Hi There is another issue with replacing a “resonant circuit” cap with just any random type of ceramic cap:
The types typically used for bypass (X5R, X7R, Z5U ….) don’t just have loss issues. They also change value with applied DC voltage. If the cap is right across a coil ( so no net DC ) this might not be a big deal. If the circuit is a bit more exotic and you have 150V DC on the cap, it is a big deal. The value of the part will be quite a ways away from that labeled value. In a bypass application, you look at the curves, work out the impact, and select a part that still has enough “C” to do the job. The part is much smaller and likely much cheaper than the alternatives. Away you go …. Bob > On Apr 5, 2024, at 2:50 PM, Jacques Fortin <[email protected]> wrote: > > Hi Jordan, > > For the ceramic caps: this is the grade of ceramic used within that counts. > As I wrote few days ago, if it is NPO grade ceramic, the capacitor is most > probably OK to use, despite that only a real-life test can confirm. > But, being just curious: can you send me a picture of the 100pF ceramic > capacitor that was installed at the C327 position ?? > > With some grades of ceramics, the dielectric losses (measured as the ESR) > are so high that they kill the Q factor of the resonant circuit, then the > gain of the oscillator stage. > And this one is running at 17MHz in the R-390A. > We know that silver mica caps are OK there, and probably polystyrene or > polypropylene ones. > But not Mylar (polyester) however. > The polyester loss factor at HF is about ten times higher than the mica or > polypropylene, so using those in a resonant circuit is excluded. > For the previously published advice you mention, about changing the mica > caps for ceramic ones, well, not sure that this was fully validated at the > time. > Like many other "recommended" modifications of the R-390A that works more or > less well... > > 73, Jacques, VE2JFE in Montreal > > ______________________________________________________________ > R-390 mailing list > Home: http://mailman.qth.net/mailman/listinfo/r-390 > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[email protected] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html ______________________________________________________________ R-390 mailing list Home: http://mailman.qth.net/mailman/listinfo/r-390 Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
