[time-nuts] Replacement A9 boards for the HP 5065A

Wayne Holder wayne.holder at gmail.com
Mon Feb 26 17:36:00 EST 2018

>> The only thing I could provok, was that almost all capacitors I
>> tried were sensitive to touch.

Most ceramic caps are sensitive to "micro phonics" via the piezoelectric
effect, which can translate mechanical stress into electrical noise.



On Mon, Feb 26, 2018 at 2:05 PM, Poul-Henning Kamp <phk at phk.freebsd.dk>

> --------
> In message <CAD2JfAjxVwO3coVTivD-Yp=XZqd78WKBDwKD0KA9fc8dq6GeOQ at mail.
> gmail.com>, paul swed writes:
> >I reverse engineered the system and guess what cap was bad? The
> integrator.
> >So not being all that smart, I hooked 2 X 10 UF caps in series. Been
> >working like a champ for 18 years.
> That capacitor isn't nearly as important as people think.
> The input signal to the integrator is continuous and jump-free and
> the relevant timeconstant is sub-second.  Dielectric absorption
> doesn't matter when there are no voltage jumps.
> We all tune the EFC of the Xtal to set the meter to zero CONTROL
> voltage, which means there is no voltage for the capacitor to leak,
> so that doesn't matter either,
> If you pick a capacitor with a couple of hundred volts rating, its
> leakage current will be less than the air and the PCB near it anyway.
> When I experimented, I could hardly find *any* property that mattered
> for that capacitor, not even the exact capacitance, because the
> adjustment procedue handles that.
> The only thing I could provok, was that almost all capacitors I
> tried were sensitive to touch.
> I didn't establish if this was mechanical (and if so if it was the
> capacitor or something else on the board) or if it was thermal
> (capacitors have astounding tempcos).
> HP tied two O-rings around the capacitor they choose, I pressume
> that is a clue that they found something similar.
> The biggest issue is probably that most of the relevant capacitors
> are square blocks, like for instance TDK/KEMET B32774D8505K.
> TDK/KEMET C4GAJUD4500AA3J could be an option, but I suspect it
> is too big to fit in the existing PCB.
> Either way, cheap and plenty available.
> --
> Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
> phk at FreeBSD.ORG         | TCP/IP since RFC 956
> FreeBSD committer       | BSD since 4.3-tahoe
> Never attribute to malice what can adequately be explained by incompetence.
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