[time-nuts] HP 5065 Rb update

[Dana Whitlow]

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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