[R-390] R-390 VFO Question
Barry
n4buq at knology.net
Wed Sep 18 13:52:54 EDT 2024
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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