[time-nuts] HP 5065 Rb update

[Dana Whitlow]

Rheostats are not necessarily stable because the current is 1st-order
dependent
on the contact resistance.

I had missed the part about the nearby lightning strike.  Could it be that
the mu-metal
shielding got partly magnetized by the high current pulse of the
lightning?  If so, there
ought to be some stability benefit from performing a degaussing operation.
If not
degaussed, the magnetization of the mu-metal will likely change over time
due to
physical knocks, changes in orientation of the Rb in the Earth's field,
temperature
changes, etc, all of which will make the Rb less stable than it could be.

Dana


On Mon, Dec 7, 2020 at 6:18 PM Bob kb8tq <kb8tq at n1k.org> wrote:

> Hi
>
> The existing c field winding is *inside* the magnetic shield on the
> physics package.
> Simply getting a second winding inside that package involves some (scary
> to most
> of us) tearing apart (an re-assembly) of the physics package.
>
> Next up, the mag field needs to be reasonably uniform. That means that the
> “new”
> winding needs to overlap pretty much all of the “old” winding. Again a bit
> of a chore.
>
> Finally, that “crude rheostat” circuit is pretty stable. The current needs
> to be stable into
> the << 100 ppm sort of range to keep the device happy. Ideally something
> that does
> a couple ppm ( 1 to 3ppm) would be the “best case” in terms of device
> stability.  Noise
> is (obviously) an issue as well.
>
> Since the sensitivity to mag field decreases with C Field current ( also
> with the assumption
> there is no stray field ….) there is a tradeoff between “tuning range” and
> required stability.
>
> One alternative is to brew up a fixed C field regulator board. Set the
> field to the lowest
> practical value using that board. Then tune the device by other means.
>
> Bob
>
> > On Dec 7, 2020, at 2:31 PM, Dana Whitlow <k8yumdoober at gmail.com> wrote:
> >
> > Paul,
> > Would it be feasible to add a smaller winding around the outside of
> > the existing C-field winding, and drive it from something other than
> > the crude rheostat circuit?  Then you could make finer adjustments
> > than the existing driver permits, and would rely only on the existing
> > driver to provide only a coarse tuning field.  Admittedly this would
> > still involve a "dangling pot", but now entirely on your own terms.
> >
> > Dana
> >
> >
> > On Mon, Dec 7, 2020 at 12:37 PM paul swed <paulswedb at gmail.com> wrote:
> >
> >> Ed
> >> Like you I don't like dangling pots either. But since I had the system
> open
> >> I took a look to see how hard that would be to accomplish. The pots on
> the
> >> digital counter and its actually quite small. The 3 leads all fit into
> an
> >> area about the size of a pencil eraser. To work on it requires removing
> the
> >> front panel. Decided to leave that alone. To risky.
> >> The systems all re-assembled stable and working about the way it always
> >> has. I did go with Bobs suggestion using a different synth count and the
> >> lower cfield current.
> >> Regards
> >> Paul.
> >> WB8TSL
> >>
> >> On Mon, Dec 7, 2020 at 1:12 PM ed breya <eb at telight.com> wrote:
> >>
> >>> OK, I see now what the circuit looks like, in the manual. The pot is
> >>> used in rheostat mode, so the specific value is needed, and it would
> >>> take some more modification of the circuit to increase the setting
> >>> resolution.
> >>>
> >>> I would recommend making a simple change to the existing pot hookup
> >>> though, which is to tie the "NC" end of the pot to the wiper - I have
> >>> this thing about not leaving dangling ends on pots used as variable
> >>> resistors. The most likely possible failure mode in a pot is an open or
> >>> intermittent wiper. In this circuit, if the wiper should open for some
> >>> reason, the circuit will go open-loop, and self-limit at around 13 mA
> >>> through the coil (or somewhat less if the coil's resistance is higher
> >>> than 700 ohms or so) - twice the normal maximum setting, or four times
> >>> the power dissipation. I don't know if this matters here, but for
> higher
> >>> reliability, tying that pot's loose end to the wiper will ensure that
> >>> this can't happen - the circuit will limit at the maximum of the normal
> >>> adjustment range.
> >>>
> >>> Ed
> >>>
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