[R-390] Carrier Meter Linearization

[Cecil Acuff]

Good Stuff Dallas...

Thanks...

Cecil...
----- Original Message ----- 
From: "Dallas Lankford" <dallas at bayou.com>
To: <r-390 at mailman.qth.net>
Sent: Friday, September 24, 2004 7:10 AM
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 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).
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