[R-390] Re: R390 panel meter resistance

Wed, 07 May 2003 21:16:21 -0400

All: Very good point, but I'd add one other piece to this.  At the point
that you set the two meters in series for a full-scale reading on the R-390
meter and note the current on the other meter, instead of disconnecting and
finding a 10-turn pot, why not just read the voltage across the R-390 meter
at that point, and then apply Ohm's law to figure out the meter resistance:

R (meter resistance) = E (voltage across meter) / I (meter current)

Dave Maples

-----Original Message-----
From: [email protected] [mailto:[email protected]]On
Behalf Of W. Li
Sent: Tuesday, May 06, 2003 11:17 AM
To: [email protected]
Subject: [R-390] Re: R390 panel meter resistance

A few days ago, a msg appeared about measuring the internal resistance
of a R390 panel meter, employing the classical method of getting a
full-scale deflection using a 1.5 volt battery and a pot; and then
adding shunting resistors to get half-scale deflection, and concluding
that the shunt resistance reflected the internal resistance of the
meter. This method appears in all ham and radio manuals, and It shall be
referred to as the "traditional" method. The traditional method is
clearly described below, and is what we were all taught.

<snip>
>1) Arrange a source of voltage with a current limiting resistor to make
the meter read full scale.  1-1/2 volt penlight cell and a 1500 ohm
resistor,  for instance, would work for a 1 ma meter movement. If you
use variable resistors, make sure you don't over drive the meter.

>2) While the circuit is operating, put resistors in parallel with the
METER until it reads half scale.

>3) The value of the parallel resistance is equal to the internal
resistance of the meter.
<snip>

Now, if you think about it, this time-honored method results in false
results, because by adding the shunt resistor across the meter, you
actually LOWER the total resistance of this simple series circuit....
and thus INCREASE the total current flow, thus falsely altering the
deflection of your R390 meter. A fresh battery is really a constant
voltage device, and not a constant current device.

One workaround is to keep this series test circuit current constant.
This method involves a second mA meter. Insert any low mA meter (say a 2
mA one) into the circuit, then connect your R-390 panel meter IN SERIES
with it and your battery and pot. Now adjust the pot to get full
deflection of the R390 meter, and note the reading on your 2 mA meter.
Remove your R390 meter, and substitute a 10-turn precision 500 or 5000
ohm pot, and adjust it alone to get the ORIGINAL READING on your 2 mA
meter. Now the resistance of the 10-turn pot is exactly the same
resistance as the internal resistance of the R390 meter, since under
this scheme the test circuit's current was kept constant.

So to get down to real-world data: (using my meters)

carrier meter = 14 ohms (traditional); 28 ohms (actual)
        1 mA full-scale

line meter = 2600 ohms (traditional); 3500 ohms (actual)
        250 uA full-scale and is an AC voltmeter in reality

I'd like to take credit, but this subject was already discussed in some
detail in the 73 Test Equipment Library volume 1 (1976) pp33-35.

Bottom-line: this is all academic if you are able to find original
meters for your unit, but is of some importance in the case you want to
adapt non-R390 meters.......

W. Li
Mercer Island, WA
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