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

[Tony Casorso]

Thanks to Barry who reminded me to switch off the HTML, I can now post this 
correctly.

You all probably remember the thread I started about my problems with a 
strong local AM station (groan). I mentioned this problem in another forum 
and was chastised and told to "fix the radio". Well, I'm still not convinced 
entirely that my radio is OK and today I swept the front end of the R-390a 
and compared it to my R-392. I have a picture of the results to share:

http://i25.photobucket.com/albums/c90/tonysradios/IMG_0856.jpg

The sweeps of the two radios are shown on the same display using the memory 
feature of the spectrum analyzer. I am sweeping from 0 to 5MHz and both 
radios are tuned to 3.3MHz. The sharper response with the lower peak is the 
R-392. The wider, taller response is the R-390a. As I mentioned last time, I 
can't find anything wrong with the R-390a. The same sort of result is 
obtained on the 1-2MHz band. I am attributing the higher response of the 
R-390a to the turns ratio of the front end RF transformers.

In both radios 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. I am using the balanced input on the R-390a.

To get the picture I am using the sweep generator to sweep at about 1 or 2 
Hz sweep rate and the spectrum analyzer in peak-hold mode.

I have checked for a proper RF ground at the AGC feed point. All resistance 
checks look correct.

I just can't believe that this is the way it is supposed to look. If 
somebody else could duplicate the R-390a result (or refute it) I sure would 
appreciate it.

Thanks,

Tony