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

[David Wise]

Might be a full-wave bridge (copper oxide or selenium stack), not just a single diode.

73,
Dave

-----Original Message-----
From: Barry [mailto:n4buq at knology.net] 
Sent: Monday, January 04, 2016 2:48 PM
To: David Wise
Cc: James Green; r-390 at mailman.qth.net
Subject: Re: [R-390] What is inside R-390 meters?

I didn't think of the possibility of a diode in the Line Level meter.  I guess I flunked the test...

73,
Barry - N4BUQ

----- Original Message -----
> From: "David Wise" <David_Wise at Phoenix.com>
> To: "James Green" <jagreen3 at sbcglobal.net>
> Cc: r-390 at mailman.qth.net
> Sent: Monday, January 4, 2016 4:42:58 PM
> Subject: Re: [R-390] What is inside R-390 meters?
> 
> 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
> 
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