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

[Bob Camp]

Hi

Another approach:

Never *assume* a ceramic is an NPO unless it is labeled as such. Just *why* folks 
would inventory (and then sell off as surplus) non-NPO 100 pf caps … who knows?

My bet: Somebody who didn’t know better found that the X7R (or whatever) parts
cost 1/3 as much as the NPO’s. They then switched all the purchase orders over to
X7R. A month or two later, you have chaos in the factory. A few weeks after that a 
whole bunch of X7R’s go on the surplus market *really* cheap. Yes, I have first hand 
experience with that happening.

Indeed the same thing applies to parts in bypass applications. Various dielectrics 
have *big* differences in a number of parameters. A Z5U or Y5V put into a location
that had an X7R just might be a bit “close to the edge” when it gets nice and hot
in the radio. The gotcha there is that the part you just pulled out probably isn’t very
clearly marked in terms of “what was it”. Simple answer: go with an X7R (if it will fit).

Bob

> On Apr 30, 2024, at 11:38 PM, Jacques Fortin <jacques.f at videotron.ca> wrote:
> 
> Hello Dave,
> 
> Thank you for all those measurement results.
> For the ceramic caps, I already mentioned that some ceramic caps (especially
> the NPO ones) can be as good as the SM ones, but some made of different
> ceramic grades are really worse, to the point that a given one prevented the
> 17MHz xtal oscillator stage to drive the R-390A first mixer with sufficient
> amplitude.
> And this was the starting point for this topic.
> I believe that Larry recommendation is: use a SM replacement cap where mica
> ones are used in the R-390A, unless you can measure that a ceramic
> replacement is good enough for the task.
> 
> 73, Jacques, VE2JFE in Montreal
> 
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