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

[Jacques Fortin]

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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