Richard -
Yes, I'm familiar with COG type caps. I had a hard time coming up with a 92 pF Dur-Mica! I have
some envelopes full of COG caps, Erie Red Caps if I remember right, but I can't find them. The DM
was the closest I could come up with 'near' 0 t/c.....
I'm not sure how much better the stability 'could' be. ~100 Hz for a variable oscillator sitting
out in the open with an A/C outlet blowing on it is no doubt a bigger delta t than it will see in a
nice warm radio, bolted to a pretty fair heatsink (steel chassis) with considerable inertia.
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>
Richard Knoppow wrote:
----- 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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