[R-390] Carrier Meter Response

[David Wise]

With all due respect to Dr. Lankford, it's the same old brute-force workaround, plus a few grace notes.

It drives me crazy to see a band-aid on top of a mistake.  Others may see it differently.

See my posts in the Pearls of Wisdom, panel_meters.pdf, from 2 Nov 2001 and 5 Nov 2001 (search for "TROUBLES ARE OVER"), and 10 Feb 2003.  A trivial wiring mod corrects the underlying design flaw, repurposing the existing R523 as a current divider.  Smooth, linear, full-turn adjustment range.  It's so good you'll fall to your knees and weep.  Well, maybe not, but you get the idea.

R537 controls meter sensitivity.  In some radios you will have to additionally touch R548 or R549.  These three are also the key to retrofitting a non-original meter movement, per Gary Gitzen's fine work starting in May 2006.

Sorry for the mangled ASCII art in my old posts - I have a sketch if there's a way to host it.  A write-up too.

Dave Wise

-----Original Message-----
From: R-390 [mailto:r-390-bounces at mailman.qth.net] On Behalf Of Perry Sandeen via R-390
Sent: Wednesday, August 27, 2014 11:51 PM
To: r-390 at mailman.qth.net
Subject: [R-390] Carrier Meter Response

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