[R-390] R-390 VFO Question
Larry Haney
larry41gm2 at gmail.com
Wed Sep 18 12:53:13 EDT 2024
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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