[R-390] No receive below 8 Mc

[Roy Morgan]

Jim,


... I recently did a shotgun replacement of all primary caps in my SW 390a
> and noticed an enormous improvement on all bands.
>

It would be interesting to know what the enormous improvement was:  maybe
you did not have before and after sensitivity measurements, or numbers
about the noise floor.  Most of us don't do such measurements,  or not very
often if at all.

I wonder if by "primary caps, where the primary is in the plate circuit of
a stage" you mean bypass caps from the B+ side to ground, or if you mean
the cap meant to tune the coil, across the primary.  And, I wonder how many
caps were found INside the coil cans.

It would seem to me that a cap across a (slug-) tuned transformer would not
see much applied voltage - though there would be B+ from the whole cap to
 the surrounding vicinity - the case or support wires or the like.  BUT,
any leakage or reduced insulation resistance in the cap would behave as a
shunt resistance across the inductor - and this could affect the RF
performance of the circuit.

Note: suggestions of long ago to clip out *resistors* in parallel with
tuned circuits in the R-390/URR (and possibly in the R-390A) to "increase
the sensitivity" and make the receiver "a whole lot hotter" were very
misguided.  If this is found in any of our radios today, re-soldering or
replacing those swamping resistors is well advised. (This is not an implied
invitation to start a discussion about the inductance of spiral-cut metal
film resistors.  Thanks.)

I would expect that mica caps in a coupling mode: that is B+ on one side
and low (maybe AGC or ground) voltage on the other side might develop small
or even miniscule leakage with age, with accompanying NOISE.

My preferred source for mica caps is: justradios.com

-- 
Regards,
Roy

Roy Morgan
k1lky since 1958