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

[Jacques Fortin]

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