[R-390] R390A Carrier Level Meter Question

[Dennis Pharr]

Thanks Harry (and all the other respondents) for the advice.  Actually, I
was aware of the meter resistance issue.  I was never intending to try to
directly substitute another 1 ma. meter movement directly into the existing
R390A bridge circuitry.  I've found a couple of matching 5 ma. meter
movementments in my junkbox that I've decided to use.   I'm planning to use
an (dare I say this) external opamp DC amplifier along the lines of the Jan
Skirrow article "Adapting Surplus Meters for the R-390A" (link below):

http://skirrow.org/Boatanchors/TechTalk2.pdf

Calibrating the Line Level VU meter will be no problem.  Trying to calibrate
the Carrier Level meter is another issue entirely - I'm not even going to
try.  The plan is to install the opamp and meters, tune to a locally strong
broadcast band station and set the pointer on the Carrier Level meter at
about 75% full scale and call it done.

Thanks again to everyone for all the advice.  This email list is the best
resource I've ever run across on any subject in amateur radio or SWL.  The
wealth of knowledge here is simply amazing.

73
Dennis Pharr
WD5JWY



-----Original Message-----
From: r-390-bounces at mailman.qth.net [mailto:r-390-bounces at mailman.qth.net]
On Behalf Of DCrespy at aol.com
Sent: Monday, August 23, 2004 5:40 PM
To: r-390 at mailman.qth.net
Subject: Re: [R-390] R390A Carrier Level Meter Question

Dennis,  
 
I assumed you were asking to decide on a substitute?  If so: the  problem is

not the full scale current, it is the internal resistance of the  meter.  
 
Most 1ma movements are 100 ohms.  The R-390A needs (if I remember  right) 18

ohms.  I have successfully used up to 38 ohm movements  with completely 
comparable results.  100 ohm movements just do  not work.  If you are
thinking of 
shunting a meter, the problem is the  same.  Most 100 microamp meters are
1000 
ohms, so shunted, you still wind  up with a 100 ohm / 1 ma meter!  The 38
ohm 
meters I have used  (International brand) had a series internal resistor,
that 
when  removed dropped the meter resistance.
 
Good luck and have fun!  73
 
Harry  KG5LO
Saline MI 
(home of a dozen meters that fit, but don't)
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