[R-390] R-390a Front End Selectivity

[roy.morgan at nist.gov]

Quoting Tony Casorso <canthony15 at msn.com>:...
> You all probably remember the thread I started about my problems with a 
> strong local AM station (groan)....today I swept the front end of the R-390a 
> and compared it to my R-392. ... The sharper response with the lower peak is
the R-392. The wider, taller response is the R-390a.

Tony,

May I paraphrase?

I have two vehicles, and the intake ports are not the same at all.  One is a
tractor  and has an intake port about three inches in diameter. The other one
is a Ferrari with eight ports, each only an inch and quarter in diameter.

Why the difference?

Well... I suggest it might be good to try the two radios each as a whole.

>  As I mentioned last time, I
> can't find anything wrong with the R-390a.

There may be nothing at all wrong with the R-390A OR the R-392.

> ... I am attributing the higher response of the 
> R-390a to the turns ratio of the front end RF transformers.

A number of factors affect the frequency response of the first stage in a radio:
 among them would be: The Q of all the tuned circuits, the coupling of the
input circuit to the secondary, The load on the secondary, the value of the
driving impedance (antenna or signal generator).
 
> I have checked for a proper RF ground at the AGC feed point. 

Its not clear to me what you mean here: The AGC feed point might have very
little to do with the RF ground used for RF input, or for the detector (the
scope probe in your case.)  when you say "...I am monitoring the grid of the
first RF stage with the power off using a 10x scope probe. The input signal is
about 160mv peak to peak for this picture."  Where is the signal taken from
thats shown in the vertical deflection of your picture?  Is that from the 1st
grid?  (I suspect so because with power off, you won't get any amplification
from the tube.)

> I just can't believe that this is the way it is supposed to look. 

I suggest you test the whole receiver for band pass/ out of band signal
rejection and the like.

Roy


Roy Morgan
13033 Downey Mill Road
Lovettsville VA 20180