[R-390] R-390 PTO Warmup Time

[Larry Haney]

Hi All, As you can imagine, there are many components that affect the
frequency stability of our 390s. Once all the oscillator support functions
(1. incorrect regulation of the +150 volt regulated supply, 2. bad tubes,
3. bad resistors, 4. bad capacitors, 5. VFO shaft grounding contact
failure, 6. bad crystal, 7. bad oven, etc.)  are working correctly, then
it's down to temperature change.  Some  years ago I did some tests and if
you leave your 390 off overnight at about 72 degrees, it will normally take
about 3.5 hours to stabilize for good cw and ssb reception, but this can be
considerably reduced.

I wrote a small doc on this subject a few years ago.  It's on our website:
R390A Ovens and Freq Stability.pdf.
https://www.r-390a.net/R390A%20Ovens%20and%20Freq%20Stability.pdf

Regards, Larry


On Thu, Jul 30, 2026 at 11:54 AM Barry via R-390 <r-390 at mailman.qth.net>
wrote:

> That makes sense.  It probably takes longer to stabilize out of the radio
> than in.  The tube by itself isn't exactly poised to be the best heat
> source for the inside components.
>
> Thanks!
> Barry - N4BUQ
>
> > Hi
> >
> > The tube has relatively little impact on the warmup process (other than
> > supplying heat, just like the rest of the radio ….). The big deal is
> typically
> > the coil. That comes both from the coil core and from the coil form /
> windings.
> >
> > I’ve run R-390’s for weeks / months at a time. It was the only way to
> keep them
> > stable enough to make me happy :)
> >
> > Bob
> >
> >> On Jul 30, 2026, at 12:59 PM, SCOTT Barry <n4buq at knology.net> wrote:
> >>
> >> Since I'm doing this with the PTO removed from the radio and the ovens
> off, I
> >> presume this is a product of just the 5749 warming up. Is that
> correct?  If so,
> >> could a solid-state replacement for that tube work here?  I know it's a
> remote
> >> cut-off, etc., but in an oscillator circuit, would that matter that
> much?
> >>
> >> I'd be hesitant to leave the radio on unattended so having an extended
> warmup
> >> like that is pretty much out of the question for me.
> >>
> >> Just thinking out loud...
> >>
> >> Thanks,
> >> Barry - N4BUQ
> >>> Hi
> >>>
> >>> Best to simply let the radio warm up for a while. The ovens will heat
> things in
> >>> a way that is not “same / same” as normal warmup. Unless things are
> really
> >>> cold, you probably will not save a lot of time.
> >>>
> >>> How long does it take for a radio to stabilize? That depends a lot on
> how picky
> >>> you are and the exact radio you happen to have. Picky folks just might
> turn it
> >>> on a few hours before they need to use it. Very picky folks might turn
> it on
> >>> the day before :) :)
> >>>
> >>> If the radio has been in storage, humidity will have had an impact. It
> will
> >>> drift longer than a radio that is used on a regular basis. Store it in
> a damp
> >>> barn for years and who knows how long you should run it to get it
> dried out. On
> >>> some stuff the answer is measured in weeks. I have gear here that the
> answer is
> >>> measured in months.
> >>>
> >>> Bob
> >>>
> >>>> On Jul 30, 2026, at 12:30 PM, SCOTT Barry via R-390 <
> r-390 at mailman.qth.net>
> >>>> wrote:
> >>>>
> >>>> While I know the manual states that the receiver should be allowed to
> warm up
> >>>> (30 minutes?), I wonder how long it should take for the PTO to
> stabilize.  I
> >>>> have the PTO out and monitoring the frequency from cold and it takes
> quite a
> >>>> while to stabilize - maybe 15 or 20 minutes.
> >>>>
> >>>> It climbs until it reaches a certain frequency and then starts very
> slowly up
> >>>> and down a few hundred Hz before settling down to a reasonably stable
> >>>> frequency.  Before I took it out, I was seeing end-to-end about
> 1,000,001.5 kc
> >>>> so I thought it may need a touch-up on that; however, I don't think I
> had let
> >>>> it warm up sufficiently because if I let it warm up a good while, it
> is much
> >>>> closer to 1,000,000kc end-to-end (approximately 1,000,000.300kc),
> hence my
> >>>> wondering about warmup time.
> >>>>
> >>>> Any thoughts on this?  Would running the oven for about 10 minutes
> and then
> >>>> turning it off help stabilize it faster?
> >>>>
> >>>> 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