[R-390] R-390A dB meter as as "S" meter

[DJED1 at aol.com]

If you look at the original Military tech manual, it has a curve of meter  
reading vs signal level.  What the meter is supposed to read is dB relative  
to 1 microvolt, just as the meter on the SP-600 does.  This means that 0 dB 
 is 1 microvolt, and 40 dB is 100 microvolts. If you define 50 microvolts 
as  S9, then 34 dB will be S9.  This ignores the issues of a receiver with a  
non-50 ohm input impedance, but this error is small compared to the 
accuracy of  the R-390A meter circuit, and entirely negligible compared to the 
average ham  signal report.  
So for practical purposes, any reading approaching 40 can be considered  
over S9, and you can easily scale readings above S9.
Unfortunately, there's no way to adjust the meter reading.  I think  the 
original specs for the meter were that the reading be within +-10 dB.   And 
I'm sure it hasn't improved with age.
Ed