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

[David Wise]

In 2007, Gary Gitzen and I came up with a Carrier circuit mod that allows retrofit with most 1mA meters.  (It can also drive an 18-ohm unit should you find one later.)  I think it's in the mailing list archive and the Pearls, but it did not make it into Y2K-R3.  I have the write-up on my hard drive.  Hey Perrier, you up for Revision 3B?

73,
Dave

-----Original Message-----
From: R-390 [mailto:r-390-bounces at mailman.qth.net] On Behalf Of Roy Morgan
Sent: Monday, January 04, 2016 3:18 PM
To: 390 list
Subject: Re: [R-390] What is inside R-390 meters?


On Jan 4, 2016, at 5:47 PM, Barry <n4buq at knology.net> wrote:

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

No 'flunking" allowed.  Only to learn a bit and increase the stored lore!

Yes, the line level meter is a rectifier type meter containing, presumably, a copper oxide full wave bridge type rectifier that is common in audio line level meters.

> ----- Original Message -----
>> From: "David Wise" <David_Wise at Phoenix.com>
>> ...
>> The Carrier meter's terminal resistance is about 18 ohms.  This is quite a
>> bit lower than most 1mA meters.

Yes, and this is the challenge in trying to drop in any of the various same-size meters of different sort folks have come across and been hopeful about.  Almost certainly any meter from some other use will have higher internal resistance, despite how urgently we all need affordable replacement meters of the same size and shape.

Note1: there is a "relatively" simple method to measure a meter's internal resistance with out putting it in danger from a normal ohm meter.  (Establish a current source to produce full scale deflection from a high resistance, then parallel the meter with a variable resistor till it reads half scale, then remove and measure the paralleled resistor.)

Note2: "True" VU meters are much bigger, are made to rather exacting specifications established decades ago, and are seldom cheap.  There is a good discussion at:
www.aes.org/aeshc/pdf/mcknight_qa-on-the-svi-6.pdf 
which states that the volume indicator standard is continued in the 1990 IEC Standard 268-17.  This and similar standards are hard to find free on the web, because standards making organizations at least *hope* to recover development costs from sales of the standards.  (They don't really, but seem to be ever hopeful.)

>> I haven't looked at the schematic for a while, but I expect the Line meter to
>> contain a rectifier,

Indeed it does.

>> because I don't think there's one external to the
>> meter.

Right, there is not one.

>>  It could also be a vane-type or dynamometer-type AC voltmeter, but I
>> doubt it.

Right again they are neither of those types.  Most vane type and especially dynamometer AC meters are as big as two fists or bigger, and heavy.

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

I'm sure I have such artwork here, if anyone can't find it "out there" on the web.

Roy

Seasons Greetings to all!

Roy Morgan
k1lky68 at gmail.com
K1LKY Since 1958

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