[R-390] 390A Line Level meter replacement..?

[Bob Weiss]

I have never taken apart an original R-390 meter due to the radium content,
but have disassembled a few DeJur VU meters of similar age and identical
packaging, in order to rescale them for use in 390s. They do indeed contain
a small copper oxide bridge rectifier, as well as a resistor.

73,
Bob Weiss N2IXK

On Sat, Oct 23, 2021 at 3:08 PM Jacques Fortin <jacques.f at videotron.ca>
wrote:

> Hi Jordan,
> To answer your questions...
> I believe that the original R-390(A) Line Level meters uses a copper oxide
> bridge rectifier mounted inside the meter.
> If not copper oxide, it have to be germanium point of contact diodes of the
> 1N34 type, or some variant of.
> From the experiments I have done with the three original Line Level meters
> I
> have in my sets, they behave as a 3920 ohms resistor at 1 kHz.
> With a 3570 ohms resistor wired in series, (the R110 in the R-390A
> schematic) they indicate 0 VU for 1.85Vpk @ 1kHz and goes full-scale at
> 2.6Vpk @ 1kHz when driven from a 50 ohms output impedance sinewave signal
> generator.
> The Audio Cyclopedia indicates that a standardised VU meter should present
> a
> load of 7500 ohms to a 600 ohms line and indicate 0 VU for 1.228V rms
> applied (+ 4 dBm)
> With the meters of the R-390s, we are not that far... 3920 + 3570 = 7490
> ohms, ~1.3 Vrms @1kHz.
> We can also estimate that the internal meter movement BEHAVES AS a ~300µA
> DC
> full-scale one.
> Let's remark here that any attempt to adapt a base meter having less
> sensitivity (taking more than 300µA for full-scale deflection) is planned
> for failure if no supplementary amplification/adaptation is used...
>
> I used a 1mA fs meter for the Line Level in my '67 EAC with an adaptation
> circuit.
> I will send you a copy.
>
> 73, Jacques, VE2JFE
>
> -----Message d'origine-----
> De : r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> De la
> part de Jordan Arndt
> Envoyé : 23 octobre 2021 09:39
> À : R-390 Forum <r-390 at mailman.qth.net>
> Objet : [R-390] 390A Line Level meter replacement..?
>
> Hi group....
>
> The 1960 S-W 390A I'm working on has no Line Level meter and I'm looking at
> re-working a couple of the correct size and style of meters that I have on
> hand to install as such.
>
> Has anyone determined whether the original meter had copper oxide
> rectifiers
> inside the meter..?
>
> Was it a single diode, a full-wave or a bridge rectifier...?  I've gone
> through the archives and haven't found a definitive answer to that
> question.
>
> One of the 390As I have has a substitute Line Level meter and a diode
> bridge
> external to the meter which works well enough for my purposes so I'd be
> happy to copy that circuit until I can find an original Line Level meter.
>
> I've already installed a replacement Carrier meter with an LM741 op-amp
> ckt.
>
> driving a substitute meter which works very well.
>
> Any info will be greatly appreciated...
>
> 73...Jordan VE6ZT
>
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