[R-390] Line Level Meter

[Sheldon Daitch]

I can't speak for the R-390 meters, but most honest
VU meters use a bridge rectifier, not a single 
diode, for rectification.  In the Simpson and 
Weston meters, at least the larger ones, like on
audio consoles, the diodes are similar to
copper oxide disks, in a four pack stack, and
the diode case can be opened, allowing the diodes
to be replaced individually.   What normally 
happens is that the meter will read 1-2 dB below
full scale with the proper input.  

We were replacing meters like this on our console 
at Greenville VOA, and ran out of meters.  I 
found some old ones, with the same problem, too
low a meter reading, and pulled one apart for 
investigation.  I think I wound up fixing 3-4
meters out of about 6-8 "dead" ones.

The smaller R-390 meters are much too small for the
larger diode bridge, and since I have never worked
on one of these meters, I really can't talk about
their specifics.

Sheldon



Roy Morgan wrote:
> 
> At 06:21 PM 8/10/2004 -0400, Bob Camp wrote:
> >... More or less you have the mechanical meter movement and some wires. In
> >some cases you have a resistor or two in there.
> 
> In the case of the line level meter, there is a rectifier, too.
> 
> >The meter movement is easy to check. If it wiggles when you rock the meter
> >back and forth in your hand then the bearings and springs are ok. If it is
> >pegged hard one way or the other you may have a problem with a spring. If
> >it is sitting mid scale and not moving then it's come off of a bearing.
> >
> >If the movement is mechanically ok then the problem often is a broken wire
> >or a failed series resistor.  ... I suppose there are audio meters with
> >diodes in them as well. Again that should be an easy fix.
> 
> Exactly, there could be a failed rectifier.  Replacement with non-selenium
> diodes will likely cause increased deflection with the correct ac voltage
> for 0 VU.  An external resistor will cure this, unless you want to change
> any you find inside the meter.
> 
> Notes:
> 
> 1) The R-390 line level meters are AC meters. They indicate 0 VU for line
> level output at the rear terminals of the radio.
> 
> 2) The pictures of line level meters I have at hand do not show a zero on
> the dial markings.. only a "VU" at the place where 0 should be which is at
> the "100" of the 0-100 dial markings.
> 
> 3) The meter movement is a DC mechanism. The meter case contains a
> rectifier (and presumably a resistor or two to set the correct
> calibration.)  The meter has an impedance of 3900 ohms.  (That is *not* the
> dc resistance of either the meter movement or the complete meter with
> rectifier.)
> 
> 4) If your meter has luminescent markings,
> 
> DO NOT OPEN IT UP AND EAT THE INSIDES.
> 
> Roy
> 
> >The one you worry about is the meter armature winding burning up. If that
> >happens then you have a bit of a problem. Other than stuff that has been
> >really nuked (as in 10 amps does not go well through a 10 micro amp meter
> >... trust me on this one) the armature rarely is the point of failure.
> >
> >If you do open up the meter you may be overcome by an impulse to eat some
> >of the glow in the dark paint. Be sure to open the meter in the presence
> >of another adult incase this happens to you. Multiple reported incidences
> >of this behavior are the most likely driver in the military's long
> >standing policy of removing meters from equipment destined for
> >disposal.  Again, you have been warned ......
> >
> >
> >         Take Care!
> >
> >                 Bob Camp
> >                 KB8TQ
> >
> >
> >On Aug 10, 2004, at 8:39 AM, Larry and Jody Cogan wrote:
> >
> >>Yesterday was an unlucky day.....found a dead line level meter on a long
> >>inactive 390A.  Hoping today will be a lucky one and some lister will have a
> >>meter they are willing to part with.  I do expect that it wont be cheap.  If
> >>anyone can help, please reply off list.
> >>
> >>Thanks,
> >>Larry
> >>
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> 
> - Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
> 7130 Panorama Drive, Derwood MD 20855
> Home: 301-330-8828 Work: Voice: 301-975-3254,  Fax: 301-948-6213
> roy.morgan at nist.gov --
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