Home: http://mailman.qth.net/mailman/listinfo/r-390
> >> >>>>>>>>>>> Help: http://mailman.qth.net/mmfaq.htm
> >> >>>>>>>>>>> Post: mailto:R-390 at mailman.qth.net
> >> >>>>>>>>>>>
> >> >>>>>>>>>>> This list hosted by: http://www.qsl.net
> >> >>>>>>>>>>> Please help support this email list:
> >> http://www.qsl.net/donate.html
> >> >>>>>>>>>>
> ______________________________________________________________
> >> >>>>>>>>>> R-390 mailing list
> >> >>>>>>>>>> Home: http://mailman.qth.net/mailman/listinfo/r-390
> >> >>>>>>>>>> Help: http://mailman.qth.net/mmfaq.htm
> >> >>>>>>>>>> Post: mailto:R-390 at mailman.qth.net
> >> >>>>>>>>>>
> >> >>>>>>>>>> This list hosted by: http://www.qsl.net
> >> >>>>>>>>>> Please help support this email list:
> >> http://www.qsl.net/donate.html
> >> >>>>>>>>> ______________________________________________________________
> >> >>>>>>>>> R-390 mailing list
> >> >>>>>>>>> Home: http://mailman.qth.net/mailman/listinfo/r-390
> >> >>>>>>>>> Help: http://mailman.qth.net/mmfaq.htm
> >> >>>>>>>>> Post: mailto:R-390 at mailman.qth.net
> >> >>>>>>>>>
> >> >>>>>>>>> This list hosted by: http://www.qsl.net
> >> >> >>>>>>> Please help support this email list:
> >> http://www.qsl.net/donate.html
> >> > ______________________________________________________________
> >> > R-390 mailing list
> >> > Home: http://mailman.qth.net/mailman/listinfo/r-390
> >> > Help: http://mailman.qth.net/mmfaq.htm
> >> > Post: mailto:R-390 at mailman.qth.net
> >> >
> >> > This list hosted by: http://www.qsl.net
> >> > Please help support this email list: http://www.qsl.net/donate.html
> >> ______________________________________________________________
> >> R-390 mailing list
> >> Home: http://mailman.qth.net/mailman/listinfo/r-390
> >> Help: http://mailman.qth.net/mmfaq.htm
> >> Post: mailto:R-390 at mailman.qth.net
> >>
> >> This list hosted by: http://www.qsl.net
> >> Please help support this email list: http://www.qsl.net/donate.html
> >> ______________________________________________________________
> >> R-390 mailing list
> >> Home: http://mailman.qth.net/mailman/listinfo/r-390
> >> Help: http://mailman.qth.net/mmfaq.htm
> >> Post: mailto:R-390 at mailman.qth.net
> >>
> >> This list hosted by: http://www.qsl.net
> >> Please help support this email list: http://www.qsl.net/donate.html
> > ______________________________________________________________
> > R-390 mailing list
> > Home: http://mailman.qth.net/mailman/listinfo/r-390
> > Help: http://mailman.qth.net/mmfaq.htm
> > Post: mailto:R-390 at mailman.qth.net
> >
> > This list hosted by: http://www.qsl.net
> > Please help support this email list: http://www.qsl.net/donate.html
>

More information about the R-390 mailing list