----- Original Message -----
From: "Garey Barrell" <[email protected]>
To: "Steve Wedge" <[email protected]>
Cc: "drakelist" <[email protected]>
Sent: Tuesday, September 13, 2011 2:39 PM
Subject: Re: [Drakelist] Well, it looks like both PTO's DO
have issues.
Steve -
OK. I fashioned a 'sorta' fiducial out of a piece of wire
and an alligator clip to the gearbox. Allowing for
parallax, lighting, reflections, 'reading' glasses, and
just plain poor close-up vision, it looks to be within a
few hundred Hz at each 100 kHz marker, and about 1.5 kHz
from end to end. Well within original specs. Of course,
with no Rubidium standard I don't 'really' know where it
is, but I think it's 'CEAPP' or 'Close Enough for All
Practical Purposes'.!
I'm sure the tubular cap had some 'secret' specification
that we'll never know, but the little Dur-Mica seems to be
holding up just fine. Nothing else has been changed, and
aside from about a 115 Hz swing from A/C breeze soak to
room temp soak doesn't seem excessive to me. I suspect
nicely bolted down to a chassis inside a nice warm
receiver or transmitter it will be just fine.
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 2-C/2-NT, 4-A, 4-B, C-Line and
TR-4/C Service Supplement CDs
<www.k4oah.com>
I wonder if the NPO characteristic is the key. The TC
of silver-mica caps varies but is around +50PPM. This is
quite low but I think an NPO cap is still less. NP0 AKA C0G
ceramic caps have very good dielectric properties but
relatively low dielectric constant (K) which makes them
physically large. Now, large is a relative term, while a C0G
cap may be large compared to some other dielectric for
surface mount purposes its small compared to an equivalent
paper or plastic cap. I think the bad reputation that
ceramic caps have in some circles is due to a confusion of
the characteristics of high-K types with NP0/C0G caps. The
high-K kind do have problems with stability, voltage
coefficient of capacitance, etc., and are used because they
can be very small and are quite suitable for some uses. For
boatanchor stuff the C0G type is best if you can find the
right values.
Silvered mica caps have a well deserved reputation for
performance and reliability, modern ones probably do not
have some of the problems of older ones such as silver
migration. BTW, I found a paper on this topic in the Bell
System Technical Journal for sometime in the 1950! I will
have to find it again.
--
Richard Knoppow
Los Angeles
WB6KBL
[email protected]
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