[time-nuts] HP 5065 Rb update

[paul swed]

Unfortunately the cfield is deep in the tube. So that would require taking
the shields and such apart. The cfield isn't just a pot its actually tied
to a feedback loops and diff amp that changes the slope to match the tube.
There are quite a few ways to improve on it. Poul wrote up a whole page on
the subject. Its a good read and quite reasonable to do.
But I was simply trying to fix an odd issue that came up. That the Rb
jumped in offset. Never really explained why it happened except for the
lightning strike some 60-100 ft away. But though it damaged ground fault
breakers and networking it had no impact to the Cesiums at all.
It did damage other stuff but pretty minor.
Its down in the 5.X E-12 region "Gud enuf".
Regards
Paul.

On Mon, Dec 7, 2020 at 3:27 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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