[time-nuts] HP 5065 Rb update

[Bob kb8tq]

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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