Moving coil meter (not true VU) and the 1176

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OneRoomStudios

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I posted this to the 1176 Rev D thread, but it's really a question about meters, so it might be more appropriate here.

I have an old meter I'm trying to use in an 1176 build. It's not a VU meter, and it doesn't react to AC. I built a (germanium) rectifier for it, added a 3K6 resistor, and it now reacts to AC, and even lands pretty darn close to 0 VU with 1.223VAC on the output. The trouble is that it's not showing GR properly. When in GR mode, it sits at -inf and doesn't move. Based on my measurements, it seems to be a 400uA meter - I measure 300 ohms resistance across the coil, and it hits 0 with around 120mV DC across it. When set to GR mode, I can measure up to 300mV going to the meter switch, but the meter shows nothing. I confirmed the issue isn't the switch (I measure continuity between poles when switched).

I'm at a loss now. The only thing I can think of is that the meter is loading down the meter driver circuit too much. Is that possible? How much DC current does a real VU meter need to hit 0 (I'm guessing 100uA)? Any ideas of what I should try next?
 

JohnRoberts

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It took me about 30 seconds to find this...
wiki said:
The original VU meter is a passive electromechanical device, namely a 200 µA DC d'Arsonval movement ammeter fed from a full-wave copper-oxide rectifier mounted within the meter case. The mass of the needle causes a relatively slow response, which in effect integrates or smooths the signal, with a rise time of 300 ms.

JR
 

Winston OBoogie

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Let's give a big, warm round of applause folks, he'll be here all week ladles and gentlespoons...  I give you...  J.R. !   

On track, fiddle with it (technical term) until it behaves.
 

radardoug

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OK, so its half as sensitive as the real thing. So you might have to fiddle an awful lot. And then buy the right meter.
Its not like they are hard to find.
 

Bo Deadly

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Remove the meter and test it separately to determine it's specific operating parameters. Meaning just put it in series with a resistor and a voltage and figure out what sort of DC is required for it to reach "0". Then add the rectifier and test it again but adjust the resistor to account for the 2 diode drops.

IOW, like other folks have said, "fiddle" with it.

And if you're tired, don't work on it.
 

PRR

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> not showing GR properly. When in GR mode, it sits at -inf and doesn't move.

You established that it displays 120mV DC. What is coming OUT of the limiter on the GR pins?

Many classic limiters output a high value (a part-volt) at idle and go down to near-zero in heavy limiting. Does yours?
 

OneRoomStudios

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Brian Roth said:
285 mV isn't enough to overcome the forward voltage drop of the rectifiers.

Bri

You're absolutely right. I misspoke/wrote in my last reply. When in Output/+4 mode, I measure 9.45V DC at the meter switch coming from the GR driver circuit - more than enough to drive the meter (I have the "0 set" turned all the way up).  When I switch into GR mode, it drops to 285mV DC. It's as if the meter is loading down the driver circuit. I think the voltage drop of the diodes is the only reason it's not going to 0V.
 
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