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

Wed Nov 3 09:10:56 EDT 2021

Hi again Jacques and the group...

The pair of 1-3/4" meters I have will not work as audio level meters without 
resorting to an op-amp circuit, and even then, I'd have little faith in 
their accuracy if I went that route.

So, thanks to Jacques info and after doing some additional reading on VU 
meters and metering and the extant circuitry in the 390A, I found and 
ordered a pair of actual VU meters which claim to act as 3900 ohm meters in 
a 7500 ohm tap across the 600 ohm line.

You can google TR-45 VU meters which are available on Amazon, which is where 
I ordered a pair. Since they were ordered from a supplier in China, I didn't 
want to send out this info until I saw some activity on the China Post 
tracking system, which occurred finally this morning.

The meters have a label attached to the meter housing showing their 
specifications for a VU circuit. Be aware that some versions of this meter 
type do not act as a 3900 ohm impedance, but rather a much lower, or in some 
cases, an unspecified value. The meters I ordered show the requirement for 
3600 ohm external circuitry to match the 7500 ohm total impedance required 
as per the 390A VU circuitry.

If you do look up these meters, you'll see that they will fit in the hole 
for the Line Level meter and the 4 panel mounting holes also match the 
original meter mounting holes.
They can also be back-lit, although I doubt I'll use that capability given 
the light off-white meter face and black pointer.

I'm looking forward to them arriving and checking out how accurate they 
are..!

In the meantime, I'm slowly working on the 1960 S-W 390A I'm resurrecting 
and there are a few mechanical parts I'll need along with a crystal or two 
for the 2nd oscillator.

More to follow....73...Jordan VE6ZT

----- Original Message ----- 
From: "Jacques Fortin" <jacques.f at videotron.ca>
To: "'Jordan Arndt'" <Outposter30 at shaw.ca>; "'R-390 Forum'" 
<r-390 at mailman.qth.net>
Sent: Saturday, October 23, 2021 1:08 PM
Subject: RE: [R-390] 390A Line Level meter replacement..?

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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