How do I get these old analog VU meters working?

Thread Starter

Bryant Arnett

Joined Jun 27, 2017
3
About 20 years ago I found these old VU meters in a dumpster. They have served as a rack space filler all these years (made my audio rack look pretty cool.) I have finally decided to try and get them working. I am hoping someone can offer some advice.

It seems obvious where the audio inputs (J3) and power (J4) are attached. However, my question is: What sort of power and what sort of audio signal do I apply?

Also, there only seems to be one adjustment possible. Any idea what that pot (P1) controls?

Thanks!!

Bryant
 

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MrChips

Joined Oct 2, 2009
19,269
Power connection on J4

COM (GND)
-V
+V
NC

For power, try -10V GND +10V for starters.
(What are the markings on those big red tantalum capacitors?)

Input on J3.
Start with something low, 10mV to 100mV audio signal.

P1 is for calibration.
 

Thread Starter

Bryant Arnett

Joined Jun 27, 2017
3
Thanks, MrChips!

I'm not too good when it comes to power. Any advice on how to easily come up with a power source? I hope to split these meters out into portable stereo boxes which could be easily plugged in.

Attached is a picture of the caps.
 

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MrChips

Joined Oct 2, 2009
19,269
The first think I would do is disconnect the circuit board and test the bare meter by itself.
What is the current or voltage to give full-scale deflection?
Is it an AC or DC meter?

I would have expected a VU-meter to already be sensitive to audio AC showing a log relationship on the meter.
Maybe this is just a standard DC meter movement and the add-on board is to amplify and convert audio to log scale.
Maybe the meter scale was just added on for this purpose.

Meter label says 1.734V, rectified AC?
EXT 3k6 Ohms. I don't know what this means.

If you have a multimeter with a resistance range you can use this to test the movement or us an AA or AAA battery. Putting a 1k-10kΩ resistor in series with the meter would give it over-current protection while doing initial testing.
 

AnalogKid

Joined Aug 1, 2013
8,100
Back in another life I helped built a TV station and designed/built/rebuilt several remote trucks.

A VU meter has a defined set of ballistic parameters. That is, the amount of needle overshoot that happens at a sharp transient increase in volume is standardized so all VU meters show the same effective volume level. The ballistics and damping are calibrated when the meter coil is loaded with a specific non-zero impedance. My guess is that it is 3.6K ohms for this meter, so you want the coil to see 3.6K ohms as the source impedance no matter what the input signal level is. A common way to do this is a variable gain opamp stage followed by a 3.6K resistor in series with the meter.

Alternatively, 3.6K might be the impedance of the meter when it is applied to an external circuit. However, the 3.6K loading impedance rings a faint bell...

I don't see any diodes on the on board, so the meter must have it built in. This means it is a true VU meter, not just a meter with a VU scale.

1.734 V = 1.228 V (0 dBu) x 1.414 (sqrt 2). It is the peak value of an input sine wave that will produce an indication of 0 dB.

https://en.wikipedia.org/wiki/VU_meter

ak
 
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Thread Starter

Bryant Arnett

Joined Jun 27, 2017
3
Thanks, AK. I am fairly certain these meters were being used as standard audio VU meters since the dumpster was behind an audio post production studio in Burbank. I'm an audio mixer and I love using actual analog VU meters. I have a pair I use now, but I am hoping to get these old ones up and running as a portable back up. I have a feeling they will work if I just plug in some audio and power them up. It seems obvious where I plug in the audio, but it's the power to the circuit board I am confused about. Thanks, again!
 

ebeowulf17

Joined Aug 12, 2014
2,940
Looks like the op amp is this, or an old version of it, anyway:

http://www.ti.com/product/TL072

Assuming the specs haven't changed, 7-36V total supply voltage (+/-3.5 to +/-18, assuming dual supply.) To my untrained eye, that looks like the only part that would be picky about supply voltage, but don't take my word for it - I still have a lot to learn here!
 

panic mode

Joined Oct 10, 2011
1,781
i would check voltage rating of those capacitors as additional insight... circuit is simple, if you post clear picture of front and back side of the PCB we can reverse-engineer it.

edit, just saw the other picture with 35V rating of capacitors. circuit probably operated from no more than +/- 12V
 
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