[R-390] R-390 VFO Question
Jim Whartenby
old_radio at aol.com
Tue Sep 17 14:24:23 EDT 2024
It appears that mounting wise, there is no physical difference between the two PTOs. Electrically, there is the screen grid of the 70H12 which is separately regulated while the 70H2 is powered by a regulated B+ supply but other then this, B+ appears to be the same. The output of the tanks is different but I am not sure it that matters since both have similar output impedances and voltage ratings.
I am not sure if the formula for the ferrite core remained the same. Perhaps there were lessons learned from the earlier builds that influenced the ferrite core formula? I am under the impression that later build R-390A/URR receivers had less of a problem with frequency effects due to aging.JimLogic: Method used to arrive at the wrong conclusion, with confidence. Murphy
On Monday, September 16, 2024 at 08:45:41 AM CDT, Barry Scott <72volkswagon at gmail.com> wrote:
Hi Larry,
I remember seeing a lot of documentation for the repair and "linearization"
of the R-390A PTOs from years ago. It's a good idea to go back and review
those. The R-390's PTO has slight differences but I think the mechanical
aspects are probably very similar to the R-390A's PTO.
Other than a cursory test, I haven't done much to check the linearity but
will probably get into that once I get the radio fully back together (which
shouldn't be long now).
Thanks again,
Barry - N4BUQ
On Mon, Sep 16, 2024 at 8:37 AM Larry Haney <larry41gm2 at gmail.com> wrote:
> 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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