[time-nuts] TAPR "PulsePuppy" Pot Selection

Mark Goldberg marklgoldberg at gmail.com
Sun Dec 24 18:05:42 EST 2017


Absolutely I see what you describe below. Bournes actually has an old paper
that describes all of these issues, but they do not seem to address them in
selection guides these days. I have no way to find out which are good until
I try 50 of them. I get a few that are just awful, basically a random
connection between the screw and the pot position.

www.bourns.com/docs/default-document-library/bourns_trimmer_primer.pdf

I have tried all kinds of tricks, going back and forth, sneaking up on it,
yes, tapping it, sweeping the wiper back and forth around the desired
setting to hopefully clean off crud from the element in that area, etc.
Some just don't want to be adjusted to a specific setting that just happens
to be on frequency. Most are fine. Some are a real pain to set where you
want.


Mark


On Sun, Dec 24, 2017 at 3:26 PM, Dana Whitlow <k8yumdoober at gmail.com> wrote:

> I think I need to clarify what I mean by "backlash".  It is not simple free
> play in
> the adjustment mechanism- it is something much more irritating, as follows:
>
> I sneak up on the desired result, but manage to overshoot slightly.  So I
> back
> off on the screw, and find that at first the result continues to change in
> the
> *original* direction (making the overshoot even worse) for a bit before
> finally
> reversing as I wanted it to.  This behavior is not conducive to having a
> good
> time making critical adjustments, nor does it lend any confidence in the
> stability
> of the adjustment in the face of handling.
>
> Dana
>
>
> On Sun, Dec 24, 2017 at 4:06 PM, Charles Steinmetz <csteinmetz at yandex.com>
> wrote:
>
> > John wrote:
> >
> > I didn't really notice much backlash, though when setting oscillators I
> >> try to approach (slowly) from one direction until it's "good enough" and
> >> then stop, to avoid that problem.
> >>
> >
> > The hot tip is not to just "sneak[] up on the sweet spot and then walk[]
> > away," as Dana put it.
> >
> > Anytime you have an adjustment with some hysteresis (classic example is
> > setting a d'Arsonville movement to zero), you want to sneak up to the
> > perfect setting and then run the adjuster *back* the way you came just a
> > touch, to leave the adjusted part on its own without any mechanical
> > connection to the adjustor mechanism.  Such contact is almost always the
> > culprit if the adjustment drifts after you set it.
> >
> > This takes some "feel" for the motion of the adjuster mechanism, but it
> is
> > well worth investing the time to learn it by repeated trials of the
> > adjuster before you leave it alone.
> >
> > Dana is spot on with his advice to tap the board (or whatever
> mechanically
> > supports the adjusted part) to make sure it doesn't drift.  If it does,
> you
> > either failed to pull the adjuster out of contact with the moving
> adjusting
> > part, or the adjusted part just can't hold its setting.  In either case,
> > better to know that now than after you button the instrument back up.
> >
> > Best regards,
> >
> > Charles
> >
> >
> >
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