[R-390] Carrier Meter Response

[Perry Sandeen via R-390]

GM List,
 
While sorting through some various *A* notes I came across this
post from Dallas Lankford. I’m reposting it as it may be useful to some.
 
From Dallas Lankford  Fri Sep 24 08:10:23 2004
Subject: [R-390]
Carrier Meter Linearization
 
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 accuracy 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 nominal)
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).
 
Regards,
 
Perrier