[R-390] Tracking down low sensitivity 0 to 8 mhz problem

[Bob Camp]

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 <jacques.f at videotron.ca> 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
> 
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