[R-390] R-390 Q of Caps and tuned circuits

[Jim Whartenby]

I would add that one should pay attention to the ceramic capacitor dielectric used in the circuit.  Some are very temperature sensitive and are a source of noise and microphonics.  See:https://www.electronics-notes.com/articles/electronic_components/capacitors/ceramic-dielectric-types-c0g-x7r-z5u-y5v.php

Also some tuned circuits use negative temperature coefficient capacitors to compensate for changes that occur over the operational temperature range.Regards,Jim
Logic: Method used to arrive at the wrong conclusion, with confidence.  Murphy 

    On Saturday, May 4, 2024 at 10:56:54 AM CDT, David Wise <d44617665 at hotmail.com> wrote:  
 
 > Any suggestions?

Nothing revolutionary, just replace paper with plastic, ceramic with ceramic, mica with mica.  Especially in the latter case use brand-new, not old or even NOS unless tested good in every possible way.

Dave Wise

PS - Any message I send that contains Larry H's email address - even in the message body - gets rejected as spam.
________________________________
From: r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> on behalf of Larry H <larry++41gm++ at ++gmail.com>
Sent: Saturday, May 4, 2024 4:57 AM
To: R-390 Forum <r-390 at mailman.qth.net>
Subject: Re: [R-390] R-390 Q of Caps and tuned circuits

Wow, this is a lot of great posts about the Q of caps.  And it is all very
good to know.  But, my original hope in posting on this subject was to try
to find a simple answer that would work all of the time to prevent using
the wrong cap in critical  circuits.  It looks like it isn't going to be
easy.  Perhaps a list of caps that have a high enough Q is necessary as
a guideline.  As I noted previously, the MIL-C-5 spec requires the Q to be
above 1,333.  Is this a good starting point?  I have seen a lot of new
ceramic caps mentioned that have a high Q, but some of them are surface
mount only, no wires attached.

Dave,  The Q info I referred to came off of the manufactures spec
datasheets, I didn't do any measurements. Your post from Apr 30, 2024,
5:30 PM has a  lot of great info in it.

There are a lot of 390 users/owners worldwide, about 700 unique users per
month on our website last year (with June having 897).  I suspect that most
of them have faced or will face a bad SM cap in a high Q circuit, as SM
caps will continually go bad.  It would be nice to come up with an easy way
to determine if a cap is an acceptable substitution or not.  I hope
something easy is possible.  Any suggestions?

Regards, Larry
-------------------------------------------------------------------------------------------------------------------------------------------------------
On Wed, May 1, 2024 at 6:28 PM David Wise <d44617665 at hotmail.com> wrote:

> That 3kV cap is definitely Type II since its capacitance changes as DC
> bias is applied.  I didn't see before, it has the circled-2 shorthand mark
> for Sprague.
>
> Dave Wise
>
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