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

[David Wise]

The Carrier meter's terminal resistance is about 18 ohms.  This is quite a bit lower than most 1mA meters.  I would expect the original meter to not have a shunt, and also use relatively heavy wire in its moving coil.  But shunt or no, any 18-ohm 1mA meter will drop into the circuit and work.  (There are at least two different circuit mods out there that will permit use of arbitrary ammeters in place of the original.)

I haven't looked at the schematic for a while, but I expect the Line meter to contain a rectifier, because I don't think there's one external to the meter.  It could also be a vane-type or dynamometer-type AC voltmeter, but I doubt it.

I have seen meter artwork scans, but I can't remember where.  Try www.r-390a.net, and this mailing list archive.

Dave Wise

-----Original Message-----
From: R-390 [mailto:r-390-bounces at mailman.qth.net] On Behalf Of Barry
Sent: Monday, January 04, 2016 11:35 AM
To: James Green
Cc: r-390 at mailman.qth.net
Subject: Re: [R-390] What is inside R-390 meters?

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