[R-390] R-390 alignment questions

[K2CBY at aol.com]

You may be deceiving yourself with respect to the sensitivity measurement. 
 
You make no mention of terminating the 8640B signal generator. 
 
To get an accurate reading, the output attenuator of the 8640B (and any  
other signal generator) has to be terminated with a specific load  resistance -- 
in this case, 50 ohms. If it is terminated with a high impedance  -- i.e., the 
unbalanced input of the R-390A -- less current is going to be  pulled through 
the series element of the attenuator, and the "multiply by"  marked on the 
skirt of the output attenuator dial isn't going to be correct.  What appears to 
be 0.2 or 0.3 uV is actually going to be higher.
 
The BALANCED input is nominally rated at 125 ohms (though this is probably  
far from constant either from one band to another or across a signle band). A  
50-ohm signal generator connected to the balanced input should therefore  
terminated by an 82 ohm resistor (with short leads) connected across the antenna  
input terminals.
 
The impedance of the UNBALANCED input is not indicated in any of the  
documentation I have, but it is supposed to be "high." I would therefore  terminate 
the signal generator with a 51 ohm resistor so that it sees a proper  load.
 
Only when the signal generator is properly terminated do the output  signal 
readings shown by the panel meter and the switch  attenuator correspond with 
reality.
 
Miles, K2CBY