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 <k8yumdoo...@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 <paulsw...@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 <e...@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 >>> >>> _______________________________________________ >>> time-nuts mailing list -- time-nuts@lists.febo.com >>> To unsubscribe, go to >>> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com >>> and follow the instructions there. >>> >> _______________________________________________ >> time-nuts mailing list -- time-nuts@lists.febo.com >> To unsubscribe, go to >> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com >> and follow the instructions there. >> > _______________________________________________ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.