[R-390] Carrier Meter Linearization

[Dallas Lankford]

TM-11-856A, among others, shows (in Fig. 41) a linear (wrt dB) carrier meter response for the R-390A.  As we all know, of course, this is not the case.  Carrier meter readings are always 20% to 30% low (20 or 30 dB less than 100 dB when a precision signal generator is used to examine the acccuracy of a carrier meter in an R-390A through its 100 dB range).  The reasons for the inaccurate carrier meter readings are incorrect values for R524, the R523 + R537 assembly, and R548.  After removing R524 and R548 and bringing insulated wires out of the IF deck so that I could vary those values, I determined that (1) R537 (22 ohms nominal) should be removed entirely (to provide more range for meter zero adjustment), (2) R524 (680 ohms nominmal) should be replaced by 649 ohms 1% (since R524 is usually higher in value than it should be, you can usually obtain 649 ohms by adding an appropriate resistor in parallel), and (3) R548 (27 ohms nominal) should be replaced by 62 ohm 1%.  R523 should probably be replaced by a 10 turn 2 watt high quality 100 ohm wire wound variable resistor to make zero adjustment easier and to improve long term zero set stability (though "zero" will still drift around a bit).