[R-390] R-390 VFO Question

Barry n4buq at knology.net
Wed Sep 18 14:51:27 EDT 2024 

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 < [ mailto:n4buq at knology.net |
> 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 <
>> > [ mailto:r-390 at mailman.qth.net | 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 <
>> >> [ mailto:n4buq at knology.net | 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 < [ mailto:n4buq at knology.net |
>> >> >>> 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 < [ mailto:n4buq at knology.net |
>> >> >>>>> 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 < [ mailto:n4buq at knology.net |
>> >> >>>>>>> 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 < [ mailto:n4buq at knology.net |
>> >> >>>>>>>>> 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 <
>> >> [ mailto:72volkswagon at gmail.com | 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 <
>> >> [ mailto:larry41gm2 at gmail.com | 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 <
>> >> [ mailto:72volkswagon at gmail.com | 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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