Help needed to understand this VU

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matta

Well-known member
Joined
Jan 11, 2005
Messages
1,640
Location
Cape Town, South Africa
Well...

I got this A*RT P*RO V*LA from a friend to look at and try save it from the
Dumpster (Sound familiar Pucho?), opened it up and it looked like a WAR
ZONE.

I have counted no less than 18 blown caps leaking goo all over the PCB,
every cap in the PSU section and the caps near the Tube heater...

I?m not even going to try and fix this as even if I recap who knows what
damaged happened in the rest of the circuit.

What I am wanting to do is salvage the push button switches, pots, VU?s and
use the case for another project, waste not, want not as my Mom used to say.

My only concern is although the VU meter says VU on the scale (so must be a
true VU right, LOL) it might actually not be. Looking at the schematic could
you give me your views on whether or not you think it is a TRUE VU of a DC
?mimicking? a VU.

Here you go:

provu.gif


Looking at it I don?t think it is because it uses a dual opamp and I can?t
help but think half of it is used to mimic a ?AC Rectifier? creating a Pseudo
VU meter, but I am a complete Newbie, so could have it COMPLETELY wrong.

The Meter itself has no markings what so ever. I?ve attached it to my G1176
and in GR mode the Meter pegs to the right (+3) with no audio in at all, and
trimming makes no difference on the meter though the G1176 unit seems to work
fine, voltages are correct, no hums, passes audio, and compresses fine,
which makes me think the meter is not a TRUE VU or something else is wrong..

I saw Jakob post this:

..feed it 775mVAC through a 3.6K resistor, and see if the needle sets around
-4dB. If so, it's right. If not, check with recorder schematics to try to
find out meter sensitivity and if they have the needed internal rectifiers..

Jakob E.

Before I desolder and try, can you guess

Thanks in advance

Matt
 
Thanks Jakob!

I meant Rectifier, not Bridge, so my assumption was correct. It is catering
for this onboard. I also highly doubt there is an rectification in the
Meter, it looks like a standard moving coil, with little room for anything
but the coil in the tiny casing.

It would probably explain the meter peaking/pegging then as well, being a DC not AC
meter?

Thanks

Matt
 
[quote author="matta"]
My only concern is although the VU meter says VU on the scale (so must be a
true VU right, LOL) it might actually not be. Looking at the schematic could
you give me your views on whether or not you think it is a TRUE VU of a DC
?mimicking? a VU.[/quote]

It is the true VU meter.

http://sound.westhost.com/project55.htm
and
datasheet for LM3915

Regards,
Milan
 
Mo,

I think with that circuit it acts like a VU, but the meter itself is NOT a
VU meter.

I don't think I can use it straight in the G1176 as I think the G1176 was
designed to work with a VU meter that contains an AC rectifier circuit, like
the Sifam AL29WF.

Am I correct? This Meter seems to have no internal rectification so I'd have
to build an external rectifier to use it in the G1176, correct?

Whereas the Sifam or say even a Modutec VU meter would have internal
Rectification and is ?plug and play?, correct?

Thanks

Matt
 
[quote author="matta"]
I think with that circuit it acts like a VU, but the meter itself is NOT a
VU meter. [/quote]
Hi Matt,
You are right.
I don't think I can use it straight in the G1176 as I think the G1176 was designed to work with a VU meter that contains an AC rectifier circuit, like the Sifam AL29WF.
Am I correct?
Yes.
This Meter seems to have no internal rectification so I'd have
to build an external rectifier to use it in the G1176, correct?
Yes, but better solution IMO is using the circuit you posted (rectifier and active time balistic) because your meter doesn't have an adequate mechanical balistic which the Sifam and the others have. Also, then you can easily add the peak reading which is sometimes very useful.
Whereas the Sifam or say even a Modutec VU meter would have internal Rectification and is ?plug and play?, correct?
Yes.

Regards,
Milan
 
Yes, but better solution IMO is using the circuit you posted (rectifier and active time balistic) because your meter doesn't have an adequate mechanical balistic which the Sifam and the others have. Also, then you can easily add the peak reading which is sometimes very useful.

Hey Mo,

Thanks for the confirming my thoughts. I think for my G1176 I'll use the Sifam, since I think it is plug an play and is quality meter.

I may just keep these around for other projects.I just wonder what kind of DC ampere meter it is? You got to love how vague chinese components are.

Cheers

Matt
 
> I just wonder what kind of DC ampere meter it is?

Put a 9V battery and a 100K resistor in series with the meter. See where the needle goes to. Change the resistor for a nice half-scale or full-scale or "0VU" indication. Do some math.
 
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