[R-390] What is inside R-390 meters?

Barry n4buq at knology.net
Mon Jan 4 14:34:58 EST 2016


Hi Jim,

The wire-wound resistance in parallel with the meter movement is a shunt.  The meter is designed such that a certain amount of current through the movement causes a full-scale reading.  Without the shunt, something most likely much less than 100 mA would cause a full scale reading.  At 100 mA, the shunt carries part of that current and the movement the rest.  With a different shunt, the meter can be used to read a different full-scale value.

I don't know if original R390 meters have shunts (or series resistors).  Aside from a possible shunt (or series resistor), I don't think there was any other circuitry in the original meters.

Regards,
Barry - N4BUQ

----- Original Message -----
> From: "James Green" <jagreen3 at sbcglobal.net>
> To: r-390 at mailman.qth.net
> Sent: Monday, January 4, 2016 12:34:38 PM
> Subject: [R-390] What is inside R-390 meters?
> 
> I have a small collection of 100 milliammeters that have the same package as
> the R-390 meters. They are made by Simpson & look to be very high quality. I
> opened one up and removed the glass and dial. Inside is the movement with a
> wire-wound resistor (I assume) in parallel with the meter movement.
> 
> Question 1: What circuit(s) would be inside for the line level meter?
> Question 2: What circuit(s) would be inside for the "S: meter?
> Question 3: Is there some site I can download the artwork for silkscreening
> the dials?
> 
> This is also a test.
> 
> 73
> 
> Jim
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