[time-nuts] HP 5065 Rb update

[paul swed]

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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