What kind of internal wiring is this? (Vintage API VU Meters)

[huggybearbass]

I have a vintage VU meter I purchased recently that has peculiar internal wiring. All of the VU meters (I only buy vintage ones) I've purchased thus far have had the standard rectifier circuitry inside that allows me to connect the positive and negative sides of a balanced cable (ignoring ground) with the 3.9k ohm resistor on the positive side and I'm off to the races. [If something is wrong about that, PLEASE tell me. I'm here to learn.] With these, I have NO CLUE what is going on with the bridged resistors.

It would be incredible to learn about what the intended connection type is for how it's currently wired, as well as (if possible) how to rewire this to act as a proper VU meter.

 

[Brian Roth]

Shooting from the hip and late in the day for me, so no actual calcs....

Perhaps a mod to cause the meter to read 0VU with a +8 dBu signal and the typical 3K9 external series resistor??

Bri

 

[Brian Roth]

One "final" thought....

Feed that meter with the typical 3k9 series resistor from a low impedance (say, 50 Ohms or less output impedance) sine wave generator. Adjust the generator signal level at 1 kHz until the meter reads 0 VU. Measure the generator's actual output voltage with a meter ACCURATE at 1 kHz. (frequency response of cheap DVMs crap out above 400 Hz or so).

Bri

 

[Brian Roth]

https://groupdiy.com/threads/vu-meter-articles-docs.85007/
Bri

 

[radardoug]

Thats not a VU meter, and looks like it dosnt have a rectifier, so all bets are off

 

[Brian Roth]

Thats not a VU meter, and looks like it dosnt have a rectifier, so all bets are off

Wow...a great point....I wasn't looking for the rectifier (Me Bad!) unless it's not visible in the pix. If NOT a "real" VU, then it's merely a milliamp meter.

However, I've seen more than a few of API-branded (NOT the same co. as the desks, etc) VU meters....seemed to be acceptable "wiggle needle" units.

Bri

 

[MidnightArrakis]

I have a vintage VU meter I purchased recently that has peculiar internal wiring. All of the VU meters (I only buy vintage ones) I've purchased thus far have had the standard rectifier circuitry inside that allows me to connect the positive and negative sides of a balanced cable (ignoring ground) with the 3.9k ohm resistor on the positive side and I'm off to the races. [If something is wrong about that, PLEASE tell me. I'm here to learn.] With these, I have NO CLUE what is going on with the bridged resistors.

It would be incredible to learn about what the intended connection type is for how it's currently wired, as well as (if possible) how to rewire this to act as a proper VU meter.

I think where the scale of this meter indicates -- 133% -- on it, that it would be much better if instead it just read -- 11 -- and call it a day!!! Those who "know" would understand what that means!!! HA!!! (Spinal Tap).....

/

 

[RoadrunnerOZ]

It’s just a 400uA Full Scale Deflection DC panel meter - you would need to calculate what resistance to feed it with after rectification according to what you want 100% to be - taking into account the 8K shunt across the terminals - this value would depend on initial voltage for full scale, then the feed series resistor as part of a voltage divider network (taking into account the resistance of the meter coil as well). I would look at the schematic for whatever gear this meter came from and work backwards from there. There are VU meter calculators online as well. 0dB for +4 should be 1.228V RMS so if that’s what you’re measuring then you need to work out what voltage across the meter without the resistors tacked across will give a reading of 100% (0dB).
You could build this - better than a diode and resistors - point to note is to use germanium diodes for meter rectifier circuits as they have a much lower forward voltage. This circuit is for a 500uA meter but would probably work fine for your 400uA one

First up I’d have a read of this article:
https://michaelfidler.com/articles/practical-vu-meter-circuits/You can bypass a lot until you get to the bit where the above diagram comes from
ps you can probably lose the 2 x 4K resistor shunt.

 

[Hubbub]

I would not "lose the 2 x 4K resistor shunt" as that and the external cap determine the fall time of the meter needle, which is needed for proper pointer dynamics. The external series resistors add in to the fray to set the rise time.

 

[RoadrunnerOZ]

Good point. However they may be an after-market fit - not necessarily part of the meter as new, but the only way to find out is to look at the original schematics for the circuit it was used in. If you use a damping capacitor you normally set the shunt resistance to get the correct ballistics if required - depends as well on the resistance of the meter itself. The resistors may be a calibration for a specific circuit.